HomeMy WebLinkAboutResolution 7478 hazard mitigationrA
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CITY OF ORONO
RESOLUTION OF THE CITY COUNCIL
No. 7478
A RESOLUTION ADOPTING NN PIN COUNTY 2024
ALL -HAZARD MITIGATION PLAN
WHEREAS, tile city of Orono has participated in the hazard mitigation planning process as
established under the Disaster Mitigation Act of 20004, and
WHEREAS, the Act establishes a framework for the development of a multijurisdictionai
County All Hazard Mitigation Plan (the Plan); and
WHEREAS, the Act as part of the planning process requires public involvement and local
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coordination among neighboring local units of government and businesses; and
WHEREAS, the Plan includes a risk assessment including past hazards, hazards that threaten
the County, an estimate of structures at risk, a general description of land uses and development
trends ; and
WHEREAS, the Plan includes a mitigation strategy including goals and objectives and an action
plan identifying specific mitigation projects and costs; and
WHEREAS, the Plan includes a maintenance or implementation process including plan updates,
integration of the plan into other planning documents and how Hennepin County will maintain
public participation and coordination; and
WHEREAS,tiie I ]an I as I een shared with the Minnesota Division of Homeland Security and
Emergency Management and the Federal Emergency Management Agency or review and
comment; and
WHEREAS, the Plan will make the county and participating jurisdictions eligible to receive
A hazard mitigation assistance grants; and
WHEREAS, this is a multi jurisdictional Plan and cities that participated in the planning process
may choose to also adopt the Plan..
NOW THEREFORE, B IT RESOLVED that the Orono City Council supports the hazard
mitigation planning effort and wishes to adopt the 2024 Hennepin County All -Hazard Mitigation
Plan.
Adopted by the City Council of Orono, Minnesota at a regular meeting held on May 28, 2024.
HENN EPIN COUNTY
2024 HENNEPIN COUNTY
MULTI -JURISDICTIONAL
HAZARD MITIGATION PLAN
VOLUME 2
Hazard Inventory
01 February 2024
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TABLE OF CONTENTS- VOLUME 2
TABLEOF CONTENTS.......................................................................................................................... 3
SECTION 1: HAZARD CATEGORIES AND INCLUSIONS...................................................................... 5
1.1. RISK ASSESSMENT PROCESS.........................................................................................................
5
1.2. FEMA RISK ASSESSMENT TOOL LIMITATIONS..............................................................................
5
1.3. JUSTIFICATION OF HAZARD INCLUSION.......................................................................................
6
SECTION 2: DISASTER DECLARATION HISTORY AND RECENT TRENDS ............................................... 9
2.1. DISASTER DECLARATION HISTORY................................................................................................9
SECTION 3: CLIMATE ADAPTATION CONSIDERATIONS....................................................................11
3.1. CLIMATE ADAPTATION...............................................................................................................11
3.2. HENNEPIN WEST MESONET........................................................................................................11
SECTION 4: COMPREHENSIVE NATURAL HAZARD ASSESSMENT PROFILES.....................................13
4.1. GEOLOGICAL HAZARDS...............................................................................................................13
4.1.1. LANDSLIDES.......................................................................................................................13
4.1.2. SINKHOLES ........................................................................................................................19
4.1.3. SOIL FROST........................................................................................................................
23
4.1.4. VOLCANIC ASH..................................................................................................................
29
4.2. HYDROLOGICAL HAZARDS..........................................................................................................
33
4.2.1. FLOODING, URBAN...........................................................................................................
33
4.2.2. FLOODING, RIVER..............................................................................................................39
4.3. METEOROLOGICAL HAZARDS.....................................................................................................45
4.3.1. CLIMATE CHANGE.............................................................................................................45
4.3.2. TORNADO..........................................................................................................................69
4.3.3. WINDS, EXTREME STRAIGHT-LINE....................................................................................81
4.3.4. HAI L...................................................................................................................................
95
4.3.5. LIGHTNING......................................................................................................................109
4.3.6. RAINFALL, EXTREME........................................................................................................117
4.3.7. HEAT, EXTREME..............................................................................................................131
4.3.8. DROUGHT........................................................................................................................143
4.3.9. DUST STORM...................................................................................................................153
4.3.10. COLD, EXTREME............................................................................................................159
4.3.11. WINTER STORM, BLIZZARD/EXTREME SNOWFALL......................................................169
4.3.12. WINDS, NON -CONVECTIVE HIGH.................................................................................185
4.3.13. ICE STORM.................................................................................................................... 205
SECTION 5: VULNERABILITY ASSESSMENT..................................................................................... 215
5.1. HAZARD RANKING MAPS..........................................................................................................215
SECTION 6: CULTURAL RESOURCE INVENTORY.............................................................................. 233
6.1. INVENTORIES............................................................................................................................. 233
6.2. NATIONAL REGISTER OF HISTORIC PLACES - HENNEPIN COUNTY ..........................................233
6.3. HENNEPIN COUNTY HISTORIC LANDMARK MAPS...................................................................241
SECTION 7: CRITICAL INFRASTRUCTURE KEY RESOURCES (CIKR)................................................... 249
7.1. CRITICAL FACILITIES INDEX........................................................................................................249
2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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5t fO'N I HAZARD CATEGORIES AND INCLUSIONS
1.1.1. Risk Assessment Process
Risk from natural hazards is a combination of hazard and vulnerability. The risk assessment process
measures the potential loss to a community, including loss of life, personal injury, property damage and
economic injury resulting from a hazard event. The risk assessment process allows a community to better
understand their potential risk and associated vulnerability to natural, intentional human -caused and
unintentional human -caused hazards. This information provides the framework for a community to
develop and prioritize mitigation strategies and plans to help reduce both the risk and vulnerability from
future hazard events.
This section describes the natural hazards that have had historical impact within Hennepin County and
assesses their associated risk with future impact. There are 19 hazards that have affected Hennepin
County and are identified and defined in terms of their range of magnitude, spectrum of consequences,
potential for cascading effects, geographic scope of hazard, historical occurrences, and likelihood of future
occurrences. There were no hazards eliminated in this revision TABLE 1.1A was created to meet FEMA
guidance.
TABLE 1.1A Bla
There were no hazards eliminated in this revision
In addition, a thorough geospatial risk analysis was conducted using locally available parcel data and
building values. Further, maps were provided where hazard boundaries and data existed. These
improvements help to provide a more accurate assessment of risk in the county to develop mitigation
actions.
1.1.2. FEMA Risk Assessment Tool Limitations
In 1997, FEMA developed the standardized Hazards U.S., or HAZUS model to estimate losses caused by
earthquakes and identify areas that face the highest risk and potential for loss. HAZUS was later expanded
into a multi -hazard methodology, HAZUS-MH, with new models for estimating potential losses from wind
(hurricanes) and flood (riverine and coastal) hazards.
HAZUS-MH is a Geographic Information System (GIS) based software program used to support risk
assessments, mitigation planning, and emergency planning and response. It provides a wide range of
inventory data, such as demographics, building stock, critical facility, transportation and utility lifeline,
and multiple models to estimate potential losses from natural disasters. The program maps and displays
hazard data and the results of damage and economic loss estimates for building and infrastructure.
However, due to the limitations of the software (only estimates losses for earthquakes, hurricanes, and
floods), Hennepin County did not use this software in 2018 or this new update in 2024. To estimate losses,
Hennepin County Emergency Management used the Hennepin County Critical Infrastructure and Facilities
Critical Facility Index (CFI) Priority Ranking Aid. This CFI was provided to municipalities, Hennepin County
Departments, and special jurisdictions to assist in identifying critical infrastructure and facilities in their
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community and estimate the potential losses. This CFI considers all hazards that were identified in the
Risk Assessment.
1.1.3. Justification of Hazard Inclusion
TABLE 1.3A provides the types of natural hazards that have been identified through analysis and
assessment.
TABLE 1.3A. Natural Hazards Bla
N tura
ype
Jusfiif ca i fi r lnciiu"sllc
Hfzd,,,
Geological
Landslide
Countywide vulnerable area, especially where
steep slopes are located, and heavy saturation
occurs.
Sink Hole
History of occurrences, poses danger to
population and property
Soil Frost
History of occurrences that have caused
infrastructure damage
Volcanic Ash
Historic volcanic eruptions (western states) have
spread ash into Hennepin County. Future
occurrences may also impact the county
Meteorological
Climate Change
There has been climate research done at the
international level through the Intergovernmental
Panel on Climate Change (IPCC) and local through
the Minnesota State Climatology Office.
Tornado
Hennepin County has a strong history of
tornadoes dating back to 1820. This hazard is a
consistent threat to both life safety and property
Winds, Extreme Straight -Line
Hennepin County has a strong history of derecho's
dating back to 1904. The Storm Prediction Center
(SPC) also highlights Minnesota as being highly
impacted by derecho activity during the summer
months.
Hail
Hailstorms occur during severe convective storms
and are an annual occurrence in Hennepin County.
Very large hail has been recorded back as far as
the National Weather Service has compiled data
(1950). These storms pose a significant threat to
people and infrastructure.
Lightning
Lightning is a regular occurrence and is associated
with thunderstorm activity. Hennepin County has
a history of lightning deaths as well as damage to
property and infrastructure
Rainfall, Extreme
Hennepin County has had a history of extreme
rainfall events, and the occurrences are becoming
much more frequent. The State Climatology
Office has published sixteen -year research
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documents on Minnesota flash floods caused by
extreme rainfall.
Heat, Extreme
Extreme heat is an annual occurrence in Hennepin
County and there have been several historic heat
waves that have caused both deaths and injuries
to our residents.
Drought
Several historic droughts have occurred across
Hennepin County dating back to 1863. These
events cause severe impacts on agriculture and
the economy as well as increasing wildfire
potential.
Dust Storm
Hennepin County has a history of dust storms
going back to the 1930's. These days' dust storms
are the cascading events of extreme drought.
Cold, Extreme
Extreme cold temperatures are an annual
occurrence in Hennepin County, with historic
outbreaks dating back to the 1800's. These events
pose significant threat to people and
infrastructure.
Winter Storm,
Hennepin County has a history of winter weather
Blizzard/Extreme Snowfall
dating back to the late 1800's. Varying degrees of
severity occur in Hennepin County due to the
different topography, with the worst conditions
occurring in western Hennepin County.
Winds, Non -Convective High
Although rare, extreme wind -producing non -
convective event may affect well over 100,000
square miles with wind damage, and may produce
extreme impacts over tens of thousands of square
miles
Ice Storm
Several ice storms have occurred in Hennepin
County dating back to the 1930's. These storms
have caused great impact to infrastructure and
people. The cascading effect of power outages is
another threat that has occurred with past ice
storms.
Hydrologic
Flooding, River
Several historic flood events have occurred due to
the Mississippi, Crow, and Minnesota River in
Hennepin County.
Flooding, Urban
Urban flooding is a consistent problem in
Hennepin County, due to torrential rainfall
associated with thunderstorm activity.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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DISASTER DECLARATION HISTORY AND RECENT TRENDS
2.1. Disaster Declaration History
One method to identify hazards based upon past occurrence is to look at what events triggered federal
and/or state Disaster Declarations in Hennepin County. Disaster Declarations are granted when the
severity and magnitude of the events impact surpass the ability of the local government to respond and
recover. Disaster assistance is supplemental and sequential. When the local government's capacity has
been surpassed, a state disaster declaration may be issued, allowing for the provision of state assistance.
If the disaster is severe enough that both the local and state government's capacity is exceeded, a Federal
Declaration may be issued, allowing for the provision of Federal disaster assistance.
It is important to note that the Federal government may issue a Disaster Declaration through the U.S.
Department of Agriculture (USDA) and/or the Small Business Administration (SBA), as well as through
FEMA. The quantity and types of damages are the determining factors. Listed below in TABLE 2.1A are
the previous Disaster Declarations that are of concern to Hennepin County. There have been six
presidential declarations since 2010.
TABLE 2.1A. FEMA Declared
Disasters (1965-2023)
Date
Disaster Type
Assistance
Disaster
.-
Number
April 7, 2020
Minnesota Covid-19 Pandemic
Individual/Public
DR-4531-MN
Assistance
March 13, 2020
Minnesota Covid-19
Public
EM-3453-MN
Assistance
November 2, 2016
Severe Storms and Flooding
Individual
DR-4290-MN
Assistance
July 21, 2014
Severe Storms, Straight Line Winds,
Public
DR- 4182-MN
Flooding, Landslides, and Mudslides
Assistance
July 25, 2013
Severe Storms, Straight Line Winds, and
Public
DR- 4131-MN
Flooding
Assistance
June 7, 2011
Severe Storms and Tornadoes
Public
DR- 1990-MN
Assistance
March 19, 2010
Flooding
Public
EM- 3310-MN
Assistance
August 21, 2007
1-35W Bridge Collapse
Public
EM-2378-MN
Assistance
September 13, 2005
Hurricane Katrina Evacuation
Public
EM- 3242-MN
Assistance
May 16, 2001
Flooding
Individual
DR- 1370-MN
Assistance
June 23, 1998
Severe Storms, Straight -Line Winds and
Public
DR- 1225-MN
Tornadoes
Assistance
August 25, 1997
Flooding
Individual/Public
DR1187-MN
Assistance
April 8, 1997
Severe Storms/Flooding
Individual/Public
DR- 1175-MN
Assistance
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August 6, 1987
Severe Storms, Tornadoes, Flooding
Individual/Public
DR- 797-MN
Assistance
July 8, 1978
Severe Storms, Tornadoes, Hail,
Individual/Public
DR- 560-MN
Flooding
Assistance
June 17, 1976
Drought
Public
EM-3013-MN
Assistance
April 18, 1969
Flooding
Individual/Public
DR- 255-MN
Assistance
April 11, 1965
Flooding
Individual/Public
DR-188-MN
Assistance
TABLE 2.1B. FEMA Declared Disasters (2019-2023)
Date Disaster Type
February 21, 2023 Severe Winter Storm
Declaration
Number
EO 23-02
April 12, 2021
Civil Unrest
EO 21-17
August 26, 2020
Civil Unrest
EO 20-87
May 28, 2020
Civil Unrest
EO 20-64
March 13, 2020
Pandemic
EO 20-01
April 11, 2019
Flooding
EO 19-30
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St to"'N �.... CLIMATE ADAPTATION CONSIDERATIONS
3.1.1. Climate Adaptation
Climate includes patterns of temperature, precipitation, humidity, wind, and seasons. Climate plays a
fundamental role in shaping natural ecosystems and the human economies and cultures that depend on
the. Climate adaptation refers to the ability of a system to adjust to climate change to moderate potential
damage, to take advantage of opportunities, or to cope with the consequences. The International Panel
on Climate Change (IPCC) defines adaptation as the "adjustment in natural or human systems to a new or
changing environment". Adaptation to climate change refers to adjustment in natural or human systems
in response to actual or expected climatic stimuli or their effects, which moderates harm or exploits
beneficial opportunities.
3.1.2. Hennepin West Mesonet (HWM)
In order to adapt to climate change, Hennepin County has built the Hennepin West Mesonet, a network
of remote sensors which provide highly accurate, near real-time measurements of weather, soil and water
conditions. Recent experiences across the Twin Cities metro area reveal a long-standing vulnerability to
dangerous weather or human -caused conditions that form very quickly without clear advance indications.
Fatal tornadoes in Rogers, MN (2006) and in North Minneapolis, MN (2011) both point to a need for more
complete and rapid surface observations from a network of sensors spread across the area. A fatal
landslide in Saint Paul, MN (2013) also shows that near real time soil temperature and saturation data
across the metro could be useful in providing alerts for evolving dangerous conditions. Other
vulnerabilities exist in our area to rapid -onset flash flooding, straight-line winds or hazardous materials
releases which require many sensors with quick detection capability to provide useful public warning or
evacuation decision -making.
The Hennepin West Mesonet delivers normal at different temporal resolutions, thus providing more
precise climate monitoring. Through climate monitoring, the HWM provides an essential service and
benefit of observing and precisely detecting impacts on the environment and ecosystems both at the
geospatial and temporal scale in Hennepin County. Archived data and current observations provide
consistent and high -quality information from decision -makers and researchers, information that can be
utilized for development of research and prediction models, improving understanding of climate
variability, advancing public climate education, and supporting development of mitigation and/or
adaptation measures for local communities.
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I St fO'N COMPREHENSIVE NATURAL HAZARD ASSESSMENTS
NATURAL HAZARD PROFILES
411;111 Hazard Assessment: LANDSLIDES
4.1.1.1. Definition. A landslide is the downward
movement of rock, soil, or other debris along a slope.
Other terms used for landslides are debris flow, earth
flow, mudslide, slump, slope failure, mass wasting, and
rock fall. The rate of landslide movement ranges from
sudden to very slow and may involve small amounts of
material up to very large amounts. The kinds of
movement include falling, sliding, and flowing. Material
can move as an intact mass or become significantly
deformed and unconsolidated. The slopes that have
landslides can range from near vertical to gently rolling
with slopes above 30% having the highest susceptibility.
4.1.1.2. Range of magnitude
Further work is needed among the Hennepin County landslide assessment team to develop range of
magnitude.
4.1.1.3. Spectrum of Consequences B211b
4.1.1.3.1. PRIMARY CONSEQUENCES:
4.1.1.3.1.1. Transportation: Mobility is frequently stopped or slowed by landslides.
When at the foot of slopes, roads and highways can be impacted by fallen rock, soil flows
and landslide debris. When routes are at the crest of slopes, surfaces may be undercut by
slides and fall away leaving voids and gaps in the road. Railroads are similarly impacted
by landslides. The practice of cut and fill in road and rail grade construction can increase
susceptibility to this problem. Besides direct damage to surface transportation routes,
secondary impacts can occur if vehicles carrying hazardous materials rupture if struck by
slides.
4.1.1.3.1.2. Electric utilities: Electric service lines often follow alongside roads, including
their routes through valleys and ravines or along the crests of slopes. This makes them
vulnerable to disruption from landslides. Cut power lines are a frequent feature of
landslide activity. Landsides impact both lines suspended from utility poles and buried
power lines.
4.1.1.3.1.3. Water, sanitary and storm sewer services: Cracked, broken or leaking water
or sewer lines often have a significant role in triggering landslides in susceptible areas.
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Inspections and maintenance of lines in vulnerable locations should be a priority to
reduce risk. Water and sewer lines are also vulnerable to damage and destruction by
landslide events.
4.1.1.3.1.4. Energy pipelines: Gas lines and other energy pipelines that pass -through
landslide susceptible areas may become weakened or severed by slide action. Damages
may be caused by direct physical impacts or by indirect transmission of stresses through
soil to the pipeline causing weaknesses or deformation of the lines.
4.1.1.3.1.5. Telecommunications: Telecommunications cables that pass -through
landslide susceptible areas may become weakened or severed by slide action. Damages
may be caused by direct physical impacts or by indirect transmission of stresses through
soil to the cable causing weaknesses or deformation of the lines. Fiber optic lines are
particularly susceptible to deformation which can cause erratic signals or total signal loss.
4.1.1.3.1.6. Structural damage: Landslides impacts to structures ranges from rapid
catastrophic destruction resulting from a landslide impact to gradual degradation of
structures from slow earth movements. Complex load factors act on structures that are
subject to landslide forces. Engineering assessment of compromised structures is vital to
both response and recovery phases of a landslide incident. Landslide impacts to
structures is both a life -safety hazard and can also be an occasion for costly property
damage.
4.1.1.3.1.7. Recreational impacts: Parks and trails are frequently placed in areas subject
to landslides. Often parks or trails are in scenic areas in ravines or valleys associated with
rivers with natural slopes being a main feature. They may also be part of former railroad
rights -of -way that have been abandoned. Human -modified slopes or other historic
disruptions of natural soils and terrain can elevate landslide susceptibility in parklands.
Slides in parks and trails is a risk to lives and safety, as well as a costly disruption to
recreation activities.
4.1.1.3.2. SECONDARY CONSEQUENCES:
4.1.1.3.2.1. Hazardous material spill or release: If cut by a landslide, pipelines may
release hazardous liquids or gasses, or polluting materials that can threaten lives, impact
property or harm the environment as a secondary hazard after the landslide.
4.1.1.3.2.2. Fire or explosion: In certain instances, landslides may trigger fires or
explosions at the site of buildings or other impacted structures, or where pipelines or
service lines carrying gas or other flammable material.
4.1.1.4. Potential for Cascading Effects
4.1.1.4.1. Life -Safety: Landslides can result in deaths and have done so in Hennepin County (1955)
and adjacent metro counties (2013). Injuries have resulted in numerous other instances, as well
as close calls. The landslide at Fairview -Riverside hospital in Minneapolis (2014) narrowly missed
pushing passing motorists on West River Road into the Mississippi River, for instance.
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4.1.1.4.2. Infrastructure Destruction: Landslides can impact many kinds of critical infrastructure.
Linear infrastructure such as roads, highways, railroads, pipelines, electric power lines and
telecommunications cables are particularly vulnerable to slides that cross their paths. Water and
wastewater infrastructure is not only vulnerable to slides as a linear system but may also help
trigger landslide activity if a break occurs in water, sewer or storm sewer lines at sites that have
other susceptibility factors. Point infrastructure located at susceptible sites anywhere between
the crest to the foot of slopes are also vulnerable.
4.1.1.4.3. Property Damage: Homes and businesses have been damaged or destroyed by
landslides in Hennepin County and surrounding counties. Lack of detailed landslide investigations
and awareness in some cases have led to development on susceptible terrain. The fact that
landslides are not covered by insurance policies has led to often catastrophic financial losses for
homeowners and businesses that are hit. Expensive litigation has also often resulted from these
incidents between property owners and cities.
4.1.1.5. Geographic Scope of Hazard Blc
Landslide activity depends on certain localized factors (see above critical values) that result in an uneven
distribution of landslides across Hennepin County. In general, Hennepin County landslide activity occurs
in the valley walls of the Minnesota, Mississippi and Crow Rivers and their tributaries. Some of the exposed
glacial sediments and bedrock layers in these valleys are unstable and subject to precipitation or spring -
induced landslides. In the interior of Hennepin County, small landslides happen in steep slopes in glacial
sediments that are found along streams, ravines, lakeshores, and wetlands. Artificially steepened slopes,
often with disrupted soils and fills, also have been sites for landslides in Hennepin County. A Hennepin
County Landslide Hazard Atlas is in development and is set for release in late 2018.
4.1.1.6. Chronologic Patterns
Further work is needed among the Hennepin County landslide assessment team to develop
Chronological Patterns
4.1.1.7. Historical Data Bld
4.1.1.7.1. HISTORICAL RECORD: Hennepin County Emergency Management commissioned an
assessment of historic landslide activity in the county using archival data and historic news
accounts. There are around two dozen landslides in Hennepin County that were documented in
written accounts including a known location and date.
• June 19, 2014 (DR-4182)
• June 1, 2014
• April 2014
• May 22, 2013
4.1.1.7.2. PRE -HISTORIC EVIDENCE: Hennepin County Emergency Management commissioned an
assessment of pre -historic landslide activity in the county using LiDAR (Light Detection and
Ranging) imagery. There are over one thousand sites in Hennepin County with landslide evidence
that have been discovered through imagery analysis.
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4.1.1.8. Future Trends Ble
4.1.1.8.1. TRENDS AND PROJECTIONS: The most significant trigger for landslide activity in
Hennepin County is precipitation. Documented trends in precipitation in Minnesota, as well as
projections into the future show an increase in overall rainfall, plus an increase in intense
precipitation events. Recent landslide activity in Minnesota and Hennepin County has risen. It
appears likely that landslide activity will continue to grow in tandem with precipitation trends.
4.1.1.8.2. EVENT PROBABILITIES: More analysis of the recently developed data is needed to
determine landslide event probabilities in Hennepin County.
4.1.1.9. Indications and Forecasting
Further work is needed among the Hennepin County landslide assessment team to develop modeling and
forecasting methods.
4.1.1.10. Detection & Warning
Additional work is needed among the Hennepin County landslide assessment team to develop detection
and warning criteria. Indications of changes in key factors will be accomplished in large part by the
Hennepin -West Mesonet network of environmental sensors.
4.1.1.11. Critical Values and Thresholds
4.1.1.11.1. Slope. Also called the angle of repose, slope is a critical factor for landslide
susceptibility. In Hennepin County, landslide activity starts to increase above 20% slope, and is
most numerous on slopes between 30-40%. Slopes may be either natural or artificially created by
human activities.
4.1.1.11.2. Soil type: Soil type is important to landslide susceptibility for several reasons.
Differences in the porosity and permeability of soils is important since it describes the degree to
which soil types will either slowly retain or quickly shed water. Other characteristics such as soil
structure may contribute to slope failure. Many soils in Hennepin have been disrupted or altered
in some way by human activities.
4.1.1.11.3. Soil moisture: Soil moisture is a critical factor in Hennepin County landslides. Among
other things, when water replaces air within soil pores, the overall weight of the soil increases.
Increasing the weight of near surface soils can increase the likelihood of the material moving
downslope and forming a landslide. The Hennepin County landslide assessment is developing
specific soil moisture criteria for alert purposes.
4.1.1.11.4. Precipitation. Precipitation is one of the most critical factors in triggering landslides
in Hennepin County. Duration, intensity, and recurrence of precipitation are important elements
in precipitation -initiated landslide events. The Hennepin County landslide assessment is
developing specific precipitation thresholds for alert purposes.
4.1.1.11.5. Springs. Springs discharge water along slopes, increasing erosion and helping to trigger
landslides. Springs in Hennepin have been mapped in detail.
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4.1.1.11.6. Bedrock. The depth from the surface to bedrock is an important factor in some kinds
of slides. Exposed bedrock is required for rock falls for instance. A shallow depth to bedrock may
also facilitate flows and other forms of slides as well.
4.1.1.11.7. Surface conditions: Vegetation on slopes usually assists in stabilizing them against
failure. Plants with deep root systems, often native species, are recommended to help slow slope
erosion. Conversely, removal of vegetation that results in bare and exposed soil increases the risk
of landslides and mudslides.
4.1.1.11.8. Soil temperature: The action of winter and spring freeze -thaw cycles seems to help
trigger some rock falls or topples. Thus, these types of landslides are the only ones that appear
to happen outside of the normal rainfall/thunderstorm season of Hennepin County. The freeze -
thaw cycles allow water, trapped in voids and crevices in rock, to expand and push rock apart,
sometimes triggering a fall.
4.1.1.12. Prevention
Further work is needed among the Hennepin County landslide assessment team to develop prevention
methods.
4.1.1.13. Mitigation
4.1.1.13.1. Avoidance (Prevention). The most effective mitigation measure against landslide
fatalities, injuries, infrastructure disruption and property loss are avoiding development and
certain human activities at sites prone to landslides. This is a preventive action. Avoidance may
be accomplished through evidence -based zoning policies that utilize local area landslide hazard
assessments that trigger site -specific landslide investigations when appropriate if development or
other uses are proposed at sites inside identified hazard zones. Specific actions include avoiding
cutting into slope sides or at the food of slopes, and not placing excessive weight on the top of
slopes by erecting structures there.
4.1.1.13.2. Education and public alerts. Education of zoning officials, landowners and need
accurate local information in order to make sound decision regarding their development and
activities in landslide susceptible terrain. A simple knowledge of landslide risk also sets the
foundation for appropriate action when a public alert is issued. Public alert thresholds, messages
and distribution methods must be developed.
4.1.1.13.3. Active mitigation methods. Geometric methods include changes in slope angle to
reduce the chances of landslides. Hydrological methods consider surface, shallow and deep -
water drainage and attempt to improve the ability of landslide -susceptible sites to drain water
effectively. Finally, mechanical methods include the use of rock anchors, netting, retaining walls,
or pilings. In general, these methods are expensive and are suitable only of sites of limited size in
areas where development is of high importance.
4.1.1.14. Response
Further work is needed among the Hennepin County landslide assessment team to develop Response
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methods.
4.1.1.15. Recovery
Further work is needed among the Hennepin County landslide assessment team to develop Recovery
methods.
4.1.1.16. References
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[� 1;2. Hazard Assessment: SINKHOLE
4.1.2.1. Definition.
A sinkhole is a bowl -shaped depression in the land
surface. Sinkholes are also called subsidence, which is a
downward settling of the surface without any horizontal
movement. Sinkholes result from natural processes
where near -surface carbonate bedrock is dissolved by
water to form underground spaces, also called voids.
These voids typically form along existingjoints or cracks in
the rock that aid the movement of water. Some voids
grow toward the surface where infiltrating surface waters
meet and flow downward into the drain of the void. This
action weakens the rock. Eventually, the weight of
overlying materials can result in a collapse. Areas
favorable for sinkhole development are called karst terrain. Certain human activities may speed up the
natural sinkhole processes in karst areas. Human activities outside of normal karst terrain can also trigger
unexpected human -caused ground collapses in materials not usually prone to sinkholes.
4.1.2.2. Range of magnitude
Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole
hazard assessment in 2020.
4.1.2.3. Spectrum of Consequences B211b
4.1.2.3.1. PRIMARY CONSEQUENCES:
Sinkholes and other land subsidence can cause significant direct damage to buildings, roads, water
supply systems and other infrastructure. The loss of land usable for farming or other development
is another consequence of sinkhole activity. Finally, groundwater contamination is a significant
consequence of karst and sinkhole activity. Subsurface water flow in karst areas creates a
situation where surface water, along with their contaminants, quickly travel deep into aquifers
without significant filtration. The problem is worsened when people use sinkholes as garbage
dumps, which was formerly a common practice in the United States.
4.1.2.3.2. SECONDARY CONSEQUENCES:
4.1.2.3.2.1. Disease. Dumping of wastes into sinkholes maybe a source of disease. A
disease outbreak in Harmony, Minnesota (Fillmore County) was traced to a sinkhole used
as a disposal point for human waste.
4.1.2.3.2.2. Dam failures. There have been instances of dams and other water -control
infrastructure being undermined by sinkholes and other karst activity.
4.1.2.3.2.3. Fires or explosions. When structures, or infrastructure such as pipelines are
impacted by sinkholes and gas lines are compromised, fires and explosions are possible.
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4.1.2.4. Potential for Cascading Effects
In Minnesota, most sinkholes are in rural areas and develop very slowly. These sinkholes are not
dangerous, and they do not cause much destruction except for the loss of crop land. When sinkholes
happen in developed urban areas however, they have the potential to be much more costly and, in some
cases, even dangerous. The active karst areas in southeast Hennepin County are in places with
concentrated developments of housing, businesses, schools and infrastructure. The potential for
destructive sinkhole events in Hennepin County has not been adequately assessed. Hennepin County
Emergency Management is initiating a study of sinkhole hazards in the county that is expected to be
complete by 2020.
4.1.2.5. Geographic Scope of Hazard Blc
The southeastern three-quarters of Hennepin County is underlain by carbonate bedrock and is karst
terrain. The western and northern limits of this area begin in the south around Excelsior and extend
northward into Medina, then eastward into Brooklyn Center. Most of this area is comprised of covered
karstwhich has overlying glacial material more than 100 feet in depth. An area with pockets of transitional
karst which has overlying glacial material between 50 and 100 feet thick is roughly bounded in the south
by Edina, west to Wayzata, and northeast to Brooklyn Center. Active karst is found in mostly along the
Mississippi River from North Minneapolis south to Fort Snelling. Scattered outlying pockets of active karst
can be found westward from Golden Valley south to St. Louis Park. Active karst areas have less than 50
feet of overlying material covering them.
Note: Other types of land subsidence are directly caused by human activities and are dealt with in the
human -caused, industrial/technological section of this hazard assessment. These include water or sewer
system breaks that cause sinkholes or collapse of underground tunnels.
4.1.2.6. Chronologic Patterns
Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole
hazard assessment in 2020.
4.1.2.7. Historical Data Bld
The Seven Oaks Park in south Minneapolis is a sinkhole. The surface depression is approximately 300 feet
wide and over 20 feet deep. The time of formation of the sinkhole is unknown but predates the
construction of the structures around it. Seven Oaks Park is located between E 341h Street and E 351h Street
at 471h Avenue South in Minneapolis (USNG 15T VK 83754 76384). Other possible sinkholes are nearby but
await more definitive confirmation.
There have been no other naturally caused incidents that are within the scope of this plan.
4.1.2.8. Future Trends Ble
Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole
hazard assessment.
4.1.2.9. Indications and Forecasting
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Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole
hazard assessment in 2020.
4.1.2.10. Detection & Warning
Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole
hazard assessment.
4.1.2.11. Critical Values and Thresholds
4.1.2.11.1. Bedrock material: Areas susceptible to sinkholes (karst terrains) are underlain by
water-soluble, but relatively impermeable bedrock such as limestone (calcium carbonate).
Soluble rocks dissolve when exposed to certain acids, including acidic water. Over time, acidic
water flowing through joints and cracks will dissolve and remove large amounts of soluble rock
creating many void spaces. In more unusual instances, sandstones or even quartzite may develop
sinkholes. In these cases, the bedrock is more permeable, but less soluble. Slower sinkhole
development may occur in these rocks.
4.1.2.11.2. Water acidity: Acidic surface water and groundwater is required for natural sinkhole
formation as the agent that dissolves soluble bedrock. Pure water has a pH of 7.0, which is neutral
— neither acidic nor base. However, water in nature is not pure. Instead, it contains natural
impurities which make it acidic. Unpolluted rainwater has a pH of around 5.6 (acidic). Rainwater
in Minnesota contains atmospheric pollutants which further lower the pH, increasing acidity.
Once at the surface, water can become further acidified by exposure to nitrogen fertilizers or
other chemicals. When this water infiltrates into the bedrock it begins to gradually dissolve any
carbonate rocks.
4.1.2.11.3. Bedrock depth: For a void to cause a collapse of the overlying surface material it must
be close to the surface. Active karst areas have carbonate bedrock less than 50 feet below the
surface. Transitional karst areas have carbonate bedrock covered by material between 50 and
100 feet. In some instances, sinkholes can occur in these conditions as well. Covered karst areas
have more than 100 feet of overburden. Sinkholes are unlikely to develop in such deep
conditions.
4.1.2.11.4. Bedrock topography. Once water penetrates the soil, it will arrive at the bedrock layer.
Typically, the bedrock is much less permeable than the overlying unconsolidated soils which
promotes lateral water flow. The water will flow according to the topography of the bedrock
finding crevices and valleys that collect water until a penetration point can be found into the
bedrock.
4.1.2.11.5. Joints, fractures, and bedding planes: These features provide easy routes for water to
travel through the rock. As water moves through this network of joints, fractures and bedding
planes, chemical action of the acidic water dissolves the bedrock. Joints and fractures are often
oriented in parallel and perpendicular patterns. Because of this, voids and sinkholes also are often
aligned to follow these patterns.
4.1.2.11.6. Water table: Fluctuations in ground water levels can affect sinkhole activity. Abrupt
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changes in ground water level can induce sinkholes. Ground water drawdown often increases
sinkhole activity.
4.1.2.11.7. Construction and development. Human development activities that add extra weight
and pressure to land surfaces by construction of new buildings and other infrastructure may
accelerate sinkhole formation. The alteration of surface and subsurface drainage flows due to
human development may also accelerate sinkhole formation by increasing the flow of water
through sinkhole drains. Water and sewer lines in karst areas are susceptible to damage from
sinkholes and other land subsidence. When water or sewer lines leak or break, the released water
may enter sinkhole systems and quickly enlarge voids, accelerating sinkhole formation.
4.1.2.12. Prevention
4.1.2.12.1. Avoidance The most effective prevention/mitigation measure against sinkhole
fatalities, injuries, infrastructure disruption and property loss are avoiding development and
certain human activities at sites prone to sinkholes. This is a preventive action. Avoidance may
be accomplished through evidence -based zoning policies that utilize local area sinkhole hazard
assessments that trigger site -specific sinkhole risk investigations when appropriate if
development or other uses are proposed at sites inside identified hazard areas. Zoning -based
measures would be challenging in Hennepin County because much of the karst areas have already
been developed.
4.1.2.13. Mitigation
4.1.2.13.1. Education. Education of zoning officials, landowners need accurate local information
to make sound decision regarding their development and activities in sinkhole susceptible terrain.
These require detailed sinkhole hazard maps. HCEM completed its Landslide Hazard Atlas to assist
in mitigation, avoidance, and planning response efforts. The atlas was release by 2020.
4.1.2.14. Response
With the completion of the Landslide Hazard Atlas in 2020. Response effort follows five key
principles: engage partnerships, have a tiered response, have a scalable, flexible, and adaptable
operational capability, unify your effort, and be ready to act. Scene stabilization will be achieved
when the immediate threat to life -safety and property damage at the scene have been stopped.
4.1.2.15. Recovery
The recovery process begins soon after the incident happens. The objective is to bring households
and communities back to normal activities post -disaster. Relief can come from a variety of ways.
Public Assistance, Individual Assistance, Emergency Repair, or Permanent Repair.
4.1.2.16. References
Hennepin County landslide Hazard Atlas. (July 2020). https://www.hennepin.us/-
/media/hennepinus/residents/emergencies/landslides/landslide-atlas-cover-contents.pdf
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41�Hazard Assessment: SOIL FROST
4.1.3.1. Definition.
Soil frost is caused when water, which is present as a
component of soil, freezes into pore ice. The depth to which
this freezing penetrates is called the deep frost. Some soils
are vulnerable to frost heaving, which is the vertical
displacement of the surface due to frost expansion or the
development of ice lenses. Melt collapse happens when the
ice lenses melt. These effects can damage roads and building
foundations and other infrastructure. Deep penetration of
frost can also have a devastating impact on critical buried
infrastructure, such as water and wastewater pipes. In
extreme cases, fire hydrants and fire sprinkler water supplies may freeze. Hard impervious frost layers in
the soil also can worsen springtime rain and snowmelt flooding by not allowing water to penetrate the
soil and increasing run-off.
4.1.3.2. Range of magnitude
Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored soil frost
hazard assessment in 2020.
4.1.3.3. Spectrum of Consequences B211b
4.1.3.3.1. PRIMARY CONSEQUENCES
4.1.3.3.1.1. Water utilities: In Hennepin County, water service lines are typically buried
between 78 to 90 inches (198.1 to 228.6 centimeters) deep. This depth is usually
protecting these lines against freezing. When particularly deep frost is formed, however,
water service lines may freeze, cutting off water services to residences, businesses, and
government facilities. Bottled water delivery is often the response of choice while
awaiting water service restoration. Water service freezing not only stops the flow of
potable water to an address, it may also interrupt fire protection systems such as
sprinklers or standpipes. Water mains, which are buried deeper than service lines, are less
likely to freeze. If they freeze, then fire hydrant services also are interrupted. Thawing
frozen water lines is difficult and time consuming. It requires special equipment and
experience. Some methods may cause structural fires. In widespread instances of frozen
water lines, service may be cut for days to weeks. Without intervention, frozen water
service lines in Hennepin County would thaw by May. Service line freezing may be
prevented by keeping a pencil -sized flow of cold tap -water always moving through the
system. Prevention is usually done at the request of the local water utility.
4.1.3.3.1.2. Wastewater services: In general, municipal sewer lines have similar depth
requirements as water service lines to prevent frost damage or disruption. Sewer lines
typically have fewer freeze problems during deep frost events than water lines, however.
Rather than frost causing problems for municipal sewer systems, a bigger issue seems to
be impacts to household septic systems.
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4.1.3.3.1.3. Energy pipelines: Gas and other pipelines are vulnerable to the effects of
frost. According to data from the Pipeline and Hazardous Materials Safety Administration
(PHMSA), 82% of cold weather failures of distribution pipelines in the US (1984 through
2014) were caused by frost heave.
4.1.3.3.1.4. Communications: Buried fiber optic cables are susceptible to impacts from
frost. This occurs when water that has infiltrated the fiber optic conduit freezes. The most
vulnerable areas where sites were cables were shallow or exposed near bridges. While
freezing has no impact on copper cables, fiber optic cables may be bent by the expansion
of the ice. Various levels of signal degradation may occur, including complete failure. As
a countermeasure, some communication companies have injected their conduit with
anti -freeze compounds.
4.1.3.3.1.5. Structural damage: Frost heave of soils can cause significant damage to
structures including cracked foundations or slabs and other effects from ground
movement.
4.1.3.3.1.6. Transportation: Roads and highways are impacted frost action. Differential
frost heaves are creating blisters in pavement that leads to cracking and potholes. Frost
can block proper drainage and lead to additional problems. Road load -bearing capacity is
affected by freeze -thaw cycles.
4.1.3.3.2. SECONDARY CONSEQUENCES:
Frost induced breaks in gas or oil pipelines can cause fires or explosions.
4.1.3.4. Potential for Cascading Effects
4.1.3.4.1. Specific sites. Deep frost can impact buried infrastructure that carry water, wastewater,
energy or communications causing service interruption by freezing or by physical damage. Frost
heaving can also cause damage to buildings and other structures. These damages are highly
dependent on localized conditions leading to impacts that area variable from address to address.
Frost depth impacts may be widespread but spotty.
4.1.3.4.2. General areas. Deep frost can create a frozen and temporarily impervious layer of soil
across wide regions which limits infiltration of snow -melt water and rainwater in springtime. This
additional runoff worsens springtime flooding across river basins and stream watersheds.
4.1.3.5. Geographic Scope of Hazard Blc
All areas of Hennepin County and the State of Minnesota are vulnerable to soil frost during winter months.
Minnesota and the adjacent state of North Dakota are the center of deep frost activity in the 48
contiguous United States. While frozen soils are routine in all parts of Minnesota, problems occur when
frost penetrates deeper than normal. The Minnesota State Building Code (MSBC) Rule 1303.1600 places
construction frost depth in Hennepin County at 42 inches (106.7 centimeters).
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4.1.3.6. Chronologic Patterns
Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored soil frost
hazard assessment in 2020.
4.1.3.7. Historical Data Bld
4.1.3.7.1. Comprehensive. Hennepin County Emergency Management (HCEM) has not yet
systematically investigated historical records of local frost depth. Precise frost measurements
using frost tubes or other sensors are unlikely to have been conducted anywhere in Hennepin
County prior to the HCEM program which started in 2015. The nearest historic soil frost records
are probably measurements taken at the University of Minnesota, Saint Paul campus. These St.
Paul records are for frost under sod. It is possible that written historical accounts of frost depth
and their effects might be found in records of municipal utility providers. These records, if
discovered, would probably be for frost under pavement which impacted water lines and other
utilities.
4.1.3.7.2. Winter of 2013-2014. The coldest Hennepin County winter since 1978-1979 occurred
in 2013-2014 with a sustained three-month cold snap. The mean temperature for the months of
December, January and February was 9.81F degrees at MSP airport. The normal for this time period
is 18.71F degrees. More snow fell than average during the period as well (57.2 inches three-month
total). Most of it fell late in the period. Frost was pushed much deeper than average. Anecdotal
reports by public work crews working on frozen water service lines reported frost as deep as 7 to
8 feet in Plymouth. Twelve cities, not including Minneapolis, provided information regarding
service interruptions. In these cities were a total of 324 water freeze up incidents, mostly service
lines. In addition, 1 hydrant froze, 2 water mains, and 4 sewer lines also became frozen. The
longest outages were over one week. Residences, businesses, care facilities, and government
buildings were impacted. In several instances, cities had to distribute bottled water to affected
residences.
There have been no other naturally caused incidents that are within the scope of this plan.
4.1.3.7.3. Pre -Historic Evidence:
Unknown. HCEM has not found any research regarding pre -historic frost depth in Hennepin
County.
4.1.3.8. Future Trends Ble
Undetermined. Climate change is having a significant impact on Minnesota and Hennepin County. Forces
generated by climate change are sometimes at odds over the net effect experienced in this area during
any winter. For instance, there has been an overall warming trend in Minnesota winters, including a
shorter winter season and higher average temperatures. More recently, prolonged outbreaks of extreme
cold air have impacted Minnesota and Hennepin County. These include the winter of 2013-2014 and the
winters of 2016-2017 and 2017-2018. These cold outbreaks appear to be related to warming in the Arctic
that has weakened the Polar Jet Stream. The weakened jet stream is less able to contain cold Arctic air in
high latitudes and block it from streaming south. Some scientists theorize that prolonged outbreaks of
extreme cold polar air may be a recurring feature of future winters in Minnesota. When coupled with low
or no -snow cover conditions, outbreaks of extreme cold may push frost deeper into the soil.
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EVENT PROBABILITIES: Unknown. Further research is needed to determine trends and probabilities of
future deep soil frost events in Hennepin County.
4.1.3.9. Indications and Forecasting
Additional study is needed to develop deep soil frost event models and forecasts for Hennepin County.
Adequate weather forecasting already exists and would certainly be a major factor in any future soil frost
forecasts. Better data on the behavior of frost in local soils under various temperature, surface material,
soil moisture and snow cover conditions is required to develop models and forecasts. Hennepin -West
Mesonet data will provide much of the needed information.
4.1.3.10. Detection & Warning
In 2015, following the disruptive winter of 2013-2014 when hundreds of water service lines were frozen,
Hennepin County Emergency Management (HCEM) began to install a network of manually read frost tubes
at locations around Hennepin County. When possible, two frost tubes were installed at the same site.
One tube was for measuring frost depth under sod, and the other for frost depth under pavement because
of the significant differences between the two. Frost tubes are usually located near a Hennepin -West
Mesonet sensor station so that weather factors can be compared to the frost depth at the site. The
measurements, taken at least weekly, can provide indications that the frost is pushing deeper than normal
and is beginning to threaten water and sewer services, fire protection capabilities, and other vital services.
When appropriate, HCEM will send out alerts to public works officials that frost may threaten their water
and sewer infrastructure.
4.1.3.11. Critical Values and Thresholds
4.1.3.11.1. Air temperature: Air temperatures below freezing (32F/OC) are required to initiate
soil frost formation. A freezing index based on degree-days of freezing may be used to roughly
estimate frost depth potential in an area.
4.1.3.11.2. Pavement. Human -made surfaces, such as concrete or asphalt roadways create ideal
conditions for exceptionally deep frost penetration into soil. The differences between frost depth
under paved roads and frost depth under natural sod is large enough to produce a few feet of
difference at the same site. Therefore, measurements should specify of they are taken under
pavement or under sod. Factors such as the thermal conductivity of pavement and the removal
of snow cover combine to push frost deep into the underlying soils. This is important because a
lot of buried infrastructure is underneath immediately adjacent to roadways, increasing their
vulnerability to frost.
4.1.3.11.3. Surface albedo: Surface albedo is the ratio of irradiance of solar energy reflected to
the irradiance of solar energy absorbed by a surface. Asphalt, dark soils, turf grasses and forests
have low albedo. Snow cover, sand, and winter prairie grasses have higher albedo. The albedo
of the primary surface is important because it influences the snow cover characteristics of the
site. Snow cover is a central factor is controlling frost depth.
4.1.3.11.4. Soil type: Different soil types freeze at different rates. Frost tends to penetrate less in
clay (heavy textured) soils and more deeply in silty or sandy (lighter textured) soils. Inorganic soils
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with >3% by weight of grains finer than 0.02 millimeter in diameter (silts, silty sands, and clays)
form frost lenses more easily and have a very high susceptibility to frost heaves.
4.1.3.11.5. Moisture content: Soil moisture effects the initial freezing of soil because of the
increased heat capacity and thermal conductivity of the soil surface. The initial freezing point of
soil is usually delayed with increasing amounts of soil moisture. As winter progresses, the soils
that have started with greater amounts of water filling pore spaces experience greater overall
frost depths due to increased thermal conductivity since air is a less efficient conductor of heat
than water. Water tables within 10 feet of the surface are a contributing factor for frost heaves.
4.1.3.11.6. Snow cover: The insulating effect of snow cover is a key factor in slowing the
penetration of frost into the soil. Each foot of undisturbed snow cover typically reduces the depth
of soil freezing by an equal amount. Snow cover is a function of the amount of snowfall received
at a location, along with the type of surface material at that location. Darker colored surfaces also
tend to help accelerate snow melting and help remove the insulating effect of snow (see albedo).
Snow removal on paved surfaces helps to push frost deeper by not allowing insulating snow cover
to accumulate.
4.1.3.11.7. Vegetative cover: Like snow, vegetation acts as an insulator to slow frost penetration
into the soil. Loose grasses or leaves can form insulating air pockets that reduce the depth that
frost can penetrate.
4.1.3.11.8. Geographic location: In general, in Minnesota the average initial soil frost date is
earlier with higher latitudes and more westerly longitudes. More northerly latitudes have longer
overall frost seasons on average. In Minnesota the change in average freezing date is about 3.3
days per degree of latitude.
4.1.3.11.9. Infrastructure condition. In general, older buried infrastructure such as service lines,
pipes and conduits are in a more deteriorated condition than newer infrastructure and are more
susceptible to damage from deep frost.
4.1.3.12. Prevention
Unknown, pending conclusion of the Hennepin County Emergency Management assessment in 2020.
4.1.3.13. Mitigation
4.1.3.13.1. Frozen water lines. Water lines can be protected against deep frost by ensuring they
are buried to the correct depth. Lines which are already installed can resist freezing by ensuring a
constant flow of a small amount of water (pencil -diameter stream from a faucet) flowing in from
the service line. Typically, water utilities will request that customers maintain running water at
addresses that have had freezing problems in the past.
4.1.3.13.2. Buildings, roads, and infrastructure. When it occurs, typical vertical ground
movement due to frost heaves and melt collapse is between 4 to 8 inches. Extreme movement
can be up to 24 inches. These ground movements are enough to cause significant damage to
human -made structures. Various mitigation measures can protect structures against frost heave
and melt collapse. Buildings which are heated rarely experience frost heave problems because of
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a portion of the heat is received by the surrounding soil which prevents ice lens formation and
heave action. For unheated structures, heaves can be prevented through keeping waters out of
freezing zone. Another mitigation method is to ensure soils surrounding structures are those less
susceptible to frost problems.
4.1.3.13.3. Distribution pipelines. Pipelines are susceptible to frost heave -produced ground
movements. Pipe materials, joining methods, soil conditions and water drainage are all important
factors in prevention of damages. In areas susceptible to frost heave damage, pipeline materials
should shift away from cast iron and threaded steel pipe and be replaced by plastic of welded
steel. Other measures can be taken to reduce the chances of frost damage to pipelines. These
include drainage to reduce water in the soil and eliminate standing water over pipelines. Soil
conditions may also be modified to reduce susceptibility to ice lens formation.
4.1.3.13.4. Flooding. Deep frost penetration can worsen spring meltwater flooding by preventing
soil absorption of snow melt or rainwater. Flood control and management measures must
consider the potential for deep frost effects in spring flood scenarios.
4.1.3.14. Response
Unknown, pending conclusion of the Hennepin County Emergency Management assessment in 2024.
4.1.3.15. Recovery
Unknown, pending conclusion of the Hennepin County Emergency Management assessment in 2024.
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4 1#""';' Hazard Assessment: VOLCANIC ASH
4.1.4.1. Definition.
Volcanic ash consists of tiny particles of jagged rock and
natural glass blasted into the air by a volcano. This ash
poses threats to human and animal health, aircraft
engines, electronics, machinery, electrical power
generation and telecommunications. Winds may carry
ash thousands of miles, impacting areas and people far
away from the volcano itself. Volcanic ash is not the
product of combustion, and thus is not like the light ashes
made by burning leaves, wood, or coal, for example.
Volcanic ash particles are hard rock fragments that do not
dissolve in water. Ash is extremely abrasive, mildly
corrosive and can conduct electricity when wet.
4.1.4.2. Range of magnitude
Unknown, pending conclusion of the Hennepin County Emergency Management assessment in 2020.
4.1.4.3. Spectrum of Consequences B2b
4.1.4.3.1. PRIMARY CONSEQUENCES
4.1.4.3.1.1. Aircraft. Aircraft in flight are particularly vulnerable to the effects of exposure
to volcanic ash. Often the ash cloud is invisible to the flight crew, and must be detected
by the odor of sulfur, or by a haze developing on the windscreen. The electrically charged
ash particles can interfere with navigational and flight instruments, and communications
equipment. The ash may clog the pitot-static system that indicates airspeed and feeds air
to several vital flight instruments. Abrasion by the jagged particles can erode leading edge
surfaces, and quickly produce a haze on windscreens so that pilots are unable to see
through them. Turbine compressor blades in jet engines can wear quickly. Finally, the low
melting temperature of volcanic ash means that the particles liquefy in the ignition
chamber of jet engines, but quickly cool in the next engine stage and end up coating
engine parts with a glaze of volcanic glass. Engines have failed from ingesting volcanic
ash. Repair costs from encounters with ash can cost millions of dollars per aircraft.
4.1.4.3.1.2. Surface transportation. At the surface, ash fall could produce hazardous
driving conditions by cutting visibilities when at least 1 millimeter (1/32 inch) of ash
accumulates on roadways. Ash fall amounts of accumulation greater than 1 mm (1/32
Inch) also obscure markings on roadways, causing confusion among drivers in the low
visibility conditions.
4.1.4.3.1.3. Human health. The main health impact of volcanic ash to people (and
animals) are to the respiratory tract and to the eyes. Ash particles less than 100
nanometers in size produce upper airway irritation. Ash particles less than 10
nanometers in size can penetrate deep into the lung and worsen the conditions of those
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with various pre-existing lung diseases. Ashes with high crystalline silica content may also
increase risk for suture silicosis. Technical analysis is required to determine silica
component of the ash.
4.1.4.3.2. SECONDARY CONSEQUENCES:
Unknown at this distance from source volcanoes.
4.1.4.4. Potential for Cascading Effects
Volcanic ash is capable of various degrees of destruction, largely based on the distance it has traveled
from the volcano of origin. Ash falling to the surface in areas near the volcano is much coarser and heavier
than the ash that winds can carry for hundreds of thousands of miles from the eruption. Since the principle
volcanic ash producing threats are located at least 800 miles west of Hennepin County, the destructive
potential is restricted to the characteristics of ash that can be wind -transported that far. The most
significant impacts at this distance involve the critical safety threat of aircraft flying through invisible high -
altitude ash clouds. Sensitive electronic devices including computers, communications equipment,
medical devices, and other critical equipment can be damaged by the abrasive and electrically charged
particles. Finally, human and animal health impacts can occur because of the effect that the irritating
volcanic ash has on the respiratory system and on eyes.
4.1.4.5. Geographic Scope of Hazard Blc
Most volcanic ash is produced during explosive volcanic eruptions. Explosive volcanoes are found along
the boundaries of Earth's converging tectonic plates that are converging, such as along the Pacific Rim,
sometimes called the Ring of Fire. Other volcanic activity is at mantle plumes, called 'hot spots, which melt
through tectonic plates. The closest volcano to Hennepin County is the Yellowstone Caldera, located about
800 miles west, in northwest Wyoming. The belt of volcanoes in the Cascade Range are about 1300 miles
west of Hennepin County in eastern Washington State. Prevailing winds from the west set up Minnesota
as a potential recipient of ash from volcanic eruptions in the western United States, Canada, and Alaska.
4.1.4.6. Chronologic Patterns
Unknown, pending conclusion of the Hennepin County Emergency Management assessment in 2024.
4.1.4.7. Historical Data Bld
Several major eruptions have occurred in North America where ash clouds traveled great distances. These
include the Spurr Volcano, Alaska (27 June 1992); Mount Saint Helens, Washington (18 May 1980) and
the Novarupta Volcano, Alaska (06 June 1912). Ash from the Spurr volcano traveled over Minnesota (see
graphic at the beginning of this section) in September 1992.
Pre -Historic Evidence
Some extremely large volcanic eruptions occurred in the geologically recent past in the Yellowstone
Super -Volcano complex in northwestern Wyoming. The United States Geological Survey estimates an
average recurrence rate of explosive volcanic eruptions at Yellowstone to be between 600,000 and
800,000 years. The pervious explosive eruptions have been the Lava Creek Eruption, Yellowstone, WY
(630,000 years ago); the Mesa Falls Eruption, Yellowstone, WY (1.3 million years ago); and the
Huckleberry Ridge Eruption, Yellowstone, WY (2.1 million years ago). Massive ash falls were generated
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by these eruptions.
There have been no other naturally caused incidents that are within the scope of this plan.
4.1.4.8. Future Trends Ble
There is no evidence that typical volcanic activity levels among the volcanoes that pose an ash fall threat
to Hennepin County are either increasing or decreasing. These volcanic events happen in geologic time
in which eruption recurrence rates of hundreds, thousands or even hundreds of thousands of years are
possible.
Event Probabilities: The United States Geological Survey (USGS) has estimated the activity level and
eruption recurrence rate of each of the volcanoes in the western United States, Canada, and Alaska.
4.1.4.9. Indications and Forecasting
Volcanic forecasting is the responsibility of the United States Geological Survey and its Volcano
Observatories. USGS scientists categorize volcanoes and estimate their explosive potential based on
evidence of past eruptions.
4.1.4.10. Detection & Warning
USGS scientists monitor precursor activity and are often able to issue alerts of impending eruptions
months or weeks prior to the event. Ash clouds are tracked by the National Oceanic and Atmospheric
Administration. The Washington Volcano Ash Advisory Center (WVAAC) is responsible to provide alert and
warning services for aviation safety. The Minneapolis Air Route Traffic Control Center (ARTCC) is served
by the WVAAC.
4.1.4.11. Critical Values and Thresholds
4.1.4.11.1. Diameter: Ash particles are less than 2 millimeters in diameter down to very extremely
small particles of less than 0.001 millimeter. Volcanic ash is lofted high into the atmosphere and
can be blown thousands of miles away from the volcano. Larger and heavier particles will fall to
Earth much more quickly than smaller and lighter particles which may remain aloft for weeks or
longer. Extremely small particles suspended in the air can be invisible to the human eye, yet
present hazards to aviation.
4.1.4.11.2. Density: Ash particles have variable degrees of density (pumice, 700-1200 kg/m3;
glass, 2350-2450 kg/m3; crystals, 2700-3300 kg/m3; and rock particles, 2600-3200 kg/m3). The
high -density ash particles are hard (5 Mohs scale). Window glass and steel have a Mohs hardness
of 5.5, for example. Ash particles have sharp edges making them very abrasive.
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4.1.4.11.3. Weight: Fallen volcanic ash is heavy and poses
a risk to buildings close to the eruption, particularly those
with flat roofs. A dry layer of ash 4 inches thick weighs 120
to 200 pounds per square yard, and wet ash weight is
usually double the dry totals. Ash weight should not be a threat
to Minnesota structures.
4.1.4.11.4. Prevailing winds. Both east -west zonal flow and
Alberta Clipper systems bring winds to Minnesota from
regions that host active volcanoes.
4.1.4.12. Prevention
Unknown, pending conclusion of the Hennepin County Emergency
Management assessment in 2024.
4.1.4.13. Mitigation
4.1.4.13.1. Avoidance. Avoidance of flight through ash clouds is vital to aviation safety. Ash cloud
alerts and warnings provide air route control centers the information they need to vector aircraft
away from ash clouds.
4.1.4.13.2. Personal protection. Personal protective equipment such as filtration masks and eye
protection from covered goggles are needed to avoid some of the health risks posed by volcanic
ash.
4.1.4.13.3. Barriers. Sealing off rooms that have sensitive electronics can be done with plastic
sheets and duct tape. Covering individual devices may also help protect them against ash.
4.1.4.14. Response
Unknown, pending conclusion of the Hennepin County Emergency Management assessment.
4.1.4.15. Recovery
Unknown, pending conclusion of the Hennepin County Emergency Management assessment.
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d111114 Hazard Assessment: FLOODING, URBAN
4.2.1.1. Definition
Urban flooding occurs when rain overwhelms drainage systems
and waterways and makes its way into the basements, backyards,
and streets of homes, businesses, and other properties. As land is
converted from fields or woodlands to roads or parking lots, it
loses its ability to absorb rainfall. Because of this, densely
populated areas are at a high risk for flash floods. The
construction of buildings, highways, driveways, and parking lots
increases runoff by reducing the amount of rain absorbed by the
ground.
4.2.1.2. Range of magnitude
The 10-year average of recent flood damages is about $20 billion. However, some years have run as high
as $40 billion.
• Deadliest Flash Flood (Dam Collapse): 1889, Johnstown Pennsylvania: 2,200 people died.
• Deadliest torrential rain flood: July 31, 1976, Big Thompson Canyon, Colorado: 143 people died
• Longest duration: 1993 61 days; The Great Midwest Flood
• Greatest USD Damage: $12 Billion 1993; The Great Midwest Flood
4.2.1.3. Spectrum of Consequences B211b
There are several ways in which storm water can cause the flooding: overflow from rivers and streams,
sewage pipe backup into buildings, seepage through building wall and floors, and the accumulation of
storm water on property and in public rights -of -way. Sometimes, streams through cities and towns are
routed underground into storm drains. During heavy rain, the storm drains can become overwhelmed and
flood roads and buildings. Low spots, such as underpasses, underground parking garages, and
basements can become dangerous.
The economic, social, and environmental consequences of urban flooding can be considerable. Water
quality issues can arise from sewer overflow's debris contamination, fertilizer runoff from agriculture
etc.... which affect public health with possible contaminated drinking water and water borne illnesses. The
cost of removal of soil from landslides, or sediment deposits from flooding can be high, as well as wildlife
habitat reconstruction as wildlife habitat can be ruined by wash out, water contaminates, oxygen loss, or
loss of access to food sources.
Chronically wet houses are linked to an increase in respiratory problems, and insurance rates and
deductibles may rise to compensate for repeated basement flooding claims. Industry experts estimate
that wet basements can lower property values by 10-25 percent and are citied among the top reasons for
not purchasing a home. According to FEMA, almost 40 percent of small businesses never reopen their
doors following a flooding disaster. Between 2006-2010 the average commercial flood claim made to the
NFIP amounted to just over $85,000. Urban flooding also erodes streams and riverbeds and degrades the
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quality of our drinking water sources and the health of our aquatic ecosystems.
4.2.1.4. Potential for Cascading Effects
Structures that encroach on the floodplain, such as bridges, can increase upstream urban flooding by
narrowing the width of the channel which can cause sediment and debris carried by floodwaters further
because the flow is occurring at a higher stage past the obstructions. This can cause channels to become
filled with sediment or become clogged with debris causing issues farther upstream from where the initial
flooding occurred.
Depending on the extent of the flooding, water quality becomes an issue because it becomes necessary
to treat contaminated runoff, but depending on the contaminants present this process can be very costly
especially when compared to its benefits. In addition to water quality in the runoff poses issues, if any
sewer or water treatment plants have been flooded, homes may now not have access to clean water or
working restrooms.
4.2.1.5. Geographic Scope of Hazard Blc
The extent of urban flooding in Hennepin County really depends on an extremely complex set of
interactions between the surface and sub -surface drainage networks and features of the environment.
Urban flooding can be small in geographic scope as in just a few streets or neighborhoods with minor
flooding damage, to large areas of entire cities being under water.
4.2.1.6. Chronologic Patterns
Urban flooding in Hennepin County typically occurs in the spring and summer months associated with
thunderstorms. Springtime urban flooding can come from both snowfall melt and runoff during the spring,
a spring thunderstorm that comes before the ground has had time to that completely preventing
infiltration, or just a normal thunderstorm (or multiple thunderstorms within a smaller period) with
excessive rainfall rates.
4.2.1.7. Historical Data Bld
Floods have been documented all the way back to 1776 in Minnesota. However official American records
don't begin until 1873. As mentioned in river flooding, of the 24 State of Minnesota Flood Declarations,
Hennepin County has been included in six, with all having urban flooding issues with road and bridge
closures. There have been no other naturally caused incidents that are within the scope of this plan.
• 1965 Flooding (DR-188)
• 1969 Flooding (DR-255)
• 1997 Severe Flooding, High Winds, Severe Storms (DR-1175)
• 2001 Severe Winter Storms, Flooding, and Tornadoes (DR-1370)
• 2010 Flooding (DR-3310)
• 2014 Severe Storms, Straight -Line Winds, Flooding, Landslides and Mudslides (DR-4182)
• 2016 Severe Storms & Flooding (DR-4290)
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4.2.1.8. Future Trends Ble
Urban flooding is a naturally occurring hazard that affects cities and regions around the world, and is
expected to become even more problematic in the future. Damages from floods are also increasing as are
the number of people who are affected by them.
Human -induced land cover change and climate change are important factors in urban flooding. Rapid
population growth and increasing migration from rural areas to cities lead to intense urbanization, which
often increases flood risk. According to recent studies, the urban heat island effect and aerosol
composition can alter the climate mechanism, which plays an important role in the storm evolution of
urbanized regions. Global warming, the other main cause of hydrologic regime change, can induce the
acceleration of the water cycle, which can consequently affect the frequency and intensity of future storm
events. Research has shown that in the future we may not necessarily see more rainfall, but more rainfall
on less days. That is to say that if the monthly average total rainfall is four inches over eight different days,
we would now see that four inches come on three or four days. So same amount of rain, just coming more
at one time.
TOO
600
500
co-
ur
499
is
6
w
300
3
z
200
188
8
--►— Drought Epidemic --e Rood Mass Movement Wet i^Mw Storm�
198'1.1983 1981.1986 1987-198,9 1997.1992 1993-1995 1996.1998 19199-2001 2002-2004 20105-20072008-2'010
4.2.1.9. Indications and Forecasting
Currently, the operational method for forecasting flash floods at the National Weather service is to utilize
the Flash Flood Monitoring and Prediction software package to compare rainfall estimates with flood -
induced rainfall accumulation thresholds, known as flash flood guidance values. The success of this
guidance depends on both accuracy of radar -estimated rainfall rates and the flash -flood guidance values.
The National Weather Service Weather Forecast Offices issues all flash -flood advisories, watches, and
warnings for their respective county warning areas. The primary indicator used by forecasters to predict
onset of flash flooding, is when radar -based rainfall estimates exceed flash flood guidance values over f 1,
3, or 6 hours. Flash -flood guidance is defined as the threshold rainfall required to initiate flooding on small
streams that respond to rainfall within a few hours.
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4.2.1.10. Detection & Warning
The National Weather Service issues flash flood advisories, watches, and warnings.
• Flood Advisory: Thunderstorms have produced heavy rainfall that may result in ponding of water
on roadways and in low-lying areas, as well as rises in small stream levels, none of which pose an
immediate threat to life and property.
• Flash Flood Watch: Atmospheric and hydrologic conditions are favorable for short duration flash
flooding and/or dam break is possible.
• Flash Flood Warning: Excessive rainfall producing thunderstorms have developed, lead to short
duration flash flooding. A warning may also be issued if a dam break has occurred.
4.2.1.11. Critical Values and Thresholds
Using thresholds for flooding indicators can be intellectual traps for the uneducated and what constitutes
an important threshold in one situation may be unimportant in another. In broad terms, moderately high
rainfall rates begin at about 1 inch per hour, and moderately long durations begin at about one hour, but
these should be considered only as the crudest of guidelines.
Conversation with the local National Weather Service in Chanhassen, MN has concluded that local
forecasters tend to look at the rainfall rate and return period more than any amount threshold. It also
depends on antecedent conditions. Consensus between the hydrologist and an operation warning
forecaster is they look for model outputs to show them at least a 10-year event as a starting point to get
flash flooding. In addition, using one particular source, they use a return period for precipitation to have
at least a 20-50-year event to get flash urban flooding in the Twin Cities Metro area.
4.2.1.12. Prevention
To improve water management and protect the sewage system from damage, cities can revamp their
underground pipe and drainage systems by separating rainwater from the sewage system. The separation
enables the wastewater treatment plant to function properly, without it being overburdened by large
quantities of storm water.
Other more obvious methods are to keep sewer systems clean of clog up with waste, debris, sediment,
tree roots and leaves.
4.2.1.13. Mitigation
Areas that have been identified as flood prone areas can be turned into parks, or playgrounds, buildings
and bridges can be lifted, floodwalls and levees, drainage systems, permeable pavement, soil
amendments, and reducing impermeable surfaces. Reducing impervious surfaces could include the
addition of green roofs, rain gardens, grass paver parking lots, or infiltration trenches.
Other mitigation strategies include developing a floodplain management plan, form partnerships to
support floodplain management, limit or restrict development in floodplain areas, adopt and enforce
building codes and development standards, improve storm water management planning, adopt policies
to reduce storm water runoff, and improve the flood risk assessment.
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4.2.1.14. Response
One of the most important things to be done during the initial response is to make sure that people are
safe. If their homes have been damages and are unlivable, finding a place for them to stay is among one
of the top priorities. Next is the access to places if roads are washed out or still underwater. One
complicated factor with flood disasters, is sometimes you do not know how bad the damage is until the
water recedes, which can take time and slow the response. Another important part of response is to make
sure water supply is available as quick as possible if there has been any contamination. The role of
Hennepin County Emergency Management is to coordinate resources that our municipalities may need
to accomplish all response needs.
4.2.1.15. Recovery
As mentioned in river flooding, recovery from floods can take weeks, to months, to years. Urban flooding
is unlike quick disasters (e.g., tornadoes) where you can see the damage immediately, sometimes with
urban flooding you must wait for the flood waters to recede to find out what damage there is to recover
from. A lot of the time, the longer the water level stays too high, the more consequences are introduced
that you must then recover from.
4.2.1.16. References
Bumsted, J. M. 1997. Floods of the Centuries. Winnipeg: Great Plains Publications.
Chang, Heejun, and Jon Franczyk. 2008. 'Climate Change, Land -Use Change, and Floods: Toward an
Integrated Assessment'. Geography Compass 2 (5): 1549-1579. doi:10.1111/j.1749-
8198.2008.00136.x.
Dartmouth.edu. 2015. 'Dartmouth Flood Observatory'. http://www.dartmouth.edu/floods/Archives/.
Doswell, Charles A., Harold E. Brooks, and Robert A. Maddox. 1996. 'Flash Flood Forecasting: An
Ingredients -Based Methodology'. Wea. Forecasting 11 (4): 560-581. doi:10.1175/1520-
0434(1996)011<0560:fffa i b>2.0.co; 2.
Gourley, Jonathan J., Jessica M. Erlingis, Yang Hong, and Ernest B. Wells. 2012. 'Evaluation of Tools Used
For Monitoring and Forecasting Flash Floods in the United States'. Wea. Forecasting 27 (1): 158-
173. doi:10.1175/waf-d-10-05043.1.
Greene, Scott, Yang Hong, Mark Meo, Baxter Vieux, Jonathan Looper, Zhanming Wan, and Amy Goodin.
2015. Urban Flooding and Climate Change. EBook. 1st ed.
http://eos.ou.edu/hazards/urbanflooding/files/Urban_Flooding_Brochure.pdf.
Huntington, Thomas G. 2006. 'Evidence for Intensification of the Global Water Cycle: Review and
Synthesis'. Journal of Hydrology 319 (1-4): 83-95. doi:10.1016/j.jhydrol.2005.07.003.
Jung, I.-W., H. Chang, and H. Moradkhani. 2011. 'Quantifying Uncertainty in Urban Flooding Analysis
Considering Hydro -Climatic Projection and Urban Development Effects'. Hydrol. Earth Syst. Sci. 15
(2): 617-633. doi:10.5194/hess-15-617-2011.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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Killen, Brian. 2015.'Urban Flooding Impacts and Solutions'. In Association of State Floodplain Managers
Conference.
Konrad, C. P. 2014. 'Effects of Urban Development on Floods'. USGS. http://pubs.usgs.gov/fs/fs07603/.
Lana, Juan. 2011. 'The Great Flood Of 1993'. Master, American Military University System.
NOAA National Severe Storms Laboratory. 2015. 'Flood Basics'.
http://www.nssl.noaa.gov/education/svrwxl01/floods/.
Ntelekos, Alexandros A., James A. Smith, Leo Donner, Jerome D. Fast, William I. Gustafson, Elaine G.
Chapman, and Witold F. Krajewski. 2009.'The Effects of Aerosols on Intense Convective
Precipitation in the Northeastern United States'. Quarterly Journal of the Royal Meteorological
Society 135 (643): 1367-1391. doi:10.1002/gj.476.
Oki, Taikan, and Shinjiro Kanae. 2006. 'Global Hydrological Cycles and World Water Resources'. Science
313 (5790): 1068-1072. doi:10.1126/science.1128845.
Sene, Kevin. 2013. Flash Floods. Dordrecht: Springer.
U.S. Department of Commerce. 1998. Ohio River Valley Flood Of March 1997. Silver Spring, MD.
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�,, Hazard Assessment: FLOODING, RIVER
4.2.2.1. Definition
River flooding occurs when river levels rise and overflow
their banks or the edges of their main channel and inundate
areas that are normally dry. River flooding can occur from
both high precipitation weather events and/or ice/snow
melt in the spring. The amount of flooding is usually a
function of the amount of precipitation in an area, the
amount of time it takes for rainfall to accumulate, previous
saturation of local soils, and the terrain around the river
system, dam failures, rapid snowmelt, and ice jams. Over
750 of Presidential Disaster Declarations result from
flooding.
River flooding is classified as Action, Minor, Moderate, or Major based on water height and impacts along
the river that have been coordinated with the National Weather Service. Action means the National
Weather Service, or a customer/partner, needs to take mitigation action in preparation for potential river
flooding. Minor river flooding means that low-lying areas adjacent to the stream or river, mainly rural
areas and farmland and secondary roadways near the river flood. Moderate flooding means water levels
rise high enough to impact homes and businesses near the river and some evacuations may be needed.
Larger roads and highways may also be impacted. Major flooding means that extensive rural and/or urban
flooding is expected. Towns may become isolated and major traffic routes may be flooded.
4.2.2.2. Range of Magnitude
• United States
o Most destructive flood: Mississippi River, 1927 (500 killed; 600,000 homeless)
o Costliest Flood: Great Mississippi & Missouri River Flood of 1993 ($30.2 billion)
• Minnesota
o Most destructive flood: 1997 Red River Flood (58 of 87 counties in Minnesota Federally
Declared Disasters)
o MN costliest flood: 1997 Red River Flood ($2 billion)
4.2.2.3. Spectrum of Consequences B2b
River flooding can affect both people and property. Losses in both wildlife and livestock can also occur,
which can drastically affect the economy. In addition, road washouts, power and water outages can also
be common with river flooding.
4.2.2.4. Potential for Cascading Effects
There is high potential for cascading consequences from river flooding. Depending on severity, there could
be public health sanitation problems, landslides, food spoilage and food production shortages from
farmland being underwater.
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4.2.2.5. Geographic Scope of Hazard Blc
River flooding occurs across all of Hennepin County. Three major rivers create Hennepin County borders
on the northwest, south and east side. Those include the Minnesota, Crow, and Mississippi Rivers. In
addition, several creeks and streams across Hennepin County have a history of flooding, which have
caused damage to property. Some of those include the Minnehaha Creek, and Nine Mile Creek. All these
rivers and creeks are susceptible to early spring snow -melt flooding as well as summer and fall storm
seasons.
4.2.2.6. Chronologic Patterns
River flooding can occur because of both snowmelt and high precipitation events which makes the flood
season start from early spring to early winter. It of course depends on how warm we start to get in the
spring how early, to when we start to get below freezing in the winter. For example, if there is more than
average snowfall/snow depth tied together a spike in temperatures during the early spring, we are melting
snow without having a fully thawed out ground, making soil impervious, which increases the runoff and
subsequently increasing chances for flooding.
4.2.2.7. Historical Data/Previous Occurrence Bld
Floods have been documented all the way back to 1776 in Minnesota. However official American records
don't begin until 1873. Minnesota has seen twenty-four Disaster Declarations due to flooding, six of which
have been in Hennepin County. There have been no other naturally occurring incidents that are within
the scope of this plan.
1965 Flooding (DR-188)
• The Mississippi River at Fridley crested at 20 ft. on April 171h, 1965, which was 4 ft. over flood
stage.
On April 15, the Minnesota River at Savage crested at 719.40 ft., over 17 ft. above flood stage (702
ft.), and 7 ft. above major flood stage (712 ft.). A day later April 16th, the Mississippi river at St
Paul crested at 26.01 ft., 12 ft. above flood stage (14 ft.) and 9 ft. above major flood stage (17 ft.).
The St Croix River at Stillwater followed suit with a record crest of 94.10 ft. on April 18, is 7 ft.
above flood stage (87 ft.) and 5 ft. above major flood stage (89 ft.).
1969 Flooding (DR-255)
• The Mississippi River at Fridley crested at 17.50 ft. on April 14, 1969, which was 1.5 ft. over flood
stage.
• Crow River crested at 16.5 ft. on April 11, 1969, which is 6.5 ft. over flood stage.
1997 Severe Flooding, High Winds, Severe Storms (DR-1175)
• The Mississippi River at Fridley crested at 17.10 ft. on April 10, 1997, which is 1.1 ft. over flood
stage.
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Crow River reached flood stage of 10 feet on 4/4/97 at Rockford which is the river monitoring
point. The river crested at 14.4 feet on 4/9/97 which was the fifth highest crest ever recorded.
The river subsided to below flood stage on 4/20/97. Substantial flooding occurred at a golf course
in the town of St. Michael. (NCDC Storm Events)
2001 Severe Winter Storms, Flooding, and Tornadoes (DR-1370)
The Mississippi River at Fridley crested twice. First at 16.60 ft. on April 15, 2001, and second at
16.40 ft. on April 281h, 2001, 0.6 and 0.4 ft. over flood stage respectively.
Four factors contributed to the flooding of 2001: significant autumn precipitation, heavy winter
snowfall, less than ideal snowmelt scenario, and record -breaking April precipitation
(http://cIimate.umn.edu/doc/journal/fIood_2001/flood_2001.htm). April 161h the Crow River at
Rockford, MN crested at 14.5 feet with a peak discharge at 13,100 ft3/s which is 4.5 ft. over flood
stage.
2010 Flooding (DR-3310)
• Crow River at Rockford reached 13.99 ft. on March 22, 2010, which was 3.99 ft. over flood stage.
2014 Severe Storms, Straight -Line Winds, Flooding, Landslides and Mudslides (DR-4182)
• Crow River at Rockford crested at 15.08 ft. on June 251h, 2014, which was 5.08 over flood stage.
4.2.2.8. Future Trends Ble
Changes in river flooding can be caused by changes in atmospheric conditions, land use/land cover, and
water management. These changes can occur in tandem, or individually which makes it difficult to
determine which factor acts as the driving force of changes in river flooding behavior. However, long-term
data does show and increase in flooding in the norther half of the eastern prairies and parts of the
Midwest. Even with data showing days with heavy precipitation increasing, this trend does not strongly
relate to changes, or increases, in river flooding. One conclusion for this is the mismatch of seasons with
which the high precipitation events occur and most likely season for flooding in most river basins within
our region$. For example, the northern Great Plains typically sees peak river flooding during spring
snowmelt, however, generally the heaviest daily rainfall events occur during the summer.
When considering the issue of future river flood hazard changes, it is important to recognize that urban
and rural land -use impacts, and water management have significant influence on river flood behavior.
While precipitation and flooding have been increasing in the northern half of the eastern prairies, general
circulation models do not show this as an area expected to have a substantial increase in runoff in the
twentieth-century or the twenty-first century forecast.
4.2.2.9. Indications and Forecasting
River Flooding typically occurs hours to days after a high precipitation event. Warnings for river floods can
often provide much more lead-time that those for flash flooding.
4.2.2.10. Detection & Warning
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The National Weather Service issues flood advisories, watches and warnings16
• Flood Advisory: Thunderstorms have produced heavy rainfall that may result in ponding of water
on roadways and in low-lying areas, as well as rises in small stream levels, none of which pose an
immediate threat to life and property.
• Flood Watch: Atmospheric and Hydrologic conditions are favorable for long duration areal or river
flooding.
• Flood Warning: Long duration areal or river flooding is occurring or is imminent, which may result
from excessive rainfall, rapid snow met, ice jams on rivers or other similar causes.
4.2.2.11. Critical Values and Thresholds
The National Weather Service uses flood categories to communicate/categorize the severity of flood
impacts in the corresponding river/stream reach. The severity of flooding at a given stage is not necessarily
the same at all locations along a river reach due to varying channel/bank characteristics or presence of
levees on portions of the reach. Therefore, the upper and lower stages for a given flood category are
usually associated with water levels corresponding to the most significant flood impacts somewhere in
the reach.
The flood categories used by the National Weather Service are:
• Minor Flooding - minimal or no property damage, but possibly some public threat (e.g.,
inundation of roads).
• Moderate Flooding - some inundation of structures and roads near stream. Some evacuations of
people and/or transfer of property to higher elevations.
• Major Flooding - extensive inundation of structures and roads. Significant evacuations of people
and/or transfer of property to higher elevations.
• Record Flooding - flooding which equals or exceeds the highest stage or discharge observed at a
given site during the period of record. The highest stage on record is not necessarily above the
other three flood categories, it may be within any of them or even less than the lowest,
particularly if the period of record is short (e.g., a few years). It is also important to note that
minor, moderate, major flood categories do not necessarily exist for all forecast points. For
example, a location with a permanent levee may begin to experience impacts at moderate
flooding level.
4.2.2.12. Prevention
Most prevention methods of river flooding fall under mitigation actions. See Mitigation below for
methods of prevention.
4.2.2.13. Mitigation
There are many ways to mitigate flooding hazards. Two techniques are hard and soft engineering
mitigation techniques. Hard engineering techniques include building dams, levees, wing dykes, and
diversion spillways. Soft engineering techniques include floodplain zoning, afforestation, wet plain
restoration, river restoration, and removal of properties in flood prone areas.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
4.2.2.14. Response
• Hennepin County Emergency Management Capabilities
• Situation monitoring Station (SMS)
• Immediate Impact Reconnaissance Teams
• Hennepin County Emergency Operations Plan
4.2.2.15. Recovery
Recovery from floods can take weeks to months to years. One complicating factor when it comes to river
flooding, is unlike quick disasters (e.g., tornadoes) where you can see the damage immediately, river
flooding you must wait for the floodwaters to recede to find out what damage there is to recover from. A
lot of the time, the longer the water level stays too high, the more consequences are introduced that you
must then recover from.
4.2.2.16. References
Bumsted, J. M. 1997. Floods of the Centuries. Winnipeg: Great Plains Publications.
Environmental Science Services Administration. 1969. ESSA and Operation Foresight. Washington D. C.:
U.S. Department of Commerce.
FEMA. 2015. "Data Visualization: Summary of Disaster Declarations and Grants I FEMA.Gov". Fema.Gov.
http://www.fema.gov/data-visualization-summary-disaster-declarations-and-grants.
Jackson, Alex. 2015. "Flood Management". Geographyas.lnfo. https://geographyas.info/rivers/flood-
management/.
National Weather Service. 2015. "NWS Flood Related Hazards". Floodsafety.Noaa.Gov.
http://www.floodsafety.noaa.gov/hazards.shtml.
National Weather Service. 2012. Hydrologic Services Program NWSPD 10-9. Department of Commerce.
Peterson, Thomas C., Richard R. Heim, Robert Hirsch, Dale P. Kaiser, Harold Brooks, Noah S.
Diffenbaugh, and Randall M. Dole et al. 2013. "Monitoring and Understanding Changes in Heat
Waves, Cold Waves, Floods, and Droughts in the United States: State of Knowledge". Bull. Amer.
Meteor. Soc. 94 (6): 821-834. doi:10.1175/bams-d-12-00066.1.
U. S. Geological Survey. 2001. Flooding in the Mississippi River Basin in Minnesota. U.S. Geological
Survey.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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d�Hazard Assessment: CLIMATE CHANGE
4.3.1.1. Definition
Climate change is a significant and ongoing change in
the long-term statistical and/or spatial behavior of
weather patterns and variables, as global temperatures
rise in response to the intensified combustion of fossil
fuels and deforestation, both of which increase
concentrations of atmospheric carbon dioxide and
other greenhouse gases. The increasing global
temperatures have, in turn, added additional moisture
to the air through higher evaporations rates, and modified patterns of global atmospheric circulation.
Climatic Background
Hennepin County has a highly variable, continental -type climate with seasonal extremes and a wide range
of weather hazards. Its position near the center of the continent, and halfway between the Equator and
North Pole, subjects it to a wide variety of air mass types throughout the year. During a single year,
Hennepin County will experience heavy snow, frigid wind chills, howling winds, intense thunderstorms,
torrential rains, and heat waves, as well as dozens of bright and sunny days.
In addition to extreme variations between our seasons, Hennepin County's climate also can include large
variations from one year to the next, or even at decadal and multi-decadal scales. The extremely dry years
of 1910, 1936, 1976, and 1988 each were followed within 1-3 years by extremely wet ones. In a six -year
span of the 2010s, Hennepin County experienced its warmest November through March on record in
2011-12, its 51h coldest on record in 2013-14, and its 4th warmest on record in 2015-16.
Climate Change in Hennepin County
In Hennepin County, climate change has meant distinct, measurable trends towards warmer, wetter, and
more humid conditions on average, even as occasional swings towards dry or cold conditions continue to
be part of the climate. As shown in TABLE 4.3.1A, county -averaged temperature and precipitation have
increased by 3.1° F and 3.0 inches, respectively since 1895. The warmest year, winter, and spring, and the
wettest summer and winter, have all occurred since the year 2000. Additionally, nine of the county's 10
warmest years and seven of the 10 wettest years from 1895 through 2023 occurred after 1970, with the
vast majority occurring after 1990.
The county's most extreme precipitation events also occurred during this period, with major flash -flooding
in 1977, 1987, 1997, 2014, and 2016. Record -level humidity extremes occurred more frequently from
2000 through 2023 than at any other time in 121 years of record.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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TABLE 4.3.1A Annual, spring, summer, fall, and winter temperature and precipitation averaged over
Hennepin County showing the 1991-2020 average values, the total change from 1895-2023, the
maximum values and the minimum values. Bold indicates occurrence since the year 2000. Data from
Minnesota DNR Climate Trends Tool (https://arcgis.dnr.state.mn.us/ewr/climatetrends/)
Average Temperature (° F)
Total
Prcc�atiorta>I1h
; ,.
Season
Change, g
Max
Min
CFtrige,
Mfx Mtn
Average,
1895-
AM e a %
1991-2020
2023
(year)
(Year)
45.15
+3.1
48.98
38.83
31.88
+3.0
41.91 12.53
Annual
(2012)
(1917)
(1991) (1910)
Spring
45.11
+2.6
52.65
37.38
8.66
+1.7
14.54 2.37
(Mar-
(2012)
(1907)
(1938) (1910)
May)
Summer
70.02
+1.7
74.57
64.43
13.11
+1.7
22.76 4.75
(Jun-
(1988)
(1915)
(2002) (1936)
Aug)
Fall
47.72
+2.6
52.74
38.62
7.55
-0.1
15.54 1.42
(Sep-
(1963)
(1896)
(1900) (1952)
Nov)
17.68
+5.0
25.39
4.42
2.57
-0.3
5.65 0.59
Winter
(Dec-
(2001-
(1935-
(2022- (1958-
Feb)
02)
36)
23) 59)
As shown in GRAPHIC 4.3.1A, confidence about the extent to which climate change has influenced
changes in the frequency or magnitude of given weather hazards in Minnesota varies considerably. Some
hazards appear strongly linked to climatic change while other hazards have yet to show any influence at
all. In general, the most notable associations include cold weather extremes becoming less severe or less
frequent, and extremes of precipitation becoming more severe or more frequent. Humid heat waves have
a moderately -strong and increasing association with climate change, because of increases in humidity.
Other common hazards, including tornadoes, hail, and strong thunderstorm winds; drought; and summer
high temperature extremes, show little or no long-term change in frequency or magnitude yet.
GRAPHIC 4.3.1A Confidence that climate change has already impacted common Hennepin County
weather/climate hazards through 2023. Provided upon request by Minnesota State Climatology Office.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
Confidence Hazard
Recent & Current Observations
cold
Rapid decline in severity & frequency
JExtreme
xtreme rainfall andl heavy
J.
Becoming (larger and mare frequent
nowfall
..--.— ..---
heat waves
�.umid
Some increase in maximum dew point and Heat
am
Index values since 1980
?Ao erltellVtOwTornadoes, hall,, thunderstorm
Int:ens4ty and frequency unchanged, but seasons
wind's
expanding aggressively
Low
Drought and dry spells
Intense & major episodes in early 2020s but no long
term trend
Lowest
Summer high temperature
Highest temperatures still yield within historical
extremes
ranges, and number of hot days not yet increasing
Worming in Hennepin County
County -averaged statistic indicate Hennepin County has warmed a total of 3.1° F since 1895, or at an
average rate of +0.24° F per decade, which exceeds global and national averages. As illustrated in
GRAPHIC 4.3.1113, using the same data source, nine of the 10 warmest years on record —including the
warmest year in 2012—have occurred since 1990.
GRAPHIC 4.3.1113 Annual temperature, averaged over Hennepin County, 1895-2023, with the trendline
showing average rate of change over the period of record. Table at right shows ten warmest years. Data
from Minnesota DNR Climate Trends Tool (https://arcgis.dnr.state.mn.us/ewr/climatetrends/).
Avg Tern�p
Average Temperature For Hennepin, January -December Year eF)
Year
Avg Ternp 2812
i,;,rl 1931
1895..
2023 1987
Trend 0 1998
2:4"N
Decade 2M
2023
20016
2921
1999
2015
4&98
48.88
48.76
48.22
47.87
7,74
47.82.
47.5
48.91
46,86
Although temperatures are increasing in every season, winter (December through February) has warmed
approximately three times faster than summer (June through August), with a total warming of 5.0° F
versus 1.7° F. Daily overnight low temperatures have also increased about three times faster than daily
high temperatures. The most extreme differences in warming rates are between winter low
temperatures, which have increased by an average of 6.4° F since 1895, and summer high temperatures,
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
which have shown very slight decreases over that same period.
Winter and nighttime -driven warming is consistent across the planet and is especially pronounced in areas
with long and severe winters —when surface heat that would normally escape into space is trapped by
the growing concentration of greenhouse gases. This warming has reduced the availability and depth of
cold air masses, such that cold air outbreaks are not as frequent or severe as they were historically, while
mild winter air masses are now more frequent and often warmer than was typical historically. For
instance, GRAPHIC 4.3.1C shows that daily minimum temperatures of -20' F or lower are now less
common in the Twin Cities than in any other period back to 1873.
GRAPHIC 4.3.1C Frequency of -20' F low temperatures in the Twin Cities. Data source: Applied Climate
Information System, accessed via https://www.dnr.state.mn.us/climate/historical/acis_stn_meta.htm1.
Number of Daily Minimum Temperatures -200 F or Lower
Twin Cities, 1873-2023
27
24
21
18
15
12
9
6
lu 1111111d I � m� 1 1$ $� mobs � mI � �1 I m� m 11� �� �m h
r� r� w m m O O rH N N M M d' Ln Ln l0 l0 F� w Oo 01 01 O rH rH N
00 00 00 00 00 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) O O O O
Across Minnesota and the region, this warming has led to far more warm records than cold records being
set. Since the year 2000, the Twin Cities airport has set 6.7 times more records for highest daily maximum
and highest daily minimum temperature, than for lowest daily minimum and lowest daily maximum
temperature (shown in GRAPHIC 4.3.111)). These recent years representjust 16% of the station history but
account for 33% of the warm records and only 5% of the cold records.
GRAPHIC 4.3.111) Number and types of daily temperature records set from 2000 through 2023 at the
long-term Twin Cities observing site, currently at the MSP airport. Source: Threaded Extremes
(https://threadex.rcc-acis.org/)
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
Twin Cities Daily Temperature Records Set from 2000-2023
(Period of Record 1873-2023)
150
136
135
120
105
105
90
75
60
45
27
30
15
0
mom
Lowest Daily Lowest Daily Highest Daily
Highest Daily
Minimum Maximum ("cold Maximum
Minimum ("warm
highs")
lows")
As noted previously, summer temperatures are increasing in Hennepin County, albeit more slowly than
winter temperatures. The average summer daily maximum or high temperature (June through August)
shows a very slight decrease over time. This observation is matched by the fact that the count of daytime
high temperatures reaching or exceeding 90' F in the Twin Cities has shown no trend since peaking in the
1930s. Meanwhile, average summer minimum or low temperatures show have increased by 3.7° F since
1895, which exceeds the rate of annual average warming for the county. Therefore, the summer warming
experienced in the county so far is attributable to warmer nights, which result in higher minimum
temperatures. GRAPHIC 4.3.1E shows summer temperature behavior over in the Twin Cities and
Hennepin County.
GRAPHIC 4.3.1E Number of 90' F days per year in the Twin Cities, 1873-2023, along with June through
August (summer) average maximum and minimum temperatures for Hennepin County, 1895-2023. Data
for Twin Cities accessed via https://www.dnr.state.mn.us/climate/historical/acis—stn—meta.html, and
for Hennepin County from https://arcgis.dnr.state.mn.us/ewr/climatetrends/.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
90-Degree Days in the Twin Cities, and Summer Average High
and Low Temperatures for Hennepin County
50 88
N 40 N 84
it 80
o
76
�- 30 ' rl 72 "=
o�i J 4"
20 J ? %lf% 1 1 ' 1 68
64
z 10 Jr % F, y �' h9 F 60
✓r 1 ,ro+v ly-•.4 rF H:! �4rr4rpppppifG it rrr r.. rJ4G4 F iF%!' i F:r� �: G�r r��.rr rr
Gorr r, (/i(r r- �/oGrG G✓fir, /araoGroG ,ry .,//. f r0000rGF iy,GGGG /rrrr �o/arr/ GGG I r.�oG%, 56
✓rr ..Ir r /( ;, r/rrGr r ✓. Ir i .rriOrrGrrGr/rrrr/:, f F,. rrrrprr ✓.roGGGG r /, (, :/r// / a GGGo. (GGrrGGrr �/,:.r.,r/r
%�GI ..1 r �/ /G �rGG� / ./err G J Ir rfr4rGr/r/r!Irl l Ir/ r9GGrGG r4 ;/ rrrr ip/ 1 r!/��i. GGGoJ44r444r44 i/,//Ir
/I ;.:.1 r ;l,rr „1.r,rr. r Ir J.rr r r, f/rrrrrrrrrr// .. I r,,,l ,rrrrr rFGr /.roGGGb / � :/./rrrr rrr..IrroGrrGr. r. /vr/r/
rF /r 1rr9G 0 Jr/ rr//rF I(frllrr / ✓GrGrF /.rrrrrrrrrrrrrrrrr�lr /rr//rFGr/rrrrr,.�/ !GGGr.F r /r//vr/ / / rGGG lrFr/r,�/ /r/r/r
52
I-, r-, 00 a) a) O O�i N N M M d' Ln Ln (p (.p F� w w m m O r H r H N
00 00 00 00 00 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) 0) O O O O
iNumber 9 0 + F Highs —Average Summer Daily High
Average Summer Daily Low High Temp Trendline
Low Temp Trendline
Although summertime high temperatures have not increased over the long-term, there have been signs
that high -humidity heat waves are now more common and severe than they were historically (see
Humidity sub -section below)
Increased Precipitation
On a county -averaged basis, precipitation in Hennepin County has increased by an average of 3 inches, or
just under 10% since 1895, with virtually all that increase occurring since 1970. As shown in GRAPHIC
4.3.11F, using the same data source, five of the 10 wettest years on record, including each of the top-3 and
four of the top-5, have occurred since 1990. Only one year since 1990 has made the list of 10 driest years
(2022 was 101h driest, not shown). The long-term Twin Cities climate station, currently at the International
Airport, set all-time annual precipitation records in 2016, and then again in 2019, and finished the 2010s
as the wettest decade on record since the 1870s.
Although at least one month from each season has increasing precipitation, the strongest seasonal
increases have been in spring and summer, whereas average precipitation during fall and winter hardly
changed or decreased slightly from 1895 through 2023. Please refer to TABLE 4.3.1A, at the beginning of
this chapter, for detailed information about seasonal precipitation in Hennepin County
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
GRAPHIC 4.3.11F Annual precipitation, averaged over Hennepin County, 1895-2023, with the trendline
showing average rate of change over the period of record. Table at right shows ten warmest years. Data
from Minnesota DNR Climate Trends Tool (https://arcgis.dnr.state.mn.us/ewr/climatetrends/).
4U
M
go
Precipitation For Hennepin, Mmary-December Year
Precwp
(in) '
Year
-- rreciip (in1991
1895- 2019
2023
'Rciid_ 0. 2002
23"� 1965
Decade
2M
1951
1977
1968
1975
1993
41.91
41.18
40.33
39.7
38,63
37.73
37.38
3TO6
36.7
3.59
Daily and multi -day extremes of rain have become more common in recent decades as well. Rainfall
records for the Twin Cities go back to 1871, but the period since 1970 dominates the heavy rain statistics,
with four of the top -six daily rainfall totals occurring during that period, including the two largest events
on record —which led to significant and even catastrophic flooding.
As shown in GRAPHIC 4.3.1G, annual precipitation and the number of days with heavy rain, or at least
one inch of precipitation, both increased during the most recent several decades.
Seasonal snowfall also has increased and remained historically high during the period of strong winter
warming and the great climatic change in Hennepin County. With snowfall records back to 1884-85, each
of the top three, tour of the top five, and 14 of the 20 snowiest seasons on record occurred after 1980.
Most recently, the 2022-23 winter was third snowiest on record in the Twin Cities, with 90.3 inches. The
period 1980-2023 represents just 32% of the station history of the Twin Cities, but accounts for 70% of
the top-20 seasonal snowfall totals.
Daily and multi -day snowfall extremes are also more common in recent decades. Eight of the 10 largest
daily snowfalls on record occurred after 1980, including each of the top four. GRAPHIC 4.3.1H shows how
days with heavy snow and seasonal snowfall have hit historical high marks only recently.
GRAPHIC 4.3.1G (top) Annual precipitation and average number of days receiving at least one inch of
precipitation, by decade in the Twin Cities. GRAPHIC 4.3.1H (bottom) Seasonal snowfall and average
number of days with at least 4 inches of snow. Data source, both graphics: Applied Climate Information
System, accessed via https://www.dnr.state.mn.us/climate/historical/acis_stn_meta.htm1.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
Twin Cities Average Annual Precipitation and Heavy Rain Days by Decade
40
9
35
34.31
8
31.50 31.14
29.88
_
29.73 29.57
7
30
26.17 26.76
27.74 26.85 27.18
27.11
�
25.17 25.72 24.97
6
u
0
2
23.88
c
}
5
Q'
20
v
4
3
D
3
0
c
a
1.0
2
D
1
0
0
1C�
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII Annual Precipitation Average Number 1-inch Precip Days
Twin Cities Average Seasonal Snowfall and Heavy Snow Days by Decade
60
71
S 50 44.91
_ 43.40
40 39,72 40.27 40.02
0 40
c
30
ouuuuuuuuuuu pppppipi
20
10
60.18
61.45
55.68
49.05
44.66
0
yo5 Lori �o�i �o`' o`' �& �05 �05 oh �05 Oy &
CO
�� �� �� �� �Q � o
Seasonal Snowfall Days with 4+ Inches
4,5
N
4 0
3.5 0-
3 E
z
2.5 ago
U
2 °1
a'
Even though periods of intense growing season drought have defined the climate of the early 2020s in
Hennepin County, these dry conditions have not reversed the long-term trend towards more
precipitation. In fact, as can be seen in GRAPHIC 4.3.1G above, even with the drought episodes, annual
precipitation during the early 2020s is still higher than every decade from the 1920s through the 1960s.
This is because the dry conditions have been episodic, generally limited to the warm season, and often
followed by very wet conditions in the cooler months.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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For instance, the six months from May through October of 2022 were the 41h driest on record in Hennepin
County, with the US Drought Monitor indicating Extreme Drought, the second -highest level, over much of
the county. A very wet period quickly followed it, however, and the six months from November through
April 2023 became the fourth wettest on record. Dry conditions set in again, with May through August
2023 ranking 3rd driest on record, followed by much -above -normal precipitation in September and
October, and then the third -wettest December on record. This oscillation between wet and dry regimes
is illustrated in GRAPHIC 4.3.11.
GRAPHIC 4.3.11 Sequential episodes of very dry and very wet conditions during 2022 and 2023 in
Hennepin County. Source: DNR Climate Trends (https://arcgis.dnr.state.mn.us/ewr/climatetrends/).
Recent Precipitation Departures from 1991-2020 Averages, and
Ranks from 1895 to 2023 Hennepin County
60% 54% 4th wettest
40
a�
20%
0%
� -20
0
-40
-60
-80
Humidity
-50% 4th driest -55% 3rd driest
June - October 2022 November 2022 - April May - August 2023 September -
2023 December 2023
Increased humidity has been notable during all seasons in recent decades. From 2000 through 2023, the
Twin Cities long-term climate station measured more daily record -high and fewer daily record -low dew
point temperatures (a measure of humidity) than any other time since records began in late 1902. Of the
14 documented days with extreme humidity yielding at least one hourly 80' F dew point reading, 10 have
occurred since 1990, and none occurred prior to the 1960s.
Even though the highest air temperatures of summer and the number of 90' or 95' F days has not
increased over the long-term, extremely humid conditions have at times combined with hot air masses to
yield unprecedented Heat Index values, which measure what the air feels during heat waves. On July 19,
2011, Flying Cloud airport measured a Heat Index of 1227, while the Twin Cities airport measured 119'F.
On August 22, 2023, another intense heat wave fueled by high moisture and dew points, sent Heat Index
values into the upper 110s F across the county, with 120' F recorded at the Hennepin -West Mesonet
stations located in Hanover and at the MSP Airport.
Record humidity has not been confined to the summer, when it is most noticeable to humans, but in fact
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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has been observed throughout the year with increased frequency during recent decades. Most notably,
in 2021 latest date a 50' F dew point had ever been recorded at the Twin Cities long-term station advanced
10 days, to December 151h, in 2021, and then 10 more days, to December 251h in 2023. The latest 60' dew
point on record was measured on November 101h of 2022. The earliest date to measure 50' F was February
20, 2017, and the earliest 60' F dew point occurred on March 17, 2012.
Increased humidity is not just a human comfort concern; it also has implications for precipitation and
severe weather frequency, because water vapor is what fuels precipitating weather systems. The high
dew points recorded on December 15, 2021, were associated with an unprecedented winter outbreak of
tornadoes and damaging thunderstorm winds in southeastern Minnesota. The December 25, 2023, high
dew points were associated with an unusually heavy December rainfall event. The 60' F dew point on
March 17, 2012, was matched or nearly matched for several more days, and fueled a rash of rare mid -
March severe thunderstorms across Minnesota.
4.3.1.2. Range of Magnitude
Climate change is unlike other hazards because it is not episodic and does not "strike." Rising global
temperatures represent a constant and increasing force that is always present, even when it is not
obviously detectable in each weather pattern or climatological data set.
The magnitude of climate change is generally measured as the total warming of the earth's atmosphere
above "pre -industrial" temperatures, with that period reflecting 1850-1900 averages in some data sets,
or simply beginning in 1880 in other data sets. These temperatures are closely, but not exclusively linked
to the global concentrations of carbon dioxide, as measured at the Mauna Loa observatory in Hawaii.
Carbon dioxide levels have increased annually for decades, but while global temperatures have increased
steadily, natural factors, like El Nino and some ocean circulation phenomena, drive normal fluctuations
the global heat content.
Virtually all data sets show that the earth has warmed between 1.1° and 1.3° C (2 — 2.3° F), and most show
a continued warming rate 0.1 to 0.2° C (.18° to 0.36' F) per decade. These warming magnitudes and rates
are smoothed to remove the influence of large short-term variations, including the world -record
temperature spikes observed in 2023, when global temperatures exceeded 1.5° C above pre -industrial
levels at times, and when the average anomaly was 1.3° to 1.54' C for the year.
Translating the magnitude of warming globally, into weather or climate impacts experienced in Hennepin
County is not straightforward. The science of "attribution," or determining how much of a given trend,
change, or event, is attributable to human -caused climate change, has largely focused on events that to
date have not included the area. These studies usually indicate that climate change is responsible for all,
or nearly all long-term warming in non -urbanized areas, and that it enhances or intensifies some types of
extreme weather events but does not "cause" them.
Given that the Twin Cities airport climate station is and has always been in an urban, built-up area, we
know that some of the temperature increase seen there is because of urban "heat island" effects and not
the changing global climate. At rural stations, and in homogenized data sets like the county -averaged one
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
referenced in other sections in this chapter, the urban warming "bias" is minimized or even non-existent.
Rural counties to the west have similar long-term temperature increases to Hennepin County. It is
therefore likely that the vast majority of the 3.1° F of average annual warming and the other seasonal
warming reported for Hennepin County results from human -caused climate change.
Applying findings from attribution studies in other areas to common hazards in Hennepin County suggests
the following:
• Climate change is likely making humid heatwaves in Minnesota more severe by increasing Heat
Index values by 4°-6° F over what would have been observed without a warmer global climate.
This also has the effect of increasing the probability of occurrence dramatically.
• Extremes of precipitation, including snowfall, may be 10-15% larger because of the higher water
content of the atmosphere due to rising global temperatures.
o Similarly, the damaging snows of December 13-16, 2022, to the north of the Twin Cities
may have had two climate changes making them more likely: 1) the increased availability
of moisture because of higher global temperatures, and 2) the winter warming that
caused the snow to be wetter, heavier, and thus more destructive.
• Out -of -season events that result from unusually warm conditions, like the severe weather
outbreak of December 15, 2021, or a record -breaking heat wave in early October of 2023, may
have been much more likely because of climate change, and therefore would have been
substantially less probable without human -caused warming.
• Any events of these types will become more probably with continued warming, and that
continued warming would make larger contributions to future events, meaning potentially
greater extremes of precipitation and humid heat waves in the decades ahead.
4.3.1.3. Spectrum of Consequences B2b
In Hennepin County, climate change has led to warmer conditions in general, especially during winter;
more precipitation, including during drought years; greater extremes of rain and snow; and more intense
humidity -driven heatwaves. Additionally, the seasonal ranges of heatwaves and severe weather events
have expanded. Even though year-to-year and multi -year variations will continue, these changes are
projected to continue as well, with an enhancement of some hazards as the world warms.
Warmer winter conditions pose some benefits for human comfort and safety but pose recreational risks
because of dangerous lake ice that may be unsuitable for fishing and ice skating. Natural systems
dependent on cold weather to keep out competitive species and predators also suffer from enhanced
winter warming, which can alter ecosystems and natural resources.
Increased rain and snow extremes mean roads and their supporting infrastructure may face increased
damages if they are not built to higher design standards. Heavy, wet snow, as occurred in the 2022-23
winter, can damage trees, knock out power, and overwhelm some structures with snow loads.
Greater precipitation totals during wet years also would imply high water levels on area lakes and streams,
increasing chances for erosion, pollution from runoff, degraded water quality, stream bank failure,
landslides, and residential flooding.
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Humid heatwaves pose significant dangers to those working, recreating, or living outside. Increases in
these dangerous conditions will affect larger proportions of the population, as the risk moves from those
most vulnerable, to the general population, and even those in excellent physical condition.
Following are some consequences expected with climate change in Hennepin County:
• Less reliable and more dangerous lake ice
• More periods of bare/snow-free ground, allowing frost to penetrate to great depths during cold
outbreaks.
• Expansion of the heavy rainfall season, leading to enhanced peak stream flows, and altered timing
of normal flow regimes.
• Increased runoff and flash -flooding as the largest events intensify and become more common.
• Water infrastructure damage from intense rainfall events
• Agricultural stress, from shifting crop ranges, heat, drought, and extreme rainfall
• More days with high water vapor content and heat index values
• Greater summer cooling costs, more days requiring cooling.
• New invasive species, both terrestrial and aquatic, especially those acclimated to warmer climates
or those that were cold weather limited.
• "Hyper -seasonality," as warm conditions develop during the "off-season," leading to bouts of
heavy rainfall or severe weather, followed by wintry conditions.
• Increase in frequency of freeze -thaw cycles, as winter is increasingly infiltrated by warm
conditions.
Some positive benefits of a changing climate might include fewer automobile accidents and damage as
more winter precipitation falls in the form of rain rather than snow or ice. However, warmer winters
doesn't necessarily mean rain instead of snow, it could mean more ice storms, which would lead to
dangerous driving conditions and power outages due to down power lines. Also, rain falling in the winter
can be disastrous if it is followed by sharply colder air and a "flash -freeze."
Additionally, summertime air temperatures are extremely likely to begin increasing in the decades ahead,
and possibly before 2030. When these hotter summers pair with normal dry swings in the climate, they
will increase drought severity and water demand, while also increasing the potential for wildfire (see
drought section of risk assessment).
Some new research (as of 2023) indicates that extreme windstorms associated with thunderstorms may
become more probable, larger, and possibly more intense as the world continues warming. These studies
indicate that, as a result, a given extreme wind event may have the ability to affect more people and more
property than in the past —not accounting for the growth and the expansion of Hennepin County's
population.
In recent years, smoke from wildfires has degraded air quality, occasionally to dangerous levels in
Hennepin County. Climate models project that wildfires and downstream smoke infiltration will become
more common as northern forests are weakened by warming winters, more severe heat waves, and even
precipitation extremes. Increased smoke particulates are a health hazard for everyone, but
disproportionally affects those with respiratory challenges, limited mobility, other health conditions, and
those who cannot shelter from the smoke.
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4.3.1.4. Potential for cascading effects
Climate change enhances some hazards, so please see chapters on Extreme Heat, Straight-line winds,
Extreme rainfall, and non -convective winds, to understand the potential cascades that climate change
may enhance or cause.
The most novel group of cascading effects to consider with climate change is when warm conditions
produce a meteorological situation previously unheard of or quite rare. Winter severe thunderstorm
events, for example, may be more likely as winters continue warming, but to occur, they would almost
certainly be accompanied by a powerful low-pressure system capable of producing plunging temperatures
and strong winds. Communities facing power outages, debris clean-up, and even search -and -rescue
operations may then have to face with cold weather hazards.
4.3.1.5. Geographic scope of hazard Blc
Climate change is a global hazard and influences weather and climate patterns in some way virtually
everywhere. In Minnesota, the greatest warming has been in the northern part of the state, and the
largest precipitation increases have been in the southeastern and central portions of the state. However,
the entire state of Minnesota, including all of Hennepin County is at risk from increased precipitation
extremes, more intense humid heat waves, and the seasonal expansion of severe thunderstorms and
heat.
4.3.1.6. Chronological patters (seasons, cycles, rhythm)
Warming is occurring year-round, though the most pronounced changes have been during winter. It
should be noted that the area's climate exhibits natural high variability, and that variability will continue,
even as Minnesota warms. It should also be noted that hazard risk does not necessarily follow the cycle
of greatest warming. For instance, damaging rains are far more likely in the summer than the winter.
4.3.1.7. Historical Data/Previous Occurrence Bld
The year 2012 may be thought of as a preview of the years and decades ahead. The 2011-12 winter was
warm and short, with bouts of 50s and 60s observed throughout Minnesota during January. March that
year saw 8 record high temperatures in Minneapolis, and 8 days above 70 degrees. Throughout the region,
March 2012 obliterated long-standing daily and monthly temperature records.
The warmth continued through the remainder of the spring and into the summer, with over 30 days above
90 degrees in parts of Hennepin County, and 2 days above 100 at MSP. This was the first summer with
multiple 100-degree readings since the summer of 1988.
Others may consider the late 2010s to be representative of the future, because:
Based on the Midwest chapter from the 2014, 2018, and 2023 National Climate Assessment, a review of
other recent research into the region, and analyses of quality -controlled, nationally standardized, and
publicly available data, the recent trends can be described as follows.
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• Bouts of extreme cold in Hennepin County and throughout Minnesota and the region are now at
an all-time low in terms of both frequency and severity. Of all changes, the loss of cold weather
extremes has the strongest link with climate change.
• Extreme rainfall episodes have become both more intense and more frequent, and Minnesota
has seen seven "mega -rainfall" events since the year 2000. Changes in extreme rainfall behaviors
are strongly linked to climate change.
• A general increase in annual and seasonal snowfall has been punctuated by an uptick in the size
and frequency of large snowfall events. This is likely related to the presence of warmer air and
more water vapor during winter, which provides more energy to passing low pressure systems
capable of producing snow.
• Severe thunderstorms and tornadoes pose challenges to long-term analyses because of changes
in reporting procedures and detection technologies over time. That said, Minnesota has been in
a pronounced severe weather lull since the summer of 2011, which followed a very active spring
and record -setting year for tornadoes in 2010. Confidence in the link between climate change and
observed severe weather trends is low. However, the severe weather season has expanded
aggressively in recent years, with record -early tornadoes in Minnesota on March 6, 2017, and
record late tornadoes (by 30 days) on December 15, 2021.
• Humid heat waves have increased in severity and frequency, in response to higher humidity.
Summertime high temperatures and the number of hot days has not changed yet.
• Despite three straight years of significant growing season drought in 2021-2023, Hennepin County
still does not have a long-term trend towards increased drought frequency or severity.
These are just some examples of the effects of climate change in Hennepin County.
4.3.1.8. Future trends/likelihood of occurrence Ble
Projections of future climates from multiple sources indicate that the area is likely to continue to see a
rapid erosion of winter extreme cold temperatures, and it is expected that Hennepin County will fail to
reach previously common benchmarks by increasingly large margins.
Extreme rainfall is projected to increase, but it should not be expected to do so on a year -after -year basis.
Instead, climate change is increasing the long-term frequency and magnitude of these events, meaning
that storms of a certain size may come every 10-20 years instead of every 50 years. By mid-century, the
area should receive an additional 3-8 days per decade with rainfall in the top 2% of the historical
distribution (GRAPHIC 4.3.1J). Thus, the expectation is that unprecedented rainfall events will occur at
some point this century, but their likelihood in the next decade will be limited by their overall statistical
rareness.
GRAPHIC 4.3.1J Average difference in number of days per year by mid-century (2040-2070) with rainfall
in upper 2% of distribution.
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Difference in Number of Days
j (Ir, 0 1.1 1.5
Snowfall extremes should continue to increase as well, although the warming of winter in general and the
effect of increased winter rains should eventually begin decreasing seasonal snowfall. However, even the
most aggressively warm model scenarios show that snow will be a major if not dominant winter
precipitation through much of the century.
Severe convective storms and tornadoes are unlikely to remain at the current low incidence rates, and a
"rebound" appears likely within the next decade, based on historical frequency alone. The association
between this rebound and climate change will remain unclear, however. It is increasingly clear that severe
convective storms will have expanded seasonal and geographic ranges. It is possible, based on new
research, that extreme straight-line thunderstorm winds will be larger and/or more intense as the climate
continues warming.
Humid heat waves have already begun increasing in response to greater available humidity. Projections
indicate that summer temperatures are likely to increase significantly in Minnesota as well during the 21st
century. It remains unclear when these trends would begin, given a lack of any recent trends toward
increasing summertime high temperatures. However, projections indicate that by mid-century, the Twin
Cities should expect 5-10 additional days per year above 95' F, which would more than double current
frequencies (GRAPHIC 4.3.1K)
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GRAPHIC 4.3.1K Difference in number of days per year by mid-century (2040-2070) maximum
temperatures above 95' F.
Like severe convective storms, drought has shown no trend towards increasing in frequency, severity,
duration, or areal coverage in recent decades. This is because the increases in precipitation have
overwhelmed even recent significant drought episodes.
Projections, however, indicate that drought will at a minimum become more severe in the future —when
it occurs. This increase would be in response to the inevitable increase in summertime high temperatures.
It remains unclear whether the actual frequency of drought conditions will increase. Projected increases
in the number of consecutive dry days during dry spells suggest that drought frequency may increase, in
the form of short, "flash" drought episodes, as have been common in the early 2020s (GRAPHIC 4.3.1L).
GRAPHIC 4.3.1L Difference in number of consecutive days per year by mid-century (2040-2070) with less
than 0.01 inches of precipitation. An increase in this variable is associated with an increase in the chance
of drought in the future.
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Projected changes in the same weather hazards that were shown and discussed previously are shown in
GRAPHIC 4.3.1M, along with confidence associated with the projections. Highest scientific confidence is
in the continued warming of winter, the continued loss of cold weather extremes, and continued increases
in extreme rainfall, leading to occasional unprecedented events. Increases heat waves are projected with
high confidence, because of both the increases in humidity already ongoing, and the increases in summer
temperature extremes projected unanimously by climate models. With these increases in heat extremes,
drought becomes somewhat more likely too, as described above; the severity of drought should increase
as summer temperatures do, but it is unclear whether drought frequency will increase. As the century
wears on, heavy snow events may continue being more extreme, but they should become less frequent
as winter warms even more. Confidence remains moderately low with severe thunderstorms in general,
even though seasonality will continue changing.
GRAPHIC 4.3.1N combines information known about observed and projected climate trends in
Minnesota.
GRAPHIC 4.3.11VI Confidence that various common Minnesota weather hazards will be impacted by
climate change through 2070.
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Extreme cold
Continued rapi�d decrease in severity aind freqUency
Extr
Extreme rainfall
Unprecedented events miolre common
Heat waives
Surnmer hiigh temperatures, maxinnurn, dew point
and heat index values al� projected to increase
frequency and duration projections Unclear
ModeraWIV Low Heavy snowfall
Greater extremes, but events less frequent as winter
rain increases
M,oclorate1V Low, Tornadoes, hail, thunderstorm
Intensity and frequency unclear but continued
winds,
seas,onial expansion aind larger "outbreaks"' possible
8RAPH|C4L3'1N Confidence that various common Minnesota weather hazards will be impacted by
climate change beyond 2O26.
Winter temperatures Increasing rapiffly, with ioss of cold extremes continued increases, with narrowing ofwinter season
Rainfall Increasing aill seasons, with more extreme, and Incresses likelybut fitning and seasonality uncertain
Snowfall Increasing, with more extreme and darraging
events
Summer temperature No liong-term trend for highternperature records,
extremes and heat waves but hot season expandingand hurnid heat waves
Increasing
Drought molo�ng-termmtrend despite intense & rrajor
mpiomdeavmearly 2mzms
Tornadoes, hailf Trends unclear,,but seasons and geographit
thunderstorm winds ranges expandfng
4.3.1.9. Indications and Forecasting
seasonal decreases |ukeiy,buusome increases
possible for extreme events
Increased severity likely as swrnmer heat increases;
projections undear for frequency and duration
projections unclearfor frequency and intensity, but,
confinued seasonal expans1ion and more "outbreaks,"
Climate change b known to beongoing and is continuously monitored by climatologists, atmospheric
scientists, chemists, biologist, physicists, oceanographers, geologists, and many others. This includes the
study of greenhouse gas concentrations, global temperatures, historical events, complex interactions
between varying earth systems, and building forecasting models to make sophisticated global, regional,
and local projections.
The state of the climate and the state of climate science are monitored and reported regularly by
thousands of scientists in an array of fields and summarized in assessment reports provided by the
Intergovernmental Panel on Climate Change (IPCC) and by the US Global Change Research Program.
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4.3.1.10. Detection & Warning
The same scientists who contribute to the body of research summarized in the national and global
assessment reports also issue statements and warnings regarding the trajectory of the climate and the
steps needed to change that trajectory, and/or to protect ourselves against potentially dire consequences
of not changing that trajectory.
While there are no warnings for climate change like tornado warnings, or flash flood warnings, the IPCC
effectively issues warnings with the release of its reports. Some scientists also often issue warnings
individually or as smaller groups. The overwhelming consensus among climate scientists is that the climate
is changing faster than we can manage and that without fast reductions in greenhouse gas emissions, we
will face severe consequences from heat waves, rising sea levels, larger storms, and greater extremes in
general.
4.3.1.11. Critical values and thresholds
Climate change is an ongoing phenomenon that manifests itself through the persistent change in the
statistical behavior of climatic variables. Although no critical values and thresholds exist in Minnesota, the
following indicators represent rare and/or uncharted territory in Hennepin County, and would indicate
climate change mileposts:
• February ice -out, Lake Minnetonka; earliest on record is March 11, 1878
• Lack of zero or colder temperature at MSP; has not happened yet, and fewest such readings was
two in 2001-02
• Winter average temperature above 27' F --has only happened once, during "year without a
winter" of 1877-78
• Low temperatures failing to reach -10' F. Previously it was -20' F, and then -157, but it we now
commonly fail to reach these thresholds.
• No subzero high temperature all winter
• Summertime minimum temperatures in excess of 80 degrees
• 90' F in March, 70' F in December or February
• Tornadoes or severe convective storms at any time from November through February
4.3.1.12. Prevention
Preventing climate change requires global coordination and massively reducing the amount of coal, oil,
and natural gas burnt for personal, municipal, industrial, and vehicular purposes. However, in the
mitigation section you will find strategies to reduce the effects as well as adaptation examples for the
changing climate.
Hennepin County has a comprehensive Climate Action Plan that includes ambitious goals to reduce
greenhouse gas emissions across the county to "Net Zero" (no emissions, or all emissions balanced by
reductions) by 2050. While this alone cannot stop climate change, it represents the type of action needed
on a larger scale to do so.
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4.3.1.13. Mitigation
In climate change studies and policy, "mitigation" refers to prevention of the climate change specifically
through reducing greenhouse gas concentrations globally. The term "adaptation" generally refers to
protecting systems and communities from the changing climate.
Hennepin County's Climate Action Plan lays out steps for not only reducing the greenhouse gas emissions
that lead to heat retention and rising global temperatures, but also to adapt the county to the changing
climate in a manner intended to improve resiliency and equity, while reducing vulnerabilities.
The plan has specific goals to:
• Protect and engage people, especially vulnerable communities.
• Enhance public safety.
• Increase the resilience of the built environment and protect natural resources.
• Reduce emissions in ways that align with core county functions and priorities.
• Partner in ways that can be most impactful.
The overall risks of future climate change impacts can be reduced by limiting the rate and magnitude of
climate change by efforts to reduce or prevent emission of greenhouse gases.
Adaptation and mitigation are complementary strategies for reducing and managing risks of climate
change. Mitigation can mean using new technologies and renewable energies, making older equipment
more energy efficient, or changing management practices or consumer behavior. It can be as complex as
a plan for a new city, or as a simple as improvements to a cook stove design. Efforts underway around the
world range from high-tech subway systems to bicycling paths and walkways. Protecting natural carbon
sinks like forests and oceans or creating new sinks through green agriculture are also elements of
mitigation. Adaptation examples are shown in Table 4.3.1113.
Table 4.3.1113.
Category
Examples
Human
Develop.
Improved access to education, nutrition, health facilities, energy, safe housing &
settlement structures, & social support structures; Reduced gender inequality &
marginalization in otherforms.
Poverty Alleviation
Improved access to & control of local resources; Land tenure; Disaster risk reduction;
Social safety nets & social protection; Insurance schemes.
Income, asset & livelihood diversification; Improved infrastructure; Access to technology
Livelihood Security
& decision- making fora; Increased decision -making power; Changed cropping, livestock
& aquaculture practices; Reliance on social networks.
Disaster Risk
Early warning systems; Hazard & vulnerability mapping; Diversifying water resources;
Management
Improved drainage; Flood & cyclone shelters; Building codes & practices; Storm &
wastewater management; Transport & road infrastructure improvements.
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Maintai ning wetlands & urban green spaces; Coastal afforestation; Watershed &
Ecosystem
Management
.
reservoir management; Reduction of other stressors on ecosystems & of habitat
fragmentation; Maintenance of genetic diversity; Manipulation of disturbance regimes;
Community -based natural resource management.
Spatial or land-
Provisioning of adequate housing, infrastructure & services; Managing development in
use planning
flood prone & other high -risk areas; Urban planning & upgrading programs; Land zoning
laws; Easements; Protected areas.
Engineered & built -environment options: Sea walls & coastal protection structures;
Flood levees; Waterstorage; Improved drainage; Flood & cyclone shelters; Building
codes & practices; Storm & wastewater management; Transport& road infrastructure
Technological options: New crop & animal varieties; Indigenous, traditional & local
knowledge, technologies & methods; Eff icient irrigation; Water -saving technologies;
Structural/Phy
Desalinization; Conservation agriculture; Food storage & preservation facilities; Hazard &
vulnerability mapping & monitoring; Early warning systems; Building insulation;
Mechanical & passive cooling; Technology development, transfer &diffusion.
Ecosystem -based options: Ecological restoration; Soil conservation; Afforestation &
reforestation; Mangrove conservation & replanting; Green infrastructure (e.g., shade
trees, green roofs); Controlling overfishing; Fisheries co -management; Assisted species
migration & dispersal; Ecological corridors; Seed banks, gene banks & other exsitu
conservation; Community -based natural resource management.
Services: Social safety nets & social protection; Food banks & distribution of food
surplus; Municipal services including water& sanitation; Vaccination programs; Essential
public health services; Enhanced emergency medical services.
Economic options: Financial incentives; Insurance; Catastrophe bonds; Payments for
ecosystem services; Pricing waterto encourage universal provision and careful use;
Microfinance; Disaster contingency funds; Cash transfers; Public -private
Institutional
Laws& regulations: Land zoning laws; Building standards & practices; Easements; Water
regulations & agreements; Laws to support disaster risk reduction; Laws to encourage
insurance purchasing; Defined property rights & land tenure security; Protected areas;
Fishing quotas; Patent pools& technology transfer.
National& government policies & programs: National & regional adaptation plans
including mainstreaming; Sub -national & local adaptation plans; Economic
diversification; Urban upgrading programs; Municipal water management programs;
Disaster planning& preparedness; Integrated water resource management; Integrated
coastal zone management; Ecosystem -based management; Community -based
Educational options: Awareness raising & integrating into education; Gender equity in
education; Extension services; Sharing indigenous, traditional & local knowledge;
Participatory action research & social learning; Knowledge -sharing & learning platforms.
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Informational options: Hazard & vulnerability mapping; Early warning & response
Social
systems; Systematic monitoring & remote sensing; Climate services; Use of indigenous
climate observations; Participatory scenario development; Integrated assessments.
Behavioral options: Household preparation & evacuation planning; Migration; Soil
& water conservation; Storm drain clearance; Livelihood diversification; Changed
cropping, livestock & aquaculture practices; Reliance on social networks.
Practical: Social & technical innovations, behavioral shifts, or institutional & managerial
changes that produce substantial shifts in outcomes.
Spheres of change
Political: Political, social, cultural & ecological decisions & actions consistent with
reducing vulnerability & risk & supporting adaptation, mitigation & sustainable
development.
Personal: Individual & collective assumptions, beliefs, values & worldviews influencing
climate -change responses.
4.3.1.14. Response
• See Hennepin County Emergency Operations Plan
4.3.1.15. Recovery
Because it is very difficult to link a specific event to climate change, it is difficult to discuss recovery as it
pertains to climate change versus each individual event as in other hazards. Please refer to the other
hazard sections to review recovery from the specific hazard.
4.3.1.16. References
Brooks, H.E. 2013. "Severe Thunderstorms and Climate Change". Atmospheric Research 123: 129-138.
doi:10.1016/j.atmosres.2012.04.002.
Climate.nasa.gov, 2016. "Vital Signs of the Planet". http://climate.nasa.gov/evidence/.
Diffenbaugh, N. S., M. Scherer, and R. J. Trapp. 2013. "Robust Increases In Severe Thunderstorm
Environments In Response To Greenhouse Forcing". Proceedings of the National Academy of
Sciences 110 (41): 16361-16366. doi:10.1073/pnas.1307758110.
Dnr.state.mn.us, 2012. "Balmy Winter in The Twin Cities 2011-2012: Minnesota DNR".
http://www.dnr.state.mn.us/climate/journal/11_12_balmy_winter.html.
Dnr.state.mn.us, 2015. "The Year without A Winter: 1877-78: Minnesota DNR".
http://www.d n r.state. m n.us/cl i mate/jou rna 1/1877_1878_wi nter. htm I.
Freshwater Society,. 2013. "157 Years of Lake Minnetonka Ice -Out History". http://freshwater.org/wp-
content/uploads/joomla/iceout/2012iceout. pdf.
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Gonzalez-Aleman, J. J., D. Insua-Costa, E. Bazile, S. Gonzalez -Herrero, M. Marcello Miglietta, P.
Groenemeijer, and M. G. Donat. 2023. Anthropogenic Warming Had a Crucial Role in Triggering the
Historic and Destructive Mediterranean Derecho in Summer 2022. Bulletin of the American
Meteorological Society, 104, E1526—E1532, https://doi.org/10.1175/BAMS-D-23-0119.1.
Harding, Keith J., Peter K. Snyder, and Stefan Liess. 2013. "Use of Dynamical Downscaling To Improve the
Simulation of Central U.S. Warm Season Precipitation in CMIPS Models". Journal of Geophysical
Research: Atmospheres 118 (22): 12,522-12,536. DOI: 10.1002/2013jd019994.
Hennepin County. 2021. "Climate Action Plan." https://www.hennepin.us/climate-action/-
/media/climate-action/hennepin-county-climate-action-plan-final.pdf
IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the
impacts of global warming of 1.5°C above pre -industrial levels and related global greenhouse gas
emission pathways, in the context of strengthening the global response to the threat of climate
change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai,
H.-O. Portner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Pean, R. Pidcock, S.
Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T.
Waterfield (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 3-24,
doi :10.1017/9781009157940.001.
IPCC, 2023: Summary for Policymakers. In: Climate Change 2023: Synthesis Report. Contribution of
Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on
Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, pp. 1-
34, doi: 10.59327/IPCC/AR6-9789291691647.001
Lasher-Trapp, S., S. A. Orendorf, and R. J. Trapp. 2023. Investigating a Derecho in a Future Warmer
Climate. Bull. Amer. Meteor. Soc., 104, E1831—E1852, https://doi.org/10.1175/BAMS-D-22-0173.1.
Marvel, K., W. Su, R. Delgado, S. Aarons, A. Chatterjee, M.E. Garcia, Z. Hausfather, K. Hayhoe, D.A.
Hence, E.B. Jewett, A. Robel, D. Singh, A. Tripati, and R.S. Vose, 2023: Ch. 2. Climate trends. In:
Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C.
Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA.
https://doi.org/10.7930/NCA5.2023.CH2
Minnesota DNR, 2021. "Mid -December Tornadoes, Derecho, and Damaging Cold Front --December 15-
16, 2021." https://www.dnr.state.mn.us/climate/journal/mid-december-tornadoes-derecho-and-
damaging-cold-front-december-15-16-2021.html
Minnesota DNR, 2022. "Blizzard, Ice, Slush Storm, and Rain, December 13-17, 2022"
https://www.dnr.state.mn.us/climate/journal/blizzard-ice-slush-storm-and-rain-december-13-16-
2022.html
Minnesota DNR, 2023. "Historic Autumn Heat, September 30 - October 3, 2023"
https://www.dnr.state.mn.us/climate/journal/historic-autumn-heat.html
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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National Climate Assessment, 2016. "National Climate Assessment".
http://nca20l4.globalchange.gov/report.
Ncdc.noaa.gov, 2016. "Global Analysis - Annual 2015 1 National Centers for Environmental Information
(NCEI)". http://www.ncdc.noaa.gov/sotc/global/201513.
NOAA, Climate.gov, 2024. "What's in a number? The meaning of the 1.5-C climate threshold."
https://www.climate.gov/news-features/features/whats-number-meaning-15-c-climate-threshold
Prein, A.F. 2023. Thunderstorm straight line winds intensify with climate change. Nature Climate Change
13, 1353-1359, https://doi.org/10.1038/s41558-023-01852-9
U.S. Department of Commerce National Oceanic and Atmospheric Administration, 2013. Regional
Climate Trends and Scenarios for the U.S. National Climate Assessment. Washington, D.C.
USGCRP, 2023: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E.
Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington,
DC, USA. https://doi.org/10.7930/NCA5.2023
Unep.org, 2016. "Climate Change Mitigation". http://www.unep.org/climatechange/mitigation/.
Wilson, A.B., J.M. Baker, E.A. Ainsworth, J. Andresen, J.A. Austin, J.S. Dukes, E. Gibbons, B.O. Hoppe, O.E.
LeDee, J. Noel, H.A. Roop, S.A. Smith, D.P. Todey, R. Wolf, and J.D. Wood, 2023: Ch. 24. Midwest.
In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C.
Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA.
https://doi.org/10.7930/NCA5.2023.CH24
World Weather Attribution, 2017. "Climate change fingerprints confirmed in Hurricane Harvey's rainfall,
August 2017." https://www.worldweatherattribution.org/hurricane-harvey-august-2017/
World Weather Attribution, 2021. "Western North American extreme heat virtually impossible without
human -caused climate change." https://www.worldweatherattribution.org/western-north-
american-extreme-heat-virtually-impossible-without-human-caused-climate-change/
World Weather Attribution, 2023. "Extreme humid heat in South Asia in April 2023, largely driven by
climate change, detrimental to vulnerable and disadvantaged communities."
https://www.worldweatherattribution.org/extreme-humid-heat-in-south-asia-in-april-2023-
largely-driven-by-climate-change-detrimental-to-vulnerable-and-disadvantaged-communities/
World Weather Attribution, 2023. "Extreme heat in North America, Europe and China in July 2023 made
much more likely by climate change." https://www.worldweatherattribution.org/extreme-heat-in-
north-america-europe-and-china-in-july-2023-made-much-more-likely-by-climate-change/
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�Hazard Assessment: TORNADO
4.3.2.1. Definition
4.3.2.2. Range of Magnitude
Tornadoes can appear in a variety of shapes and sizes ranging from large wedge shapes with a diameter
greater than a mile down to thin rope like circulations. The strongest tornadoes can have wind speeds
more than 200 mph. Tornado wind speeds are estimated after the fact based on the damage they
produce. Tornadoes are characterized on a scale of 0 (weakest) to 5 (strongest) according to the Enhanced
Fujita (EF) Scale. The original Fujita Scale was devised in 1971 by Dr. Ted Fujita of the University of Chicago.
The scale gives meteorologist the ability to rate from FO to F5 based upon the type and severity of damage
that the tornado produced. At that time, there were very few actual measurements of tornado wind
speeds that he could relate to the damage, but Dr. Fujita used them together with a lot of insight to devise
approximate wind speed ranges for each damage category.
In subsequent years, structural engineers have examined damage from many tornadoes. They use
knowledge of the wind forces needed to damage or destroy various buildings and their component parts
to estimate the wind speeds that caused the observed damage. What they found was that the original
Fujita Scale wind speeds were too high for categories F3 and higher, which may have led to inconsistent
ratings, including possible overestimates of associated wind speeds.
With these inconsistent ratings in mind, a panel of meteorologists and engineers convened by the Wind
Science and Engineering Research Center at Texas University devised the new Enhanced Fujita Scale,
which became active as of February 1, 2007. The EF Scale incorporates more damage indicators and
degrees of damage than the original "F" Scale, allowing more detailed analysis and better correlation
between damage and wind speed. You can see both scale charts below TABLE 4.3.2A.
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TABLE 4.3.2A Fujita Scale
FujiFujita Scale
Enhanced uj ta Scale"'
�.
73-4112 mph
i
fii
IMF /
The follow are records from around the County as well as Hennepin County.
Maximum wind speed
• United States
o 318 MPH (Moore, OK, May 3, 1999)
• Hennepin County
o 166-200 (estimated)
Maximum width
• United States
o 2.6 miles (El Reno, OK Tornado, May 31, 2013)
• Hennepin County
o 880 Yards (St. Louis Park, May 22, 2011)
Longest track
• United States
o 235 miles (Tri-State Tornado, March 18, 1925)
• Hennepin County
o Hennepin: 70.9 Miles (June 23, 1952)
Fastest forward motion:
• United States
o 73MPH (Tri-State Tornado, March 18, 1925)
• Hennepin County
o 30 MPH (Champlin -Anoka Tornado, June 181h, 1939)4
Largest outbreak
• United States
o 211 tornadoes in 24 hours (SE US outbreak, April 27, 2011)
• Hennepin County
o 3 tornadoes in 3 hours (May 6, 1965)
Longest duration
• United States
o 3.5 hours (Tri-State Tornado, March 18, 2915)
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Greatest pressure drop.
• United States
o 100 milibars (Manchester, SD, June 24, 2003). *An unofficial drop of 194 millibars
was noted from the Tulia, TX tornado on April 21, 2007.
Costliest tornado
• United States
o $2.9 billion (Joplin, MO, May 22, 2011)
Deadliest tornado
• United States
o 695 killed (Tri-State Tornado, March 18, 1925)
Deadliest modern-day tornado
• United States
o 158 killed (Joplin, MO, May 22, 2011)
Deadliest tornado outbreak
• United States
o 747 killed (Tri-State Outbreak, March 18, 1925)
Deadliest modern-day outbreak
• United States
o 324 killed (SE US Outbreak, April 25-28, 2011)
4.3.2.3. Spectrum of Consequences B2b
The consequences from tornadoes can range from minor damage and injuries to complete destruction
and death. Please see the chart below (TABLE 4.3.2B) that correlates the EF rating scale with the expected
damage seen.
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TABLE 4.3.2B EF Rating Scale
Minor' damage. shungles blown off or parts of a
roof peeled off, clarnage to,giutters/sidint
branches broken off trees, shallow rooted trees
EF-0 EN toppled. I i
Moderato damage more significant real
�� // / damage wundows broken exterior doors
%/f , o
%� ip � � %F� � damaged or lost, mobile homes overturned or
bad!y damaged.
Considerable da,rnage roofs torn off well
constructed homes, homes shifted off their
EF-2 111-135 Mph foundation, mobde homescompletelly
destroyed, large trees snapped or uprooted,.
f.'.ars can be tossed.
....... ............ ""..... ____._... ....____... ......... ............... .......... ............................
''Savers' damage° entire stories of well
constructed homes destroyed, significant
EF- '1 k ( �� damage done to large b0diimgs, homes with
weak foundations can be !blown away„ trees
begin to Hose their bark.
Extreme' damage: Well constructed homes are
F 4 leveled, bars are t1hrown significant distances,
top story exteHor walk of rnasonry, buiidings
would likely collapse.
homes are swept away, steel" reinforced
concrete,wucture,5 are critically damaged,
high-rise buildings susta�n severe structural
I/ stripped of biranches and snapped.
4.3.2.4. Potential for Cascading Effects
Beyond the destruction and lives that tornadoes leave behind, there are many cascading events or hazards
that can follow. If a tornado takes out a power source and there is expected extreme temperatures to
follow, you have now increased the number of people vulnerable to extreme heat or cold event
consequences. A lack of power impacts the ability of people to remain warm or cool and may also disable
medical equipment. If a tornado disrupts farming is, anyway, this can lead to food shortages and/or
disrupt the food chain. As debris is deposited anywhere and everywhere from a tornado, this can lead to
water contamination, and a fire hazard with lumber from houses, buildings and trees amongst damaged
power lines and gas leaks.
Another consequence is the economy impact. Indirect losses that occur from the destruction of a tornado
are hard to estimate directly after an event. Losses could include lost production, sales, incomes and labor
time, increased commute times and transportation costs from goods having to be rerouted, decreased
tourist activity, and utility disruptions. Some people might lose their jobs all together. The decreased
economic activity also results in lost taxable receipts and uses up federal disaster relief funds to help the
clean-up, repair, and replacing of loss assets. Loss of production an also result in surging prices due to
shortages. A well-known example of this occurred when refineries were affected by a tornado in the
southern United States in 2011, which caused gas prices to rise.
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4.3.2.5. Geographic Scope of Hazard Blc
The United States has the highest incidence of tornadoes worldwide, with more than 1,000 occurring
every year. This is due to the unique geography that brings together polar air from Canada, tropical air
from the Gulf of Mexico, and dry air from the Southwest to clash in the middle of the country, producing
thunderstorms and the tornadoes. The illustration below (GRAPHIC 4.3.2A) provides all tornadoes that
have occurred from 1950-2012 as plotted by the Storm Prediction Center.
GRAPHIC 4.3.2A National Tornado Occurrence Map 1950-2012
U.S. Tornado Map
yews 1950 to 20,12.
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The illustration below (GRAPHIC 4.3.2113) provides all tornadoes that have occurred from 1820-2014 as
listed by Hennepin County Archives.
GRAPHIC 4.3.2113 Hennepin County Tornado Occurrence map 1820-2014
4.3.2.6. Chronological Patterns
Tornadoes can occur during anytime of day and any time of year. However, most tornadoes have occurred
in the afternoon hours and during the months of May through August. The graphic below (GRAPHIC
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4.3.2C) shows the tornado reports nationally from 1950-2014. You can see in the chart that tornadoes
occur (and are reported) more typically starting in April through September with the greatest months
being June and July. These two months are typically identified as Minnesota's tornado season.
GRAPHIC 4.3.2C
700
600
500
Va
.4..a
c> 400
n
Oj
300
4t
Number of Tornado Reports Per.....Moil 1950 .....2014
Row) IDa�ta, National Chrna�ft IDa�ta CerAer, IMIN 1DNIR 201.4�
200
100
0
lan Feb IMar Afar May .1 un Dull Aug Sep OCL NOv IDec
Month
4.3.2.7. Historical Data/previous occurrence Bld
Native peoples in tornado -prone areas such as Hennepin County experienced tornadoes and developed
oral traditions to explain them. The first written record of an American tornado is from July 8, 1680, in
Cambridge, MA. The first officially recorded tornado in Minnesota was sighted near Fort Snelling in
Hennepin County on April 19, 1820. Because tornadoes are more numerous in the United States than any
other nation, tornadoes have been studied here more than anywhere else. In 1882, the U.S. Army Signal
Corps assigned Sgt. John Finley to investigate weather conditions that form tornadoes. Technology limits
made the early understanding of tornado anatomy difficult. The adoption of radar revolutionized the
study and forecasting of tornadoes. The first US Weather Bureau radar in Minnesota was installed at the
Minneapolis -Saint Paul International Airport in the early 1960s. Air Force meteorologists issued the first
tornado forecast in March 1948. The US Weather Bureau followed suit by 1952. Important advancements
in understanding tornadoes were made by Theodore Fujita who studied tornado formation and damage
across the Midwest in the 1960s and 70s. Modern era radar was installed at the Twin Cities office of the
National Weather Service in 1996.
In Minnesota and Hennepin County, the record of tornado sightings encompasses nearly 200 years from
records kept at Fort Snelling. The local newspaper record, which often contain notices of weather events,
goes back over 160 years. In general, early reports are incomplete and may contain some factual errors.
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As settlement and population density increased human interactions with tornadoes also increased.
Reports became more numerous. GRAPHIC 4'3'20and GRAPHIC 4'3'2Edepict standardized and reliable
tornado data in Minnesota and in Hennepin County extending back to 1950. Advanced technology has
made detection easier and resulted inmore reports ofweak tornadoes.
* May 22,2011
* May 6,ly65
There have been no other incidents identified.
GRAPHIC 4.3.211)
Minnesota Tornadoes Since 1950
1111111111 F Scale l�ffEFScale
800
686
700
oOO
468
500
- 400
0
300
E 211 207
� 200
1.00
26
689 33 S 6 O
O INNER Ell.�~ 0m __ —
FO/EFO 1-1/EF1 F2/EF2 F9/EF9 1-4/EF4 1-5/EF5
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GRAPHIC 4.3.2E
l...iennelpiin County Tornadoes 1950
1111111111 F Scale lij� EF Scale
9 8
8
Va
OJ
O
5
0 4
4
3
rtt 3 2 2
2. 1
U
NVU�IPIRUk VU�PRUk
FO/onl-0 1-1./EFI. 1-2./onl-2. 1-3/El-3 1-4/El-4 1-5/El-5
i.:m,/ l-5
4.3.2.8. Future Trends Ble
When looking at trends of tornado occurrences, one must keep in mind how reporting has changed over
the last decade as well as population increase. With more people covering a larger geographical area than
100 years ago, there is bound to be more reports of tornadoes occurring because people are there to see
them. There seems to be no trend since 1954 of the occurrences of F1 and stronger tornadoes and
increase in tornado reports results from an increase in the weakest tornadoes, F0. If just looking at
stronger events being reported, you can run into the problem of changes in tornado damage assessment
procedures in trend identification.
Taking out changes in population and reporting measures, there is less trend in the number of tornadoes
per year, as in there doesn't seem to be a growing number of tornadoes each year, or less for that matter.
Research does show there seem to be more extreme swings in tornadoes per year. While years have
always varied in terms of number of tornadoes, they generally fell between a certain range. In the past
decade however, researchers have started seeing toad counts that have deviated well outside of that
range. Another trend researchers are seeing is the number of tornado days seems to be decreasing, while
the number of tornadoes per day has been increasing.
Researchers have also been looking into trends on when the 'tornado season' starts. The average start
days of tornadoes is March 22nd, and that has not changed (tornado season start is defined as first 50
tornadoes of F1/EF1 strength have been reported). However, there have been later and early starts to the
season in recent years. Seven of the 10 earliest tornado starts have occurred since 1996, and four of the
latest starts occurred between 1999 and 2013 of 60 years of records.
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4.3.2.9. Indications and Forecasting
National responsibility for developing tornado indications and forecasts rests with the National Oceanic
and Atmospheric Administration/National Weather Service's Storm Prediction Center (SPC) in Norman,
Oklahoma. The SPC issues daily Convective Weather Outlooks. These outlooks give general categories that
explain the chances/risk of tornadoes each day. As conditions look to develop more favorable for tornadic
storms to occur, the SPC will issue Mesoscale Discussions (MDs). MDs contain a graphical depiction of the
mesoscale convective developments, an area affected line, concerning line, valid time, a summary
paragraph summary, and a paragraph for a technical discussion. There are five categories of concern
issued with the MD:
• Severe Potential...
Watch Unlikely (5 or 20%)
• Severe Potential...
Watch Possible (40 or 60%)
• Severe Potential...
Watch likely (80 or 95%)
• Severe Potential...
Tornado Watch likely (80 or 95%)
• Severe Potential...
Severe Thunderstorm Watch Likely (80 or 95%)
• Severe Potential...
Watch Needed Soon (95%)
After an MD is issued, SPC will monitor conditions and if tornadic potential still is likely, they will issue a
tornado watch. A tornado watch is issued when atmospheric conditions are favorable for the
development of severe thunderstorms capable of producing tornadoes. On average, Hennepin County is
included in 4 tornado watches each year. In addition to the SPC's information about potential for
tornadoes, the National Weather Service Forecast Office will issue Hazardous Weather Outlook (HWO)
based on their thoughts for the potential of tornadoes occurring. In this discussion, they will highlight the
best time, and generally geographic location for storms to occur.
4.3.2.10. Detection and Warning
National responsibility for detection and warning of tornadoes falls on the local National Weather
Service's Weather Forecast Offices (WFO). The local WFO for Hennepin County is in Chanhassen, MN. One
of the systems the WFO uses to detect tornadoes is RADAR. There are two RADAR sites that the
Chanhassen WFO uses, the NEXRAD WSR-88D and the Terminal RADAR. The NEXRAD WSR-88D is located
at the Chanhassen WFO office, and the Terminal RADAR is in Woodbury and is used daily for incoming
aircraft. There are many different products that the NWS can use from these RADARS that help them
detect whether a storm has a tornadic signature to it.
Another avenue that the WFO uses are spotter reports, or reports from emergency managers. In the
metro region, there is an organized amateur radio group called Metro SKYWARN that teach SKYWARN
spotter classes to amateur radio operators so they can make reports directly to the local WFO. Hennepin
County Emergency Management also trains internal SKYWARN spotters to report to the Hennepin County
Emergency Operations Center during activations or directly to the local WFO.
If the WFO sees evidence that there is a tornado either on the ground, or the potential, they will issue a
tornado warning. A tornado warning means a severe thunderstorm has developed and has either
produced a tornado or radar has indicated the presence of atmospheric conditions conductive to tornado
development. On average, Hennepin County is in a tornado warning between 30 and 45 minutes a year.
Once a tornado warning has been issued, there are a variety of notification systems that notified
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automatically in which they then send off the notification of tornado warning as well: Wireless Emergency
Alerts (WEA), Outdoor Warning Sirens, Digital Message Signs, IPAWS, and NOAA Weather Radios. In
addition to the automatic notification, television and radio station may also begin to broadcast the
warning information.
4.3.2.11. Critical Values and Thresholds
According to NOAA, there is no single critical threshold values to confirm or predict the occurrence of
tornadoes of a particular intensity without looking at damage. The critical values of the F & EF tornadoes
scales can be seen above in the Range of Magnitude section.
4.3.2.12. Prevention
There is nothing you can do to prevent a tornado from occurring. However, you can prevent some of the
consequences from occurring by being prepared. It is crucial to always be aware of the weather forecast
and if there is a possibility of severe weather. Further, having multiple methods of receiving weather alerts
from official sources is also important.
4.3.2.13. Mitigation
While there is no way to prevent a tornado from occurring, you can prevent some of the consequences
from occurring by being weather aware for life safety, build safe rooms for sheltering or retrofit walls to
safe room standard. Here are some of the ideas from the FEMA Mitigations Handbook
Education and Awareness Programs:
• Conduct outreach activities to increase awareness of tornado risk and impacts.
• Educate citizen through media outlets.
• Conducting tornado drills in schools and public buildings
• Teaching schoolchildren about the dangers of tornadoes and how to take safety precautions.
• Distributing tornado shelter location information
• Supporting severe weather awareness week
• Promoting use of National Oceanic and Atmospheric Administration (NOAA) Weather Radios.
Construction of Safe Rooms:
• Requiring construction of safe rooms in new schools, daycares, and nursing homes.
• Encouraging the construction and use of safe rooms in homes and shelter areas of manufactured
home parks, fairgrounds, shopping malls, or other vulnerable public structures.
• Encouraging builders and homeowners to locate tornado safe rooms inside or directly adjacent
to houses to prevent injuries due to flying debris or hail.
• Developing a local grant program to assist homeowners who wish to construct a new safe room.
Require Wind -Resistant Building or Retrofitting Techniques:
• Structural bracing
• Straps and Clips Anchor Bolts
• Laminated or impact -resistant glass.
• Reinforcement pedestrian and garage door
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4.3.2.14. Response
Hennepin County Emergency Management Capabilities
• Situation monitoring Station (SMS)
• HCEM Immediate Impact Reconnaissance Teams
• Mutual Aid
4.3.2.15. Recovery
There are two types of recovery, short term, and long term. Initial short-term recovery can be getting the
power back on or cleaning up debris. There are many things to consider when talking about long-term
recovery. Depending on the extend of the tornado and location, large, wooded areas can pose a fire
threat, so damaged trees and branches need to be managed. Another important consideration is business
recovery. It tookJoplin 3 years to be able to re -build their hospital and high school. Other businesses have
been shown the struggle for one or more years after a disaster. Another consideration of recovery is the
mental recovery of not only victims, but of the rescue workers that responded and helped during the
initial short-term recovery process.
4.3.2.16. References
Brooks, H. E., G. W. Carbin, and P. T. Marsh. 2014. 'Increased Variability of Tornado Occurrence in the
United States'. Science 346 (6207): 349-352. doi:10.1126/science.1257460.
Kunkel, Kenneth E., Thomas R. Karl, Harold Brooks, James Kossin, Jay H. Lawrimore, Derek Arndt, and
Lance Bosart et al. 2013. 'Monitoring and Understanding Trends in Extreme Storms: State of
Knowledge'. Bull. Amer. Meteor. Soc. 94 (4): 499-514. doi:10.1175/bams-d-11-00262.1.
Metro Skywarn. 2015. 'Metro Skywarn'. https://metroskywarn.org/.
National Centers for Environmental Information. 2015. 'Severe Weather Data I National Centers For
Environmental Information (NCEI) Formerly Known As National Climatic Data Center (NCDC)'.
Ncdc.Noaa.Gov. http://www.ncdc.noaa.gov/data-access/severe-weather
U.S. Department of Commerce, Weather Bureau. 1946. Climatological Data, Minnesota. National
Oceanic and Atmospheric Administration, Environmental data and Information Service, National
Climatic Center.
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�Hazard Assessment: WIND, EXTREME STRAIGHT-LINE
4.3.3.1. Definition
Extreme straight-line winds are thunderstorm winds
that exceed 70 mph and can reach or exceed 100
mph. Along with damage potential to trees, power
lines, vehicles and structures, these winds pose risks
to life and safety.
Most thunderstorms produce gusty winds from
downdrafts of air flowing from the tops of the storm.
Some thunderstorms produce winds of 58 mph or
stronger, officially making them "severe" by National
Weather Service standards.
Occasionally, severe thunderstorms will produce
destructive winds that far exceed the 58-mph
threshold. These winds are often referred to as "straight-line winds," to differentiate them from the
cyclonic, turning winds of a tornado. Extreme straight-line winds can indeed produce tornado-like
damage.
Extreme thunderstorm winds can be highly localized, or widespread along an arc of storms extending
dozens of miles or concentrated locally in numerous individual cells within a line or cluster of storms. The
duration of straight-line winds at a given location can be as brief as 30 seconds or can last upwards of 30
minutes. The storms producing the extreme winds may cover just 30 miles, or they may track for hours
and cover hundreds of miles.
The latter case represents an important class of extreme thunderstorm winds called "derechos." A
Derecho is an extreme, widespread, and long-lived windstorm, usually associated with bands of rapidly
moving showers or thunderstorms variously known as bow echoes, squall lines, or quasi -linear convective
systems. If the swath of wind damage extends for more than 240 miles, includes wind gusts of at least 58
mph along most of its length, and several, well -separated 75 mph or greater gusts, then the event may be
classified as a derecho.
In general, derechos follow two basic types: Progressive Derechos tend to form on the northern edge of a
steamy air mass, and the derecho is usually associated with one primary, very intense thunderstorm cell
that follows the boundary of the hot air. These derechos have the greatest potential for catastrophic
damage, and given enough instability, there is almost no limit to the intensity of their thunderstorm winds.
Serial Derechos, by contrast, tend to form to the west of warm and unstable air masses, often along cold
fronts, and often in the presence of very fast winds aloft. These instances lead to long, arcing, fast-moving
lines of storms with many different cells, any of which can harness the strong winds aloft and produce
damaging winds. These derechos can produce widespread damage because of all the "candidate" storm
cells, but they generally lack the destructive potential of progressive derechos.
Hennepin County has been affected by numerous extreme straight-line windstorms, including derechos.
Every decade from the 1950s through the 2010s had multiple extreme thunderstorm wind events within
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the county.
4.3.3.2. Range of magnitude
Maximum wind speeds:
• Hennepin:
20, 1951
1980
o Measured 100 mph, Wold-Chamberlain Field (MSP), July
o Measured 86 mph at Flying Cloud Airport, on 15 July
o Estimated over 100 mph on July 3, 1983
• Other Twin Cities Metro:
• Minnesota:
• Region:
0 110 mph sustained, gust 180 mph, St. Paul, Aug 20, 1904
0 121 mph, Donaldson, MN, September 1, 2011
0 117 mph, Alexandria, July 19, 1983
0 128 mph (Northeast of Madison, WI May 31, 1998)
0 126 mph, Atkins, IA, August 10, 2020 (140 mph estimated
from damage surveys)
Maximum width: 100 miles (Kansas —The "Super Derecho of May 8, 2009)
Longest track: 1300 miles (The Boundary Waters -Canadian Derecho July 4-5, 1999)
Longest duration: 22 hours (The Boundary Waters -Canadian Derecho July 4-5, 1999)
Costliest US Derecho: $7.5 Billion (The Iowa -Midwest Derecho of August 10, 2020)
Deadliest US Derecho: 73 killed (The "More Trees Down" Derecho July 4-5, 1980)
4.3.3.3. Spectrum of Consequences B211b
Extreme thunderstorm winds and derechos are most common in the warm season and pose risks to those
involved in outdoor activities. Campers or hikers in forested areas are vulnerable to being injured or killed
by falling trees. Boaters risk injury or drowning from storm winds and high waves that can overturn boats.
Trees around lakes pose risks to walkers, joggers, and cyclists. At outside events such as fairs and festivals,
people may be killed or injured by collapsing tents and flying debris. Additionally, anyone caught outside
may be injured by flying debris. Any person without adequate shelter is at significant risk in extreme
thunderstorm winds.
Occupants of cars and trucks also are vulnerable to being hit by falling trees and utility poles. Further, high
profile vehicles such as semi -trailer trucks, buses, and sport utility vehicles may be blown over.
Even those indoors may be at risk for death or injury during derechos. Mobile homes may be overturned
or destroyed, while barns and similar buildings can collapse. People inside homes, businesses, and schools
are sometimes victims of falling trees and branches that crash through walls and roofs; they also may be
injured by flying glass from broken windows. Finally, structural damage to the building itself (for example,
removal of a roof) could pose danger to those within.
Throughout Hennepin County, and especially in suburban and urban areas, electrical lines are vulnerable
to high winds and falling trees. In addition to posing a direct hazard to anyone caught below the falling
lines, wind damage to the power infrastructure can result in massive, long-lasting power outages.
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Hundreds of thousands of people may lose power for a week or more, as happened most recently in 2013.
In addition, unlike the localized damage produced by a tornado, often covering the equivalent of one
square mile, extreme thunderstorm wind damage can be widespread, affecting tens or even hundreds of
square miles within the county. As a result, repairs often require substantial effort, with additional delays
related to shortages in supplies.
Extreme straight-line winds also can expose socio-economic vulnerabilities among Hennepin County's
diverse and growing population. Derechos and severe thunderstorms can strike quickly, posing serious
challenges to the elderly, or anyone with limited mobility who is caught outside. Those new to the region
who are unfamiliar with severe weather, how to access information about it, and how to respond, may
be caught off -guard and unprepared for the dangerous winds. Language barriers also may prevent some
people from getting vital information as the storm is approaching. Anyone without adequate shelter will
be subject to all the risks of being outside during dangerous thunderstorm winds. In general, extreme
thunderstorm winds pose greater threats to disadvantaged populations that may lack the resources
others have to anticipate, plan for, seek shelter from, and recover from extreme straight-line winds.
4.3.3.4. Potential for cascading effects
• Flash Flooding - On occasion, the convective system responsible extreme wind damage will stall,
back -build, or regenerate, producing excessive rainfall. In other cases, the storm may simply
unload enormous quantities of rainfall. On July 1, 1997, a complex of thunderstorms produced
80-110 mph winds and extensive damage from Wright into western Hennepin County, while
dropping 3-5 inches of rain in 60-90 minutes over much of the area. The rains flooded every type
of road in the county, submerging vehicles and significantly delaying emergency vehicles deployed
to respond to the extreme wind event.
• Power Outages and Arctic Outbreaks — Dangerously cold air had never been considered a serious
concern in relation to extreme thunderstorm winds and derechos, which tend to form during the
warm season. On December 15, 2021, however, a historic outbreak of intense thunderstorm
winds and tornadoes struck southeastern Minnesota, knocking out power for 1-3 days as
temperatures in the 10s F settled into the region.
Any extreme straight-line wind occurring outside the usual warm season, and particularly
between November and March, may pose significant cold weather risks in its aftermath. Without
power, electrical baseboard heat will not operate, nor will many appliances, security systems,
electronic devices, or lights.
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Power Outages and Intense Heat — Some of the most intense summer thunderstorm winds and
the explosive class of "progressive derechos" tend to occur on the fringes of major heat waves.
The heat and deep moisture often pool near the boundary that promotes the development of
thunderstorms, and those ingredients
act to fuel
hintensification
te lthe storms and thedevelopment of
destructive winds.rY"; it iiU1y
When thunderstorm winds damage the
electrical infrastructure during or prior
to intense heat waves, residents are left
without the benefit of air-conditioning
while having to deal with intense heat.
This sort of cascading effect occurred in
the Ohio Valley and eastern US on June
29, 2012, when a derecho traveled for
700 miles, impacting 10 states and
Washington, D.C. An estimated 4 million
customers lost power for up to a week.
The region impacted by the derecho was
also during a heat wave, which claimed
34 lives in areas without power following
the derecho.
This map illustrates the large-scale meteorological
environment favorable for progressive and serial
derechos on the northern or western fringe of a high-
pressure area associated with a major heat wave over
central and eastern United States.
Wildland Fires — Extreme straight-line winds and derechos can obliterate millions of trees across
miles of forest due to the extreme winds associated with them. This increases fuel loads on
forests and escalates the risk of wildland fire.
Tornadoes — Extreme straight-line winds and tornadoes can and do occur with the same
convective system at times. In addition to the December 15, 2021, event discussed above,
damaging straight-line winds and tornadoes also occurred near each other in or close to Hennepin
County on July 3, 1983, July 1, 1997, and September 21, 2005.
The tornadoes may occur with isolated supercells ahead of the derecho producing squall line, or
they may develop from storms within the squall line itself. Tornadoes have occurred with serial
derechos, as on December 15, 2021, and on May 12, 2022, in southwestern Minnesota, and they
have also occurred with progressive derechos, as on July 3, 1983.
Blizzards — It has yet to be documented in Minnesota, but any cold -season derecho is likely to be
associated with a vigorous low-pressure system and it would be possible for not just cold air, but
intense snow and wind, to follow damaging thunderstorms within 6 to 48 hours.
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4.3.3.5. Geographic Scope of Hazard Blc
Hennepin County is within a high -frequency corridor for extreme thunderstorm winds and derechos that
covers much of the eastern half of the US.
Every part of the county has experienced r5,
significant damage from unusually intense'
thunderstorm winds. Within the county,µ
there are no favored areas. Winds estimated°�k ��� �� ,�
h d
to 80 mph hit downtown Minneapolis in Aprils
of 1986, tearing a hole in the roof of the�w��
Metrodome. Winds at least that strong winds
have hit ever corner of the count with 100
Y Y,
mph winds measured at the international
airport in 1951, and winds likely well over 100 „�„�
mph striking the northern suburbs in July of
1983.F
Nationally, derechos most commonly occur
along two axes. One track parallels the "Corn4�
Belt" from the upper Mississippi Valley
southeast into the Ohio Valley; the other Approximate number of times "moderate and high intensity"
extends from the southern Plains northeast (MH) derechos affected points in the United States during the
into the mid -Mississippi Valley. During the years 1980 through 2001. Areas affected by 3 or more derecho
cool season (September through April), events are shaded in yellow, orange, and red.
derechos are relatively infrequent but are
most likely to occur from east Texas into the southeastern states. Although derechos are rare west of the
Great Plains, derechos occasionally do occur over interior portions of the western United States, especially
during spring and early summer.
The highest annual frequencies of occurrence appear along the "Corn Belt," from Minnesota and Iowa
into western Pennsylvania, and in the south-central states, from eastern parts of the southern Plains into
the lower Mississippi Valley. However, the frequencies vary by season. During the warm season (May
through August), derecho events are most frequent in the western part of the Corn Belt. During the
remainder of the year (September through April), the maximum frequencies shift south into the lower
Mississippi Valley
4.3.3.6. Chronologic patterns (seasons, cycles, rhythm)
Extreme straight-line winds and derechos in the United States are most common in the late spring and
summer (May through August), with more than 75% occurring between April and August. The seasonal
variation of derechos corresponds rather closely with the incidence of thunderstorms. However, as noted
above, Minnesota (and neighboring states) experienced extreme straight-line winds qualifying as a
derecho on December 15, 2021.
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4.3.3.7. Historical data/previous occurrence Bld
The Independence Day Derecho of
1977
Although it did not affect Hennepin
County, the "Independence Day
Derecho of 1977" formed over west
central Minnesota on the morning of
Monday, July 41h. As the derecho
moved east-southeast, it became
very intense over central Minnesota
W f
f '-REST Bx-Wr,
r
rS
wr
COT
MN D T'
wp
c Dr .�
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off
around midday. From that time through the afternoon, the derecho produced winds of 80 to
more than 100 mph, with areas of extreme damage from central Minnesota into northern
Wisconsin.
The derecho continued rapidly southeast across parts of Lower Michigan during the evening,
producing winds up to 70 mph and considerable damage before finally weakening over northern
Ohio around 1:30 AM on Tuesday, July 51h. This event was notable for affecting recreationist and
travelers out enjoying the Independence Day holiday.
West Metro to Northern Wisconsin Derecho of 1983
On July 3, 1983, between 12:30
and 13:20 local time, a complex of
extremely severe thunderstorms
affected a southwest to northeast
swath of Hennepin County.
Damage was most extensive from
eastern Lake Minnetonka,
through Maple Grove and
Champlin. The storms continued
into Anoka County and produced
the Twin Cities area's most recent
EF-4 tornado in Andover (most
recent as of January 2024).
Champhno
Dayton, Maple
f-,...
Grove, 7 houses
w crsawu
destro ed 4
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ecei
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moderate to
severe damage
FC, DRAW
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boats
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,
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Scattered wind damage
MEN MURK
Champlin: shopping i
maH severegy
d,arnaged; cars
thrown through
parking lot, $8.5 M'
(2015 USD) damage
r•r. tWTHQrvw,r
Extensive/continuous �T
Extreme straight-line winds
waned damage'
caused significant damage in a
southwest -to -northeast swath across Hennepin County. The storm complex raced northeastward
into Wisconsin during the next few hours, and aerial surveys conducted by the University of
Chicago found over 150 linear miles of continuous EF-1-equivalent straight-line wind damage,
with pockets of EF-2 damage —stretching from Carver County to Ashland, Wisconsin. The National
Weather Service Issued "Very Severe Thunderstorm Warnings" for the storm, to indicate winds in
excess of 75 mph, and sirens sounded throughout Hennepin County.
This storm remains (as of 2024) the most destructive severe convective storm event in the Twin
Cities Metropolitan Area, since the May 6, 1965, tornado outbreak.
2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
The 1-94 Derecho of 1983
Around dawn on the morning of Tuesday, July 19, 1983, well north of warm/stationary front over
South Dakota and northern Iowa, a bow echo moved out of northeast Montana and began
producing damaging winds in northwest North Dakota. This would be the beginning of a
noteworthy progressive derecho event that would move across the northern Great Plains and
upper Mississippi Valley and reach the Chicago metropolitan area by late evening.
As the convective system's cold
pool continued to deepen and
elongate east -southeastward with
the mean cloud -layer flow, it
ultimately reached the warm front
as that boundary advanced slowly
north across eastern South Dakota
and southern Minnesota. This
meeting occurred during the early
afternoon over west central
Minnesota, and likely accounts for
the appreciable increase in storm
strength observed around that time
as the convection became surface
based. At this time the storm
system also expanded in scale, evolving into a squall line with two and sometimes three bow echo
segments as it continued across Minnesota and later Wisconsin, with Interstate 94 near its central
axis.
The path of the 1983 1-94 Derecho as it crossed over six states on
July 19, 1983.
Winds over 100 mph were recorded at the airport in Alexandria, Minnesota, Minnesota, where
planes and hangers were damaged and destroyed. The storm continued to produce much damage
as it moved east-southeast across south central and southeast Minnesota; approximately 250,000
customers lost electrical power in the Minneapolis/St. Paul area, a record at that time. Thirty-four
people were injured in Minnesota and Wisconsin from this storm. Of these injuries, 12 were from
mobile homes being blown over, and eight were from falling trees.
The Northwoods/ "Right Turn" Derecho of 1995
During the late afternoon of Wednesday,
July 12, 1995, thunderstorms formed over
southeast Montana and began producing
winds that damaged homes and barns. As
the storm system moved east across North
Dakota, vehicles were overturned, and a
grain bin was destroyed. Measured winds
reached 70 mph at Bismarck, ND. As the
system approached Fargo during the early
morning of July 13th, it became a well-
defined bow echo storm with measured
winds of 91 mph at the Fargo airport. The
_ "JULY r
03
Mr ND 1AM �.N rm7
lawn
C BST CCY '�+
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C07 11RM nl l'i t_ +Mro
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tDT
three derechos to occur on consecutive days across
Northern Minnesota.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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derecho was becoming a "high end" event.
The derecho took a track similar to one of the previous nights, producing significant damage for
the second night in a row from southeast North Dakota eastward across northern Minnesota to
western Lake Superior. Damage was extreme across Minnesota, with over five million trees blown
down and many buildings damaged, and some destroyed. Six campers were injured from the
falling trees during the pre -dawn hours. Trucks with plows were needed to clear many of the
roads, and some areas were without power for a week. Damage totaled well over $30 million in
1995 dollars.
Extreme Thunderstorm Winds and Other Hazards, July 1, 1997
A complex of very intense thunderstorms moved out of South Dakota during the afternoon and
approached the Twin Cities during the early evening, producing multiple tornadoes rated up to F-
3 (now EF-3), along with destructive winds that spread from central Minnesota into Wright,
Sherburne, Hennepin, and Anoka counties and beyond. Although not long enough to qualify as a
derecho, this storm was as destructive over a path that was over 100 miles long and 10 miles wide
in some areas.
Wind gusts estimated from 85 to 110 mph damaged small airports and planes; destroyed homes
and garages; snapped or uprooted tens of thousands of trees; flipped trailers and mobile homes;
blew down headstones in cemeteries; and produced over 100,000 power outages in the western
and northern Twin Cities area, including Hennepin County.
The storms also produced extreme rainfall rates, exceeding the threshold for 200 or even 500-
years storms at the 1 and 2-hour duration, as 3-5 inches of rain occurred in 60-90 minutes. The
rains overwhelmed drainage capacity across Hennepin County and stranded or submerged
vehicles on parts of Interstates 94, 394, 494, 694, 35-W, along with parts of US Highways 10, 169,
and 212, and literally dozens of other state, county, and smaller roads. The intense flash -flooding
hampered emergency responses in the parts of the county damaged by winds.
Hail Derecho, May 15, 1998
A severe squall line developed in western Texas around midnight and raced northeastward,
making it to south-central Kansas by daybreak, southwestern Iowa by mid -morning, and the Twin
Cities area by 16:00 local time. The storms produced widespread damaging wind along the 1000-
mile-long track, and reached peak intensity in Iowa, Minnesota, and Wisconsin, with fast-moving
tornadoes and 1-2" hail driven by 60-80 mph winds.
This was an unusual extreme wind event, qualifying easily as a derecho, but not fitting easily into
the "progressive" or "serial" categories. This is among the only known damaging thunderstorm
events in Minnesota history to have originated in Texas.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
The storms produced a record number of power outages in Minnesota (the record has since been
broken twice), and snapped or uprooted thousands of trees in Hennepin County alone (with
estimates of over 1000
trees killed in Ramsey
f
(� a � r mp i r � 7d "" %%%/i I � / �%✓//»✓r r ,�u /t , , r i
i / u , rrr ear l ✓ / IJJJ(v
County). A tornado tracked
,� �l���l f���� �� 1�% ��/i� r� ill/ f �'/�i�j���l�lU f/ 1✓�Iri
from Roseville into Blaine,
at an estimated speed of 80
iti//���r�% po9� / / f� y l>t: m>✓ w fj ( I !f r U�; /i �i/ flu /yl / Amy
mph, causing significant ,„� //1/,� tl�,,, �� /✓ � om rr j" /ia �,, , 1 ,f, �rr �6
damage to homes. The
majority of the damages %0 �1 e // f %°/l, l�, f���,���r
, �� „ %/ a>l�ni�ai�r�nU rarf rl l (J� 1 / � ��y�r iX
however, were from wind -
driven hail, which broke
0 ��rrif/
windows, damaged roofs, %✓e� �9���iul�;�i� �'� l//� �ul�U,,
bent garage doors, and
forced automobile Radar at 16:25 local, as bowing hail core entered central Twin Cities on May
dealerships in Bloomington 15, 1998
to submit claims for their
entire outdoor inventories.
The compound hail and wind damage from this storm produced over a billion dollars, adjusted for
inflation, in home, automobile, and business property insurance claims.
The largest hail reported in the Twin Cities was 2 inches, and most reports were in the 1-1.5" range.
However, the intense straight-line winds turned the hail into dangerous projectiles, and produced
far more damage than would normally be expected.
The Southern Great Lakes Derecho of 1998
During the early evening of Saturday, May 30,
tornado -producing supercells over eastern
South Dakota merged and became a squall
line that moved east into southern
Minnesota. As the squall line crossed
southern Minnesota it evolved into a bow
echo system that expanded in scale and
raced east across the southern Great Lakes
before finally dissipating over central New
York after sunrise on Sunday, May 31st. This
bow echo system produced one of the most
dangerous and costly derecho events in the
history of the Great Lakes region. The
"Southern Great Lakes Derecho of 1998"
adversely affected millions of people on the
weekend after Memorial Day. Many
casualties and record amounts of damage
occurred.
2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
The bow echo system began to produce significant wind damage over south-central Minnesota
about 10 p.m. Saturday evening. As the system moved rapidly eastward it grew south into
northern Iowa and caused damaging winds over most of southeast Minnesota and northeast
Iowa. Many trees and power lines were blown down and several farm buildings were damaged or
destroyed.
The most intense damage occurred near the northern end of the bow echo system in Minnesota,
from Sibley and McLeod Counties eastward across southern portions of the Minneapolis/St. Paul
metropolitan area. Along this band, winds greater than 80 mph were measured; in some areas,
estimated speeds reached 100 mph. Tens of thousands of trees were blown down, 500,000
customers lost power, two semi -trailer trucks were overturned, two apartment building roofs
were blown off, and 100 boats were destroyed. In addition, over 100 homes were destroyed or
badly damaged, and over 2000 others received some damage. Twenty-two people were injured,
and damage to property was estimated to be about $48 million in 1998 U.S. dollars ... with $35
million dollars of that damage occurring in Dakota County alone.
In summary, while crossing southern Minnesota and northeastern Iowa, the derecho event
caused about $50 million in 1998 U.S. dollars of damage, left about 600,000 customers without
power, and injured twenty-two people. In some areas, power was not restored until nearly a week
after the event.
Boundary Waters —Canadian Derecho
On July 4, 1991, a major derecho in
the BWCAW, known as the
®,10-33 %
Superior" National Forest
Boundary Waters -Canadian
034-66%
July 4, 1999, Storm Blow own
Derecho, lasted for more than 22
6T-100%
'
hours traveled more than 1,300
,w ,,. a �,..a�, ` ➢� � . �e�� � .
miles and produced wind speeds
averaging nearly 60 mph, peaking at
1 0 ,e
80-100 mph. The blowdown caused
p
widespread devastation with
casualties both in Canada and the
United States. The storm front
Figure 2. Percentage of trees blown down in Superior National
Forest in northeast Minnesota on July 4, 1999. Scale: 1 " =15
initiated as large complex of
miles. (Courtesy of USDA Forest Service, Superior National
thunderstorms in South Dakota. The
Forest)
storm moved west to east snapping
tree trunks in half that pulled power lines
down with them in Cass, Crow Wing, Itasca and Aitkin
Counties. After blowing down trees on 1,300 acres on the Chippewa National Forest and dropping
heavy rains that eroded 9,000 acres of
shorelines, the storm continued into northeast Minnesota.
The storm entered the Arrowhead region of northeastern Minnesota in the early afternoon. Here,
winds of 80 to 100 mph resulted in injuries to about 60 canoe campers and damage to tens of
millions of trees within 477,000 acres of forest land on the Superior National Forest in the course
of leveling a swath 30 miles long and 4 to 12 miles wide. The storm affected approximately
477,000 acres (16 percent of the Superior National Forest). The BWCAW sustained the heaviest
damage in a line from Ely to the end of the Gunflint Trail.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
Other Notable or Recent Extreme Thunderstorm Wind Events
o September 21, 2005 (Hennepin County) —Large, slow -moving supercell thunderstorms
produced large hail, tornadoes, and extreme downburst winds in Anoka and northern
Hennepin County, with wind gusts estimated up 100 mph in Brooklyn Center, where a
man was killed by falling trees.
o September 20, 2018 (southern Minnesota) —A line of fast-moving thunderstorms, like a
serial derecho but not traveling far enough to qualify, produced nearly continuous and
severe damage as tornadoes and straight-line winds ravaged communities in south-
central and southeastern Minnesota, including Waseca, Owatonna, Faribault, Northfield,
and Cannon Falls. National Weather Service surveys indicated straight-line winds exceed
100 mph.
o July 19 (central Minnesota and July 20, 2019 (southern Minnesota) —An intense heat
wave with Heat Index values to 115' F fueled a derecho that tracked 490 miles from
central Minnesota into Michigan. The next day, as the heat dome settled southward,
another derecho tracked 860 miles from western South Dakota, through southern
Minnesota, Wisconsin, and northern lower Michigan, crossing the damage path of the
previous day's extreme winds in Wisconsin.
o August 10, 2020 (Iowa and Midwest) — One of the most extensive and destructive
mainland storm events in US history, an extreme derecho tracked from the
Iowa/Nebraska border to the Indiana/Ohio border, reaching maximum intensity in
eastern Iowa, where winds gusted over 100 mph over an unusually large area, with 80-
120 mph gusts lasting over 30 minutes in areas near Cedar Rapids.
o December 15, 2021 (Southeast Minnesota and Midwest) — By far the latest -in -the -
season severe weather outbreak in Minnesota, this serial derecho traveled from southern
Nebraska into Wisconsin, producing widespread 75 mph winds and 22 tornadoes across
south-central and southeastern Minnesota, damaging buildings and homes, uprooting
trees, and knocking out power. One man near Rochester was killed by straight-line winds.
This event set a record back to 2004 for most reports of hurricane -force (74 mph) wind
gusts. The storms were followed quickly by a strong cold front the dropped temperatures
into the 20s and 10s F, as extreme non -convective winds associated with a powerful low-
pressure area spread over the region.
o May 12, 2022 (Corn Belt into western Minnesota)— Another powerful serial derecho with
wind gusts of 85 to over 100 mph required just six hours to track from southern Nebraska
to the Brainerd Lakes area of Minnesota. This massive event produced a dust storm from
the dry conditions in western and central Minnesota, along with extensive damage to
towns and rural properties. As of October 2023, this event was estimated to have
produced over three billion dollars in damage across the region.
4.3.3.8. Future trends/likelihood of occurrence Ble
For decades, the science was inconclusive about the connection between climate change and extreme
thunderstorm winds or derechos, suggesting and trends in the frequency or intensity of these dangerous
hazards would be short-lived and attributable primarily to "normal" variations in weather and climate
patterns.
Recent research, however, has suggested that a warming climate can influence the size and/or intensity
of derechos and other extreme thunderstorm wind events. Physical modelling simulations of the August
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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2020 derecho in Iowa revealed that while the storm would not necessarily have produced stronger winds
in a warmer world, the likelihood of a stronger nearby heat wave would have allowed the damaging winds
to cover more area and last longer.
Another investigation of extreme straight-line wind occurrences showed an observed increase both their
intensity and their areal coverage in the United States as the climate has warmed and theorized a 7.5%
increase in intensity for each additional degree C (1.8 degrees F) of warming.
Similarly, a study of a lethal 2022 Mediterranean derecho showed that the marine heat wave in its vicinity
that helped fuel it was itself made substantially more likely and more intense by rising global
temperatures. This marine heatwave contributed substantial intensity and wind energy to the
thunderstorm complex, which simulations showed would have been of "ordinary" strength in the absence
of climate change.
Taken together, these studies suggest that the changing climate can make extreme straight-line
thunderstorm winds and derechos larger, longer lasting, and in some cases, more intense. As the climate
continues warming, therefore, a given extreme straight-line wind event may be more likely to affect
Hennepin County and neighboring areas.
4.3.3.9. Indications and Forecasting
National responsibility for developing tornado indications and forecasts rests with the National Weather
Service's Storm Prediction Center (SPC) in Norman, Oklahoma, and the local National Weather Service
office in Chanhassen.
4.3.3.10. Critical Values & Thresholds
Winds in a derecho must meet the National Weather Service criterion for severe wind gusts (greater than
57 mph) at most points along the derecho path. Most other extreme straight-line wind events are well
above this threshold as well. In stronger derechos, winds may exceed 100 mph.
Based on current warning criteria and analysis of local and regional storm events, the following thresholds
apply:
• 58+ mph: Entry level for "severe." Some damage to trees and powerlines.
• 70+ mph: outdoor warning sirens activated in Hennepin County; significant tree and electrical
infrastructure damage, with structural damage possible.
• 80+ mph: Wireless Emergency Alerts (WEAs) triggered; structural and vehicular damage likely;
risks from airborne debris.
• 100+ mph: tornado-like damage expected, with secondary damage from debris -bombardment.
4.3.3.11. Preparedness
Hennepin County Emergency Management employs meteorologists who monitor the potential for
extreme straight-line winds and communicate with an array of county personnel as conditions warrant.
Those planning to be outdoors for a significant length of time must be aware of the weather forecasts,
especially if well -removed from sturdy shelter. Preparation means staying "connected" via television,
radio, NOAA Weather Radio, or social media. Extreme straight-line winds rarely occur without warning,
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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although warning lead times may be comparatively limited during the early stages of storm development.
Emergency water and food supplies, can openers, batteries, and flashlights should be on -hand in case of
power disruptions.
4.3.3.12. Mitigation
Education and Awareness Programs
• Educating homeowners on the benefits of wind retrofits such as shutters and hurricane
clips.
• Ensuring that school officials are aware of the best area of refuge in school buildings.
• Educating design professionals to include wind mitigation during building design.
Structural Mitigation Projects — Public Buildings & Critical Facilities
• Anchoring roof -mounted heating, ventilation, and air conditioner units
• Purchase backup generators
• Upgrading and maintaining existing lightning protection systems to prevent roof cover
damage.
• Converting traffic lights to mast arms.
Structural Mitigation Projects — Residential
• Reinforcing garage doors
• Inspecting and retrofitting roofs to adequate standards to provide wind resistance.
• Retrofitting with load -path connectors to strengthen the structural frames.
4.3.3.13. Recovery
Recovery from extreme straight-line winds can take weeks as power outages from these storms can be
extensive. A widespread event, or one in densely populated areas, may require search -and -rescue
operations, which can be hampered when fallen trees or downed power lines block critical routes. Utility
and infrastructure repair needs can exceed local resources and staff availability. Homes and businesses
often require extensive repairs, bottlenecking the supply of contractors who provide such work, and
opening the door to out-of-state and even predatory contract services who exploit the desperation and
confusion often associated with disaster recovery.
Hennepin County Emergency Management Capabilities:
• Situation Monitoring Station (SMS)
• Virtual Situation Monitoring Station (VSMS)
• Damage Assessment Teams.
Hennepin County Emergency Plans:
• Hennepin County Emergency Operations Plan
4.3.3.14. References
2023. Gonzalez-Aleman, J. J., D. Insua-Costa, E. Bazile, S. Gonzalez -Herrero, M. Marcello Miglietta, P.
Groenemeijer, and M. G. Donat. Anthropogenic Warming Had a Crucial Role in Triggering the Historic
and Destructive Mediterranean Derecho in Summer 2022. Bulletin of the American Meteorological
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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Society, 104, E1526—E1532, https://doi.org/10.1175/BAMS-D-23-0119.1.
2023. Lasher-Trapp, S., S. A. Orendorf, and R. J. Trapp. Investigating a Derecho in a Future Warmer
Climate. Bull. Amer. Meteor. Soc., 104, E1831—E1852, https://doi.org/10.1175/BAMS-D-22-0173.1.
2023. Prein, A.F. Thunderstorm straight line winds intensify with climate change. Nature Climate Change
13, 1353-1359, https://doi.org/10.1038/s41558-023-01852-9
2022. Minnesota DNR. "Destructive thunderstorms, May 12, 2022."
https://www.dnr.state.mn.us/climate/journal/destructive-thunderstorms-may-12-2022.htmI
2021. Minnesota DNR. "Mid -December Tornadoes, Derecho, and Damaging Cold Front --December 15-
16, 2021. https://www.dnr.state.mn.us/climate/journal/mid-december-tornadoes-derecho-and-
damaging-cold-front-december-15-16-2021.html
2020. National Weather Service, Quad Cities. "Midwest Derecho - August 10, 2020, Updated: 10/8/20 12
pm." https://www.weather.gov/dvn/summary_081020
2019. Minnesota DNR. "Concentrated Thunderstorm Wind Damage, July 20, 2019."
https://www.dnr.state.mn.us/climate/journal/concentrated-thunderstorm-wind-damage-july-20-
2019.html
2019. Minnesota DNR. "Extreme Heat and Big Storms, July 19, 2019."
https://www.dnr.state.mn.us/climate/journal/extreme-heat-and-big-storms-july-19-2019.htmI
2018. National Weather Service, Twin Cities. "September 20, 2018, Tornado Outbreak and Widespread
Damaging Wind (updated 11/15)." https://www.weather.gov/mpx/20180920_Severe_Weather
Date unknown. National Weather Service, Storm Prediction Center. "About Derechos."
http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm. Retrieved 2015.
January 2013. "NOAA Service Assessment. The Historic Derecho of June 29, 2012." U.S. Department of
Commerce.
2007. Mosier, Keith. "After the Blowdown: A Resource Assessment of the Boundary Waters Canoe Area
Wilderness, 1999-2003." United States Department of Agriculture.
2002. Sanders, Jim. "After the Storm. A Progress Report from the Superior National Forest." United
States Department of Agriculture.
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�Hazard Assessment: HAIL
4.3.4.1. Definition
Hail is precipitation that is
formed when updrafts in
thunderstorms carry raindrops
upward into extremely cold
areas of the thunderstorm where
they are continuously lofted and
form into hail. They eventually
become heavy and fall to the
ground. Hail can cause billions of
dollars of damage to structures,
cars, aircraft, and crops, and can
be deadly to livestock and
people.
Damaging hail is associated with
severe thunderstorms, and is
often found in proximity to
strong winds, torrential rainfall,
and even tornadoes.
Large hail, source NSSL (http://www.nssi.noaa.gov/education/svrwxl0l/hail/)
Supercell thunderstorms are responsible for most Minnesota hail reports more than 1.5 inches in
diameter, and nearly all reports in excess of 2.5 inches. These supercell thunderstorms may or may not be
tornadic at the time of hail production. Damage becomes significantly more likely as hail size increases
because the impact factor increases exponentially with incremental growth (Table 4.3.4A).
Table 4.3.4A Hail diameter and impact. From Marshall et al. (2001).
..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
Haii Diameter
1„
211
311
Impact (foot-ibs)
<1
22
120
4.3.4.2. Range of magnitude
Largest hail stones reported.
• Hennepin:
0 3-inch diameter, Minneapolis, August 11, 2023
0 3-inch diameter, Independence, August 5, 2019
0 4-inch diameter, Bloomington, Richfield, South Minneapolis, July 8, 1966
• Adjacent counties:
0 4-inch diameter, Delano, Wright County, August 5, 2019
0 4.25-inch diameter, New Prague, Scott County, August 24, 2006
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0 4-inch diameter, northern Anoka County, June 14, 1981
0 4-inch diameter, Zimmerman, Sherburne County, August 27, 1990
• Minnesota:
0 6-inch diameter, between Edgerton (Pipestone County) and Chandler (Murray County),
July 4, 1968
0 6-inch diameter, near Worthington, Nobles County, July 28, 1986
• US: Record diameter of 8" recorded at Vivian, SD, on July 23, 2010.
Costliest hail event
• May 15, 1998: $950 million USD in 1998 dollars (-3.1 billion in 2023) from damages in Minnesota
resulting from hail, straight-line winds, and isolated tornadoes. Vast majority of losses were from
wind -driven hail, which destroyed thousands of new and used vehicles, roofs and siding on
thousands of homes.
4.3.4.3. Spectrum of consequences
2
Hail over one inch in diameter may produce small "dimples" or "pocks" on vehicle exteriors. At 1.5 inches,
damage to roofing materials becomes common. At sizes greater than 2", windshields and rear windows
are often cracked or shattered, vehicle bodies damaged badly, residential windows may be broken,
residential siding welted, and many varieties of roofs badly damaged (Table 4.3.4113) for an example of roof
damage thresholds).
Although fatalities are uncommon, injuries to the head, shoulders, back, and arms are not. Severe bruising,
often in multiple locations, is the most typical type of injury. Drivers and passengers of vehicles also may
have cuts and lacerations from flying glass.
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Table 4.3.4113. Damage onset thresholds for various roofing materials. From Marshall et al. (2002).
Ty le o 1 Rwoofing,
Hailstone Size
rod gt,..
�
...,
�fl��
3-tab asphalt shingles
l OO
215
30 r'. Lanii_iiated shingles
1.25
32,
Cedar, shhrigles
L25
32
edjur i cediir shakes
1.50
38
Fibeu- enienit tiles
-50
38
ojxrete tiles
1.75
44
u It-t p gr°ayel roofing
.
Large hailstorms also tend to halt traffic and may require snow removal equipment to clear area roads. An
early morning hail event in November of 1999 caused traffic jams and spinouts in Eden Prairie, and
snowplows were needed to clear over 2 inches of accumulated hailstones from 1-494. `
Although the human toll from hail tends to be much lower than from tornadoes and straight-line winds,
hailstorms are often costlier, because of the costs associated with cosmetic damages to residences,
vehicles, and businesses. Severe crop damage is also common, with soybeans and corn especially
susceptible to damages from wind-blown hail. Hail rarely causes infrastructural damage.
4.3.4.4. Potential for cascading effects
The consequences of hail are generally limited to the duration of the hail event, providing few options for
cascading effects. However, large, and damaging hail events tend to be associated with strong or severe
thunderstorms that produce or can produce other convective weather hazards, which can exacerbate or
compound the impacts. The large hail core in a tornado -producing supercell thunderstorm is often very
near the tornado itself. Thus, hail damage victims are at risk of becoming tornado victims as well. High
situational awareness is therefore required during large hail. Any person caught outside during a hailstorm
is also at significant risk from excessive rainfall and lightning. Any building or vehicle with shattered
windows is also more susceptible to flying debris through those now open windows as well.
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4.3.4.5. Geographic scope of hazard
1c
Minnesota is north and east of the
spatial hail frequency maximum within
the US, which stretches from
southwestern South Dakota, into
Nebraska, Kansas, Oklahoma, Colorado,
and Texas.
Within Minnesota, hail tends to be�
most common in the southern and
western portions of the state although 1 �
large and damaging hail has been
observed in every county. The map of Average number severe hail days, 2003-2012, from storm Prediction
all known 4" hail reports since 1955
Center WCM Page.
does show a preference for western
and southern Minnesota, but also
shows a clustering of reports near the Twin Cities, where more people are available to observe and report
hail.
4.3.4.6. Chronologic patterns (seasons, cycles, rhythm)
Most years, Hennepin County sees at least one large hail
event. The seasonal hail threat coincides with the
thunderstorm season, generally from April through
September, with a notable peak in frequency in June
and July. Severe hail has been reported as early as
March in Hennepin County, and as early as February in
greater Minnesota. Hail was observed with
thunderstorms in the Twin Cities on December 16,
2015, though no damage was observed. Damaging hail
in Hennepin County has been reported in November
and has occurred several times during October.
Minnesota 4"+ Hall Deports, 1 -2014
N VNR Srare Qunarc o gv Office
11 µl
a
d
1
ire
4.0
,
Flail Diameter
4im"b"
It
MNDNIR
4"+ hail reports in Minnesota, from DNR
State Climatology Office
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4.3.4.7. Historical (statistical) data/previous occurrence
May 6, 1965: Most widespread, intense, and long-lasting hail event on record in Twin Cities. Although
May 6, 1965, is best known for its devastating tornadoes in the Twin Cities, the storms also produced
destructive hail for an unusually long duration and over an unusually large area. Hail the size of ping
pong balls, golf balls, tennis balls and baseballs were
reported throughout the evening, in association with both
the tornadic storms and the many non-tornadic
thunderstorms cells. The largest hail stones were reported
in Hennepin County, generally inside what is now the 494 ,
694 corridor. Hail reports were received before the first �
tornado confirmations, and well after even the last �_f"' -�`r
suspected tornado, and the hail event lasted- „
«M,
approximately six hours. Many areas were hit by
tornadoes early in the evening, and destructive hail later"r .
r
in the evening,and some locations were hit by three
distinct waves of hail larger than golf balls. Locations in )
Hennepin County reporting golf ball or larger hail include
Minneapolis, Bloomington, St. Louis Park, New Hope,,.,,,p
Brooklyn Center, Maple Grove, Brooklyn Park, Edina,'
Deephaven, Crystal, and Eden Prairie.
May 15, 1998: Derecho hailstorm
A severe squall line developed in western Texas around
midnight and raced northeastward, making it to south-
central Kansas by daybreak, southwestern Iowa by mid- Wind (blue), hail (green), and tornadoes
morning, and the Twin Cities area by 16:00 local time. The (red) reported on May 15, 1998. Generated
storms produced widespread damaging wind along the from Severe Plot 3.0 (see references).
1000-mile-long track, and reached peak intensity in Iowa,
Minnesota, and Wisconsin, with fast-moving tornadoes and 1-2" hail driven by 60-80 mph winds.
The storms produced a record number of power outages in Minnesota (the record has since been
broken twice), and snapped or uprooted hundreds of trees in Hennepin County alone (with estimates
of over 1000 trees killed in
Ramsey County). A tornado
tracked from Roseville into
Blaine, at an estimated speed of
80 mph, causing significant
damage to homes. Most of the
damages however, were from
the hail, which broke windows,
damaged roofs bent garagef/,
doors and forced automobile -
�i r ,
Intl
r r ICJ � 9 bY'�iri/
dealerships in Bloomington to
r
submit claims for their entire
outdoor inventories. I Radar at 16:25 local, as bowing hail core entered central Twin Cities
• •
2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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The largest hail reported in the Twin Cities was 2 inches, and most reports were in the 1-1.5" range.
However, the intense straight-line winds turned the hail into dangerous projectiles, and produced far
more damage than would normally be expected.
August 6, 2013: The National Night Out Storm
Radar and report -based hail tracks. Source Minnesota State Climatology Office
On an evening when many Minnesotans were outside at neighborhood block parties, a powerful
supercell thunderstorm moved across central Minnesota into western Wisconsin, producing a large
swath of severe weather. Most reports were concentrated just south of the 1-94 corridor, and the storm
caused extensive damage to crops and vehicles.
The National Night Out storm had
less wind but somewhat larger hail
than the May 15, 1998, storm.
Winds were generally confined to
65 mph or less, but hail sizes were
typically 1.5 - 2 inches in the core
of the storm, which covered the
southwestern third of Hennepin
County. Damage to roofs and
vehicles was common from Maple
Plain, through the Lake
Minnetonka area, into Eden Prairie
and Bloomington. Damages were
not quantified locally, but Aon-
Benfield counted $1.25 billion in
damages from storms over the
northl ern and centraUS on August
Damage to squad car. Image courtesy Eden Prairie Police Department
5-7, noting that Minnesota and
Wisconsin were hardest -hit.
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An additional significant hail event occurred on August 11, 2023. However, that incident did not have as
high of impacts as the other events already described.
4.3.4.8. Future trends/likelihood of occurrence Ble
Research into hail frequencies in a changing climate has been somewhat limited, though modelling efforts
have suggested that the frequency of hail may decrease at the expense of more days with straight-line
winds, because the atmosphere may favor higher instability but lower -shear profiles as the equator -to -
pole temperature gradients weaken (Brooks 2013). Other research has suggested there may be fewer hail
days, but more significant events on the days with hail. The bottom line is that significant hailstorms, some
significant, are still to be expected into the future.
4.3.4.9. Indications and Forecasting
Like other severe weather hazards, national responsibility for hail monitoring and forecasting lies with the
National Weather Service's Storm Prediction Center (SPC) in Norman, Oklahoma. The SPC uses three
different "products" that detail in anticipation of a severe weather event:
Convective Outlooks are spatial products that assign risk categories for severe weather and
quantify the varying risk for hail (and other hazards) each day, along with an explanation of the
basis for the risk categories assigned. Outlooks are issued for Day 1 (day of), and days 2-8. Only
Day-1 outlooks contain hail -specific probabilities. "Day 1" outlooks are issued at 01:00, 08:00,
11:30, 15:00 and 20:00 (all times CDT). For Day 1, risk categories include Marginal, Slight,
Enhanced, Moderate, and High. These risk categories are assigned based on the probabilities of
severe weather (or a particular hazard) occurring with 25 miles of a point. (As shown in Table
4.3.4C)
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Table 4.3.4C
Day IIIIIII� 1g
I
outlook III I
Probability
10% w,ith
Significant Severe
III �;! M �
11SLGT
Significant Severe
pp
% with
Significant �r evere
1
45%45%
with
Significant Severe
.
W.'Ii lu �! A'9
II
IJJ
V� . Severe
Significant
JJ
SPC probabilistic risk table with corresponding outlook categories
Risk categories and probabilities are displayed on maps as color contours. The image below shows
the slight risk and probabilities of specific hazards at the 15:00 CDT outlook, just hours ahead of the
National Night Out storm.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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Convective outlook (upper left), tornado (upper right), severe wind (lower left), and hail probabilities on
august 6, 2013. From SPC's severe weather events database.
Mesoscale Discussions (MDs) are used to identify a particular area of concern within a risk area,
often when storms have developed or are expected to, and to communicate the possibility that a
watch may be issued. The MD will be tagged with a statement of likelihood regarding the issuance
of a Watch, as follows:
Severe Potential... Watch Unlikely (5 or 20%)
Severe Potential... Watch Possible (40 or 60%)
Severe Potential... Watch likely (80 or 95%)
Severe Potential... Watch Needed Soon (95%)
MDs can also be used to communicate additional concerns or trends during an ongoing event. Like
Convective Outlooks, MDs are both graphical and textual. The following MD graphic was issued
after the 15:00 CDT Convective Outlook, in anticipation of a watch issuance.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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..,
1"h���
ti. � _ b
11 4�uuu Ilu Illrll II II'ullllu ��ull �uuu1111 ��;u ou II � hull' �uuu II'rill II uuulluu
11� II II 11111111 10119! Io011 I � I1111111 i'! ll �IIIIII 11"BII11V11V1111
G7
iu
"I 10 0 E jrimi III�� o err �i
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r iii....
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, a r ��h
f
�i �h�h n h,lkk �blllh hlhl� 1008�r
vu m
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F. 1010 +urliG>�y�P �Vll �1y� J1JJ li 1 ili f0 P>rrrrr i �l ��lniiii�i �� � �°
I
1010 �rir
7
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SPC MCD #1638
Mesoscale Discussion graphic issued in anticipation of National Night Out severe weather event
Watches are issued when atmospheric conditions are favorable for the development of severe
weather. They are more geographically specific than Convective Outlooks, and they have defined
geographic boundaries, as well as start and end times. Typically, a watch will cover about 50,000
square miles --slightly more than half the size of Minnesota --and will last between 5 and 8 hours.
Tornado watches are used when conditions favor development of tornadoes, in addition to other
forms of severe weather. Severe thunderstorm watches are used when the tornado risk is
relatively low and hail or strong winds are expected. Large hail can be expected with both types
of watches, and neither connotes a greater or lesser risk of hail.
The National Night Out hail even was initially covered by a Tornado Watch, which was replaced by
a Severe Thunderstorm Watch after a few hours, when it became apparent there was not enough
low-level moisture or shear to produce tornadoes, but plenty instability aloft and mid -level shear
to produce large hail and strong winds. Below is the Severe Thunderstorm Watch outline with
radar overlay.
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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�f f
� f 0ON)ri'Jof', 9
err
ftjV*Q al I�gW.uG,.Kr7 �'�I �,PZF1 ki' Y
�4
o �
JI
NO
f "y"�YY 1✓,M V+N��F poi `�I
........................ ,,,,, .,,,,"m ,, ,,,,,,,,,,�C�i..,,� ,, ,.,,,, ,,,......
Severe Thunderstorm Watc i 422 Valid from 735 Puntil 200 AM CDT
4yP"n!�, ,N�r1 ,.ryV,,,i m, & r1,� 9i.I.�L�e� .wd✓� �" i�,e'�: 'M �Ff f? (°J, il1f(. la RbTC
Severe thunderstorm Watch outline with radar overlay on August 6, 2013
In addition to the SPC's information and products, the local National Weather Service Forecast Office
issues a Hazardous Weather Outlook (HWO), generally 1-2 times per day, as situations warrant, to share
thoughts about the potential for severe weather, including hail. These outlooks often discuss likely timing
and locations.
4.3.4.10. Detection & Warning
Local responsibility for detecting and warning citizens about severe hail lies with the National Weather
Service forecast office in Chanhassen. The primary means to communicate urgent storm location and
timing information is with Severe Thunderstorm and Tornado Warnings. These warnings indicate that
severe weather is imminent and will be affecting the warned area for a specified period of time. As with
watches, hail can be expected in both Severe Thunderstorm and Tornado Warnings, and neither is a better
indicator than the other of hail risk.
The NWS uses a combination of trained spotters and radar to detect severe hail. NWS Chanhassen has a
RADAR site for remote monitoring of hail -containing storms --the NEXRAD WSR-88D in Chanhassen.
Numerous tools and algorithms enable NWS staff in Chanhassen to use this system for identification of
severe hail in thunderstorms.
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Spotter reports, reports from emergency managers, and increasingly, reports from social media also help
forecasters in Chanhassen assess the severity of ongoing storms.
4.3.4.11. Critical values and thresholds
The National Weather Service considers hail to be
severe if it equals or exceeds one inch in diameter.
The NWS will issue a severe thunderstorm warning
with a "Considerable" tag when hail is expected to
be 1.75 inch in diameter or greater or will issue a
severe thunderstorm warning with a "Destructive"
tag when hail is expected to be 2.75 inches in
diameter or greater which would trigger a Wireless
Emergency Alert for those in the warning area.
Because impact increases exponentially with
incremental increases in hail size, larger hailstones
pose a significantly greater risk to safety and
property. Therefore, spotters are trained to use
common objects to make estimates about the size
of hailstones. It should be noted that few hailstones
are ever measured. Instead, they are often
observed, compared to the common objects, and
then the size is inferred from the size of the stated
objects. Thus, reported hail sizes are almost always
crude estimates. Table 4.3.411) summarizes the
common objects used as hail size references, along
with the approximate diameter. The diameters, and
often not the common objects, will be preserved in
the Storm Events Database.
4.3.4.12. Prevention
Table 4.3.4D
poi "Nii lmwslu 4� w,ei s a
bb
< 1A
< 0.64
< 34
< 39
Pea,
IA
0.64
24
35
marble
JJ2
1.,3
35
56
dime
V10
1.6
38
6
penny,
314
13
40
64
nickel
71'
.2
46
74
quarter
1
3.5
49
79
half dollar
1 174
3.3
154
67
wainut
1 11 '
3.6
60
97
golf ball
1374
4A
64
103
bon egg
2
5.1
60
111
teirnniis ball
2 V2
6.4
77
124
seball
3 314
7.,0
31
13
tea oup
3
7.6
34
135
grapefruit
4
10,.1
98
153
sottbamll
4112
11A
103
166
Hailstorms cannot at present be prevented and should be considered an occasional risk within Hennepin
County.
4.3.4.13. Mitigation
Hailstone size comparisons of commonly reported
The risks of being killed or injured by hail are reference objects.
greatest when hail is very large and/or wind driven. Thus, awareness of conditions that could lead to severe
weather and hail, and having a plan of retreat if storms approach is of primary importance.
As with all storms, the safest place to be when it's hailing is inside, in a sturdy structure, away from
windows. Even though cars often lose windows and contain some flying glass, they may be safer than
being outside, if the travel distance to the vehicle is reasonable. If no shelter or vehicle is available, retreat
to lower ground, if possible, stay away from trees, which pose a lightning risk, and by covering the head
to avoid potentially lethal impacts from large hail.
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On the road, many drivers make choices that ultimately compromise the safety of other motorists. Driving
into hail at highway speeds increases a hailstone's momentum (and therefore impact) substantially. Thus,
if it begins hailing while driving, slow down and look for potential shelter options off the road. There may
be none, but slowing down will reduce the impact of hail to the vehicle, reducing the risk for damage, and
potential injury from shattered glass. If slowing down does not adequately reduce the risks, pull
completely off the road, never under an overpass, and stop.
4.3.4.14. References
Aon-Benfield, August 2013 Global Catastrophe Recap,
http://thoughtleadership.aonbenfield.com/Documents/20130904 if august global recap.pdf
Brooks, H. E., 2013: Severe thunderstorms and climate change. Atmos. Res., 123, 129-138.
Changnon, S. A., D. Changnon, and S. Hilberg, 2009: Hailstorms across the nation. Illinois State Water
Survey, Champaign, IL,
Doswell, C. A., H. E. Brooks, and M. P. Kay, 2005: Climatological Estimates of Daily Local Nontornadic
Severe Thunderstorm Probability for the United States. Wea. Forecasting Weather and
Forecasting, 20, 577-595, doi:10.1175/waf866.1.
Hail Basics. NOAA National Severe Storms Laboratory,
http://www.nssl.noaa.gov/education/svrwxl01/hail/ (Accessed February 25, 2016).
Hail Size as Related to Objects (Storm Prediction Center),
http://www.spc.noaa.gov/misc/tables/haiIsize.htm (Accessed February 25, 2016).
Marshall, T.P., R. Herzog, S. Morrison, and S. Smith. 2002. Hail Damage Threshold Sizes for Common
Roofing and Siding Materials. 21st Conf.Severe Local Storms. American Meteorological Society.
Minnesota DNR, National Night Out Storm: August 6, 2013.
http://www.dnr.state.mn.us/climate/journa1/130806 national night out storm.html
NCDC, Storm data and unusual weather phenomena with late reports and corrections: May 1998,
volume 40.
NCDC, Storm Events Database: https://www.ncdc.noaa.gov/stormevents/
NOAA/NWS, JetStream - Thunderstorm Hazards: Hail
http://www.srh.noaa.gov/wetstream/tstorms/hail.htm
Storm Prediction Center, National Severe Weather Database Browser (Online SeverePlot 3.0),
http://www.spc.noaa.gov/climo/online/sp3/plot.php
Storm Prediction Center (SPC), Storm Prediction Center WCM Page, http://www.spc.noaa.gov/wcm/
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THIS PAGE WAS INTENTIONALLY LEFT BLANK
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d11;1 ,, Hazard Assessment: LIGHTNING
4.3.5.1. Definition
Lightning is one of the oldest observed natural
phenomena on earth. It has been seen in volcanic
eruptions, extremely intense forest fires, surface nuclear��„������
detonations, heavy snowstorms, in large hurricanes, and
most commonly, thunderstorms. Lightning is essentially
an electrical current where electrostatic discharges
between the cloud and the round other clouds within a
g io�ll i i
cloud, or with the air. Within a thunderstorm, many small,,,,,
bits of ice (frozen raindrops) bump into each other as they
move around in the air. Those collisions create an electric charge. The positive charges, or protons, form
at the top of the cloud and the negative charges, or electrons, form at the bottom of the cloud. Since
opposites attract, that causes a positive charge to build up on the ground beneath the cloud. The ground's
electrical charge concentrates around anything that sticks up, such as metal conductors, tall buildings,
people, or trees. The positive charge coming up from these points eventually connects with the negative
charge reaching down from the clouds, and that is when you see the lightning strike.
4.3.5.2. Range of Magnitude
The magnitude of lightning is incredibly variable from storm to storm. Typically, when discussing
magnitude of lighting, one is concerned mostly with lighting strikes that hit the ground. GRAPHICS 4.3.5A
and 4.3.5113 are using data from the National Climatic Data Center, which show the reported costs from
lightning for the past 10 years.
GRAPHIC 4.3.5A
Total Damage Cost from Lightning Per Year: Nation Wide
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GRAPHIC 4.3.5113
3.5
3
2.5
c 2
0
1.5
1
0.5
0
Total Damage Cost from Lightning Per Year: Minnesota
- 10 Year Average = 1.21
2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014
4.3.5.3. Spectrum of Consequences B2b
Lightning strikes are the leading causes of wildfires and have been responsible in the past for some of the
most devastating fires in the southwest United States. According to Storm Data, Minnesota ranks 281h in
the United States in lightning deaths from 1959-2012. Lightning is not only a threat to public safety, but
also a threat for public and private structures because of the large number of structural fires started from
lightning each year. Lightning can have direct and/or indirect effect, depending on whether it strikes a
structure directly or not. The effects depend greatly on the conductivity of the materials the electricity
travels through.
Material
Consequence
Electrical voltages created by electrical discharges dissipated in the ground
Electrical
that is struck by lightning.
Substantial damage and injuries from fires, burns, and destruction caused by
Thermal
a major release of heat.
Forces of attraction occur between parallel conductors that are traversed by
Electrodynamic
currents in the same direction create mechanical stresses and strain.
The lightning current induces extremely high voltage and an extremely strong
Electromagnetic
electromagnetic field that generate very powerful electric pulses that can
damage sensitive electronic devices.
Electrochemical
Corrosion due to currents circulating through buried conductors
Acoustic (Thunder and
Windowpanes can be shattered a few meters from the point of impact.
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Pressure Waves)
From simple dazzling to being struck dead by lightning, with a range of effects
Physiological
in between: Nervous shocks, various forms of blindness, deafness, blacking
out, and momentary or prolonged comas.
A common misconception of people being killed from lightning is because they were struck. Most lightning
injuries and deaths are causes by mechanisms other than direct lightning strikes. Only 3-5% of lightning
strike victims take a direct strike. 3-5% of lightning victims are contact injuries where the person is
touching or holding an object to which lightning attaches, such as indoor wired telephones or plumbing
that transmits current to the person. 30-35% of injuries are caused by a side flash, also called splash. Side
flashes occur when lightning hits an object such as tree or building and travels partly down that object
before a portion jump to a nearby victim. Most injury (50-55%) come from ground current. Ground
current arises because the earth is not a perfect conductor. Ground current effects are more likely to be
temporary, slight, and less likely to produce fatalities. However multiple victims and injuries are more
frequent from ground current. Another 10-15% of injury occur from upward leaders. Upward leaders are
upward discharges of lightning, which almost always occur from towers, tall buildings, or mountain tops.
A direct consequence to the body is an intense shock can severe impair most of the body's vital functions.
Cardiac arrest is common. Commonly when there is a strike that affects the heart directly, there is a
massive shutdown. With every beat the heart depolarizes and changes its electrical signal. In addition,
people can develop problems days, weeks, or months after being struck or being close to a lightning strike.
4.3.5.4. Potential for Cascading Effects
Lightning strikes that hit the ground, called cloud to ground strikes, can have a vast array of consequences.
One of the most common cascading events is when a lightning strike causes a fire to start, which can then
spread to homes, or produce wildland fire. Another consequence would be if lightning strikes a
transformer and people are without power for days, those people could be at risk for heat illnesses if hot
and humid conditions persist.
When lightning strikes a building, transients are generated on adjacent power, data, telephone and/or RF
lines. As these transients pass through electronic equipment on their way to earth, they can cause both
immediate damage and longer -term component degradation. However, the problem goes far beyond a
direct strike. Today our electronic systems are intrinsically connected to the outside world, not only by
mains power cables, but also through data and telephone lines, RF feeders, etc. Transient over voltages
from lightning activity up to 1 km away can destroy equipment inside a building, even when the building
itself has not been struck. As transients can be induced onto any conductive cable -overhead or
underground, the power, data, telephone, or RF lines leaving a building to join the main network or even
running between buildings can provide a way in for transients looking for a path to earth. Lightning simply
striking the ground, or even cloud -to -cloud lightning, induces a transient overvoltage on those cables,
allowing access directly into the electronic heart of that theoretically protected building. The following is
a list of possible secondary consequences following a lightning event.
• Downtime and disruption
• Hardware damage
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• Software corruption
• Data loss
• Lost production
4.3.5.5. Geographic Scope of Hazard Blc
As mentioned, lightning is one of the oldest observed natural phenomena on earth and has been seen in
many different types of natural phenomena. This means lightning occurs across the world, including the
United States, and of course, Minnesota. Individual lightning strikes are relatively small in geographic
scope. However, when an area has a storm filled with lightning, or multiple storms filled with lightning,
you can have a large geographic area being affected all at the same time. Graphic 4.3.5C shows Flash
Density map from Vaisala which shows the flashes per square mile per year for the entire United States.
Graphic 4.3.5C
4.3.5.6. Chronologic Patterns
Lightning can happen any time of year, however it is more prominent with spring and summer months as
this is when most of the convective weather occurs.
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S 0 N D
Month
4.3.5.7. Historical Data Bld
Lightning is a usual occurrence in thunderstorms across the State and Hennepin County each year. Every
year, about four percent of Minnesota structural fires are caused by natural events, one can infer these
natural events to be lightning related. The National Climatic Data Center states that there have been
$700,000 dollars in damage and 6 injuries due to lightning strikes in Hennepin County since August of
1995. From 1959-2014, Minnesota has had 64 lightning fatalities in the state.
Historically, data shows us that leisure -related activities are the greatest source of lightning fatalities.
From a study that looked at lightning deaths from 2006 through 2013, fishing contributed to the most
lightning deaths with 11% of all deaths.
See GRAPHIC 4.3.5D for the top 11 activities that contributed most the lightning deaths during this period.
This is consistent with a study that was published in 1999 that looked at lightning casualties and damages
from 1959 to 1994 in the United States.
There have been are no other lightning related incidents that are within the scope of this plan.
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GRAPHIC 4.3.5D Lightning Fatalities
4.3.5.8. Future Trends Ble
Some studies have shown changes in lightning associated with seasonal or year-to-year variations in
temperature, but there have not been any reliable studies conducted to indicate future trends of
occurrence until recently. A study looked at two variables, precipitation, and cloud buoyancy and how
they might be a predictor of lightning (see more in the indications and forecasting section for predicting
and forecasting lightning). The scientists found that on average, climate models predict a 12 percent rise
in cloud -to -ground lightning strikes per temperature degree increase in the contiguous U.S. This is roughly
a 50 percent increase by year 2100 if earth continues to see the expected seven -degree Fahrenheit
increase in temperature. While this is a step into looking into the future trends of lightning as our climate
continues to change, less is known about the exact locations on where strikes will increase.
4.3.5.9. Indications and Forecasting
"Lightning is caused by the charge separation within clouds, and to maximize separation, you have to lift
more water vapor and heavy ice particles into the atmosphere" (Romps, 2014). It is known that the faster
the updrafts, the more lighting, in addition, the more precipitation, the more lighting. Howfast the updraft
of the convective clouds is determined by the convective available potential energy (CAPE) which is
measured by radiosondes, balloon -borne instruments, released by each weather forecast office (WFO)
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twice a day. CAPE is essentially how potentially explosive the atmosphere is. In essence, where forecasters
see high CAPE values, and high-water vapor content in the atmosphere is where expected lightning and
thunderstorms are to occur.
4.3.5.10. Detection & Warning
Currently, there are no official alert or warning products that are issued by the National Weather Service
for just lightning. There are, however, certain programs that can be used that have lightning detection.
One of the leading lightning detection companies across the United States is Vaisala. Vaisala's Global
Lightning Dataset was first launched in September 2009. However, currently there is no way to receive
lightning detection data from Vaisala, or other detection sources, without a paid subscription to a specific
service. There are also very few, if any, sources that will give you the distinction between cloud to ground
lightning, intra-cloud, and cloud to air lightning, partly because the science is just starting to understand
how to detect the difference. Hennepin County has installed lightning sensors at select mesonet stations
in the Hennepin West Mesonet network which detect lightning strikes within a 20-mile radius. These
sensors can provide some information on how close lightning is to cities in Hennepin County.
4.3.5.11. Critical Values and Thresholds
Although there are not watches or warnings for lightning, by using the detection services that available,
one can watch how lighting within a storm is changing. In general, if lightning activity is increasing within
a storm, one can infer that the storm is strengthening. If lighting activity is decreasing, one can infer that
the storm is weakening.
4.3.5.12. Prevention
You cannot prevent lightning from occurring, but you can prevent some of the consequences by being
aware of when thunderstorms are forecasted as well as being aware of the potential cascading
consequences that can accompany the lightning. If a person sees lightning or hears thunder, they should
go inside immediately.
4.3.5.13. Mitigation
While there is no way to prevent lightning from happening, there are mitigation strategies to help protect
from the effects of lightning. First is protecting critical facilities and equipment by installing protection
devices such as lightning rods and grounding on communications infrastructure, electronic equipment,
and other critical facilities. Another way to mitigate for lightning is through educational and awareness
programs. Developing brochures to hand out at festivals, or with monthly water bills is one of the popular
strategies. Additionally, teaching schoolchildren about the dangers of lightning and how to take safety
precautions is another way to reach the parents at home as well.
4.3.5.14. Response
Quick response when it comes to effects from lightning is crucial. When someone is struck or is affected
by a near strike, ground current, first aid and CPR is crucial. However, CPR must continue for a long time
because it takes a long time for the heart to beat again, the diaphragm to function, and even longer for
the brain to reboot and control vital organ functions. People who go into cardiac arrest from
lightning have a 75 percent mortality rate. Quick response is also needed when lighting causes a fire.
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Whether it is a structure fire or grass/wildland fire, the more spread, the more damage. Please see the
Wildland Fire section of this hazard assessment for more information about response.
4.3.5.15. Recovery
Assessing the damage is the first part of the recovery process. People who are victims of a strike or near
strike ay not ever fully recovery and may continue to have issues the rest of their lives. However, the faster
the treatment they can get immediately, the faster recovery they will see.
4.3.5.16. References
Holle, Ronald L. 2012. 'Recent Studies of Lightning Safety and Demographics'. 2012 International
Conference on Lightning Protection (ICLP). doi:10.1109/iclp.2012.6344218.
Jensenius Jr., John. 2014. 'A Detailed Analysis of Lightning Deaths in the United States from 2006
through 2013'. National Weather Service Executive Summary.
LA3pez, RaA°I, and Ronald Holle. 1995. 'Demographics of Lightning Casualties'. Seminars in Neurology 15
(03): 286-295. doi:10.1055/s-2008-1041034.
Romps, D. M., J. T. Seeley, D. Vollaro, and J. Molinari. 2014.'Projected Increase In Lightning Strikes In
The United States Due To Global Warming'. Science 346 (6211): 851-854.
doi:10.1126/science.1259100.
Vaisala.com. 2015.'Vaisala -A Global Leader in Environmental and Industrial Measurement'.
http://www.vaisala.com/en/Pages/default.aspx.
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11;1 Hazard Assessment: RAINFALL, EXTREME
4.3.6.1. Definition
Extreme rainfall leads to flash
flooding, infrastructural and
property damage, and even loss
of life. Although the definition
varies by application, extreme
rainfall events are generally
understood to have rates that
meet or exceed a given
threshold, often tied to storage
or drainage capacity.
In forecasting applications, extreme rainfall drives the issuance of National Weather Service flash -flood
products based on "flash -flood guidance," which is a changing, location -dependent value that utilizes
pre-existing soil moisture and land cover conditions. Unsaturated soils and ample vegetation require
higher precipitation rates to trigger flash -flooding than saturated soils, denuded vegetation, or impervious
surfaces.
Extreme rainfall also is critical to hydrologic design of roads, trails, culverts, retention and detention ponds,
dams, and other types of infrastructure. Engineers and planners design these facilities to withstand all but
some small percentage of all heavy rainfall events. For instance, many non -critical features like small roads
and trails are designed to withstand a storm that has a 10% probability in any given year (also known as
the 10-year storm). More critical features will often be designed for 100-year rainfall events --those that
have a 1% probability in any given year. NOAA Atlas 14 contains the most recent scientific estimates of
rainfall amounts for durations from 5 minutes to 60 days, and with recurrence intervals of 1 through
500-years.
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4.3.6.2. Range of magnitude
I� aXIMirm'FA`In,"
R rnf 11 c ara i, irY,,,
H' r rYeparYCoattf ,,,,,,,,
M irYhsesixJ a '
Official: 10.00 inches, MSPJuly
Official: 15.10 inches, Hokah,
23-24, 1987
Aug 18-19, 2007
Unofficial: 12.75 inches,
Unofficial, La Crescent, 17.21
Bloomington, July 23-24, 1987
inches, August 18-19, 2007
13.80" MSP JulY 20-24, 1987
17.45 inches, Hokah, August 18-
22, 2007
I%titihaC,� ���
17.90 inches, MSP, July 1987
23.86 inches, Hokah, August
2007
4.3.6.3. Spectrum of consequences (damage scale, common impacts and disruptions, response needs)
2
The most dangerous result of extreme rainfall is flash flooding, which has numerous consequences, arises
from a combination of factors, and is covered in greater depth as its own chapter within this assessment.
Other severe hazards are not related to directly flooding. Following is a brief annotated list of common
consequences resulting from extreme rainfall:
• Injury, drowning, death: those unable to get to higher ground, and those stuck in vehicles that
either failed to navigate or are unaware of high water are at significant risk. Flooded roads,
particularly at night, are especially dangerous.
• Infrastructure damage: roads, culverts, drainage basins, bridges, and even dams can succumb to
the direct force of heavy flowing water, and to erosion from the ground below. Sewer and
wastewater systems may overflow.
• Stalled, stranded, or damaged vehicles. Many vehicle batteries die in high water, causing vehicles
to stall. Parked vehicles in low-lying areas may also be inundated and stranded. Water frequently
gets inside the vehicles, damaging the electronics and the interior.
• Structural failure: eroding soils from a heavy rain may undermine the structural integrity of houses
and buildings, resulting in complete or partial collapse.
• Water damage. Water enters sub -grade floors through small openings and in extreme events can
accumulate to inches or even feet on the lowest levels, as municipal sewer systems exceed
capacity and water backs up into residential lines. Electrical equipment becomes susceptible to
damage, and interior materials may be compromised and may develop dangerous mold or mildew.
• Crop damage: it is common for major extreme rainfall events to damage agricultural fields, often
wiping out an entire season's worth of crops.
• Water quality: extreme rainfall washes high level of compounds into area waterways, which may
exceed allowable contaminant thresholds for days or even weeks after a major event.
• Recreational loss: extreme rainfall events target the lowest areas first, meaning that lakes and
rivers are susceptible to overflow. No -wake laws impede water sports, and overflowing streams
and rivers can produce dangerous conditions for canoeing and other human -powered water
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activities. Trails and paths near lakes and rivers are often flooded, preventing bicycling, jogging,
and walking. Recreational departments will require extra labor hours to return recreational
resources to proper working conditions.
4.3.6.4. Potential for cascading effects
Most cascading effects associated with extreme rainfall are identical to those associated with flash -
flooding and urban flooding.
Extreme rainfall hazards can easily be compounded by other pre-existing hazards, as well as hazards that
develop after an event. In many cases, extreme rainfall --especially of shorter durations --occurs with severe
supercell thunderstorms, squall lines, and mesoscale convective systems. Almost by definition, these
systems are multi -hazard events. Thus, straight-line downburst winds, large hail, tornadoes, and frequent
lightning are often associated with the same storms that produce extreme rainfall rates. Power may be
out, which complicates efforts to remove water using sump pumps. This was the case in June of 2013,
following a major wind event in the Twin Cities. The July 23-24 super storm produced record -setting and
basement -inundating rainfall from storms that also produced heavy damage from tornadoes. There were
instances during the evening in which tornado warnings and Flash -Flood warnings were in effect for the
same area simultaneously. Seeking shelter in a basement posed flood -related risks.
Extreme rainfall also can play a role in tree mortality, and associated damages to public sidewalks, personal
property, and electrical systems. On June 21, 2013, a major tree fall event that was also the largest
weather -related power outage in state history, resulted notjust from the prolonged downburst winds, but
also from intense rains that fell both earlier in the day, and during the storm. Though the winds were 50-
60 mph with some higher gusts for over 10 minutes in many places, they produced far more damage than
would be expected at those speeds. The severity of tree damage likely resulted from the saturated soils,
which provided less resistance than normal, allowing trees to become "loose" and eventually topple.
Whether short or prolonged in duration, extreme rainfall is often associated with summerlike air masses.
Thus, extreme rainfall may occur before, during, or after an extreme heat event. Similarly, extreme rainfall
can occur during drought conditions, as was the case in 1987.
Additional specific cases of high -impact multi -hazard extreme rainfall events will be outlined in the
Historical (statistical) data/previous occurrence section.
4.3.6.5. Geographic scope of hazard Blc
Extreme rainfall rates may cover between 50 and 1500 square miles at a time. After accounting for
movement, the total area affected by rainfall more than 3 inches may cover thousands of square miles,
with hundreds of square miles receiving over six inches of rain. In exceptionally rare cases, 6-inch rainfall
totals may cover an area greater than 1,000 square miles --approximately the size of two Twin Cities area
counties. The Minnesota State Climatology Office has documented 12 of these "mega" rainfall events in
Minnesota since the mid-1800s. These events are always associated with catastrophic damage and often
loss of life.
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4.3.6.6. Chronologic patterns (seasons, cycles, rhythm)
Extreme rainfall has been observed from April through November, but peak probabilities are generally
from June through August, and to a lesser extent, September. The frequency of 3 and 4-inch rainfall peaks
during July.
Twin, Cities Heavy Rainfall's by Month, 1 71-2015
2
" events
26
i ,, " events
" events
24
20
16
12
6
........... w ... ......
1 ... ;ems .... .v....... w . .
Jan Feb Mar Apr May Jul' Jul
Aug
Sep Oct Novi De
Graphic of 2, 3, and 4-inch daily rainfall totals in Minneapolis since 1871.
Like other convective weather hazards, extreme rainfall goes through more and less active periods.
Hennepin County has at times gone many years between major events. 2014, 2002, and 1997, on the
other hand, are relatively recent examples of years with multiple extreme events in the county.
4.3.6.7. Historical (statistical) data/previous occurrence Bld
NOAA Atlas 14 is the definitive source for extreme rainfall estimates and contains the most recent scientific
estimates of rainfall amounts for durations from 5 minutes to 60 days, and with recurrence intervals of 1
through 500-years. The following table is for a point selected in central Hennepin County. The top row
contains recurrence intervals (or return periods), and the left column is storm durations. The value in bold
where they intersect is the likely amount in inches expected for a storm of that duration, at that recurrence
interval. The values in parentheses represent the 90% confidence range around the bold value Example:
For 24-hour rainfall at a 100-year recurrence interval is estimated to be 7.34 inches, and is 90% likely to
be between 5.55, and 9.65 inches.
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TABLE 4.3.6A is derived from a statistical technique that utilizes data from multiple stations and is based
on observations.
TABLE 4.3.6A Precipitation frequency estimates for a point in central Hennepin County
The 100-year recurrence value for 24-hour rainfall is the most frequently cited value, and indeed, many
structure are designed for such an event. It is, however, important to note that shorter durations of
excessive rainfall can also overwhelm systems, and many have therefore been designed for 1, 3, or 6-hour
thresholds. Structural, civil, and hydrological engineers can provide further information on exceedance
thresholds used for infrastructure elements. Additionally, heavy rainfall over longer durations can
overwhelm systems, even when exceptional hourly rainfall rates are lacking.
Extreme rainfall, therefore, should be anticipated on a variety of timescales, and not just measured by
daily or 24-hour rainfall only. Radar estimates and automated rain gauges help forecasters understand
rainfall quantities for shorter and longer durations, and noteworthy rainfall events of many duration -
magnitude combinations have affected Hennepin County.
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July 23-24,1987, Super Storm
The heaviest rainfall ever officially recorded at a Twin Cities weather station fell between about
18:00 CDT on 23 July and about 02:00 CDT on 24 July 1987. During this eight -hour interval,
observers at the Twin Cities International airport station measured an even ten inches of rain (9.15
inches of which fell in a five -hour period). In addition to the heavy rainfall, the 23-24 July storm
spawned an F3 tornado near Goose Lake in Hennepin County and produced extensive damage in
Maple Grove and Brooklyn Park. Damage in other areas was extensive, largely the result of flooded
homes and businesses, ruptured storm sewers, and washed out or inundated streets and
highways. Two flood related deaths were reported, and property damage was estimated to be in
excess of $30 million (1987 dollars).
The 23-24 July storms occurred along an outflow boundary that had separated extremely warm,
moist air to the south and east and much cooler, drier air immediately to the north and west. The
interaction of these air masses produced intense thunderstorms with extremely heavy rainfall
over the southwestern portion of the Twin Cities on 20-21 July 1987, two days prior to the 23-24
July outbreak. Rainfall amounts during this event included 3.83 inches at the Twin Cities airport
station, 9.75 inches near Shakopee and 7.83 inches at the neighboring community of Chaska.
The 23-24 and 20-21 July storms, together with the rainfall produced by thunderstorms earlier
and later in the month, brought unprecedented July rainfall to the Twin Cities area.
The International airport station recorded 17.91 inches, approximately six times the July normal.
An unofficial monthly total of 19.27 inches was recorded in west Bloomington.
Ironically, July 1987's excessive rainfall came in the middle of a prolonged period of subnormal
precipitation. Precipitation had been below normal for every month from October 1986 through
June 1987 and, following about six weeks of wet weather in July -August 1987, the drought
returned. Extreme dryness prevailed during much of the ensuing year with a near record dry June
and record warmth during the summer of 1988.
July 1,1997, Derecho and Flood
An intense mesoscale system containing supercells and a fast-moving squall line tore through the
central and northern Twin Cities area during the evening, producing extensive wind damage and
catastrophic flooding. Numerous tornadoes rated up to F3, were reported from the Willmar area,
through Wright and Sherburne Counties. Non-tornadic winds more than 100 mph knocked out
power, severely damaged structures, and snapped and uprooted trees in Wright, Anoka,
Sherburne, and northern Hennepin counties.
As the storm complex moved into the central portions of the Twin Cities, it produced some of the
heaviest one -hour rainfall ever measured in Minnesota. 3-4 inches fell within one hour over the
central and eastern parts of Hennepin County, as well as adjacent portions of Ramsey and Anoka
counties. 1-35 and 1-94 were closed south of downtown Minneapolis and standing water more
than 10 feet in some areas prompted boat rescues in south Minneapolis and Richfield. Edison High
School in northeast Minneapolis sustained major flood damage, and hundreds of homes and
residential complexes were severely damaged by inundation.
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Late May through June 2014 - repeated/persistent heavy rainfall events
A persistently wet pattern punctuated by
numerous heavy rainfall events during June
2014 led to significant flooding and
estimates of approximately $12 million in
damage throughout Hennepin County. The
greatest impacts tended to be focused near
water bodies and low-lying areas.
Numerous stations in Minnesota reported
record monthly rainfall for June.
May 31- June 2: 2-4 inches of rainfall was
common over the county, with 4.3"
reported at Flying Cloud. This was part of a
nearly statewide heavy rainfall event. Lake
Minnetonka rose to its highest levels in 109
years following this event.
June 6-8: A scattered rainfall event, with up
to 2 inches in western Hennepin County,
and an isolated 3-inch report near
Independence.
June 14-16: 2-3 inches throughout the county. Levels began rising rapidly along many waterways.
June 18: Isolated reports of up to 1 inch in association with a major event concentrated over
southern MN, and in advance of the more significant event on the following day.
June 19: Major, long -duration intense rainfall event, with waves of heavy precipitation throughout
the day. Flooding became common and widespread. 3-5 inches were common throughout the
county, with 4.13 reported at MSP—the heaviest daily total since October 2005. 5.47" was
reported by CoCoRaHS in Eden Prairie. Seven-day rainfall amounts of 4-8 inches were common
across the county, with even more to the south and west.
Municipalities, school districts, and other public interests within Hennepin County reported losses
and expenses more than $12 million USD (2014). The following list is not exhaustive, but rather
representative of the scale and impact of damage from the excessive rainfall.
• Bloomington, $265-270k: parkland damage; destruction of warming house
• Eden Prairie, $360-370k: pipe ruptures damage to Duck Lake Trail, Eden Prairie Road,
recreational trails, sewers, and banks of Riley Creek
• Golden Valley, $90-95k: unspecified damages to roads, sewers, culverts
• Greenfield, $20-25k: roads, sewers
• Hennepin County Sheriff's Office, $26k: water patrol docks and one boat damaged.
• Hopkins School District, $5k: washouts at High School, West Jr. High, Gatewood
Elementary, and Eisenhower
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• Minneapolis Park Board, $6.8M: Mudslide behind Fairview -Riverside affecting 100' x 250'
slope and exposing facility oxygen tanks and require extensive re -engineering and
restoration.
• Minnehaha Creek Watershed District, $180k: Lake Minnetonka reached record high water
mark of 931.11 feet, and Minnehaha creek exceeded 100-year flow at Hiawatha (with 893
cu ft.). The entire creek watershed was severely impacted, as were many of the MCWD's
capital projects.
• Minnetonka, $55k: unspecified damages to municipal property
• Minnetonka Independent School District, $NA: Destruction/failure of retaining wall at
high school.
• Mound, $1M: unspecified damages to streets, culverts, sewers, parks, and infrastructure
• Orono, $150k: severe damage to Starkey Road and Balder Park Road
• Park Nicollet Methodist Hospital, $3.6M: Drainage system destroyed; sunken grade
creating sinkhole risk; low-lying electrical circuitry inundated and damaged, pumping,
sandbagging and dewatering required; barriers construction.
• Richfield, $70-75k: Power failure at sanitary lift station, damage to pumps, trails and paths
inundated, littered with debris, and damaged.
• St. Louis Park, $50-55k: severe damage on Louisiana Ave
• Wayzata, $70-75k: city marina flooded and damaged; culverts damaged, requiring
emergency repairs.
August 18-20, 2007 - worst
rainfall event on record in MN
Perhaps the most extraordinary
precipitation event in
Minnesota's modern history
shattered Minnesota's 24-hour
rainfall record. The 15.10" total
recorded at 8:00 AM on Sunday,
August 19, 2007, near Hokah in
Houston County is the largest
24-hour rainfall total ever
measured at an official National
Weather Service observing
station in Minnesota, breaking
the old record of 10.84 inches
by an astonishing 39%.
Rainfall Totals for Southern Minnesota
August 18through August 20 (8:00 AM CDT), 2007
0 1 2 3 4 5 6 7 8 101214 inches
State Climatology Office- DNR Waters
Rainfall totals for entire 3-day rainfall event in southern Minnesota in
august of 2007. In most areas, 80-90% of the totals came within the
first 24 hours of the event.
The storm also obliterated the state's "unofficial" rainfall record, when a non -National Weather
Service rainfall observer near La Crescent (Houston County) reported 17.21 inches for the 24-hour
period ending 7:00 AM, Sunday, August 19. This is the largest 24-hour value in the Minnesota State
Climatology Office database and broke the previous statewide non-NWS observer record 12.75"
by a margin of 35%. Both new records far exceeded expected totals, even for record -breaking
events, and are so large, a true return period estimation is virtually impossible.
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The combination of huge rainfall totals and a very large geographic extent, make this an
extraordinary episode. The area receiving six or more inches during a 24-hour period during this
torrent encompassed thousands of square miles- the largest such area known to the Minnesota
State Climatology Office.
There have been no other incidents that are within the scope of this plan.
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4.3.6.8. Future trends/likelihood of occurrence Ble
The 2023 National Climate Assessment indicates that winter and spring precipitation is expected to
increase, while summer and fall precipitc
2070), the latest science suggests that
rainfall events that would ranking in
the top 2% for the period 1981-2010,
will become more common. Most of
Minnesota can expect, on average, an
additional day per year with these
events, which amounts to an
approximate doubling infrequency.
4.3.6.9. Indications and Forecasting
The Chanhassen Office of the National
Weather Service is the local authority
for extreme rainfall monitoring and
forecasting, and uses flash flood Additional days per year with upper 2% rainfall events by mid-century
(2041-2071). Source, 2014 National Climate Assessment, Midwest Chapter.
guidance, based on soil moisture and
land cover conditions, to evaluate whether expected and/or ongoing heavy rainfall poses a significant
flooding risk. Additionally, NOAA's Weather Prediction Center (WPC) has a legacy of advanced hydro -
meteorological monitoring and prediction and offers Excessive Rainfall Outlooks and Mesoscale
Precipitation Discussions that are comparable to the severe weather products offered by the Storm
Prediction Center. Unlike the Storm Prediction Center, however, the WPC does not issue Watches of any
sort.
Forecasters monitor and analyze numerical weather models and other predictive tools to ascertain
potential extreme rainfall and associated flash flooding threats. The following sequence of
products may then be used in an idealized situation, though it should be noted that extreme
rainfall threats may appear of disappear at any step in this timeline:
4+ days out: Chanhassen NWS Office highlights threat for heavy or extreme rainfall and flash flooding
potential in Hazardous Weather Outlook products.
1-3 days out: WPC issues Excessive Rainfall Outlook, indicating Marginal, Slight, Moderate, or High Risk of
excessive rainfall, according to the following probabilities:
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Current/valid Excessive Rainfall Outlooks can be found at: http://www.wpc.ncep.noaa.gov /qpf/ excess
rain.shtml
Within 48 hours: Chanhassen NWS Office issues Flash Flood Watch, based on combination of expected
precipitation and local Flash Flood Guidance values.
Important: In early spring 2018, the NWS will no longer use Flash Flood Watches, and will
instead consolidate them into generic Flood Watches, as part of its Hazard Simplification
process: https://www.weather.gov/news/170307-hazard-simplification
Within 1-6 hours: WPC issues Mesoscale Precipitation Discussion to highlight emerging flooding potential
from expected, developing, or ongoing thunderstorm and rainfall activity. These discussions are only used
for large areas of concern (generally the size of 25 or more Minnesota counties) and do not pertain to
highly localized extreme events.
Each discussion includes an annotated graphic indicating the area of concern, and a brief text discussion
focused on the mesoscale features supporting the anticipated heavy rainfall. The potential for flash
flooding within the area of concern will be highlighted by one of three headlines:
FLASH FLOODING LIKELY High confidence exists that environmental conditions are favorable, or
will become favorable, for heavy rainfall that will result in flash flooding.
FLASH FLOODING POSSIBLE Environmental conditions are favorable, or will become favorable, for
heavy rainfall, but there are questions about how the event will evolve and/or whether flash
flooding will occur.
FLASH FLOODING UNLIKELY High confidence exists that environmental conditions are
unfavorable, or will become unfavorable, for heavy rainfall that will result in flash flooding.
Once event has begun: Chanhassen NWS Office issues Flash Flood Warning, based on combination of
precipitation received, further precipitation expected, soil conditions, and stream levels and flow. A Flash
Flood Warning is issued when flash flooding is occurring or is imminent. These warnings differ from Severe
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Thunderstorm and Tornado warnings, in that they are not issued in advance of the parent thunderstorm(s),
but instead after the storm has begun, ideally in advance of the flash -flooding itself. The behavior of
approaching storms is erratic enough that pre -storm lead time for flash -flood warnings would lead to high
false alarm rates.
Flash Flood Warnings are issued as polygons that attempt to match the spatial extent of the true threat
(as opposed to covering entire counties). Like Severe Thunderstorm warnings, they may cover slivers of
counties, or multi -county swaths. The warning period depends on the duration of the event itself, but
Flash Flood Warnings may continue for several hours after the precipitation has subsided.
4.3.6.10. Detection & Warning
The Chanhassen NWS Office and North Central River Forecast Center (adjoining the Chanhassen office)
monitor local flood conditions using a combination of manual and remotely sensed information. Key
warning detection and decision sources include but are not limited to:
• Radar -estimated precipitation, which can be used in conjunction with flash flood guidance values
to determine flood potential.
• Automated, real-time stream gaging, which indicates the level and flow of critical streams.
• Real-time, manual, or automated rainfall reports
• Radar and local meteorological trends, indicating potential for storms to continue and/or
redevelop in or near affected areas.
• Reports from spotters, emergency managers, first responders, the media, and the public
• Images or videos shared via social media or other means.
The Chanhassen NWS Office will issue a Flash Flood Warning if the forecasters determine that information
from the above and other detection sources indicate that flash flooding is occurring or is imminent in each
area.
4.3.6.11. Critical values and thresholds
Unlike other weather hazards, Watch and Warning thresholds for flash floods vary with the pre-existing
meteorological conditions. Conditions with saturated soils and high or overtopped streams require
substantially less precipitation to generate flash -flooding than conditions with low soil moisture and low
stream levels. Although some anticipated precipitation amounts may suggest to forecasters that flash
flooding is possible, irrespective of soil conditions, the Watch and Warning thresholds are generally
determined on a case -by -case basis, by considering the Flash Flood Guidance for the area(s) of concern.
Flash Flood Guidance (FFG) values estimate the average amount of rainfall (in inches) for given a duration
required to produce flash flooding in the indicated county or area. These values are based on a
combination on current soil moisture conditions and land cover considerations, and therefore change in
response to the local hydro -climatic situation. Throughout much of Hennepin County, and especially in
urban areas, less rainfall is required to produce flash flooding than in many neighboring areas, because of
the county's high concentration of impervious surfaces.
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Current flash -flood guidance for 1, 3, and 6-hour rainfall can be found at:
• https://www.weather.gov/ncrfc/LMI_ROF_NFP_FlashFloodGuidance
4.3.6.12. Prevention
To improve water management and protect the sewage system from damage, cities can revamp their
underground pipe and drainage systems by separating rainwater from the sewage system. The separation
enables the wastewater treatment plant to function properly, without it being overburdened by large
quantities of storm water.
Other more obvious methods are to keep sewer systems clean of clog up with waste, debris, sediment,
tree roots and leaves.
4.3.6.13. Mitigation
Areas that have been identified as flood prone areas can be turned into parks, or playgrounds, buildings
and bridges can be lifted, floodwalls and levees, drainage systems, permeable pavement, soil
amendments, and reducing impermeable surfaces. Reducing impervious surfaces could include the
addition of green roofs, rain gardens, grass paver parking lots, or infiltration trenches.
Other mitigation strategies include developing a floodplain management plan, form partnerships to
support floodplain management, limit or restrict development in floodplain areas, adopt and enforce
building codes and development standards, improve storm water management planning, adopt policies
to reduce storm water runoff, and improve the flood risk assessment.
4.6.3.14. Response
One of the most important things to be done during the initial response is to make sure that people are
safe. If their homes have been damages and are unlivable, finding a place for them to stay is among one
of the top priorities. Next is the access to places if roads are washed out or still underwater. One
complicated factor with flood disasters, is sometimes you do not know how bad the damage is until the
water recedes, which can take time and slow the response. Another important part of response is to make
sure water supply is available as quick as possible if there has been any contamination. The role of
Hennepin County Emergency Management is to coordinate resources that our municipalities may need
to accomplish all response needs.
4.6.3.15. Recovery
As mentioned in river flooding, recovery from floods can take weeks, to months, to years. Extreme
rainfall/flooding is unlike quick onset disasters (e.g., tornadoes) where you can see the damage
immediately, sometimes with excessive rainfall/flooding you must wait for the flood waters to recede to
find out what damage there is to recover from. A lot of the time, the longer the water level stays too high,
the more consequences are introduced that you must then recover from.
4.6.3.16. References
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24-hour Minnesota Rainfall Record Broken August 19, 2007. (n.d.). Retrieved March 30, 2016, from
http://www.dnr.state. mn.us/cli mate/journal/24hour_rai n_record. html
25th Anniversary of the 1987 Twin Cities Superstorm: July 23-24, 1987. (n.d.). Retrieved March 30, 2016,
from http://climate.umn.edu/doc/journal/870723_24_superstorm.htm
Heavy Rain and Tornadoes: June 19, 2014. (n.d.). Retrieved March 30, 2016, from
http://www.dnr.state.mn.us/climate/journal/140619_heavy_rain_tornadoes.htmI
Heavy Rains Fall on Southeastern Minnesota: August 18-20, 2007. (n.d.). Retrieved March 30, 2016, from
http://www.dnr.state.mn.us/climate/journal/ff07O820.html
Heavy Rains of May 31-June 2, 2014. (n.d.). Retrieved March 30, 2016, from
http://www.d n r.state. mn. us/cl i mate/jou rnal/heavyra i n 140531_140602. htm I
Historic Mega -Rain Events in Minnesota. (n.d.). Retrieved March 30, 2016, from
http://www.dnr.state. mn.us/cli mate/summaries_and_publications/mega_rai n_events.html
Historic Rainfall and Flooding of August 18-20, 2007. (n.d.). Retrieved March 30, 2016, from
http://www.weather.gov/arx/augl907
Minnesota Flash Floods. (n.d.). Retrieved March 30, 2016, from
http://www.dnr.state.mn.us/climate/summaries_and_publications/flash_floods.html
National Climate Assessment. (n.d.). Retrieved March 30, 2016, from
http://nca20l4.globalchange.gov/report/regions/midwest
PFDS (Precipitation Frequency Data Server/NOAA Atlas 14): Contiguous US. (n.d.). Retrieved March 30,
2016, from http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=mn
Record -Setting Rainfall in June 2014. (n.d.). Retrieved March 30, 2016, from
http://www.dnr.state.mn.us/climate/journal/140630_wet june.html
Winkler, J. A., Andresen, J. A., Hatfield, J. L., Bidwell, D., & Brown, D. G. (n.d.). Climate change in the
Midwest: A synthesis report for the national climate assessment.
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�.7,,,;< Hazard Assessment: HEAT, EXTREME
4.3.7.1. Definition
4.3.7.2. Range of Magnitude
The magnitude of extreme heat can vary greatly. You can have extreme heat events where you have
shorter periods (3-5 days) with much higher -than -normal temperatures, or you can have longer periods
(2-3 weeks) with temperatures only 5-10 degrees higher than normal temperatures.
• Hottest Heat Wave on record MN: July 18, 2011
• Longest Heat Wave on record MN: June 3-10, 2021
• Most Recent Heat Wave for Hennepin County: August 251h, 2013
• Deadliest MN Heat Wave: August 4-8, 2001; 5 fatalities
4.3.7.3. Spectrum of Consequences B2b
Extreme heat can be just as deadly as other natural hazards by pushing the human body beyond its limits.
Under normal conditions, the body's internal thermostat produces perspiration that evaporates and cools
the body. However, in extreme heat and high humidity, evaporation is slowed, and the body must work
extra hard to maintain a normal temperature. Most heat disorders occur because the victim has been
overexposed to heat or has over exercised for his or her age and physical condition. Effects can be seen
through just a few people or by many depending on extent the temperatures rise above normal, or other
hazards occurring simultaneously. People most at risk include elderly and very young persons, chronically
ill patients, socially isolated people, urban residents, and people without access to air conditioning.
There are different conditions, or disorders, related to extreme heat illnesses: heat stress, heat
exhaustion, heat stroke, and hyperthermia. Heat stress is the perceived discomfort and physiological
strain associated with exposure to hotter than normal environment, especially during physical activity.
Heat exhaustion is a mild -to -moderate illness due to water or salt depletion resulting from exposure to
extreme heat conditions or strenuous physical activity. Heat stroke is a severe illness resulting from
exposure to environmental heat, or strenuous physical exercise during extreme heat conditions. Heat
stroke is characterized by a human body core temperature greater than 1040F along with central nervous
system abnormalities such are delirium, convulsions, or coma. Heatstroke can have a fast onset and poor
survival rate.
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4.3.7.4. Potential for Cascading Effects
One complicating factor when discussing impacts of extreme heat, is extreme heat doesn't necessarily
immediately impact people when it sets in, instead it is when the periods of extreme heat last for days
and weeks that it takes its toll on people. Additionally, when overnight air temperatures do not cool below
70 degrees F, it does not give people's bodies a break from the heat. An additional complicating factor is
when extreme heat conditions are paired with another hazard. For example, if severe thunderstorms
affect an area and knock out power right before extreme heat sets in, you now have additional people
exposed to extreme heat without working air conditioners. Extended durations of extreme heat can also
exacerbate drought conditions and can also lead to excessive power consumption needs causing the
potential for brown- and black -outs, which would only make the exposure conditions worse.
Extended periods of extreme heat also contribute to wildfire hazard through a process wherein natural
materials, particularly sand and bare soil absorb solar radiation, holding the heat very near the surface,
resulting in extremely high surface temperatures. The hot surface heats the overlying air, which rises,
carrying the heat upward. The extremely hot surfaces generate strong updrafts, essentially creating local
winds that dry surrounding vegetation, increase fuel temperatures, and intensify and spread wildfires.
The dry vegetation, high fuel temperatures, and high winds increase the static electricity, increasing the
potential for spontaneous combustion, particularly during prolonged periods of drought. Extreme heat
temperatures can also force the closure of airports due to the lack of sufficient air density for take -offs
and landings.
4.3.7.5. Geographic Scope of Hazard Blc
When this hazard happens, it can be as small as a local hazard, or countywide with areas of highest
concern in the largest metropolitan areas because of the Urban Heat Island (UHI). Urban heat islands are
large metropolitan urban areas that are warmer in temperature than surrounding rural areas because of
pavement, blacktop, and buildings. The University of Minnesota conducted a study showing the Twin
Cities metro area temperature differences in 2011.
Graphic 4.3.7A illustrates measured temperature differences of up to 10 degrees just within Hennepin
County.
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Graphic 4.3.7A
TOIU A Urban, Heat Island
Sept. 75 4am, COT
% '. Low ' 30 4 F
4.3.7.6. Chronologic Patterns
While the definition of extreme heat indicates an extended period where temperatures are above average
high temperature, you typically see extreme heat as an issue during the summer months of May through
September in Hennepin County.
4.3.7.7. Historical Occurrence Bld
There have been several past instances of extreme heat in Hennepin County. The earliest records of
extreme heat include the Dust Bowl of the 1930's. The Dust Bowl years of 1930-36 brought some of the
hottest summers on record to the United States, especially across the Plains, Upper Midwest, and Great
Lake States. For the Upper Mississippi River Valley, the first few weeks of July 1936 provided the hottest
temperatures of that period, including many record highs.
Two consecutive heat waves occurred in 1999. The first was on July 23-25, 1999, when a massive upper
ridge over the central U.S. enabled heat to build into Minnesota. Heat indices ranged from 95-110 on the
23rd, 90-105 on the 241h, and climaxed at 95-116 on the 251h. One death resulted from the heat wave after
a man fell asleep inside a closed vehicle on the 251h. The second heat wave of 1999 occur less than a week
later for central and south-central Minnesota. This heat wave lasted from July 291h, 1999, through July
301h, 1999. This heat wave was stronger with heat indices climbing to the 95-114 range with lows in the
70s and dew points in the middle 60s to 70s which produced heat indices 70-85 even in the morning hours.
In 2001, there were another two heat waves, one that was from July 30 through August I", and a second
from August 41h through August 81h. The July 30th-August V heat wave is commonly known for the heat
wave where Minnesota Vikings football player Corey Stringer collapsed on the football field around
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midday on July 31 in Mankato and was taken to the hospital. Mr. Stringer died early on August 1", 2001.
The second heat wave of 2001 came just three days later and persisted for five days. This heat wave
produced five fatalities all within Hennepin County. Hot weather and tropical -like humidity pervaded the
region, as virtually all stations registered highs in the 90s all five days. Minneapolis -St. Paul (MSP) reached
98 or 99 three straight days (August 5-7) when highs were 98, 99 and 98 respectively; the highs at MSP on
August 6 and August 7 set records. A few noteworthy heat indexes, including the highest known value
around Minnesota for each day, are:
• August 4 - 110 at Morris (Stevens County), 107 at Redwood Falls (Redwood County), and 102 at
MSP.
• August 5 - 114 at Alexandria (Douglas County) and Morris (Stevens County), 110 at Maple Lake
(Wright County) and Montevideo (Chippewa County), and 107 at Mankato (Blue Earth County)
and at MSP.
• August 6 - 118 at Rush City (Chisago County), 114 at Redwood Falls (Redwood County), 110 at
Faribault (Rice County), and 109 at MSP.
• August 7 - 117 at Morris (Stevens County), 116 at Redwood Falls (Redwood County), 109 at MSP,
and 107 at Staples (Todd County).
• August 8 - 102 at Little Falls (Morrison County) and Staples (Todd County), 100 at Appleton (Swift
County), and 95 at MSP.
Another heat wave occurred in 2005. High temperatures at Minneapolis -St. Paul International Airport
remained at or above 90 degrees for 9 consecutive days between July 9th and 17th. This extended period
of hot weather set a record for the 3rd longest streak of at or above 90-degree highs since 1891 in the
Twin Cities. On July 12th, a laborer putting up a fence in Arden Hills in Ramsey County suffered severe
heatstroke. He collapsed at the work site and was rushed to a local hospital. His body temperature
reached 108.8 degrees, but miraculously he survived after receiving intensive medical attention. He
awoke from a medically induced sedation 24 hours after falling ill and made a full recovery.
Two heat waves occurred in 2011, one in June and one in July. The June heat wave occurred on June 7",
where it broke the all-time true temperature record for the day at 103°F. This was the warmest day in the
Twin Cities in almost 23 years, when July 31, 1988, had a high of 105 degrees. The second heat wave of
2011 occur in July as a large ridge of high pressure expanded across the Upper Midwest and allowed for
a stagnant pattern, and eventually oppressive heat and humidity to develop. The heat wave broke records
for temperature and dew point, and even heat indices across the region. Maximum heat index values of
115 to 125 were common. A record high minimum temperature was set on July 18th, when a low
temperature of 80 degrees was recorded at Minneapolis - St. Paul International Airport. The previous
record was 78 degrees which was set in 1986. A record high minimum temperature was also set on July
20th, when a low temperature of 80 degrees was recorded. The previous record was 76 degrees which
was set in 1901, 1935 and 1940. A total of 44 fans were treated at Target Field (32 treated in their first aid
facilities and more than a dozen treated in their seats). The heatwave led to record power demand. Xcel
Energy set a record with the highest one -day peak demand ever of a little more than 9,500 megawatts on
Monday, July 18th. The heat affected turkeys in southwest Minnesota, where 50,000 turkeys died due to
heat related causes near Redwood Falls. In addition to the turkeys that died, several news articles had
references to heat related deaths to livestock in southern and western Minnesota, but the articles were
not specific for counties. The heat and humidity were also blamed for road buckling on 1-94 in Minneapolis.
Two lanes of northbound 1-94 at Lowry Ave, and two lanes of eastbound 1-94 at 49th Ave, were closed
because of buckling pavement.
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The most recent heat wave occurred in 2013 specifically August 251h through August 271h. A large ridge of
high pressure built across the central part of the United States during the last week of August. Heat and
humidity increased across the Upper Midwest starting the weekend of August 25th and lasted until the
latter part of the week with a string of 90+ afternoon temperatures, combined with dew points in the 70s,
caused heat indices to rise above 100 degrees from Sunday, through Tuesday, August 27th. In the Twin
Cities metro area, heat indices remained above 80 degrees overnight, and afternoon heat indices
continued above 100 degrees through Thursday afternoon, August 29th. The Minnesota State Fair was
going on during the time. 216 people required treatment at medical stations at the fair for heat related
illnesses, 10 of whom were transported to local area hospitals. In addition, several record high
temperatures were observed, and a dew point temperature of 77 degrees on August 27th at 3:00 PM tied
the MSP high dew point temperature record set on August 27, 1990. It also tied the record for highest
dew point ever during the State Fair (77 degrees - August 28, 1955, and August 27, 1990). Minneapolis
schools canceled all outdoor after -school athletics practices during this period. The August 26th high of
96 degrees in the Twin Cities broke the 94-degree record set in 1948. In Hennepin County, from the 25th
through the 29th, there were 28 people who were treated for heat related illnesses, either as walk-ins at
emergency rooms, or transported by ambulance to hospitals.
There have been no other incidents that are within the scope of this plan.
4.3.7.8. Future Trends Ble
Numerous studies have documented that human -induced climate change has increased the frequency
and severity of heat waves across the globe. While natural variability continues to play a key role in
extreme weather, climate change has shifted the odds and changed the natural limits, making heat waves
more frequent and more intense. In an unchanging climate both new record highs and new record lows
are set regularly, even while the total number of new records set each year may decrease as time goes
on. Sixty years ago in the continental United States, the number of new record high temperatures
recorded around the country each year was roughly equal to the number of new record lows. Over the
past decade, however, the number of new record highs recorded each year has been twice the number
of new record lows, a signature of a changing climate, and a clear example of its impact on extreme
weather.
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ICI' 51 011001
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tl 95 s data frorn KeeW et Sul.. aM other data, from AA,
4.3.7.9. Indications and Forecasting
Heatwaves are most common in summer when high pressure develops across an area. High pressure
systems can be slow moving and persist over an area for a prolonged period such as days or weeks. Not
all high-pressure systems bring heat waves. However, high pressure that is combined with high
temperatures and high dew points are those that bring the extreme heat events. Typically, with high
pressure, you have clear skies, which allows strong solar inputs as well. There has been a study done in
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which showed local evaporation also plays a role in causing high moisture values near the surface.
4.3.7.10. Detection & Warning
The two crucial values for the National Weather Service issuing excessive heat products are described
below in the definitions of advisory, watch, and warning criteria.
• Excessive Heat Advisory: The heat index will reach 95 °F for at least three hours one day. The
forecast maximum Wet Bulb Globe Temperature will reach 85 for three hours one day. The heat
index will reach 95 °F for two days in a row, along with an overnight low no cooler than 73 'F.
• Excessive Heat Watch: A possibility the heat index will reach 100 °F for one day and/ro the forecast
maximum Wet Bulb Globe Temperature could reach 87 for one day, and/or the heat index could
reach 100 °F for two days in a row, along with an overnight low no cooler than 73 'F.
• Excessive Heat Warning: Maximum heat index at MSP Airport reaches 100 °F or greater for at
least 1 day. The forecast maximum Wet Bulb Globe Temperature will reach 87 for one day. The
heat index will reach 100 °F for two days in a row, along with an overnight low no cooler than 73
'F. Advisory conditions for at least four consecutive days.
4.3.7.11. Critical Values and Thresholds
The heat index is what gives us the critical values for indications and warnings. The Heat Index is
sometimes referred to as the "apparent temperature". The Heat Index, given in degrees Fahrenheit, is a
measure of how hot it feels when relative humidity is added to the actual air temperature.
Temperature (7)
�a
Likelihood of Heat Disorders with Prolonged Exposure or Strenuous Activity
Caution Extreme Caution M Danger M Extreme Danger
Another measurement that is used to describe how the human body reacts to extreme heat is the Wet
Bulb Globe Temperature (WBGT). This is different from the heat index because it factors in wind and solar
radiation along with temperature and humidity. The WBGT parameter has been used by the military for
heat safety since the 1950s as it is a better representation for individuals who are active in the heat, since
wind and sun factor into how out body cools itself off. Many athletic associations including the sports of
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running, football, tennis, and soccer have used the WBGT as well. The critical values used by the military
can be seen below.
Applies to overa ge sired, heat-acchinrated soldier wrairing BDU, hot wenther (Seas rB VED 5,07 for furdier l d'ance)
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4.3.7.12. Mitigation
There are many ways to mitigate for extreme heat events. Mitigating from the health effects of extreme
heat can be having air conditioning, cities opening cooling centers, or adjusting work ours for those
individuals who work primarily outside. There are some energy efficiency measures in houses and small
commercial buildings can help to keep the indoor environment within comfortable temperature
conditions without use of air conditioning during extreme heat events such as: roof deck insulation, wall
insulation, high performance windows, and building orientation.
Mitigation strategies that require coordination and construction include shading of buildings, asphalt and
other dark surfaces with trees can reduce the UHI effect. Solar panels placed on canopies over parking
lots and other paved surfaces can also shade and reduce the UHI effect. Direct shading of buildings also
reduces heat in buildings in the event of power outages in an extreme heat event. However, tree planting
requires adequate space, water, and maintenance, and the correct selection of trees. Another mitigation
strategy is the management and restoration of parks in urban areas increases vegetated areas, which can
help reduce heat island effects. Increasing recreational and riparian spaces in urbanized areas has many
additional benefits including health benefits from air and water quality improvements. Additionally, there
are pavements that have technologies to reduce heat island effects. The pavements reflect more solar
energy, enhance water evaporation, are more porous, or have been otherwise modified to remain cooler
than conventional pavements.
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Education about extreme heat can also be a strategy.
TABLE 4.3.7A White -Newsome et al (2014) describe educational strategies in their four -city study:
TABLE 4.3.7A Four City Study
- .11111111ZHISM11612
• Revisit framing of heat warnings
Detroit
• Invest in full scale public relations campaign to educate residents on heat and
health.
• Educate grade school students about climate change.
• Ensure that county summer campaign includes a heat health component.
• Develop messages that connect climate change to everyday life
• Identify strategies to prevent oversaturation of messaging (e.g., home -based
care providers have many health messages to deliver)
New York
• Using focus groups, determine how and where to best promote cooling centers
to a greater diversity of vulnerable persons.
• Make health messages that apply to everyone.
• Consider additional risk factors in messaging, such as obesity and risk aversion
• Revisit messaging about where to go (e.g., ride public transportation, cooling
centers, mall) during heat waves.
• Educate people to participate in traditional cooling behaviors.
Philadelphia
. Increase messaging to encourage buddy systems or checking on loved ones.
• Consider use of social media or partnerships with GenPhilly
(http://www.genphilly.org) to remind younger generations to check on
vulnerable family members
• Create clearinghouse of projects and materials
• Develop —check on your neighbor1l programs or messaging.
Phoenix
. Work with Salvation Army on trainings for social service providers
• Improve collective definitions of heat wave.
• Partner with academics to better translate study findings
4.3.7.13. Response
There are many things an individual can do to respond to extreme heat events. The following list is from
the American Red Cross:
• Listen to a NOAA (National Oceanic and Atmospheric Administration) Weather Radio for critical
updates from the National Weather Service (NWS).
• Never leave children or pets alone in enclosed vehicles.
• Stay hydrated by drinking plenty of fluids even if you do not feel thirsty. Avoid drinks with caffeine
or alcohol.
• Eat small meals and eat more often.
• Avoid extreme temperature changes.
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• Wear loose -fitting, lightweight, light-colored clothing. Avoid dark colors because they absorb heat
from the sun.
• Slow down, stay indoors, and avoid strenuous exercise during the hottest part of the day.
• Postpone outdoor games and activities.
• Use a buddy system when working in excessive heat.
• Take frequent breaks if you must work outdoors.
• Check on family, friends and neighbors who do not have air conditioning, who spend much of
their time alone or who are more likely to be affected by the heat.
• Check on your animals frequently to ensure that they are not suffering from the heat.
As an Emergency Management agency, opening cooling centers to the public, adjust cooling center and
homeless shelter hours to account for those at need during non-traditional open hours are all response
strategies used. Many time neighborhood networks are also unofficially activated to check on their elderly
and vulnerable populations.
The City of Chicago stated that one of the biggest changes after the 1995 Chicago Heat Wave has been
technology. Chicago now has implemented a 311-center phone number to reach City Hall. Someone in
another state with an elderly mother living alone in Chicago can call the 311-center, and a well-being
check will be conducted by the appropriate agency. This allows the city to be more proactive that reactive
when it comes to calls about extreme heat illnesses.
4.3.7.14. Recovery
Like many other weather -related disasters, recovery from an extreme heat event is not fast. As
mentioned, consequences from extreme heat can begin to show after the extreme heat has subsided so
checking on vulnerable populations as part of the response, also carries over to the recovery process. It's
important to acclimatize to changes in temperatures. So as the body has started to get used to extreme
heat once the temperature drops back down can have effects as well. Giving the human body time to
adjust to these shifts is important to remember for workers who may spend most of their day outside.
4.3.7.15. References
Bernard, Susan M., and Michael A. McGeehin. 2004. "Municipal Heat Wave Response Plans". Am J Public
Health 94 (9): 1520-1522. doi:10.2105/ajph.94.9.1520.
Bouchama, Abderrezak, and James Knochel. 2003. "Heat Stroke". The New England Journal of Medicine
346 (25): 1978-1988.
Climate Communication Science & Outreach. 2015. "Climate Communication I Heat Waves: The
Details". https://www.cIimatecommunication.org/new/features/heat-waves-and-climate-
cha nge/heat-waves-the-deta i Is/.
Ksi.uconn.edu. 2015. "Wet Bulb Globe Temperature Monitoring I Korey Stringer Institute".
http://ksi.uconn.edu/prevention/wet-bulb-globe-temperature-monitoring/.
Kunkel, Kenneth E., Stanley A. Changnon, Beth C. Reinke, and Raymond W. Arritt. 1996. "The July 1995
Heat Wave in the Midwest: A Climatic Perspective and Critical Weather Factors". Bull. Amer.
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Meteor. Soc. 77 (7): 1507-1518. doi:10.1175/1520-0477(1996)077<1507: tjhwit>2.0.co;2.
Minnesota, University. 2011. "Islands in the Sun I Institute on the Environment I University of
Minnesota". Islands.Environment.Umn.Edu. http://islands.environment.umn.edu/.
Ncdc.noaa.gov. 2015. "Storm Events Database - Event Details I National Climatic Data Center".
http://www. ncdc. noaa.gov/stormevents/eventdeta i ls.jsp?id=473157.
Weather.gov. 2015. "Twin Cities, MN". http://www.weather.gov/mpx/.
White -Newsome, Jalonne, Marie S. O'Neill, Carina Gronlund, Tenaya M. Sunbury, Shannon J. Brines,
Edith Parker, Daniel G. Brown, Richard B. Rood, and Zorimar Rivera. 2009. "Climate Change, Heat
Waves, and Environmental Justice: Advancing Knowledge and Action". Environmental Justice 2 (4):
197-205. doi:10.1089/env.2009.0032.
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�Hazard Assessment: DROUGHT
4.3.8.1. Definition
A generalized definition of drought is a period of
abnormally dry weather sufficiently prolonged for the
lack of water to cause serious hydrologic imbalance in
. u
the affected area. In easier to understand terms, a
drought is a period of unusually persistent dry weather���
that persists long enough to cause serious problems '
such as crop damage and/or water supply shortages
If the drought is brief, it is known as a dry spell, or
partial drought. A partial drought is usually defined as
more that 14 days without appreciable precipitation,
whereas a drought may last for years. Another type of drought is a flash drought, which is a "rapid onset
or intensification of drought [... ] set in motion by lower -than -normal rates of precipitation, accompanied
by abnormally high temperatures, winds, and radiation" (MIDIS, 2024). When a drought begins and ends
is difficult to determine because rainfall data alone won't tell you if you are in a drought, how severe your
drought may be, or how long you have been in drought.
The most used drought definitions are based on meteorological, agricultural, hydrological, and
socioeconomic effects:
1. Meteorological — A measure of departure of precipitation from normal. Due to climatic
differences, what might be considered a drought in one location of the country may not be a
drought in another location.
2. Agriculture — Refers to a situation where the amount of moisture in the soil no longer meets the
needs of a particular crop.
3. Hydrological — Occurs when surface and subsurface water supplies are below normal.
4. Socioeconomic— Refers to the situation that occurs when physical water shortages begin to affect
people.
4.3.8.2. Range of Magnitude
The severity of the drought depends upon the degree of moisture deficiency, the duration, and the size
of the affected area. The magnitude of a considered drought event corresponds to the cumulative water
deficit over the drought period, and the average of the cumulative water deficit over the drought period's
mean intensity.
• Most Severe Drought: 1030-1936 Dust Bowl or 'Dirty Thirties'
• Longest Drought: 1944-1950s: Southwestern United States
• Costliest: Second to the Dust bowl that is estimated to have cost $1 billion in 1930's money is the
drought of 1989 and 1999. It is estimated the drought costs somewhere between $80 and $120
billion worth in damage.
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4.3.8.3. Spectrum of Consequences B2b
Drought impacts are wide -reaching and may come in different forms, such as economic, environmental,
and/or societal. A reduction of electric power generation and water quality deterioration are also
potential effects. Drought conditions can also cause soil to compact, decreasing its ability to absorb water,
making an area more susceptible to flash flooding and erosion. A drought may also increase the speed at
which dead and fallen trees dry out and become more potent fuel sources for wildfires. An ongoing
drought which severely inhibits natural plant growth cycles may impact critical wildlife habitats. Drought
impacts increase with the length of a drought, as carry-over supplies in reservoirs are depleted and water
levels in groundwater basins decline. Impacts from drought can also be exacerbated because of dust
settling on snow, which causes increased solar energy absorption. As a result, snowmelt takes place earlier
in the season and runoff magnitudes increase.
The impacts related to early runoff pose problems for many important sectors in Minnesota including
agriculture, recreation, tourism, and municipal water supplies. Reservoirs may also be at capacity during
these constrained runoff periods, causing spills to be necessary. Ideally, to avoid releases of water
downstream, water is captured over a longer timeframe with gradual melting of snowpack. Alternatively,
dust produced from the hardening and drying of bare soil can also be exposed as vegetative cover
decreases due to extended periods of drought.
Although droughts can be characterized as emergencies, they differ from other emergency events in that
most natural disasters, such as floods or forest fires, occur relatively rapidly and afford little time for
preparing for disaster response. Droughts typically occur slowly, over a multi -year period, and it is not
obvious or easy to quantify when a drought begins.
4.3.8.4. Potential for Cascading Effects
As mentioned, there are many different consequences that can occur from drought. Since droughts
typically occur over longer time periods of months, seasons, and years it's possible to start with a few
consequences initially, but as the drought persists or worsens, your consequences can start to multiply.
This can happen within just the drought hazard itself, but another aspect is adding another hazard on top
of or as result of the drought. For example, in drought conditions that have persisted for many months, if
you have a rain event occur over a short period of time, the ground will not be able to absorb the moisture
quick enough creating a flash flood event. Another common cascading event is the threat and increase of
wildfires due to the dry conditions.
4.3.8.5. Geographic Scope of Hazard Blc
Due to natural variations in climate and precipitation, it is rare for all of Minnesota to be deficient in
moisture at the same level at the same time. However, single season droughts, and different magnitudes
and intensity over some portions of the State are quite common. In addition, it is typical for all of Hennepin
County to be within a drought at the same time, although possible to have part of Hennepin County in a
higher level of drought category than another part of the county.
4.3.8.6. Chronologic Patterns
Drought can occur any time of year, however people mostly think of its effects in the spring and summer
months. The onset of summer drought intensity can, and typically, begins with the prior fall and winter
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being drier than average.
4.3.8.7. Historical Data Bld
Perhaps the most devastating weather driven event in American History, the drought of the 1920's and
1930's significantly impacted Minnesota's economic, social, and natural landscapes. Abnormally dry and
hot growing season weather throughout the better part of two decades turned Minnesota farm fields to
dust and small lakes into muddy ponds. The parched soil was easily taken up by strong winds, often
turning day into night. The drought peaked with the heat of the summer of 1936, setting many high
temperature records that still stand today.
One of the most significant droughts to affect the County was the drought of 1976-1977. The 1976-77
drought was widespread and by some measures was exceeded only by the severity of conditions during
the 1930's. In spring of 1976, the general lack of precipitation was statewide. Shallow residential and
farm wells began to go dry in June. Some municipalities also were affected. Precipitation continued to
be much less than normal for the rest of 1976 and gradually returned to normal during the summer of
1977. Minnesota's State Climatology Office records show the precipitation total for the Twin Cities to be
16.50 inches, well below the 27-inch average (based on the Twin Cities Monthly & Yearly Twin Cities Total
Average).
Another severe drought that had an impact on Hennepin County was the drought of 1988. A nationwide
event, the Drought of 1988 intensified in June with Minneapolis receiving only 0.22 inches of rain, making
it the driest June ever recorded in the metro area. The June average temperature for Minneapolis was
74.4 degrees Fahrenheit, which equaled the second warmest June ever. Statewide temperatures ranged
from 6 to 9 degrees above normal. By the end of June most of the state was classified as either in "severe"
or "extreme" drought.
The drought continued into July with temperatures six degrees above normal in Minneapolis and rainfall
1.5 to 3 inches below normal. Soil moisture levels reached record lows at most University of Minnesota
Experiment Stations. In the Minneapolis area, maximum temperatures of 90 degrees or greater were
recorded 17 days, a record high for July. Most locations reported maximum temperatures exceeding 100
degrees at least once during the month.
By August, the drought began to subside but not after severe agricultural damage was caused and several
records were broken across Hennepin County and the State of Minnesota including:
• June precipitation averaged 1.40 inches statewide, replacing the old record low of 1.50 inches set
in 1910.
• May through August average temperature at 69.7 degrees was nearly 2 degrees higher than the
old record set in 1936.
• Minneapolis -St. Paul Airport had 44 days with 90 degrees or more. The old record has been 36
days in 1936.
• The Palmer Drought Index dropped below -7 in northwest Minnesota for the first time since
record keeping began at the turn-- of -the -century. The old record had been -6 in September 1934.
• Groundwater levels throughout the state reached new record low levels.
• The Mississippi River at St. Paul reached low levels previously experienced only in 1934 and 1976,
prompting the first total sprinkling ban in Minneapolis and St. Paul.
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There have been no other incidents that are within the scope of this plan.
4.3.8.8. Future Trends Ble
In the past few years, there have been several studies published that show to have conflicting conclusions
when it comes to trends in past drought occurrence and how the future looks. Part of this is because of
the different definitions of drought. Because of the different definitions, a small reduction in the mean of
one parameter, can translate into a much larger increase in drought on the other parameters, or
definitions.
Many of the computer modeling have shown increased trends in drought occurrences across much of the
northern hemisphere. However, results of satellite -based studies along with other observation -based
studies conclude there is no significant trend in areas with drought in the past three decades.
4.3.8.9. Indications and Forecasting
Drought intensity categories are based on five key indicators and numerous supplementary indicators.
The accompanying drought severity classification table shows the ranges for each indicator for each
dryness level. Because the ranges of the various indicators often don't coincide, the final drought category
tends to be based on what most of the indicators show. The analysts producing the final determined
category also weighs the indices according to how well they perform in various parts of the country and
at different times of the year.
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4.3.8.10. Detection & Warning
At present, the best approach for predicting the development, intensification, and demise of a drought is
a two -fold strategy that combines the monitoring of both local water and climate conditions and large-
scale wind patterns, including the comparison of current conditions to historical analogues, with the
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interpretation of computer forecasts. This strategy is employed by both the monthly and seasonal drought
outlooks, which are issued monthly by the National Oceanic and Atmospheric Administration, National
Weather Service, and Climate Prediction Center as an operational effort geared toward infusing such
advances into drought predictability. Although predicting drought on any scale remains a challenge,
progress in understanding global -to -regional scale climate -system phenomena provides hope for
improving drought prediction at longer lead times.
Early warning of drought onset, and characterization of its evolving environmental and economic impacts,
can be further enhanced using regional -scale early warning systems that promote sustained partnership
networks linking meteorological and climatological information providers to water, agriculture, and other
private and public management communities.
4.3.8.11. Critical Values and Thresholds
According to the Minnesota Statewide Drought Plan, there are five drought phases/triggers that follow
closely to the drought intensity categories. TABLE 4.3.8A describes the drought triggers from the
Minnesota Drought Plan. These triggers are based on conditions for the different watersheds across the
state.
TABLE 4.3.8A Drought Triggers
Drought Phase/Triggers
Conditions
Non -Drought Phase
A signification portion of the watershed is not under drought
conditions according to the U.S. Drought Monitor.
Drought Watch Phase
A significant portion of the watershed is "abnormally Dry" or in a
"moderate Drought".
Drought Warning Phase
A significant portion of the watershed is in a "Severe Drought", or
from public water suppliers using the Mississippi River, the average
daily flow at the USGS gage near Anoka is at or below 2000 cfs for
five consecutive days.
Restrictive Phase
A significant portion of the watershed is in an "Extreme Drought",
or for public water suppliers using the Mississippi River, the
average daily flow at the USGS gage near Anoka is at or below
1500 cfs for five consecutive days.
Emergency Phase
A significant portion of the watershed is in an "Exceptional
Drought", or highest priority water supply needs are not met, or
there are threatened or actual electricity shortages due to cooling
water supply shortages, or for public water suppliers in the Twin
Cities, the average daily flow of the Mississippi Rover UGSG gage
near Anoka is at or below 1000 cfs for five consecutive days.
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4.3.8.12. Mitigation
Even though you can't prevent a drought from occurring, they are hard to predict, or how long they will
last, there are ways you can protect from some of the consequences.
• Monitor Drought Conditions: this can provide early warnings for policymakers and planners to
make decisions through actions including:
• Monitor Water Supply: This can save water in the long run though the following actions:
• Develop a drought emergency plan.
• Develop criteria or triggers for drought -related actions.
• Develop agreements for secondary water sources that may be used during drought conditions.
• Rotating crops by growing a series of different types of crops on the same fields every season to
reduce soil erosion.
• Practicing contour farming by farming along elevation contour lines to slow water runoff during
rainstorms and prevent soil erosion, allowing the water time to absorb into the soil.
• Using terracing on hilly or mountainous terrain to decrease soil erosion and surface runoff.
• Planting "cover crops," such as oats, wheat, and buckwheat, to prevent soil erosion.
• Using zero and reduced tillage to minimize soil disturbance and leave crop residue on the ground
to prevent soil erosion.
• Constructing windbreaks to prevent evaporation from reclaiming salt -affected soil.
• Collecting rainwater and using natural runoff to water plants.
• Encourage farmers and agriculture interests to obtain crop insurance to cover potential losses
due to drought.
4.3.8.13. Response
When drought occurs, the water supplier and community must take action to reduce the demand for
water. While increasing water supplies would be of benefit, most such remedies require more than five
years to plan and construct new reservoirs, canals, and/or groundwater sources. Reducing water demand
can result in significant positive effects within only a few days.
Voluntary action from water users can result in up to 25% water use reduction for short periods of time.
Mandatory restrictions have resulted in as much as a 40% reduction of water use. This savings effect is
directly related to a) the public's belief that the emergency is real; b) the public clearly understands the
actions required to reduce water use; and c) the active enforcement of mandatory water use restrictions.
It is very important for water suppliers to understand the public seldom sustains the voluntary water
conservation levels more than a few months. Drought response actions, even mandatory water use
restrictions are designed to be suspended once the drought is deemed over. Drought response programs
and water efficiency programs are two very different actions for two different problems.
Water efficiency programs are designed to effect long-term (even permanent) water use reductions;
drought response is designed to solve short term water supply deficits. Water efficiency programs can
reduce the impact of subsequent droughts, but water efficiency strategies continue beyond the term of a
drought. Water efficiency planning is usually based on the economics of avoided costs or least cost
planning. Drought response is meant to solve an emergency supply shortfall; thus, does not always need
to be justified by avoided costs.
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4.3.8.14. Recovery
Like all disasters, recovery from drought can takes months to years to return to a state of normalcy. On
August 7, 2012, President Barack Obama called for an "all hands-on deck" approach to the drought at a
White House Rural Council meeting. At the same meeting, the President asked that the USDA take the
lead in coordinating the Federal effort to help with drought response and recovery.
To support this collaboration across multiple federal agencies, the concepts and organizing principles of
the National Disaster Recovery Framework (NDRF) were leveraged to promote a more integrated and
cohesive response to drought. Based on the input received in the Drought Recovery Regional Meetings,
the NDRF team identified "big bucket" issues to organize Federal resources identified across all applicable
departments and agencies. These included technical assistance, grant programs, loan programs, and
information resources.
TABLE 4.3.8B shows resources for short-term and long-term recovery. The short-term section provides
links to agencies providing relief resources and information. The long-term recovery section is geared
more toward information to aid in mitigation and adaptation, but long-term recovery resources are also
listed.
TABLE 4.3.8B Aaencv and Recovery Support
Agency
Short Term Recovery
• The Natural Resources Conservation
Long Term Recovery
• Crop Insurance
Service
• Risk Management Agency
• The Rural Development Program
• Natural Resource Protection/Private
• The Farm Service Agency
Lands
• Crop Production Losses
• Agricultural Water
U.S. Department of
• Disaster Assistance
Enhancement Program
Agriculture provides
financial and technical
Programs
• Emergency Watershed
assistance to drought
• Natural Resource Protection/Private
Protection - Floodplain
affected areas and
Lands
Easement
services
• Environmental Quality
• Watershed Protection and
Incentives Program
Flood Prevention
• Emergency Watershed
• Wetlands Reserve Program
Protection
• Conservation Technical
• Community Water and Wastewater
Assistance
• Community Water and Wastewater
• The Recovery Act
• DOI's Bureau of Reclamation
• The Drought Water Bank
administers the WaterSMART and
water and Energy Efficiency
Grants that aims to make more
efficient use of existing water
supplies through water conservation,
Us Department of
efficiency, and water marketing
Interior
projects. Funding is also available to
promote water use efficiency
program projects like rebate
programs, irrigation system
upgrades, water conservation
education programs and to address
and improve Best Management
Practices.
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• EPA works with states to manage
programs that provide financial
assistance for projects that protect
Environmental
public health and water quality. EPA
Protection Agency
also manages the WaterSense
Program, which helps consumers
identify water -efficient products,
practices and programs.
National Oceanic and
• Endangered Species Act
• Endangered Species Act
Atmospheric
• NIDIS
• NIDIS
Administration
Small Business
• Economic Injury Disaster Loans
• Economic Injury Disaster Loans
Administration
4.3.8.15. References
ClimateStations.com. 2015. 'Graphical Climatology of Minneapolis (1820-Present)'. Climatestations.Com.
https://www.climatestations.com/minneapolis/.
Damberg, Lisa, and Amir AghaKouchak. 2013. 'Global Trends and Patterns of Drought from Space'.
Theoretical and Applied Climatology 117 (3-4): 441-448. doi:10.1007/s00704-013-1019-5.
National Drought Mitigation Center. 2015. 'Drought in the Dust Bowl Years'. Drought.Unl.Edu.
http://drought.0 nl.edu/DroughtBasics/DustBowl/Droughti ntheDustBowlYears.aspx.
Robbins, William. 1989. 'Drought -Stricken Areas Find Relief After Rains'. The New York Times.
Seneviratne, Sonia I. 2012. 'Climate Science: Historical Drought Trends Revisited'. Nature 491 (7424):
338-339. doi:10.1038/491338a.
The National Drought Mitigation Center. 2015. 'United States Drought Monitor > About USDM >
Classification Scheme'. Droughtmonitor.Unl.Edu.
http://droughtmonitor.unl.edu/aboutus/classificationscheme.aspx.
Trenberth, Kevin E., Aiguo Dai, Gerard van der Schrier, Philip D. Jones, Jonathan Barichivich, Keith R.
Briffa, and Justin Sheffield. 2013. 'Global Warming and Changes in Drought'. Nature Climate
Change 4 (1): 17-22. doi:10.1038/nclimate2067.
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:: Hazard Assessment: DUST STORM
4.3.9.1. Definition
A dust storm is a strong, violent wind that carries fine
particles such as silt, sand, clay, and other materials,
often for long distances. The fine particles swirl
around in the air during the storm. A dust storm can
spread over hundreds of miles, rise over 10,000 feet,
and can have wind speeds of at least 25 miles per
hour. Dust storms usually arrive with little warning
and advance in the form of a big wall of dust and
debris. A common name for dust storms is Haboob,
which comes from Arabic word habb meaning wind.
4.3.9.2. Range of Magnitude
There are two main kinds of dust storms; one where the dust is carried along the surface, and the other
where dust is lifted high into the atmosphere. Each of these dust storm types can happen individually, or
together at the same time. If these two types of storms happen together at the same time, there is the
potential for greater magnitude of consequences versus each type individually. Below are a few examples
of dust storms from the National Climatic Data Center that have occurred in the United States since 1950.
• Most Recent, Minnesota: May 12, 2022: Blowing dust ahead of a serial derecho (a type of fast-
moving extreme thunderstorm wind) spread from eastern Nebraska to Sioux Falls, SD, and up
through western Minnesota, dropping visibility below % mile, with zero visibility reported in
places. A lighter wave of blowing dust entered the western Twin Cities area, including Hennepin
County.
• Longest Distance: May 17, 2001, Dust from a storm in China traveled across the ocean and
deposited dust from Alaska to Florida.
• Most Costly: June 101h, 2013, Humboldt, Nevada, $1.5 million Property Damage
• Deadliest: October 13, 2009, SW S.J. Valley, 3 fatalities
4.3.9.3. Spectrum of Consequences B211b
Dust storms can have environmental, health, social, and economic consequences. Health consequences
include poor air quality due to the increase in breathable suspended particles in the air which can be
almost an instant consequence with people choking on dust or a consequence from particles suspended
over time. Environmental consequence can be dust deposition on the landscape which can cause drying
of leaves, and negative growth of plant and damage to crops. Some of the social impacts can be road and
aviation accidents due to the poor visibility. Economic impacts can include damage to structures, and
roads, costs associated with cleaning of infiltrated dust inside the houses and buildings, costs associated
with accidents, material, crop, and production loss. On 75 million acres of land in the United States alone,
wind erosion is still a dominant problem, with four to five million acres moderately to severely damage
each year.
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AREAS WHERE WIND EROSION OCCURS ON CROPLAND
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Many believe that dust storms are not a worry for urban areas. However urban communities are not
immune to the harmful effects of dust storms either. One thing that is a concern when a dust storm hits
a town or city is power outages and infrastructure damage. Anyone of these two things could have a
negative result for a business. Also, there could be extensive damage to computers and communications
equipment from the buildup of dust. The dust particles can get into buildings and businesses and work
their way inside computers and telecommunications equipment, ruining the delicate technologies on the
inside. Again, with many businesses today being dependent on technologies such as computers and
communications equipment, this could have a negative impact on commerce.
Additionally, vulnerable populations within urban or other populated areas may experience
disproportional consequences from dust storms. For instance, those without shelter would have little to
protect themselves from the airborne particulates and may suffer more frequent or acute respiratory
distress. Those with limited mobility may find it similarly difficult to seek shelter. In all cases, persons with
respiratory conditions like asthma, the elderly, infants, and anyone with compromised health may bear a
greater cost from dust storms than the general population.
4.3.9.4. Potential for Cascading Effects
The immediate economic impact of dust storms is significant, but it doesn't rival major natural disasters
that destroy entire cities. For instance, the damage due to dust storms in China averages at about $6.5
billion per year. A single major earthquake can do damage five times that figure. However, experts argue
that the real economic impact of dust storms, particularly those that originate in areas of desertification,
is difficult to pin down because of the long-term consequences they have on the livelihood of people who
live in the area. When dust storms kick up in agricultural dry lands that are degraded, they remove the
topsoil, which causes further desertification. As a result, farmers are forced to watch the topsoil, and their
livelihood, literally blow away. This cycle, if gone unchecked, threatens to displace whole communities in
some regions.
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4.3.9.5. Geographic Scope of Hazard Blc
The winds involved with dust storms can be as small as "dust devils" or as large as fast moving regional air
masses. Dust storms occur most frequently over deserts and regions of dry soil, where particles are loosely
bound to the surface. Dust storms don't only happen in the middle of the desert, however. They happen
in any dry area where loose dirt can easily be picked up by wind. Grains of sand, lofted into the air by the
wind, fall back to the ground within a few hours, but smaller particles remain suspended in the air for a
week or more and can be swept thousands of miles downwind. Dusts storms can reach as high as 10,000
feet with an aerial coverage on the leading edge that can stretch for hundreds of miles. However, on
average, they only travel around 25 to 50 miles.
4.3.9.6. Chronologic Patterns
Dust storms are not common around Minnesota, but they can happen any time of year, and have occurred
in the past. They are most common in desert regions, including the US Southwest and often are triggered
by downdraft winds from monsoon thunderstorms. They are slightly more common during the afternoons
and evenings than at cooler times of day, but only because of the importance of thunderstorms, which
tend to be most numerous and most intense during afternoons or evenings. Otherwise, diurnal cycles of
heating and cooling have no effect on dust storm behavior or probability.
In Minnesota, dust storms are most likely during persistently dry conditions, and/or when dry and loose
soil is also unprotected by mature vegetation. Because the growing season features higher rates of
moisture conduction between plants and soils, and because the same plants will shield underlying soils
from wind erosion, dust storms will tend to favor the pre -green -up periods of Late March into May, or
late September into early November.
GRAPH 4.3.9A shows the critical wind erosion period in Minnesota. It shows that March, April, and May
are the periods of the year where agricultural fields are particularly vulnerable to wind erosion, and to
extension dust storms, due to higher wind speeds with direction of prevailing wind than normal and low
vegetative cover on fields.
GRAPH 4.3.9A Critical wind erosion
30
25
20
1.5
1.0
5
U
lan
Percent of All EirOsive Will
it ire ire e 1po II it , MN
Feb (March Apr
May .1 un
Dull
Aug
Sep
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IDec
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4.3.9.7. Historical Data Bld
The "Dust Bowl" era of the 1930s was so named because of massive dust storms that frequently ravaged
the Plains during that extraordinarily dry period. During this period, Minnesota saw some of the worst
dust storms in its history. In 1934, dry conditions combined with high winds to produce thick dust on five
or more dates at the end of the month. February had at least six more dust storm dates, followed by 15
dates in March, and 19 dates in April, with the worst of the dust storms occurring on May 9-10.
Meteorologists at the time reported these latter dust storms were likely the most severe of their kind ever
experienced in the area, with extreme soil erosion exposing and subjecting new seed to the strong winds.
The most recent severe dust storm clipped western Minnesota and hit much of South Dakota head-on
during a severe weather outbreak on May 12, 2022. Intense downburst winds generated by severe
thunderstorms advanced well ahead of the storms at speeds of 60-80 mph. The region had been quite
dry, and soils were loose and unprotected by vegetation. As a result, a huge cloud of thick dust raced
north northeastward across the region, dropping visibilities to zero in spots, especially in Nebraska and
South Dakota. Visibility below a quarter mile was common in western Minnesota. A lighter cloud of
blowing dust moved into Hennepin County during the evening, though visibility was hardly reduced, and
no impacts were reported.
There have been no other incidents that are within the scope of this plan.
4.3.9.8. Future Trends Ble
There is no current research available on the direct effects of future climate conditions on the incidence
of dust storms. However, because drought conditions have the effect of reducing wetlands and drying
soils, droughts can increase the amount of soil particulate matter available to be entrained in high winds,
where agriculture practices include tilling. This correlation between drought conditions and dust storms
means that an increase in future droughts could increase the incidence of dust storms, even though the
drought is not directly related to the directly to the dust storm.
4.3.9.9. Indications and Forecasting
Dust storms move quickly. Other than seeing a wall of brown dust approaching in the distance, there is
not much warning before a dust storm arrives. However, they usually precede thunderstorms. So if
conditions have been dry, and one can see a large cumulonimbus cloud and feel the wind is picking up,
one can expect dust to be blowing with the possibility of dust storm type reduced visibilities and
consequences. Dust storm events are caused by different weather systems showing different intensities
and identifiable characterizes in observational systems.
There are four dust storm generation types: frontal, meso- or small-scale, disturbances, and cyclogenesis.
Key features of cold front -induced dust storms are their rapid process with strong dust emissions and a
large, affected area. Frontal dust storms typically last 3-5 hours with wind speeds of 36-83 mph and
typically affect an area of 7,700 to 77,000 square miles.
Meso- or small-scale dust storms are the most common type of dust storm including thunderstorms,
convections along dry lines, gusty winds cause by high pressure, and more. The most common occurrence
are thunderstorms in which the organized outflow from the downdrafts of decaying thunderstorms blows
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dust plumes. These storms can typically last 2-5 hours with winds from 53 to 78 mph. They produce the
highest level of particle emission over a limited area, typically 2,000 to 6,000 square miles.
The third type of dust storms are caused by tropical disturbances. These typically show strong
concentration of dust in the air and last longer than frontal and meso- or small scale at 3-7 hours with
wind speeds 30 to 58 mph. The typical area covered is just 200 to 4000 square miles.
The last type of dust storm occurs from cyclogenesis which is the development of strengthening or a lower
pressure area. Dust storms from cyclogenesis typically last longer than the others at 4-21 hours with wind
speeds 38 to 65 mph because cyclogenesis tends to be stationary. These storms typically affect and area
of 4000 to 31,000 square miles.
4.3.9.10. Detection & Warning
As mentioned earlier, there is not a lot of indication for dust storms besides knowing the current
conditions that may present the storm from occurring. However, with each of the types of dust storms
mentioned above, there is never always a dust storm when those conditions are present. The National
Weather Service in Chanhassen does not have a specific definition for when they would issue a blowing
dust advisory or dust storm warning. In fact, The NWS Office in Chanhassen has never issued a blowing
dust advisory or dust storm warning. However, the Grand Forks National Weather Service has.
4.3.9.11. Critical Values and Thresholds
The blowing dust advisory conditions, visibilities at or below 1 mile, and dust storm warning, visibilities
less than % mile, are the two critical values when it comes to warning the public for public safety concerns.
Among those concerns are health concerns when dust particles are inhaled. The particles that are small
enough to be inhaled are known as PM10 which are particulate matter less than 10 microns in size or
smaller.
4.3.9.12. Mitigation
The effects of sand and dust storms can be reduced by using several health & safety measures and
environmental control strategies. Large-scale sand and dust storms are generally natural phenomena,
and it may not be always practicable to prevent it happening. However, control measures can be taken to
reduce its impacts.
To reduce the consequences of dust events that may not reach dust storm criteria, cities can take
appropriate control of dust raising factors such as increasing the vegetation cover where possible using
native plants and trees as buffer. These can reduce wind velocity and sand drifts at the same time of
increasing the soil moisture.
Some health and safety measures that should be taken to minimize the adverse impacts due dust storms
can be alerting vulnerable populations, using dust masks, and restricting outdoor activities and staying
inside when dust storms are occurring.
Mitigation strategies to reduce wind erosion from dust storms are lumped into two major categories:
reduce the wind force at the soil surface and create a soil surface more resistant to wind forces. Some of
these strategies are standing residues, planting perpendicular to prevailing winds, windbreaks, grass
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barriers, strip cropping, or clod -producing tillage.
4.3.9.13. Response
One of the most important things to be done during the initial response is to make sure that people are
safe. The role of Hennepin County Emergency Management is to coordinate resources that our
municipalities may need to accomplish all response needs.
4.3.9.14. Recovery
It is important to note that conditions and consequences from a dust storm may linger longer that one
can see to the naked eye. There may be lingering dust in the air after a dust storm so the first step to
recovery is to continue to avoid breathing in outdoor air for hours after a storm passes. From an
emergency management perspective, assessing the amount of property damage, preparing a list of
specific damage to property and buildings, and agriculture damage are top on the list to start the recovery
process.
4.3.9.15. References
Lei, H., and J. X. L. Wang. 2014. 'Observed Characteristics of Dust Storm Events Over The Western United
States Using Meteorological, Satellite, And Air Quality Measurements'. Atmospheric Chemistry and
Physics 14 (15): 7847-7857. doi:10.5194/acp-14-7847-2014.
Oregon Partnership for Disaster Resilience. 2012. State of Oregon Natural Hazards Mitigation Plan.
http://www.oregon.gov/LCD/HAZ/docs/OR_NHMP_2012.pdf.
Stefanski, R, and M V K Sivakumar. 2009. 'Impacts of Sand and Dust Storms on Agriculture and Potential
Agricultural Applications of A SDSWS'. IOP Conf. Ser.: Earth Environ. Sci. 7: 012016.
d o i :10.1088/ 1755-1307/7/1/012016.
Tatarko, John. 2004. Wind Erosion: Problem, Processes, and Control. Ebook. 1st ed.
htt p://www. n res. usda.gov/Internet/FSE_DOC U M E NTS/n res 142 p2_019407. pdf.
W. A., Mattice. 1935. 'Dust Storms Novemeber 1933 to May 1934'. Monthly Weather Review 63 (2): 53-
55
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�C Hazard Assessment: COLD, EXTREME
4.3.10.1. Definition
The term extreme cold can have varyingdefinitions
in hazard identification. Generally, extreme cold
events refer to a prolonged period (days) with
extremely cold temperatures. An extreme cold event ' �'
is when temperatures are dangerously lower than
historical averages and pose risk to people, animals,
and critical infrastructure (CISA, 2024). The extreme
cold definition also depends on the area you live. In
southern regions relatively unaccustomed to winter weather, near freezing temperatures could be
considered extreme cold. In the North, extreme cold can mean temperatures well below zero.
When defining extreme cold one also must mention wind chill. The wind chill temperature is an apparent
temperature, or how cold it feels to people outside. Wind chill is based on the rate of heat loss from
exposed skin caused by wind and air temperature. As the wind increases, it draws heat from the body,
driving down skin temperature and eventually the internal body temperature.
Wyk$ YpW
4.3.10.2. Range of Magnitude
• Lowest Temperature in MN: -60'F (Feb 2, 1996: St. Louis County)
• Lowest Temperature in Hennepin County: -41'F (Jan 21, 1888)
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• Lowest Wind Chill in MN: -71 OF with new formula and -100 OF with old formula (Jan 9-10, 1982)
• Lowest Wind Chill in Hennepin County: -6-73 OF with the new formula and -87 OF with the old
formula. (Jan 22, 1936)
• Lowest Maximum Temperature for Hennepin County: -20 (Jan 15, 1988)
• Longest period temperature below 32°F in Hennepin County: 66 Day 16 Hours (8PM Dec 18, 1977,
through 11 AM Feb 23, 1978)
• Longest Period temperature below 0°F in Hennepin County: 7 Days 18 hours (8 PM Dec 31, 1911,
through 10 AM Jan 8, 1912)
4.3.10.3. Spectrum of Consequences B211b
Extreme cold temperatures have well known impacts on human health. On average, the United States
sees 29 cold weather -related fatalities each year. In 2019, there were 62 cold -related deaths in Minnesota
(MN DPH, 2019).
Human and animal exposure to cold temperatures, whether indoors or outside, can lead to serious or life -
threatening health problems such as hypothermia, cold stress, frostbite or freezing of the exposed
extremities such as fingers, toes, nose, and ear lobes. Hypothermia occurs when the core body
temperature is less than < 95°F. If persons exposed to excessive cold are unable to generate enough heat
(e.g., through shivering) to maintain a normal core body temperature of 98.6°F, their organs can
malfunction. When brain function deteriorates, persons with hypothermia are less likely to perceive the
need to seek shelter. Signs and symptoms of hypothermia (e.g., lethargy, weakness, loss of coordination,
confusion, or uncontrollable shivering) can increase in severity as the body's core temperature drops.
Extreme cold also can cause emergencies in susceptible populations, such as those without shelter, those
who are stranded, or those who live in a home that is poorly insulated or without heat (such as mobile
homes). Infants and the elderly are particularly at risk, but anyone can be affected.
Damage to structures due to extreme cold events is relatively low. Freezing pipes can be the largest
problem. Extended periods of cold weather can increase the potential for frost depth problems. The depth
to which soils freeze and thaw is important in the design of pavements, structures, and utilities. Increased
depth of frost can also delay the frost thaw in the spring which would cause those in agriculture a later
start to their season, which may lead to less yield of crops. Broken water mains can put significant
demands on municipal public works departments.
4.3.10.4. Potential for Cascading Effects
Extremely cold temperatures often accompany a winter storm, so individuals may have to cope with
power failures and icy roads. Although staying indoors as much as possible can help reduce the risk of car
crashes and falls on the ice, individuals may also face indoor hazards. Many homes may become too cold
either due to a power outage or because the heating system is not adequate for the weather. The use of
space heaters and fireplaces to keep warm increases the risk of household fires and carbon monoxide
poisoning.
During cold months, carbon monoxide may be high in some areas because the colder weather makes it
difficult for car emission control systems to operate effectively. Carbon monoxide levels are typically
higher during cold weather because the cold temperatures make combustion less complete and cause
inversions that trap pollutants close to the ground reducing air quality.
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4.3.10.5. Geographic Scope of Hazard Blc
Extreme cold is typically associated with the northern states in the winter. However, extreme cold
conditions can occur as far south as Texas. As mentioned in the definition, the social impact or where/how
the public is accustomed to cold weather plays a factor in what is called extreme cold for a specific
geographical area.
GRAPHIC 4.3.10A shows an example from 2014. You can see extreme cold apparent temperatures for
most of the central United States.
GRAPHIC 4.3.10A
MeW, 09A
AppH r nt Tempe,raittire, (" )
4.3.10.6. Chronologic Patterns
Extreme cold outbreaks occur most commonly during the December, January, February months of the
year.
4.3.10.7. Historical Occurrence Bld
Extreme cold is a regular occurrence in Minnesota and in Hennepin County. There have been no incidents
that are significant enough to be included in this plan.
GRAPHICS 4.3.10B and 4.3.10C shows historically the frequency of lows at or below -10°F and highs at or
below 0 degrees in Hennepin County.
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GRAPHIC 4.3.10113
Frequency of Lows At or Below -10 Degrees,
Minneapolis
E
A,T
Year
GRAPHIC 4.3.10C
Frequienicy of Highs At or Below 0 Degrees,
minneapolis
Highs at Or
Below 0
10 Yr ,Avg
Highs at or
Below 0
r�
Year
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What is the coldest wind chill ever seen in the Twin Cities or Minnesota? The answer can be a little tricky
because in November 2001 the formula on how to calculate the wind chill was changed. Perhaps the
coldest wind chill the Twin Cities has ever seen was -67°F with the new formula (-87°F with the old
formula) back on January 22, 1936. The temperature was -34°F with a wind speed of 20mph. All traffic in
the Twin Cities was severely impacted and several fatalities were caused by the cold. Without a lengthy
state-wide wind record, it is difficult to say when the coldest statewide wind chill was. There are some
candidate dates though besides January 22, 1936. On January 9th and 10th, 1982 temperatures of -30°F
and winds of around 40mph were reported in Northern Minnesota. This would translate to -71°F by the
new formula (-100°F by the old formula.)
A few other notable extreme cold events are:
1989 Feb 3:
• At 6:00 AM in the Twin Cities the air temperature was -22°F with a wind speed of 17mph,
creating a wind chill temperature of -49°F (by the 2001 formula).
1994
• On January 13, 1994, an arctic air mass settled over Hennepin County. From January 13
to January 19, true air temperatures dropped from -10°F on January 13 to -27°F on
January 19. The high temperature on January 18 was -16°F. Morning air temperature
readings were -26°F in the Twin Cities at gam with a wind chill temperature of -48°F (by
the 2001 formula). The University of Minnesota on the Twin Cities campus closed on the
18th due to the cold and Governor Arne Carlson closed all public schools.
1996
• On January 31, 1996, some of the coldest weather to ever hit Hennepin County settled
over the area and remained entrenched through February 4. Minneapolis set three new
record low temperatures as well as Minnesota recording the coldest day on record on
February 2. A mean temperature of -25°F was measured that day with a high of -17°F and
a low of -32°F. This was within two degrees of tying the record low temperature set in the
Twin Cities and the coldest temperature recorded this century. On the same date that
the Minnesota state record minimum temperature record was set on February 2, 1996 (-
60°F near Tower), Governor Arne Carlson cancelled schools for cold a second time. In the
Twin Cities at 6am February 2, 1996, the air temperature was -30°F with a wind chill
temperature of -48°F (based on the 2001 formula).
• Another extreme cold event took place on December 24, 1996. A strong low-pressure
system that deposited heavy snow over northern Minnesota also brought down very cold
Canadian air. Temperatures fell to 15 to 35 degrees below zero. In addition, the high
temperature on Christmas Day in Minneapolis was only -9°F. Combined with the record
low temperature that morning of -22°F, the mean temperature for Christmas Day was -
16°F. Christmas Day, 1996 set a record for being the coldest Christmas Day on record for
the Twin Cities metro going back to when modern day records began in 1871. The
temperature in Minneapolis fell to -27°F.
2004
• The first wind chill warning that was issued for the Twin Cities under the new wind chill
temperature formula established in 2001 was the arctic outbreak of January 29-30, 2004.
The coldest wind chill observed in the Twin Cities during that period was -430F at 8:00 AM
on January 30, 2004.
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2006
• In the wake of a winter storm on February 17, 2006, strong high pressure moved in and
created strong winds and dangerous wind chills. The coldest wind chill seen at the Twin
Cities International Airport was -34°F. The coldest wind chill found statewide was -54°F at
Thief River Falls.
2014
• Governor Mark Dayton cancelled K-12 public schools statewide on Monday January 6th,
2014, due to extreme wind chills that were forecasted well in advance. The coldest wind
chill temperature in Minnesota was -63°F at Grand Marais Airport at 9:00 AM with a -31°F
air temperature and a 21mph wind. The coldest wind chill temperature in the Twin Cities
was -48°F at 5:00 AM with an air temperature of -22°F and a 15-mph wind. Many schools
also cancelled classes the following day as well. The wind chill at 4am January 7th was -
28°F at the Twin Cities International Airport with an air temperature of -14°F and a wind
of 6mph. Statewide the coldest wind chill was -50°F reported at Duluth at 4:00 AM with
an air temperature of -23°F and a west wind of 16mph.
• Schools were cancelled at many locations again on Thursday, January 23. The coldest wind
chill in the Twin Cities on January 23 was at 2:00 AM with a wind chill of -37°F with an air
temperature of -14°F and a NW wind of 15mph. The coldest statewide wind chill was -
51°F at Park Rapids at 6am with an air temperature of -33°F and as wind of 6mph.
• Schools were cancelled for a fourth day across the Twin Cities on January 27 as well.
Classes were also canceled for the day for the University of Minnesota. The coldest wind
chill in the Twin Cities was -39°F at 4:00 AM (-13°F air temp and wind NW 20mph). The
coldest wind chill statewide was -53°F degrees at the Grand Marais Airport at 8:00 AM (-
26°F air temp, wind NE 16mph).
• Schools were cancelled once more across the Twin Cities on Tuesday January 28th.
University of Minnesota classes were cancelled in the morning. The coldest wind chill in
the Twin Cities was -29°F at gam with an air temperature of -12°F and a wind speed of
8mph. The coldest wind chill in the state was -52°F at Fosston at 7:00 AM with air
temperature of -33°F degrees and a wind speed of 7mph from the south.
4.3.10.8. Future Trends Ble
In Minnesota, there are climate change signals showing the loss of formerly normal cold temperatures.
That is saying that the coldest day of the year has warmed by about 8°F since the early 201h century and
the 15 coldest days have warmed by about 7° F over the same period.
GRAPHIC 4.3.1011) shows this warming period of coldest temperatures from about 1970 forward. This
means the coldest high temperatures have warmed dramatically since 1970 and are now warmer than at
any other time on record. In addition, the high temperatures at or below zero have become much less
common in recent years and may soon be the exception, rather than the rule.
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GRAPHIC 4.3.1011)
MY r
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4 �1is 1930 1 ?.I;1 1970 I s"P'i �"'0Nf;"
Winter(111F), °Yer-rw' BpgrnPvri
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LOWS
10 Y1. '.wj
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While temperatures during our winter months seem to be warming, and as mentioned high temperatures
at or below zero have become much less common in recent years, this does not mean we will not be
seeing any extreme cold events in the future.
4.3.10.9. Indications and Forecasting
The National Weather Service is responsible for forecasting all extreme cold events for Hennepin County.
Typically, extreme cold events occur when a continental polar or continental arctic air mass makes its way
down over Minnesota. These are air masses that originate over the ice and snow-covered regions of
northern Canada and Alaska where long, clear nights allow for strong cooling of the surface. Extreme cold
typically occurs with or following a low pressure. As the system passes off to the east, continental polar
or continental arctic air gets pulled down on the backside of the low pressure.
4.3.10.10. Detection & Warning
The National Weather Service issues Wind Chill Advisories, Watches, or Warnings based on the following
forecasted criteria:
• Wind Chill Advisory: Widespread wind chill values around -25°F to -34°F are expected.
• Wind Chill Watch: Widespread wind chill values around -35°F or colder are possible.
• Wind Chill Warning: Widespread wind chill values around -35°F or colder are expected.
• Extreme Cold Watch: The possibility of wind chill or air temperatures colder than -35 'F.
• Extreme Cold Warning: Wind chills or air temperatures colder than -35 °F are expected.
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4.3.10.11. Critical Values and Thresholds
Depending on where you live in the state, there are different critical values that related to the advisories,
watches, and warnings listed above. The critical wind chill values for Hennepin County are -25°F and -35°F.
It is at -250F that exposed skin can start to see frostbite in 30 minutes of being outside. At -35°F, it can
take only 10 minutes for exposed skin to be susceptible to frostbite.
4.3.10.12. Mitigation
Education and Awareness Programs
• Educating the public regarding the dangers of extreme cold and steps they can take to protect
themselves when extreme cold occurs.
• Organize outreach to vulnerable populations, including establishing and promoting accessible
heating centers in the community.
• Encourage utility companies to offer special arrangements for paying heating bills.
• Educate homeowners and builders on how to protect their pipes including locating water pipes
on the inside of building insulation or keeping them out of attics, crawl spaces, and vulnerable
outside walls.
• Informing homeowners that letting a faucet drip during extreme cold weather can prevent the
buildup of excessive pressure in the pipeline and avoid bursting.
4.3.10.13. Recovery
Depending on the consequences that occurred during the extreme cold event, recovery can be short or
long. Recovery time from frostbite depends on the extent of tissue that was affected. It can take
sometimes up to three months to determine the extent of the damage. When it comes to recovery from
deep frost depth, it can take months to years to recover from consequences of broken water mains or
broken roadways, or crop yield.
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4.3.10.14. References
Dnr.state.mn.us,. 2016. "Minneapolis/St. Paul Climate Data - Extremes: Minnesota DNR".
http://www.dnr.state.mn.us/cIimate/twin_cities/extremes.htm1.
Kunkel, Kenneth E., Roger A. Pielke, and Stanley A. Changnon. 1999. "Temporal Fluctuations in Weather
and Climate Extremes That Cause Economic and Human Health Impacts: A Review". Bull. Amer.
Meteor. Soc. 80 (6): 1077-1098. doi:10.1175/1520-0477(1999)080<1077:tfiwac>2.0.co;2.
Medina -Ramon, M, and J Schwartz. 2007. "Temperature, Temperature Extremes, And Mortality: A Study
of Acclimatisation and Effect Modification In 50 US Cities". Occupational And Environmental
Medicine 64 (12): 827-833. doi:10.1136/oem.2007.033175.
Nws.noaa.gov,. 2016. "NWS Weather Fatality, Injury and Damage Statistics".
http://www.nws.noaa.gov/om/hazstats.shtml.
U.S. Department of Health and Human Services Centers for Disease Control and Prevention,. 2014.
Deaths Attributed to Heat, Cold, And Other Weather Events in The United States, 2006-2010.
Young, B.A. 1981. "Cold Stress as It Affects Animal Production". Journal Of Animal Science 52 (1): 154-
163.
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Hazard Assessment: WINTER STORM, BLIZZARD, EXTREME SNOWFALL
4.3.11.1. Definition
Winter storms produce intense
snowfall rates and/or large
accumulations that can
immobilize entire regions and
paralyze cities, stranding
commuters, closing airports,
stopping the flow of supplies,
and disrupting emergency and
medical services. The weight of
snow can cause roofs to
collapse and knock down trees
and power lines. Homes, farms,
and businesses may be isolated
for days. The cost of snow
removal, repairing damages, Cars on Excelsior Boulevard after 1940 "Armistice Day Blizzard." Courtesy W
and the loss of business can MN Historical Society
have severe economic impacts
on counties and municipalities. In Hennepin County, virtually all winter storms are generated by the
convergence of moisture and cold temperatures associated with low-pressure systems.
Blizzards represent the most dangerous class of winter storms, combining strong winds with falling or
freshly fallen snow to reduce visibility for a period of time. Technically, they are defined as three hours or
more of sustained winds or frequent gusts of 35 mph or higher in falling or blowing snow, and visibilities
reduced to a quarter mile or less. The strong winds create deadly whiteout conditions that bring traffic to
a standstill, enabling the wind -driven snow to form dangerous drifts that are impossible for many vehicles
to pass. In addition, the strong winds are often accompanied by falling temperatures and low wind chills,
subjecting stranded motorists to life -threatening conditions that may persist for 24 hours or more. Lastly,
the strong winds of blizzards exert additional stress upon structures if they were already straining under
the load of heavy snow.
All winter storms have some combination of cold air, moisture, and lifting mechanisms that turn the
moisture into precipitation. Most winter storms affecting Hennepin County are associated with
extratropical cyclones (low-pressure systems). Typically, the heaviest snow and blizzard conditions are
found on the left side of the path of the storm system.
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Typical weather pattern associated with major winter storms in Minnesota and Upper Midwest. Source
NOAA, http://www.nws.noaa.gov/os/winter/resources/Winter_Storms2008.pdf
Unfortunately, blizzards are not consistently tracked and are difficult to diagnose retroactively.
Moreover, most major winter storms in Hennepin County have not prompted Blizzard Warnings. In fact,
one of the last NWS-issued Blizzard Warning in Hennepin County was on November 1-2, 1991, during the
infamous Halloween Blizzard. However, many winter storms have produced Blizzard warnings in
neighboring counties, along with winds in Hennepin County that significantly compounded the impacts
from accumulating snow. Therefore, to avoid confusion and the misattribution of impacts, in this report, a
blizzard is any accumulating snow event known to have a significant wind -driven and blowing snow
component.
While many winter storms produce sleet and/or freezing rain, Hennepin County Emergency Management
recognizes these as distinct hazards and will cover them separately.
4.3.11.2. Range of Magnitude
A given location in Hennepin County sees 24-hour snowfall totals over six inches once or twice per year
on average, though there have been years with five or more such events. Blizzards, on the other hand,
recur approximately once every 3-4 years in western and northwestern parts of the county, and every 6-
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8 years inside the 494-694 loop. It should also be noted that blizzard conditions can occur without large
snowfall accumulations. These "ground blizzard" situations are most common in rural Minnesota, but can
occur in open areas of Hennepin County, west of the 1-494 corridor, and especially west of MN highway
101.
Calendar -day .
..
..
2-day snowfall..
3-day snowfall•:
5-day snowfall•:
total
November 1991
(nearMonthly
. Lake .
994
Snowstorm total
47" (near FInland, Lake County
01/06-08/1995
Monthly total
.. (Collegeville)
• •
4.3.11.3. Spectrum of Consequences B211b
Outdoorlife safetyhazards: Severe winter storms and blizzards are often accompanied by falling
temperatures and dangerous wind chills. Persons caught outside unprepared can face
disorientation, frostbite, hypothermia, and death. A quarter of winter storm casualties occur
among those caught outside in the storm.
Poweroutoges/utilities: Heavy snow can cause power outages from direct loading on electrical
wires, and more commonly from indirect sources, for example when tree limbs become
overloaded with snow and fall onto wires. Heavy, wet snow can cause widespread power outages,
and strong winds exacerbate this impact. The duration of service outages is typically related to
the complexity and magnitude of the outage pattern, along with the ability of crews to get to
repair sites. Thus, high -volume, heavy, wet, wind -driven snow events are associated with higher
outage numbers and longer service delays.
Structural failure: Heavy snow will can cause roof collapse, not just at residences, but at larger
commercial facilities as well. Large roof spans lacking consistent support are especially vulnerable.
The former Hubert H Humphrey Metrodome Stadium in Minneapolis failed three separate times
from excessive snow loads causing the Teflon canopy to tear.
Transportation: By far the greatest and most common impacts from winter storms in Hennepin
County are to the transportation infrastructure, but there is no strict threshold above which heavy
snow is guaranteed to produce a particular impact. Stranded vehicles and snow removal costs
increase with greater accumulations, but accidents and spinouts are often a function of prior road
conditions, driver preparedness and awareness, and the consistency of the accumulating snow.
For instance, from January 31- February 2, 2004, a well -forecast series of winter storms produced
widespread 8-11" snowfall totals across the Twin Cities, but a relatively small impact, owing to
preparedness, and the generally fluffy nature of the snow. By contrast, a much smaller event on
March 8 that same year, produced only 1-3 inches, but did so unexpectedly and within a 2-hour
window. This "surprise" event caused hundreds of spinouts and accidents and forced the closure
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of the 1-94 exit at Highway 280.
The NWS estimates that 70% of winter storm related casualties result from vehicular accidents.
Heavy snow impedes traffic, creates hazardous travel conditions, and requires plowing and
surface treatment to keep roads passable. It also significantly reduces visibilities, which
compromises driver reaction times. In blizzard conditions, the effect of wind further restricts
visibilities, often to zero, and can easily disorient drivers. Stranded drivers and those forced to
leave their vehicles because of accidents are often directly exposed to the harsh conditions
outside their vehicles and can quickly find themselves in a life -threatening situation.
Airports frequently experience significant delays, and it is common for all runways to close for a
time during major winter storms.
4.3.11.4. Potential for cascading effects
Heavy snow and blizzard conditions can occupy a large portion of any strong, cold -season extratropical
cyclone, and as a result can precede, follow, or be accompanied by a wide range of weather conditions.
Situational awareness is key to understanding if and how the effects of winter storm conditions will be
compounded by the following hazards.
Flooding: Unusually intense and/or repetitive snowfalls can drain local governments of their
resources, as crews put in long hours to maintain roads, and clear debris. As the heavy snow melts,
it poses flooding risks for area streams, basements, low-lying intersections, and other areas prone
to ponding. Heavy rainfall events falling onto or just after the melting of a large snowpack pose
immediate flooding threats, as soil storage capacity is often very limited. In April of 2001, heavy
rains in southern Minnesota caused considerable flooding, after an unusually long and snowy
season left a large snowpack and saturated soils.
Extended power outages: A severe winter storm that knocks out power becomes much more
dangerous as the time to restore service increases. This is especially true of storms that are
followed by a rapid drop in temperatures. Residences and facilities dependent on electrical power
for heating or heat distribution can become dangerously cold within hours of power loss.
Sometimes a heavy snowfall event or blizzard occurs shortly after a major ice storm. In these
cases, the ice produces the initial critical loading, but then the snow and/or wind acts as the "final
straw," resulting in severe and widespread power outages. In these situations, the snowstorm or
blizzard is just another link in a chain of cascading hazards already in progress.
Overexertion: Snow removal after a major event often results in a casualty spike related to
overexertion resulting from attempting to dislodge stranded vehicles and clear snow from
sidewalks and driveways. It is a major cause of winter -related fatalities in the US.
Severe weather: In rare situations, a major winter storm can follow a significant severe weather
event. An infamous tornado -blizzard combination affected Janesville, WI on November 11, 1911.
The tornado killed nine people and was followed almost immediately by a historic cold front that
brought blizzard conditions within a couple hours of the tornado's passage, as temperatures fell
from the 60s and 70s into the teens. On April 26, 1984, a strong, killer tornado hit Minneapolis
and St. Anthony, and was followed within three days by up to 10 inches of snow. Most recently,
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on March 31, 2014, a confirmed tornado struck near St. Leo in Lyon County MN, while a Blizzard
Warning was already in effect.
4.3.11.5. Geographic Scope of Hazard Blc
A given winter storm may affect several hundred
thousand square miles over a period of days, and often
will have an instantaneous footprint of 50,000 square
miles, under which dangerous winter weather
conditions are occurring. The swath of all precipitation
including rain and thunderstorms may cover an area
the size of several Midwest states.
Winter storms have occurred in virtually every part of
the US, except for coastal southern California, parts of
the Sonoran Desert, and southern Florida. The most
severe winter storms are found in the Central and
Northern Plains, and downwind of the Great lakes, and
along the East Coast. Comparatively, Minnesota
experiences storms that generally produce lesser
snowfall totals and/or weaker winds.
4.3.11.6. Chronologic patterns (seasons, cycles, rhythm)
Extent of precipitation associated with major winter
storm on December 11, 2010
Winter storm season in Minnesota extends from late October through April, with peak frequencies from
late -November through mid -March. Historically, February has had the fewest major snowstorms.
However, since 2004, February has become remarkably more active, while March has become less so.
4.3.11.7. Historical data/previous occurrence Bld
The Twin Cities has had dozens of major winter storms since the late 191h century, with 25 calendar -day
snowfalls of 10 inches or greater, and 26 two-day totals of at least 12 inches (TABLE 4.3.11A).
TABLE 4.3.11A Historical 2-day snowfall totals of 12" or greater in the Twin Cities. Events in bold are known
blizzards in Hennepin County since 1940.
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1/22/1917
16.0
3/4/1985
16.7
3/29/1924
12.0
3/31/1985
14.7
3/13/1940
15.6
12/1/1985
15.9
11/12/1940
16.7
11/1/1991
26.7
3/23/1952
14.1
11/30/1991
14.3
3/12/1962
12.7
3/9/1999
16.0
3/18/1965
12.2
12/11/2010
17.1
3/23/1966
13.6
2/21/2011
13.8
Additionally, some smaller snowstorms have also produced blizzard conditions in Hennepin County.
Notable recent examples include March 1-2, 2007, and February 21, 2014, when 6-12 inches of snow were
finished off with 25-40 mph winds. Following are more detailed accounts of some of the area's most
noteworthy winter storms.
The Armistice Day storm of November 11, 1940
is the defining blizzard of the 20th century in Minnesota and remains the storm against which all
other blizzards in this state are compared. It was a high -impact, high -mortality blizzard affecting
a huge swath of Minnesota, Wisconsin, Iowa, and the Dakotas.
The storm began as a low-
pressure area over Colorado on
the morning of November 10,
which then swung
northeastward and intensified
rapidly as it passed over La
Crosse and eventually Lake
Superior on the 12th.
Initially warm conditions gave
way to rapidly falling
temperatures, and rain turning
to extremely heavy windswept
snow. Winds were sustained
above 30 mph over much of
Minnesota, with gusts exceeding
65 mph in some areas. Snowfallr,°,.�a.-
rates at times were as high as Surface pressure chart on November 11, 1940
three inches per hour. Snowfall
totals of 15-25 inches were common across Minnesota, including Hennepin County.
The long duration of the storm, combined with its rapid onset and its severity contributed to
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extreme losses, including 49 deaths in Minnesota alone- many of whom were stranded motorists
who could not navigate the enormous snow drifts that were up to 15 feet high in open sections
of Hennepin County. Over a dozen of the dead were hunters who were dressed for pleasant
weather and were caught off -guard and stranded on islands in the Mississippi River. One train
derailed, two were involved in a head-on collision, and one could not complete its route because
of the snow. The regional death toll exceeds 150, with many of the non -Minnesota deaths coming
from numerous capsized Great lakes vessels.
"Storm of the Century'; January 10-12, 1975.
Formed by a then -record -setting low pressure system, this storm only produced 4-8" of snow in
the Twin Cities but hit areas to the west and north much harder. There, hurricane -force winds
gusts and blinding snowfall were common, with accumulations of up to 27 inches and drifts of 10-
20 feet in open country. Ice accumulated over one inch in parts of southwestern and southern
Minnesota, and the combination of ice, heavy snow, and severe winds produced thousands of
power and telephone outages.
The storm claimed the lives of 35 Minnesotans, 21 of whom suffered heart attacks. The Red Cross
provided food and shelter to over 17,000 people. Despite the heavy losses, the storm was well
anticipated, and forecasts are credited with keeping the casualty toll in check.
Back -to -Back Record -Breakers, January 20-22, 1982.
A low-pressure system interacting with an exceptionally air mass in retreat produced a broad
swath of heavy snow over much of Minnesota on January 20. Widespread daily totals of 10-20
inches were common, and the Twin Cities recorded 17.1", which broke the all-time daily snowfall
record that had been set during the Armistice Day storm.
As the storm wound down and exited the region on the 21", a more potent low-pressure system
emerged from the Colorado Plains. This system intensified and moved into the region on the 22nd
producing heavy snow, sleet, ice, thunder, and blizzard conditions, prompting the closure of
interstates 90 and 35 for part of the day. Snowfall totals of 10-20 inches were again common, this
time over an even larger area. The Twin Cities recorded 17.2" on the 22nd, breaking the all-time
snowfall record that had been set just two days earlier.
The extreme snow loads from these storms —in many cases greater than 30 inches —caused many
residential and commercial roof failures.
"Wall of White" blizzard, February 4, 1984.
A fast-moving low-pressure system and cold front charged through Minnesota, producing 2-4
inches of light powdery snow and sustained winds more than 40 mph, with gusts as high as 75
mph.
The snow and wind were unexpected and moved southward at up to 50 mph. The sudden onset
of the blizzard caused severe traffic problems in rural areas, where visibilities fell to zero and snow
drifts covered many roads. Cars stalled in the snow, spun out, and motorists who ventured out
were subjected to subzero temperatures and 40-60 mph winds.
The storm killed 21 people in a matter of hours, almost all from exposure, and almost all of whom
had been in stranded vehicles. This storm remains the most lethal single weather event in
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Minnesota in the last 50 years.
Thanksgiving weekend Blizzard, 1985.
An unusually prolonged and widespread winter storm produced several waves of heavy snow over
Minnesota, Iowa, Wisconsin and the Dakotas between November 281h and December V, 1985.
In the Twin Cities, at
least 5 inches on three
consecutive days, with
each consecutive day
producing more snow
than the last —this
behavior is
unprecedented in the
area's recorded history
and resulted in three-
day totals in excess of
20 inches.
Although the snow
during the first two
days of the storm was
very heavy, it fell in
light winds as a cold air
mass remained in place
over the region. The
final wave of snow,
however, was
KAN'SAS ,. MIS5OUM
associated with a
powerful and
intensifying low +0R04
pressure system, and R0,d
10
produced a slight
warm-up, followed by Snowfall pattern, From Nov 28 — Dec 1, 1985, modified from original, courtesy of
strengthening winds NOAAACDC, December 1985.
and rapidly falling
temperatures. The large geographical reach of this storm system overwhelmed Minnesota's road
networks, and many state highways and local roads became impassible and had to be closed.
Thousands of travelers hoping to get into or out of Minnesota we forced to remain in place into
the following work week.
Halloween Blizzard, October 31— November 2,1991.
A low-pressure system dove into southern Texas from eastern Colorado, picked up copious
moisture from the Gulf of Mexico, and then proceeded on a north-northeast path, nearly
following the central portion of the Mississippi River, before passing through Wisconsin and out
over Lake Superior. This scenario and trajectory produced a historic period of heavy snow in the
Twin Cities and much of eastern Minnesota, followed by intense winds and plummeting
temperatures.
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The snow began around noon in the Twin
Cities and intensified throughout the day.
Five to 10 inches had already fallen by the
end of the day, and intense snowfall
continued throughout the overnight period.
By daybreak on November I", most of the
Twin Cities area already had well over a foot
of snow on the ground, with heavy snow still
falling. Many areas experienced a decrease in
snowfall intensity beginning in the late
morning, but snow nevertheless continued to
accumulate at a rate of an inch every 2-3
hours throughout the afternoon and into the
evening.
Winds had picked up during the morning also,
and increased throughout the day, with
sustained speeds between 20 and 30 mph
with many gusts above 40 mph in the Twin
Cities. By mid -evening, another band of
heavy snow spread across the area, as winds
reached peak speeds of 25-40 mph with gusts
as high as 50 mph. Whiteout conditions
permeated the entirety of Hennepin County
during this period.
Snowfall totals from Halloween Blizzard. Courtesy of
Minnesota DNR State Climatology Office
Snow continued at a lighter pace into the 2nd and even the 3rd of November, but most of the
snow had fallen, with 25-30" totals falling on through the event.
The storm prompted school closings on both Friday November 1, and Monday November 41h in
some districts, as snow removal efforts were significantly behind schedule. The storm broke daily
and all-time snowfall records in the Twin Cities, and in its aftermath, the earliest subzero
temperatures on record were observed.
Dome Teflon Roof #3 Snowstorm and Blizzard, December 10-12, 2010.
A very potent winter storm developed over South Dakota and Nebraska on Friday, December
10th, then strengthened as it moved into Iowa through Saturday, December 11th. Moisture
surged into the Upper Mississippi River Valley ahead of the system on Friday, and precipitation
pushed into the region during the overnight hours. Both coverage and intensity increased during
the day on Saturday, and winds increased to 25-40 mph with higher gusts by afternoon.
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Snowfall totals from December 10-12, 2010, storm. Courtesy of NWS
Chanhassen
Very heavy snow accompanied this system, with widespread totals between 12 and 24 inches.
The Twin Cities recorded 17.1 inches, making it the fifth largest snowstorm on record, and the
largest in December. For the third time in 30 years, the excessive snow load ripped and then
collapsed the Teflon roof of the Metrodome.
There have been no other incidents that are within the scope of this plan.
4.3.11.8. Future trends/likelihood of occurrence Ble
Research on the future of winter storms in Minnesota is lacking, but recent trends indicate a tendency
towards increases in the size of the largest snowfall events. However, this increase is not yet statistically
significant.
Climate change on one hand is causing a rapid warming of winter, and on another hand is putting more
water vapor into the atmosphere. Therefore, it is plausible that snowstorm intensity could increase, even
as seasonal snowfall decreases. However, using data from the Twin cities and Minnesota in general, there
is no evidence that seasonal snowfall is decreasing, even though significant winter warming is well
underway. It is possible that the current trend of an increase in high -end snowfall events will continue.
Using the Twin Cities snowfall record from 1900-2015, a daily snowfall ofjust of six inches can be expected
annually. The 10-year snowfall amount for a calendar day is just over 12 inches. These values can be
analyzed for durations of up to 7 days and return periods of up to 100 years.
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Snowfall amounts for a given event duration and return period, based on Twin Cities data from 1900-
2015.
Using the same data somewhat differently, we can assess the expected frequency of common daily
snowfall amounts.
Frequency with which a daily snowfall total at a point in Hennepin County will equal or exceed a given
amount:
4.3.11.9. Indications and Forecasting
The Twin Cities/Chanhassen forecast office of the National Weather Service is the official forecasting
authority for major winter weather events affecting Hennepin County. High -intensity winter storms are
usually well anticipated by the numerical weather prediction models, often up to a week in advance, and
forecasters tend to have high awareness of potentially dangerous winter conditions two days or more
before they develop.
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Warning Products
Remarks
Sustained wind or frequent gusts greater than
A major life safety hazard is ongoing or
or equal to 35 mph accompanied by falling
imminent. Danger is greatest for those
and/or blowing snow, frequently reducing
traveling or caught outdoors. Maybe issued
visibility to less than 1/4 mile for three hours
2-4 times per year in open areas of for
or more
southern and western Minnesota. Very rare
in Hennepin County, one was November 1-2,
1991.
Significant and dangerous winter weather is
This product spans a large range, from
expected, generally within 24 hours. Six or
heavy snow events with little or no wind, to
more inches of snow, not to exceed 48 hours,
major wind -driven events that produce
half an inch of sleet and/or forecaster
near -blizzard conditions. Typically, 2-4
discretion: a combination of snow, sleet,
issued for Hennepin County per winter.
freezing rain, blowing snow, and/or wind
leading to significant impacts.
The occurrence of snow squalls (short bursts
quick onset snow band with intense
of intense snow) meeting or exceeding one or
snowfall with potential impacts.
both of the following conditions:
• Visibility 1/4 mile or less in
snow with sub -freezing road
temperatures. Often
accompanied by wind gusts
greater than 30 mph.
• Plunging temperatures
sufficient to produce a flash
freeze, along with a significant
reduction in visibility from
falling and/or blowing snow.
Additional factors to consider:
• Time of day.
• Highways and interstates
impacted.
These are polygon -based warnings that last
usually an hour or less. Larger and longer
events are covered by Winter Storm
Warnings.
Severity tags:
• General (no tag): Used
frequently. Snow squall
conditions are expected or
observed, but mitigating
actions, combined with societal
context, will reduce the threat
to safe travel.
• "SIGNIFICANT" tag: Used only
when suspected or observed
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Warning Products
Remarks
conditions, both meteorological
and non -meteorological,
suggest a substantial threat to
safe travel, such that WEA is
warranted to alert all devices in
the path of the squall.
Watch Product Name
Significant and dangerous winter weather is
As certainty about an event approach, it
possible, generally within 72 hours. Blizzard
may be "upgraded" to a warning. Many
conditions with visibility less than a quarter
become lower -standing Advisories, and
mile due to falling and/or blowing snow and
about 1/10 Watches end up with no
frequent wind gusts to 35 mph, for three
Warning orAdvisory product.
hours or more. Six or more inches of snow
with an event, not to exceed 48 hours in
length. A quarter inch of ice. A half inch of
sleet. Forecaster discretion: a combination of
snow, sleet, freezing rain, blowing snow
and/or wind leading to significant impacts.
Advisory Product Name
Winter weather that causes inconvenience
but is not dangerous if proper caution is
exercised. 3-6 inches of snow. Bowing snow,
causing local visibility reductions. Less than a
half inch of sleet. Less than a quarter inch of
ice. Forecast discretion: a combination of light
snow, sleet, freezing rain, blowing snow,
and/or wind leading to impacts.
In ideal situations, progression of NWS products used will include a Hazardous Weather Outlook, Watches,
and then Warnings or Advisories.
4.3.11.10. Critical Values & Thresholds
The baseline for a winter storm product (i.e., Watch or Warning) is generally 6 inches in 12 hours or 8
inches in 24 hours. The baseline for an Advisory is generally 3 inches in 12 hours. However, NWS
forecasters may issue Watches, Warnings and Advisories at lesser thresholds if other hazards or concerns
warrant a different standard.
4.3.11.11. Preparedness
Before the storm strikes, homes, offices, and vehicles should be stocked with an emergency kit.
At home or work, primary concerns are primary concerns are loss of heat, power and telephone service,
and a shortage of supplies in prolonged or especially severe and disruptive events. Essential supplies
include:
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• Flashlight and extra batteries
• Battery -powered NOAA Weather Radio and portable radio to receive emergency information.
• Extra food and water such as dried fruit, nuts and granola bars, and other food requiring no
cooking or refrigeration.
• Extra prescription medicine
• Baby items such as diapers and formula
• First -aid supplies
• Heating fuel
• Emergency heat source: properly ventilated fireplace, wood stove, or space heater
• Fire extinguisher, smoke alarm; test smoke alarms once a month to ensure they work properly.
• Extra pet food and warm shelter for pets
• Back-up generator (optional) but never run a generator in an enclosed space.
• Carbon monoxide detector
• Outside vents should be clear of leaves, and debris, and cleared of snow after the storm.
In vehicles, the supplies in GRAPHIC 4.3.11A are essential for winter storm survival.
GRAPHIC 4.3.11A Source: NWS Winter Storm Safety (http://www.nws.noaa.gov/om/winter/before.
shtml)
ID I
i
Cell Phone First Aid Kit .dumper Cabdes Spare Tire Flares
Charger
ImI LDIIN6 AN EMERGENCY SUPPLY KITFOR YOUR CAR
IFull Tank
of Gas,
Wader
dant Sand or
Kitty Litter
Mittens,
Hat, Bopts Flashlight u�o hoveV Hlarh�ets Tows dope
WaTmii lothand Brush�J,ifNlf i
If traveling on the road for a significant length of time, be aware of the weather forecast, especially if you
will have long drives with large distances between towns. Stay "connected" via television, radio, NOAA
Weather Radio, or social media. Major winter storms rarely occur without warning, although road travel
can subject motorists to rapidly changing, sometimes unexpected weather conditions. Thus, check
forecasts throughout your route each day before your leave, and plan accordingly.
4.3.11.12. Mitigation
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Education and Awareness Programs
• Vehicle fleet crews and others who spend substantial time on the road should be familiar
with NWS warning products, jurisdictions, and be familiar with how to obtain pertinent
information. All professional drivers should carry winter weather survival supplies.
• Homeowners and commercial properties should be aware of snow load safety and best
practices for preventing roof damage. See FEMA document P-957, "Snow Load Safety
Guide" (January 2013)
• Members of the general public should understand the risks posed by winter storms, and
should review the information available at https://dps.mn.gov/divisions/hsem/weather-
awareness-preparedness/Pages/wi nter-storms.aspx.
4.3.11.13. Recovery
Recovery from a major snow event can take days, or even weeks if it is complicated by a combination of
cold weather, power outages, fallen trees, ice, or snow. In forested areas, logging activities may be
significantly impacted, and fuel loads may exacerbate the potential for wildland fire. In addition to power
outages, persistent wind loading on structures has at times caused gas line ruptures.
4.3.11.14. References
Minnesota DNR State Climatology Office, 751h Anniversary of the Armistice Day Blizzard,
http://www.dnr.state.mn.us/climate/journal/armistice_day_blizzard.html
Minnesota DNR State Climatology Office, Tornado of March 31, 2014,
http://www.dnr.state.mn.us/climate/journal/tornadoesl40331.html
National Weather Service, Winter Safety Home Page, http://www.nws.noaa.gov/os/winter/
National Weather Service, Winter Storms: The Deceptive Killers, ARC 4467 NOAA/PA 200160, 12 pp.
Available at http://www.nws.noaa.gov/os/winter/resources/Winter_Storms2008.pdf
National Weather Service- La Crosse Forecast Office, Armistice Day Storm - November 11, 1940,
http://www.weather.gov/arx/nov111940
National Weather Service -La Crosse Forecast Office, Blizzard/Winter Storm of December10-12, 2010,
http://www.weather.gov/arx/decillo
Schwartz, Robert M., and Thomas W. Schmidlin. "Climatology of blizzards in the conterminous United
States, 1959-2000." Journal of Climate 15.13 (2002): 1765-1772.
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Hazard Assessment: WINDS, NON -CONVECTIVE HIGH
4.3.12.1. Definition
Non -convective high winds are rare, long-lasting,
sustained events that can pose significant life
safety risks and produce widespread damage over
a large area, while originating from sources
unrelated to thunderstorms (i.e., not related to
tornadoes or thunderstorm downbursts). In the
Upper Midwest and most of the US, they form in
association with intense and/or rapidly intensifying
mid -latitude cyclones (low pressure systems).
"Wake lows" developing behind thunderstorms
have been observed to produce relatively
prolonged bouts of non -convective strong winds in
Minnesota --sometimes resulting in damage-- but
these events are best considered within the
spectrum of consequences and cascading effects
resulting from derechos and other severe
thunderstorms events.
The most common scenario in Minnesota, occurring 1-3 times per year on a statewide basis, is for a
prolonged (multi -hour) period of sustained 30-45 mph winds, with frequent gusts to 60 mph, and isolated
gusts as high as 70 mph. These events tend to result in sporadic minor structural damage, and occasionally
cause isolated injuries or even deaths.
A more dangerous class of events occurs roughly once or twice per decade in Minnesota, and produces a
pocket of enhanced wind speeds, often sustained above 45 mph for several hours, with gusts exceeding
hurricane force. These events produce massive wind loadings that can result in significant infrastructural
and property damage, and the most extreme among them yield death and injury rates that resemble
those of tornado outbreaks.
Unfortunately, the meteorological differences between these two classes of events are quite subtle, and
identifying the potential for the higher -impact extreme cases remains a forecasting challenge. In fact,
every instance of them on record in the Upper Midwest has been under -forecast, in some cases
significantly. Like derechos, there is no specific National Weather Service warning product for them. Most
events in Minnesota have occurred during High Wind Warnings, within lower -priority Wind Advisories,
and even during Blizzards Warnings. Those latter cases will be considered under Blizzards and will be
discussed only briefly here.
Further complicating matters, no standardized database or method for cataloging non -convective
extreme winds exists. Therefore, precise statistics on areal extent, duration, and total impact are lacking.
4.3.12.2. Range of magnitude
Maximum event (Hennepin): measured gust 89 mph at MSP on October 10, 1949
Maximum event (non -Hennepin): measured 100 mph at Rochester on October 10, 1949
Maximum duration: 36 hours, Wisconsin, October 26-27, 2010
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Maximum sporadic wind damage footprint: 1000 mi long x 450 mi wide, November 10, 1998,
and October 26-27, 2010
Maximum extreme wind damage footprint (MN): 400 mi long x 200 mi wide, October 10, 1949
Summary of typical versus extreme non -convective wind events
Event
Frequency
Maximum
Maximum
Damaging
Extreme
Footprint
Type
per decade
sustained
wind
wind
wind
winds
gusts
duration
duration
(mph)
(mph)
(hr)
(hr)
High
10-30
30-45
55-70
4-8
NA
Isolated minor
Wind
structural damage
covering an area the
size of MN.
Injuries/deaths in 5-
10% of events
Extreme
1-2
45+
75-100
6-24
3-6
Isolated minor
Wind
structural damage
covering several
states. Significant
infrastructural and
property damage
covering dozens of
counties. Numerous
injuries/deaths per
event common.
4.3.12.3. Spectrum of Consequences B211b
Non -convective winds killed nine Minnesotans between 1980 and 2005, with several other deaths
possible between 2006 and 2014. Estimates suggest 20-40 additional deaths occurred between 1940 and
1979. Thus, with at least 30 deaths (and possibly as many as 55) since 1940, non -convective extreme winds
clearly present a life safety risk on par with those of tornadoes and convective storm hazards.
Research has shown that non -convective wind fatalities are like derecho fatalities, in that the majority of
them occur outdoors, in boats, or in vehicles. Only 5% of documented US non -convective wind deaths
between 1980 and 2005 occurred within structures. By contrast, over 70% of tornado -related deaths
occur within buildings or homes, illustrating that people are less likely to seek shelter during non -
convective high winds than during tornadoes.
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Sources and locations of US non -convective wind fatalities, modified from Ashley and Black 2008 (see references)
Unlike derechos, the peak frequencies of non -convective extreme winds occur during the mid -spring and
especially mid -fall transition seasons. This timing minimizes the number of outdoor recreational activities
and reduces the potential exposure to wind -related hazards. The notable exceptions are 1) Minnesota's
fishing opener, typically during the first half of May, at the end of the spring risk period, and 2) Minnesota's
hunting seasons, which span the heart of the peak risk in October and November.
Boaters face substantial risks during non -convective high wind events. The reduced friction of open water
often increases wind speeds and wave heights and threatens to capsize boats. Once overturned or
submerged, a boat's occupants will be subject to the seasonally cold water, which poses serious risks for
hypothermia and eventual drowning. Given the harsh conditions, rescue operations can be difficult, if not
impossible. Several of the known deaths during the Armistice Day storm of 1940 were from skiffs that
capsized in the 40-60 mph winds, hours before snow began to fall.
The prolonged nature of non -convective high wind events means that hunters and others spending time
outdoors face extended risk exposure from falling trees. In urban or built-up areas, falling trees and power
lines are the most typical sources of risk. During extreme events, urban inhabitants can be injured or killed
by flying debris. In rural areas, outbuildings are often damaged, and barns frequently collapse.
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Occupants of cars and trucks also are vulnerable to being hit by falling trees and utility poles. Further, high
profile vehicles such as semi -trailer trucks, buses, and sport utility vehicles are frequently blown over
during sustained non -convective wind events.
Though they only make up 5% of the 1980-2005 deaths shown above, construction sites may make larger
proportional contributions during periods of high economic growth, when the number of large projects
multiplies. Workers have been and can be blown from ledges or scaffolding and bombarded by loose
materials.
Because they are so rare, the Twin Cities area has not experienced the consequences of a major non -
convective wind event in several decades. Examination of the event in 1949, combined with what is known
about derechos, suggests that a current -era repeat would be catastrophic. The total population exposed —
outdoors, on the streets, in traffic —would likely be several times larger than in 1949. Power disruptions
would cover the entire metropolitan area, and thousands of roads and street segments would be blocked
by fallen trees, wires, and utility poles. The breadth of an extreme system, acting on our complex and
dense concentration of overhead distribution feeders, would necessitate a massive temporary workforce
to restore service after an event. Outages would likely last days, which could be particularly dangerous if
winter conditions followed the high winds.
4.3.12.4. Potential for Cascading Effects
Non -convective high winds can occupy a large portion of any strong extratropical cyclone, and as a result
can follow, precede, or be accompanied by a wide range of weather conditions. The parent intense low-
pressure systems frequently produce severe thunderstorms and tornadoes in areas that are later affected
by the non -convective high or extreme winds. In some cases, the dangerous winds stretch far
northwestward, into the portion of the cyclone where heavy snow is falling or has fallen. In these
situations, severe blizzard conditions develop, and the winds function as one of many mutually enhancing
hazards.
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Phase 1 Phase 2 Phase 3
Non -hazardous "°"011llllll Non -hazardous
weather lulu weather
IIIII
The four generalized scenarios in which non -convective extreme winds most frequently occur in the Upper Midwest. It
should be noted that a single system may produce different scenarios at different locations. The Armistice Day storm
1940 generated each of the four scenarios listed.
Considering that thunderstorm hazards tend to be distributed in the southeast quadrant of a cyclone, that
blizzards tend to occupy the northwestern quadrant, and that any system capable of both will tend to
move northeastward through the region, it is unlikely that any given location will experience severe
thunderstorms, non -convective extreme winds, and blizzard conditions from the same system. However,
a powerful system on November 11, 1911, did just that, producing killer tornadoes in Iowa, Wisconsin,
Illinois, and Missouri, followed by record -setting temperature drops of 60-80 degrees in 6-10 hours with
blizzard conditions and wind gusts as high as 75 mph. This event is a true singularity in the central US, in
that nothing else like it has ever been recorded.
Perhaps the most common scenario for any one location in the Upper Midwest is that the extreme winds
follow a period of inclement but otherwise non -hazardous weather and are followed by a return to non-
hazardous weather as well.
The scenario a given event follows is determined by both relative position with respect to the center of
low pressure, and the depth of cold and/or warm air and moisture available to the system as it moves
through the region. Those factors, in turn, influence the likelihood of cascading effects. In Scenario 1, the
primary impacts are damage and power outages, and weather conditions in the storm's wake generally
will not further escalate the situation. In all other scenarios, there is some potential for combinations of
the following cascading effects.
Severe weather — Virtually all known non -convective extreme wind -producing systems in the
Upper Midwest have also produced severe weather hazards somewhere within the storm's warm
sector, which is in its southeast quadrant. Incidentally, concentrations of a system's most extreme
non -convective winds typically follow the cold front into the southeast quadrant as well. Thus, if
a sufficiently intense system produces tornadoes or straight-line winds (both of which can form
in the high -shear environments of these systems if enough instability is present), some of the
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areas affected will be at risk for non -convective high or extreme winds, generally beginning 6-24
hours after the severe weather. This occurred in south-central and southeast Minnesota on
December 15, 2021, when severe thunderstorm winds to 75 mph or greater knocked out power
and were followed by non -convective winds of 60-80 mph several hours later.
In these situations, any debris generated by the severe weather will have the potential to become
airborne and further scattered by the non -convective winds, prolonging the hazard exposure by
hours. Moreover, the sustained wind loadings will further weaken or damage already -
compromised structures, causing the potential for further collapse. The winds will also threaten
to blow down trees and power structures previously spared. Lastly, these intense non -convective
winds will add a layer of danger to ongoing search and rescue operations.
Blizzard — Although the very strongest winds tend to wrap into what had been the warm sector
and are often removed from the area of heavy snow, the broad area of strong and even dangerous
winds can reach back into areas experiencing (or previously experiencing) winter weather
conditions. In these cases, the wind hazards are compounded by falling temperatures, reduced
visibilities, and slippery or obstructed roads. Winds combined with heavy snowfall can knock
down trees, power lines and power poles, blocking streets and cutting some residents off from
their communities.
Cold — Even areas that do not experience blizzard conditions may see rapid temperature drops
behind the cold front. Because these events usually occur during the transition seasons, the
extent and depth of the cold air tend to be minimized. However, temperatures can fall near or
below zero, and wind chill temperatures can fall to -25 or lower. The cold weather risks are
greatest in areas that had lost power or utility service from extreme winds, as frostbite and
hypothermia become serious concerns.
Flash Flooding — Most of the systems capable of extreme winds move quickly enough that
precipitation amounts are kept under 2 inches. However, there have been instances of prolonged
heavy rainfall and at least minor flooding, raising the possibility of a joint flood/non-convective
wind disaster at some point in the future, though none have been recorded in Minnesota. The
force of moving water combined with sustained strong winds would easily overwhelm stranded
vehicles and would significantly hamper rescue operations.
Wildland Fires —The swaths of trees toppled by non -convective high winds can increase fuel loads
on forests and escalating the risk of wildland fire. Additionally, although most non -convective
wind systems produce some precipitation, many of the extreme winds come through "dry," and
even in fair conditions. If the system passes through during a drought or other condition with
unusually dry vegetation, the winds could easily enhance wildfire risk. Any existing fires would
have the potential to spread rapidly and uncontrollably.
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4.3.12.5. Geographic Scope of Hazard Blc
A typical extreme wind -producing non -
convective event may affect well over
100,000 square miles with wind damage U ,,
and may produce extreme impacts over
e ONAM ,
tens of thousands of square miles. The h �� e /NG W
wo "n
total footprint may resemble those of �y ort lip
:i %/%i/ i;
derechos, but the time signature is very
Ni/ 10ai� different because non -convective events
i
y al, 1
often affect large areas simultaneously „ a „ �,It ,/
and for much longer durations than 110 W/ �Io a
PR
convective weather systems.
I viol a ,%
M11111111 > oot i
Non -convective extreme winds have been'"�
recorded in every state, but their impacts
are greatest in heavily populated areas,
even though their frequencies and
magnitudes may be greatest on the open Number of non -convective high wind fatalities in the lower48
Plains of the central US. The highest death United States during the period 1980-2005. Source:
rates per unit area are found in the
http://earthzine.orgl2011/06/04/death from -a -clear -blue -sky -
extreme -non -convective -high -winds/ (modified from Ashley and
northeastern US, between Maryland and Black2oos)
New York state, where "nor'easters" can
expose large, dense populations to hurricane -force (or greater) winds, and along the Pacific coast. Death
rates in these regions are 10 times higher than in Minnesota and the Upper Midwest, because of higher
frequencies of intense low-pressure systems, the complex topography found between the mountains and
coasts induce wind -enhancing terrain effects, and the much greater population concentrations.
Within the Midwest, Minnesota appears to lie on the northwestern side of a risk corridor, which
maximizes near Chicago.
4.3.12.6. Chronologic patterns (seasons, cycles, rhythm)
Non -convective extreme winds associated with strong low-pressure areas are most common during the
fall and spring transition seasons, when the polar jet stream's mean track is near the Upper Midwest and
when continental temperature gradients are strong. Although strong cyclone development is more
common in spring than in fall, the conditions favoring explosive intensification are more common during
autumn, and thus, October and November have by far the highest frequency for non -convective extreme
winds.
4.3.12.7. Historical data/previous occurrence Bld
The record of non -convective extreme wind events in Minnesota is incomplete, owing to the lack of
adequate instrumentation, documentation, and categorization. Knowing the true frequency of extreme
winds in Minnesota would help estimate the likely recurrence of impacts on the modern landscape and
population. The following events are those known to have produced significant non -convective wind
impacts in Minnesota and the surrounding region.
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The Armistice Day storm of November 11, 1940
Is best remembered as high -impact, high -mortality blizzard, but the extreme winds prior to the
snow were responsible for much of the cascading disaster that followed. Extreme non -convective
winds capsized skiffs used by hunters in southern Minnesota, and produced impossible navigation
on the Mississippi River, which forced at least 12 hunters to shelter on islands, where they
ultimately froze to death. The winds wrecked large vessels on Lakes Michigan and Superior,
resulting in 59 fatalities. From Minnesota east into Michigan and Ohio, winds were sustained at
35 mph or greater for several hours, with many stations recording average speeds more than 50
mph. Gusts of 70-80 mph are believed to have been common throughout the region. The
strongest winds were over
Wisconsin Illinois and western
Michigan, to the south and °
southeast of the intensifying low-
pressure center. The winds blew
down utility poles, and cut power
and communications to much of z,
Minnesota, Wisconsin, Illinois, -pr
and
dangerous Michigan, Brous situation creating as
g
temperatures fell into the teens . k .,"L._
and single digits. ° r f
The event produced all four en
extreme wind scenarios a %""',
g described previously in different
..A
parts of the region. Across much Surface weather map, Nov11, 1940. Shaded area represents
of Wisconsin, Lake Michigan and region of wind impacts. Dark area represents hurricane force
Lower Michigan, the dangerous, wind gusts. Modified from La Crosse NWS.
prolonged winds of 40-60 mph
(gusting up to 80 mph) were the only significant hazard posed by the storm. Over Iowa and Illinois,
tornadoes and severe thunderstorms swept through the area during the morning, and then non -
convective sustained winds of 25-45 mph (gusting 55-70 mph) blew for 8-12 hours following the
passage of the strong cold front. Over western Iowa, much of Minnesota, northwestern Wisconsin
and the eastern Dakotas, non -hazardous weather gave way to strong winds gusting up to 70 mph,
severe blizzard conditions, and dramatically falling temperatures; these conditions stranded and
killed at least two dozen motorists. Lastly, the central and western Dakotas had wind gusts to 65
mph, little or no snowfall, but dangerously cold temperatures.
On October 10,1949
The most severe non -convective wind event on record in Minnesota struck most of the state and
produced over 75,000 square miles of derecho-level damage. Minneapolis recorded seven
straight hours of sustained winds above 40 mph, three hours of sustained winds above 50 mph,
and two hours of gusts exceeding 75 mph, including a maximum gust of 89 mph. In Rochester, a
100-mph wind gust was recorded. Boat works facilities were demolished on Lake Minnetonka, as
well as numerous other Minnesota lakes; docks were destroyed, and sailboats were piled onto
the shores of Minneapolis lakes; windows were blown out of homes, storefronts, and office
buildings; and many brick buildings partially collapsed. In downtown Minneapolis, large
signboards were twisted, the 65-foot chimney of the Sheridan Building fell onto and severely
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injured several people, and workers
on upper floors of the Foshay Tower
fell ill from motion sickness due to the
extreme swaying of the building. The
n�
winds inflicted destruction or severe
damage upon barns, windmills, waters
towers, and elevators
grain
241
throughout rural Minnesota. The�10�
event claimed 27 lives region -wide
(four in MN), and severely injured
r
hundreds (at least 100 in MN). Many
of the casualties were caused by
1 "',
blunt trauma from flying or falling
� a
objects, and lacerations from flying
(°
glass. Northern States Power counted
approximately 4800 broken lines and
600 broken poles in southern
Surface weather map, Oct 10, 1940. Shaded area represents
Minnesota alone. An additional 48
region of wind impacts. Dark area represents hurricane force
broken poles were counted in the
wind gusts ModifiedfromDaily Weather Map s
Fergus Falls area. In some areas,
outages lasted into early November. Losses exceeded
$100 million USD (2014) at a time when
there was far less infrastructure and property than there is today.
This storm system produces a band of occasionally heavy rain that in some cases fell into the
howling winds, producing visibilities near zero at times. The rain itself otherwise had a marginal
impact (no significant flooding, no damage), and although severe weather was reported well to
the south of the region, no other significant hazards preceded or followed the extraordinary winds
in Minnesota and the Upper Midwest.
On November 10,1998,
An explosively intensifying low pressure system tracked from Kansas to western Lake Superior,
producing a wide array of dangerous weather conditions, punctuated by a deadly, long-lasting
bout of non -convective extreme winds. The storm set the statewide low-pressure record (at the
time), with 962.7 millibars registered at both Albert Lea and Austin.
Although most of Minnesota had widespread 30-50 mph winds, with gusts up to 75 mph, the most
devastating winds stretched from central Iowa, through the majority of Wisconsin, and into Upper
and western Michigan. These areas experienced up to 18 hours of sustained 35-50 mph winds
with frequent gusts of 65-75 mph, and many gusts exceeding 85 mph, including a 93-mph gust
recorded at the La Crosse NWS office. Wind gusts exceeded 85 mph over far southeastern
Minnesota.
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The winds resulted in 10 deaths, 34
serious injuries, and at least $50
million USD (2014) in damages.
Wisconsin was hardest hit, but
impacts were severe in Minnesota,
where a school bus was blown of the
road, and hunters in the Paul Bunyan
State Forest were stranded in heavy
snow and high winds because dozens
of fallen trees blocked all possible
exits. Near Foxhome in northwestern
MN, 27 consecutive power poles were
snapped.
The Milwaukee and Green Bay, WI
National Weather Service offices
collected detailed information on the
storm. Some of the worst impacts (all
Wisconsin) included:
Surface weather map, 12:00 PM CST, Nov 10, 1998. Shaded
area represents region of wind impacts. Dark area represents
hurricane force wind gusts. Base map generated from
Plymouth State Weather Center.
• Green Lake Co: barn leveled on outskirts of Berlin. Shingles ripped off business in Green Lake.
Light poles bent by wind in Berlin.
• Sauk Co: Shed demolished in Baraboo area. Tree fell on trailer near Lake Delton. Many trees
and power lines downed in eastern part of county near Wisconsin River, causing 1000
outages.
• Columbia Co: 50-year-old woman killed when blown into Wisconsin River, where extreme
winds created powerful undercurrent. Semi -truck tipped over on 1-94. Columbus, a home's
brick chimney damaged, and roof of balcony ripped off.
• Iowa Co: elderly man near Cobb suffered head injury after being knocked down by a gust of
wind. Semi -truck driver injured when vehicle flipped over by wind gust on Highway 80, just
north of Stephens. Five other semi roll-overs in county. Apartment building and hotel in
Dodgeville sustained roof damage. New home under construction demolished. Barn collapsed
in rural Hollendale. New building destroyed near Spring Green.
• Dane Co: 87-year-old man died after car blown into him on north side of Madison. Capitol
Square business had window blown in. Several businesses in Mt. Horeb sustained wind
damage. Roof torn off multi -unit apartment building in Manona, and 4 other nearby buildings
also damaged. Two businesses in Stoughton damaged. 12 semi -trucks flipped over in 10-min
period on 1-90/94, and several more on US18/151 and Hwy 51. Several barns in county
damaged. Moored boats on Lake Kegonsa were pushed into each other, resulting in damage.
• Lafayette Co: Large portion of Darlington High School roof ripped off. Elsewhere in county, 5
farm buildings destroyed, 15 more damaged. Five homes in county sustained damage due to
fallen trees, and 1 business suffered structural damage. Several county roads blocked by tree
debris.
• Green Co: Semi roll-overs reported on US 11/81, and Hwy 81 in town of Monroe. Airplane
flipped over at Brodhead airport. Silo roof blown off on County M. Damage inflicted on county
salt sheds in New Glarus and Brodhead. Approx. 5000 customers without power at one time.
• Rock Co: Beloit, 25 large trees knocked down, damaging several homes. 1/3 of Janesville
Parker High School roof torn off. Evansville, two businesses with blown -in windows, and siding
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peeled off on 5 other buildings. Edgerton, 2 homes sustained damage from fallen trees, 5
businesses lost siding. Approx. 14,000 county electrical customers without power.
• Fond du Lac Co: City of Fond du Lac, sheet metal and siding on a church steeple peeled off by
the wind, over 100 homes damaged. Eden, shed blown away. Two semis flipped by wind on
Hwy 41, and cars pushed or blown into ditch. Oakfield, roof of pig barn ripped off. 2800 county
electrical customers without power.
• Sheboygan Co: woman in Sheboygan injured by flying glass debris after window blown out of
a business. Two other city businesses suffered roof/sheet metal damage. Barn near Plymouth
leveled. Semi -truck tipped over on Hwy 23 west of Sunset Rd. Three homes in Sheboygan Falls
damaged by felled trees.
• Dodge Co: scattered damage reported in all parts of county. Juneau, roof was ripped off
business building. Three semi -trucks flipped over. Approx. 2000 county customers were
without electrical power at one time. Multiple -vehicle accident near intersection of Hwy 151
and 16-60 due to vehicles being pushed sideways by gusts.
• Washington Co: Approx. 8000 customers lost electrical power. Two semi -trucks flipped over
on Hwy 45, resulting in closure of road. County 911 center logged 54 calls for damage
assistance. Barn blown down on Hwy 28 near Kewaskum. Several schools closed early.
• Ozaukee Co: Siding ripped off several homes and telephone poles snapped in Port
Washington. Belgium, about 1/4 of roof was torn off building under construction. Several
schools closed early in Mequon and Thiensville.
• Jefferson Co: Ft. Atkinson woman injured after when blown into side of her home. Semi -truck
driver injured when truck flipped over on 1-94 near Hwy 26 interchange. Another semi
overturned by a gust on US 18 near Hwy 89. At least 17 homes in county sustained damage
from tree debris. Many acres of corn crop flattened. Barn blown across Hwy 106 east of Ft.
Atkinson. Approx. 6000 customers lost electrical power. Concrete wall of new grocery store
In Ft. Atkinson, blown down.
• Waukesha Co: Two women injured in Muskego when tree fell on car. New Berlin man injured
after motorized garbage cart rolled over by a wind gust. Hwy J, Pewaukee, driver injured after
tree fell on car. Approx. 15,000 customers lost electrical power. Semi -truck flipped over by
gust on 1-94 near Hwy 83 interchange. At least 3 barns in county were badly damaged. In both
Muskego and Sussex, two new walls at school construction sites toppled. Construction site on
Hwy 36 near Burlington badly damaged. Several boats damaged on county lakes due to large
waves.
• Milwaukee Co: 87-year-old man fell face -first onto sidewalk when door he was opening blown
from his hand; went into coma and died November 16. Southridge Mall, woman sustained
head injury when blown over in parking lot. Hundreds of trees uprooted across county,
damaging dozens of homes, apartments, and businesses. 20,000 customers lost electrical
power. Traffic lights knocked out of service at 75 intersections. A train sustained damage from
tree debris while moving through northern part of county. Significant damage to gates,
ground equipment, and signs at General Mitchell Int'I Airport.
• Walworth Co: Semi -truck driver injured after vehicle flipped over on Hwy 11 near Racine Co.
line. Roof damage to at least 6 businesses and nursing homes in county. Semi -truck rollover
on 1-43 near the Hwy X interchange resulted in spilled fuel that closed road. Several
Whitewater buildings and a stadium damaged. Walls blown down at construction sites in East
Troy and Elkhorn.
• Racine Co: Woman injured when traffic signal light blew onto her vehicle. Racine, woman
injured when tree fell on home. Police officer injured by flying debris while out on a call.
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Construction wall blown down. Brown's Lake, shed destroyed. Several other homes and
businesses sustained damage from trees.
• Kenosha Co: 16-year-old boy electrocuted in Bristol as he tried to escape after a wind gust
toppled a live electrical line on his car. Near Salem on Hwy 50, small car partially airborne by
wind gusts and blown into ditch. Semi -truck was flipped over on 1-94.
• Brown Co: Kaukauna, several dozen homes evacuated when top of water tower holding
225,000 gallons blew off. Green Bay, Interstate 43 Tower Bridge closed because of multiple
semi blow-overs.
The record -breaking extra -tropical cyclone October 25-27, 2010
This system brought a widespread severe weather event and serial derecho to the lower -Midwest,
followed by a massive, 2-day non -convective high wind event that stretched from the Dakotas
and Nebraska to Michigan. The sea -level pressure of 955.2 millibars at Bigfork, MN shattered the
previous state record set by the November 10, 1998, storm system. The reading at Bigfork is also
the lowest on record anywhere in the Central US and is a mere 0.2 millibars from the record for
contiguous US.
Despite the extraordinarily low pressure, the enormous area occupied by non -convective high
winds, and the unusually long duration, this event lacked the wind severity of those in 1949 and
1998. 60 mph gusts were observed at most stations in the storm's 8-state footprint, but not a
single station recorded an 80-mph gust. The winds produced nearly 500,000 power outages (at
one point or another), toppled thousands of trees and power lines, but produced fewer casualties
(2 fatalities and 8 injuries), and less property and infrastructural damage than the other systems.
This result is not well understood, because wind speed and impacts tend to be highly and strongly
correlated with the strength of the cyclone, as represented by its lowest sea -level pressure. It is
possible that this event, for a currently unknown reason, failed to produce or incorporate the
dynamical and mesoscale features that typically produce extreme winds in high -intensity
systems.
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Locations of non -convective 58 mph or greater gusts, cyclone center, and other hazards. Courtesy NWS Duluth.
The October 2010 event was also
unusual because it produced pockets
of excessive rainfall. Typically, strong
regional winds aloft with these
systems prevent thunderstorms from
training and ensure that precipitation
is not prolonged. Thus, the highest
precipitation total is usually kept
below 2 inches. In this case however,
numerous clusters of thunderstorms
formed just east of the advancing low
center, producing widespread heavy
rainfall. As the cyclone reached peak
intensity, its forward motion slowed
dramatically, and heavy stratiform
precipitation (eventually changing to
heavy snow) impacted many of the
same areas that received repetitive
thunderstorms. Portions of northeast
Minnesota received over four inches
Rainfall associated with October 25-27 non -convective high wind event.
Courtesy NWS Duluth.
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of precipitation, with isolated reports of over 5 inches, resulting in flooded intersections, submerged
roads, and minor damage to businesses and residences. The locations receiving the heaviest rainfall were
in the same position with respect to the cyclone center as areas that often receive the most intense non -
convective winds; fortunately, however, this storm did not produce such winds, and there were few or no
compound flooding/extreme wind effects.
December 15, 2021
An unusual winter situation unfolded
during this evening as a muggy airmass
and a developing cyclone produced
intense thunderstorms that raced
northeastward from Nebraska into
southeastern Minnesota, producing 22
tornadoes in the state, along with
extensive straight-line wind damage.
After the storms cleared the area, the
intensifying low-pressure system"
responsible for them approached, with E e-like eature seen in eastern Nebraska on December15 2021
Y J` , as
an "eye -like" center of circulation and severe thunderstorms advance through southeastern Minnesota and
a large area of strong non -convective intense non -convective winds move northeastward with the circulation.
winds. The winds moved into the same
areas damaged by the severe thunderstorms. Rochester, for instance, recorded 77 mph wind gusts with
the severe thunderstorms, and then three hours of 55-70 mph non -convective gusts, with another peak
of 77 mph just before midnight local time. Throughout southern Minnesota, non -convective wind gusts
reached 60-75 mph, producing tens of thousands of power outages as a much colder air mass settled into
the region.
The non -convective winds were quite strong, especially considering the severe weather barrage they had
followed, but the peak winds remained below the levels of those witnessed in 1949 and 1998, likely
because this cyclone was not quite as intense, and because it was still gaining strength as the strongest
winds passed through Minnesota.
4.3.12.8. Future trends/likelihood of occurrence Ble
Non -convective high winds are relatively rare, occurring, on average, fewer than three times per year in
Minnesota. Extreme events are even rarer, and only affect some part of the state approximately once or
twice per decade. Open areas of the state in the west and south are more conducive to extreme
thunderstorm winds than other areas, but extreme non -convective winds do not appear to follow that
pattern. If anything, extreme winds, and especially the impacts of them, are slightly more common in the
hilly and tree -filled eastern parts of the state than on the open prairies.
The frequency of non -convective extreme wind in Minnesota is directly tied to the frequency of intense
mid -latitude or extratropical cyclones. Unfortunately, the physical link between explosive cyclogenesis
(the process that leads to intense low-pressure systems) and human -caused climate change, is not well
understood, so research into the future of these systems has been inconclusive, with results depicting all
possible scenarios.
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Consultation of all available research suggests that extreme non -convective winds have a frequency like
high -end tornado events, with recurrence intervals on the order of multiple decades within Hennepin
County.
4.3.12.9. Indications and Forecasting
Forecasting authority for non -convective high wind events rests with local National Weather Service
forecast offices. High -intensity mid -latitude cyclones are usually well anticipated by the numerical
weather prediction models. As a result, forecasters tend to have high awareness of potentially strong
winds 2 days or more before they develop. In ideal situations, progression of NWS products used will
include a Hazardous Weather Outlook, High Wind Watch, and High Wind Warning. In some cases,
damaging and even deadly winds have arisen within Wind Advisories.
Despite high awareness of strong regional wind potential, most non -convective high wind events in the
region, and all extreme events, have been under -forecast. As a result, the impacts have come as surprises.
An after -action report from the disastrous 1949 event concluded that forecasters had "little evidence by
which the severity might have been forecast." Although forecasting techniques have improved
dramatically since that time, underestimation is still a concern. The November 10, 1998, event forecast
products made no mention of winds exceeding 65 mph, yet there were dozens of separate instances of
winds exceeding 80 mph throughout the region. Even the lower -impact, October 2010 event had dozens
of gusts exceeding the maximum thresholds named in forecast products. The forecasting challenges arise
from a combination of low event frequency, low priority (when compared with other hazards), and limited
understanding of the latest research.
Recently, mechanisms contributing to cyclone -related, non -convective extreme winds have become
better understood. Events with extreme winds share the following commonalities:
Intense cyclone. The strongest 5% of cyclones in the Upper Midwest have minimum sea -level
pressure of 980 millibars or lower and produce strong regional winds. Both the likelihood and
coverage of high and extreme winds increase as the minimum pressure drops, with 972 millibars
serving as a threshold below which both are almost guaranteed.
The first indicator that extreme winds are possible is the forecast of a sub-980 millibar
cyclone within the region. The lower the forecast minimum pressure, the greater the
potential for impacts. Potential can be ascertained several days in advance.
Cyclone passes north or northwest of area. Although non -convective strong and high winds can
be distributed widely throughout the cool side of any intense cyclone, the most extreme winds
tend to be found to the south of the center of low pressure, especially in cyclones whose
minimum pressure is below 972 millibars. This is most likely within 300 miles of the cyclone, but
distances vary depending on the circulation structure. For example, the October 1949 event had
its maximum impact area 150-300 miles southeast of the low, versus 25-150 miles to the south
of the low in the November 1998 event.
The second indicator that extreme winds are possible is if the sub-980 millibar cyclone is
forecast to pass northwest or north of the area. The nearer the cyclone (to the
north/northwest), the greater the potential for impacts, especially if the minimum
pressure is forecast below 972millibars. Potential can be ascertained 1-3 days in advance.
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The third indicator that extreme winds are possible is the formation of a sting jet or a
mesoscale dry hook (or both), which can be detected on satellite products.
TABLES 4.3.12A and 4.3.12B can be used as guides for anticipating non -convective wind impacts, based
on pressure ranges, distance from the cyclone, and location relative to the cyclone.
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TABLE 4.3.12A
a, >980
41
972-980
1 a E <972
Nearest distance to cyclone center
> 500 m i 1 300-500 m i
Low Low Low
Low Low Mot
< 300 m i
Low I Low I Low
No I Yes I No I Yes I No Yes
Does cyclone pass northwest or north of area?
Likelihood and coverage of high wind impacts, given cyclone intensity, distance, and location.
TABLE 4.3.12B
>980
972-980
1 a E <972
Nearest distance to cyclone center
> 500 m i 300-500 m i < 300 m i
Low
Low Low Low Mac
Low Mad Mad
No I Yes I No I Yes I No Yes
Does cyclone pass northwest or north of area?
Likelihood and coverage of extreme wind impacts, given cyclone intensity, distance, and location.
4.3.12.10. Critical Values & Thresholds
Because duration is such an important component of the wind loadings and total impacts, no firm
thresholds have been determined for non -convective wind speeds. However, research has shown that
some impacts emerge when gusts exceed 60 mph. When gusts exceed 75mph, impacts are often
widespread, and casualties tend to increase dramatically.
4.3.12.11. Preparedness
If planning to be outdoors for a significant length of time, be aware of the weather forecast, especially if
you will be well -removed from sturdy shelter. Stay "connected" via television, radio, NOAA Weather
Radio, or social media. Non -convective high wind events rarely occur without warning, although warning
lead times may be comparatively limited during the evolution of an extreme wind episode. Because
protracted and extensive electrical and communication disruptions may occur, set aside emergency water
and food supplies, can openers, batteries, and flashlights.
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4.3.12.12. Mitigation
Education and Awareness Programs
• Field construction crews, public works employees, and those who work or spend
significant time outdoors should be educated about these risks.
• Members of the public should understand the risks posed by non -convective wind events.
• Educating homeowners on the benefits of wind retrofits such as shutters and hurricane
clips.
• Ensuring that school officials are aware of the best area of refuge in school buildings.
• Educating design professionals to include wind mitigation during building design.
Structural Mitigation Projects — Public Buildings & Critical Facilities
• Anchoring roof -mounted heating, ventilation, and air conditioner units
• Purchase backup generators
• Upgrading and maintaining existing lightning protection systems to prevent roof cover
damage.
• Converting traffic lights to mast arms.
Structural Mitigation Projects —Residential
• Reinforcing garage doors
• Inspecting and retrofitting roofs to adequate standards to provide wind resistance.
• Retrofitting with load -path connectors to strengthen the structural frames.
4.3.12.13. Recovery
Recovery from non -convective high winds can take weeks and may be complicated by a combination of
cold weather, power outages, fallen trees, ice, or snow. In forested areas, logging activities may be
significantly impacted, and fuel loads may exacerbate the potential for wildland fire. In addition to power
outages, persistent wind loading on structures has at times caused gas line ruptures.
4.3.12.14. References
Ashley, W. S., & Black, A. W. (2008). Fatalities associated with nonconvective high -wind events in the
United States. Journal of Applied Meteorology and Climatology, 47(2), 717-725.
lacopelli, A. J., & Knox, J. A. (2001). Mesoscale dynamics of the record -breaking 10 November 1998 mid -
latitude cyclone: A satellite -based case study.National Weather Digest, 25(1/2), 33-42.
Knox, J. A., Frye, J. D., Durkee, J. D., & Fuhrmann, C. M. (2011). Non -Convective High Winds Associated
with Extratropical Cyclones. Geography Compass, 5(2), 63-89.
Knox, J. A., & Lacke, M. C. (2011). Death from a clear blue sky: extreme nonconvective high winds.
Earthzine.org (http://earthzine.org/2011/06/04/death-from-a-clear-blue-sky-extreme-non-
convective-high-winds/)
Lacke, M. C., Knox, J. A., Frye, J. D., Stewart, A. E., Durkee, J. D., Fuhrmann, C. M., & Dillingham, S. M.
(2007). A climatology of cold -season nonconvective wind events in the Great Lakes region. Journal
of Climate, 20(24), 6012-6022.
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Minnesota State Climatology Office, Minnesota DNR, (2021). Mid -December Tornadoes, Derecho, and
Damaging Cold Front --December 15-16, 2021. https://www.dnr.state.mn.us/climate/journal/mid-
december-tornadoes-derecho-and-damaging-cold-front-december-15-16-2021.html
Minnesota State Climatology Office, Minnesota DNR, (2015). Anniversary of October 10, 1949
Windstorm. https://www.dnr.state.mn.us/climate/journal/491010_windstorm_anniversary.htmI
NOAA, National Climatic Data Center (1998). Storm Data and Unusual Weather Phenomena November
1998, V 40 no 11.
NOAA, National Climatic Data Center (2010). Storm Data and Unusual Weather Phenomena October
2010, V 52 no 10.
National Weather Service, Marquette MI. Storm Warning: Advancements in Marine Forecasting since
the Edmund Fitzgerald, http://www.weather.gov/mqt/fitz_gales
National Weather Service, La Crosse WI. Armistice Day Storm - November 11, 1940,
http://www.weather.gov/arx/nov111940
National Weather Service, Duluth MN. The North American Extratropical Cyclone of October 26-27,
2010. http://www.weather.gov/dIh/101026_extratropicaIIow
Vose, R. S., Applequist, S., Bourassa, M. A., Pryor, S. C., Barthelmie, R. J., Blanton, B., ... & Young, R. S.
(2014). Monitoring and understanding changes in extremes: Extratropical storms, winds, and
waves. Bulletin of the American Meteorological Society, 95(3), 377-386.
Williams, D.T (1949). A brief meteorological summary of the October 10, 1949, Windstorm at
Minneapolis, MN. Minnesota State Climatology Office event archives.
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3d Hazard Assessment: ICE STORMS
4.3.13.1. Definition
Ice storms are major winter weather events that produce
accumulations of ice, either from rain falling in sub -freezing
surface temperatures, or from heavy sleet.
In Minnesota and Hennepin County, ice storms form most
commonly ahead of a warm front, resulting in warm air being
lifted over colder air in place, producing precipitation that is
warm enough for rain but then freezes on contact with sub-
freezing objects. When the front is associated with strong low
pressure, the precipitation can be quite heavy, with rapid ice
accumulations. With weaker systems or when the front is
stationary, it may produce sustained light to moderate
precipitation for many hours. Either situation can lead to ice -related impacts.
Significant ice storm damage in southwestern
Minnesota in April 2013. Courtesy MPR.
If the layer of freezing air near the surface is deep enough, the precipitation will fall as sleet instead of
freezing rain. The granular nature of sleet generally makes it less of a damage and safety hazard than
freezing rain, but sleet is nevertheless often a part of major ice storms.
4.3.13.2. Range of magnitude
Magnitude of ice accumulation is rarely measured, and most accounts are purely anecdotal. Severe ice
storms in Minnesota have been reported to leave a glaze up to 3 inches thick.
4.3.13.3. Spectrum of consequences B2b
Heavy accumulations of ice can bring down trees, topple utility poles, and damage communications
towers, disrupting power and communications for days, while utility companies make extensive repairs.
Ice also damages roofs, gutters, and downspouts, and falling tree limbs often cause devastating secondary
damages to structures and vehicles.
Even small ice accumulations can be extremely dangerous for motorists and pedestrians, and ice storms
often result in increased accidents, falls, and injuries. The following categories represent the most
common and severe consequences for ice storms:
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Outdoor life safety hazards
If associated with a severe winter
weather system, heavy snow, strong
winds, falling temperatures and
dangerous wind chills may follow the
�w
ice storm. Persons caught outside
unprepared can face disorientation, 26
frostbite, hypothermia, and death.
25% of winter storm casualties occur ,r
among those caught outside in the
storm.
Power/utilities
Ice storms can cause power outagespo,aadipuraw�lNrmu'ro'Car0
mm
from direct loading on electrical wires, 40 ale
and more commonly from indirect sodxt
sources, for example when tree limbs 28 10 1) 4 11"
Temperature profiles associated with freezing rain. Source: Midwest
become overloaded with ice and fall Regional Climate Center.
onto wires. Ice accumulations greater
than a quarter inch can cause
widespread power outages, and strong winds exacerbate this impact. The duration of service outages
is typically related to the complexity of the outage pattern, along with the ability of crews to get to
repair sites. Thus, prolonged ice storms with strong winds are associated with higher outage numbers
and longer service delays.
Structural damage
Ice storms can damage roofs at residences, and at larger commercial facilities as well. Large roof spans
lacking consistent support are especially vulnerable. Secondary damage from falling ice -coated tree
limbs is especially common. These falling limbs are often significantly heavier because of the ice and
can break windows and damage downspouts and gutters. In if the rain is especially heavy, ice can
penetrate vulnerable locations in roofs, deforming them and often leading to significant water damage
to plaster and drywall materials inside the structure.
Transportation
Ice storms are especially dangerous to the transportation. Major ice storms can paralyze the entire
transportation system, including public transportation and airports. Spinouts and accidents frequently
number in the hundreds. However, most large ice storms are anticipated, and road treatments are
possible ahead of time. Smaller events from freezing drizzle only cause minor ice accumulations, but
when unforeseen, can be devastating. A thin glaze from freezing drizzle on November 20-21, 2010,
resulted in several hundred reported accidents, and at least two fatalities.
4.3.13.4. Potential for cascading effects
Extended power outages
An ice storm that knocks out power becomes much more dangerous as the time to restore service
increases. This is especially true of storms that are followed by a rapid drop in temperatures. Residences
and facilities dependent on electrical power for heat distribution can become dangerously cold within
hours of power loss.
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Moreover, it is not uncommon for a major ice storm to be followed by or transition to a heavy snowfall
event or blizzard. In these cases, the ice produces the initial critical loading, but then the snow and/or
wind acts as the "final straw," resulting in severe and widespread power outages. In these situations, the
snowstorm or blizzard is just another link in a chain of cascading hazards already in progress.
Flooding
Depending on hydrological and meteorological conditions, ice storms may prime areas for both flash -
flooding, and river flooding. Flash -flood scenarios unfold when the glaze of ice is especially thick,
temperatures rise to slightly above freezing, and a period of heavy thunderstorms or heavy rain occurs
before the ice can melt. Because of ice restricting flow into storm sewers, falling rain can lead to rapid
ponding on roads and low-lying areas. If the storm water infrastructure is not obstructed, a heavy glaze on
the land will prevent absorption by soils, and will direct falling rain directly into area streams, which may
rise rapidly. It should be noted that these scenarios to date are extremely rare, and reports in Minnesota
have been highly localized.
River flooding can occur after a major ice storm if a large snowpack had been present and/or additional
rain falls over a large area. The melted snow would be the initial cause of rising river levels, which would
then be exacerbated by rain falling over ice, and to a lesser extent by the melting ice itself. Like flash -
flooding, these situations are not common and would require a convergence of many factors. The main
risks would occur during the late winter snowmelt period.
Severe weather
In rare situations, it is possible for ice storms to follow or be followed by a significant severe weather event.
November, March, and April are currently the most likely months. Power outages and compromised
communications from ice storms may limit situational awareness needed to heed severe weather
warnings. A direct hit by a major severe weather event on an area recently affected by an ice storm would
further complicate damages and compound clean-up efforts. Similarly, an ice storm following a damaging
severe weather event would threaten to worsen the impacts significantly, with additional tree, power,
structural, and interior damage possible.
4.3.13.5. Geographic scope of hazard Bic
Most major ice storms in Minnesota affect thousands to tens of thousands of square miles --generally an
area the size of 10-20 southern Minnesota counties. There have been larger events, and ice storms in the
central and southern US often cover 50-100 thousand square miles at a time, with total footprint of up to
250 thousand square miles in some cases.
The State Climatology Office has noted that historically, ice storms have tended to favor higher terrain
locations just inland from the north shore of Lake Superior, and along the Buffalo Ridge in southwestern
Minnesota. While ice storms have affected every part of Minnesota, these areas have elevated
frequencies.
4.3.13.6. Chronologic patterns (seasons, cycles, rhythm)
GRAPHIC 4.3.13A shows the peak months, historically, for ice storms in Minnesota are January and April,
but the main season should be considered November through April. Rare ice storms have occurred in
Minnesota in October and May.
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GRAPHIC 4.3.13A
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by 'tate
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........... .,
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4.3.13.7. Historical (statistical) data/previous occurrence Bld
Most parts of Minnesota
average between 3 to 5 days
of exposure per season.
Approximately 6 to 9 hours
of that time includes
freezing rain. It should be
noted that freezing rain and
drizzle can occur while
transitioning between rain
and snow weather patterns.
The frequency of true ice
storms, however, is much
ro
lower. Thirty ice stormsOA
affected Minnesota in the 20
� t
winter seasons between
1995-96 and 2014-15,
yielding an approximate TNr�tyucaala�nrrtnagllrrUm'+�7praYderwi6r€ruazrnr9trn,&Sa+dare'1,941'�2fJtrrc�d.P'npmnGlp.gurq��'ara'Y4'�rP24t3'.
frequency of 1.5 per year.
However, ice storms can be highly episodic and clustered in time, with no ice storms in five of those years
(25%), and six events during the 1996-97 winter alone.
The following noteworthy ice storms affected various parts of Minnesota:
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Feb. 22, 1922.
Blizzard, ice and thunderstorms across Minnesota, with winds hitting 50 mph in Duluth while
thunderstorms were reported in the Twin Cities. Heavy ice over southeast Minnesota with 2 inches
of ice on wires near Winona. Over two inches of precipitation fell in many areas. This was also one
of the largest ice storms in Wisconsin history with ice four inches in diameter on telegraph wires.
One foot of ice -covered wire weighed 11 pounds.
Jan. 9-10,1934.
Sleet and ice storm over southwest Minnesota. Hardest hit was Slayton, Tracy, and Pipestone. The
thickest ice was just east of Pipestone with ice measuring 6 to 8 inches in diameter. At Holland in
Pipestone County 3 strands of #6 wire measured 4 % inches in diameter and weighed 33 ounces
per foot. The ice was described as: "very peculiar in formation being practically round on three
sides, the lower side being ragged projectiles like icicles: in other words, pointed. The frost and ice
were wet, not flaky like frost usually is. In handling this, it could be squeezed into a ball and did
not crumble."
March 3-5,1935.
Called "the worst ice storm in Duluth's history," the area covered by this storm was centered on
Duluth and extended up the Lake Superior coast to Beaver Bay, and east to Ashland, WI. The worst
of the storm extended about 40 miles to the west and south of Duluth. The storm began in the
evening of March 3, with rain and wet snow falling at the Duluth Weather Bureau, and a
temperature of 26 degrees. By morning the snow stopped but the rain continued. Ice had
accumulated to % inches by 11 AM and %e inches at 4PM, at which point the lights started going
out. By the morning of the 5th, ice coatings were measured at 1.5 inches and Duluth was virtually
cut off from the outside world, except for short wave radio. A local ham radio operator sent the
Duluth Weather Bureau reports. Four streetcars had to be abandoned in the storm, three of them
in the western part of the city. A heavy salt mixture and pick axes were used to try to free the stuck
streetcars. A one -mile stretch of telephone poles along Thompson's Hill was "broken off as if they
were toothpicks" due to the ice. A Duluth, Masabi & Northern Railway engineer estimated up to
7 inches of ice on cables in Proctor. 75% of shade trees were reported ruined in Moose Lake, with
thousands of trees stripped of their limbs. Hibbing also had damage due to ice with the breaking
of large and small branches. The Portal Telephone Company in the city of Superior, Wisconsin
noted ice from % to 1 % inches in diameter.
Nov. 10-11, 1940
(Armistice Day Storm). This destructive storm also produced up to % inch of ice on wires with ice
thickness to 1 inch in Pine City and Lake Benton. Combined with fierce winds, damage to power
poles was widespread. In correspondence with M.R. Hovde, the meteorologist in charge of the US
Weather Bureau Office, Northwestern Bell reported:
• Northwestern Bell and Tri-State Telephone & telegraph Company Repairs and
Replacements. $79,000 total estimated cost.
• Thickness of ice on wires- Generally 1/8-to-1/2-inch diameter. 1 inch in diameter in two
small areas.
• Time ice first began to form- Early morning of November 11, 1940
• Length of time ice remained on wires- About 24 hours.
• Locality of heaviest ice formation- 1-inch diameter in small area near Pine City. 1-inch
diameter in vicinity of Lake Benton.
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• Approximate number of wires down -1600
• Approximate number of poles down -2400
• Extent of delay of service- Average 18 hours for toll and 36 hours for exchange lines out
of service.
• Remarks: The above covers damage to both Northwestern Bell and Tri-State Telephone
Company plant in Minnesota. The greatest damage was in the area about 20 miles east
and west of a line from Sandstone to Albert Lea.
Jan. 14, 1952.
Glaze, sleet and ice storm across Minnesota from St Cloud south into Iowa. 1,100 Northwestern
Bell telephone wires down. The Buffalo Ridge in the Pipestone area the hardest hit with % inches
of solid ice on Northern State Power wires with icicles to 3 inches. Northwestern Bell reported ice
to 1 % inches of ice on their wires in the same area. Thunder and a shower of ice pellets
accompanied the storm in New Ulm and Mankato. Minneapolis General Hospital treated 81
victims of falls on icy streets.
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Southwest Minnesota Ice
Storm, April 9-11, 2013.
A slow -moving low-pressure
system pumped copious
amounts of moisture up into a
subfreezing air mass, resulting
in up to 48 hours of nearly
continuous freezing rain in
southwestern Minnesota,
eastern Nebraska,
northwestern Iowa, and
eastern South Dakota. Just
north of the freezing rain,
heavy, wet snow accumulated
6-14 inches. In southwestern
Minnesota, hundreds of trees
and power poles were snapped
by the ice, which accumulated
to nearly 1" thick near Worthington. Extensive secondary damage occurred to residences and
vehicles, as tree limbs snapped off and crashed through windows. Power outages lasted days in
some areas. Governor Dayton issued Executive Order 13-03, to authorize state assistance for
recovery efforts in southwestern Minnesota.
There have been no other incidents that are within the scope of this plan.
4.3.13.8. Future trends/likelihood of occurrence Ble
Little is known about future trends with respect to ice storm activity. On one hand, damaging ice storm
frequency may decrease, as more and more winter events fall as above -freezing liquid. Another argument
is that more events that would have been snowstorms will contain freezing rain, and hence, more ice
storms. Yet another line of reasoning suggests that increased wintertime moisture will result in more heavy
precipitation events, including heavy rain and freezing rain. The topic has received little research attention,
so there is virtually no "consensus" about what is likely to happen.
4.2.13.9. Indications and Forecasting
The Twin Cities/Chanhassen forecast office of the National Weather Service is the official forecasting
authority for major winter weather events affecting Hennepin County, including ice storms. High -intensity
winter storms are usually well anticipated by the numerical weather prediction models, often up to a week
in advance, and forecasters tend to have high awareness of potentially dangerous winter conditions two
days or more before they develop. The potential for significant ice accumulation 1-3 days out is also
monitored by the Weather Prediction Center, at NOAA/NWS headquarters.
4.3.13.10. Detection & Warning
Warning authority for ice storms also lies with the Twin Cities/Chanhassen forecast office of the National
Weather Service. An urgently severe ice storm will be covered by an Ice Storm Warning, which indicates
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over a quarter inch of ice accumulation is expected. These situations may lead to damage and power
outages, in addition to dangerous or impossible travel.
If a severe ice storm is expected with other winter hazards, especially snow, the NWS may cover all hazards
under a Winter Storm Warning. Similarly, lesser ice accumulations with lighter accumulating snow may be
covered under a Winter Weather Advisory.
4.3.13.11. Critical values and thresholds
Ice storm or Winter Storm Warnings will be issued when over % inch of ice accumulation is expected.
Damage to trees, along with power outages, increase dramatically after %" of ice accumulation.
4.3.13.12. Preparedness
Because ice storms are likely to disrupt power and disable local transportation routes, before the storm
strikes, homes, offices, and vehicles should be stocked with needed supplies. At home or work, primary
concerns are loss of heat, power and telephone service, and a shortage of supplies in prolonged or
especially severe and disruptive events.
Essential Supplies
• Flashlight and extra batteries
• Battery -powered NOAA Weather Radio and portable radio to receive emergency information.
• Extra food and water such as dried fruit, nuts and granola bars, and other food requiring no
cooking or refrigeration.
• Extra prescription medicine
• Baby items such as diapers and formula
• First -aid supplies
• Heating fuel
• Emergency heat source: properly ventilated fireplace, wood stove, or space heater
• Fire extinguisher, smoke alarm; test smoke alarms once a month to ensure they work properly.
• Extra pet food and warm shelter for pets
• Back-up generator (optional) but never run a generator in an enclosed space.
• Carbon monoxide detector
• Outside vents should be clear of leaves, and debris, and cleared of snow after the storm.
4.3.13.13. Mitigation
Education and Awareness Programs
• Vehicle fleet crews and others who spend substantial time on the road should be familiar with
NWS warning products, jurisdictions, and be familiar with how to obtain pertinent information.
All professional drivers should carry winter weather survival supplies.
• Members of the general public should understand the risks posed by winter storms, and should
review the information available at https://dps.mn.gov/divisions/hsem/weather-awareness-
prepared ness/Pages/wi nter-sto rms.aspx.
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4.3.13.14. Recovery
Recovery from a major ice storm can take days, or even weeks if it is complicated by a combination of
other weather hazards. In forested areas, logging activities may be significantly impacted, and fuel loads
from fallen trees may exacerbate the potential for wildland fire. In addition to power outages, persistent
wind loading on structures, associated with powerful winter storms, has at times caused gas line ruptures.
4.3.13.15. References
Changnon, S. A., & Karl, T. R. (2003, 09). Temporal and Spatial Variations of Freezing Rain in the
Contiguous United States: 1948-2000. Journal of Applied MeteorologyJ. Appl. Meteor., 42(9),
1302-1315. doi:10.1175/1520-0450(2003)0422.0.co;2
Homeland Security and Emergency Management. (n.d.). Retrieved April 11, 2016, from
https://dps. mn.gov/divisions/hsem/weather-awareness-preparedness/Pages/winter-storms.aspx
Ice Storm - Southwest Minnesota: April 9-10, 2013. (n.d.). Retrieved April 11, 2016, from
http://www.dnr.state. mn.us/climate/journal/130410_wi nter_storm.htmI
Ice Storms. (n.d.). Retrieved April 11, 2016, from http://mrcc.isws.illinois.edu/living_wx/icestorms/
North Shore Ice Storm: March 23-24, 2009. (n.d.). Retrieved April 11, 2016, from
http://climate.umn.edu/dOc/journaI/Ice_stormO9O323-24.htm
Overview of Extensive Ice Storms in Minnesota, retrieved from
http://files.dnr.state.mn.us/natural_resources/climate/summaries_and_publications/ice_storms
_in_minnesota.pdf
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5t to",N VULNERABILITY ASSESSMENT
After hazards were identified, they were given a ranking of "high", "medium" or "low". This was based
on their probability of occurrence, their impact on population, critical infrastructure, and the economy.
Each participating municipality may have differing degrees of risk exposure and vulnerability compared to
others due their geographic proximity to the hazard. However, many of the hazards are countywide risks
due to their size and their impacts, and because not all are geographically specific. Under each map
portion is a hazard ranking justification statement of why the hazard was given the ranking it received.
5.1 Hazard Ranking Maps Blb
The following pages provide hazard rankings (in alphabetical order) for the following hazards:
GRAPHIC 5.1A
Blizzard
212
GRAPHIC 5.1B
Climate Change
213
GRAPHIC 5.1C
Drought
214
GRAPHIC 5.111)
Dust Storms
215
GRAPHIC 5.1E
Extreme, Cold
216
GRAPHIC 5.1F
Extreme, Heat
217
GRAPHIC 5.1G
Extreme, Rainfall
218
GRAPHIC 5.1H
Flooding, River
219
GRAPHIC 5.11
Flooding, Urban
220
GRAPHIC 5.1J
Hail
221
GRAPHIC 5.1K
Ice Storm
222
GRAPHIC 5.1L
Lightning
223
GRAPHIC 5.1M
Tornado
224
GRAPHIC 5.1N
Winds, Non -Convective
225
GRAPHIC 5.10
Winds, Straight -Line
226
GRAPHIC 5.1P
Winter Storm
227
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GRAPHIC 5.1A Blizzard
w
'Reg— Daytnn
�k P,
_
RrLr
.�
6.�
lden
.� 'Go
VaNl" o...._,
Orarucuad 3
=n ka
rMluau�ur "
d aurxcwrw o
„ r
_
High Eden
�w
Hazard Ranking liusfification:
Occurrences, „ impacts and land me were all used'in the imethcdcdogy fair- rarn,ki�neg Bliiazaurds as a: hazard in
Hje nnepdn sty., Henneon County has winter storms with high, uiwinuds occur each year, andbeing eing blimard� are
flie most dangerouscass � �unN�r sauursdccma urtlhip ..0 In addbw rm, blinard conditibns occuiTing eadi year in Min , the M= Hennepin County. Land Gover map was used to detem7dine. areas
of HeTmbepin Cbuntywitharfficial suflFace. These drtificial surfaces are stimn l� assiodated with. nrdnere buildings
and higher, populated areas are. The areas °emu Mess than 5 arttifidad surfaces were ranked at higher risks for
bibizzards because they are rnore su:sceptble to high winds and blowingsmw with lfties Lem than 14 mi%e, It
is touu al�tho u e not irnpossible, to get visibliity less than V4 with anweas: dhat are built! 4p more because of
building Ibloding viindand hdowing snow
Hennepin i jauntyMulti-Jurisdictlon � ra a.s 5
Hazard IMiitiigation Plan 20,24 Wes
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GRAPHIC 5.1113 Climate Change
Hazard IMankfing lusUficfflon,
Imipacts, its kwig lasting consequemes, dwnd time onset were the priimary medxmds, used in ranking climate
change as ffd' iim Henrm-a iwn Couu w, Unilike other hazards dim ater change! is slmm onset dM we will not sn
dew to iday consequences, but! [ong term consequences. Reawxting behind why bermise nF thnis slow onset that
dimkrke change acium Hennepin n iyr is, ranked s: nw&um is Ibecause of the iirmpacts �wmd cnn: es as it
applies to uatdn;er lam• within Hennepin County.
Hennepin C)aunly i du ,
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GRAPHIC 5.1C Drought
'77"77�
r
Ch,
Hazard Rankkg
Hi4h
Miediuim
kO'p
' '�.
Hazard Ranktng lusbficaUon
Occurrences anid limpads were bier primimry nwffiads used to, rank Drauloit as hazard across Hennepin Cbuinty.
Every singk year, pat if not all of Hennepi� County is: induded wftNn,a k-w-4,of drought acrnrdmig to, die Lhifted
se Dwaujht monitau; Drought can have impacts on many aspects across dw county, wivich is what shafted the
basetine at medium ranting for this hazard. The. westem side of thie cmnbl is Tnurh more agiimikure hewq +an
the east, side, wfikh can have inore, iffnparks and consequences Ibecause of droughtwha its v*Oat gave ffie west
side of Henna n County a high ranking.
Hennepin County Multi-Jurisdictibn 0 2.5 5
HazaTd Miti4ation Plan �20124
FtEnrepinj cot m, Eynerwricy Piumgmert 111 63he�dmm i. ki1k* L' eqn Wig wmj ki 11.441
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GRAPHIC 5.111) Dust Storms
jI Rogers
�w
.
UII
a
®...
Lake
Gin
_.__.__........................ ___. _, a _VII
�Nr
ruA IRE
__ _. __ , ____ ____.____.... E _ ................. __ _
RchfiEM
- ie;
Hazard Ranking Ch' aen� . w ....
High Ede Raide
HazaA Rankiiingi ➢u itiifii bi
Occunmrre-sand hi the, primary m °rased 11m, rank Du:st Sbwms as a Ord acruss Hennepin
Caunty, Dust sl=sare rare acricss IFin nneorn CI. AYNnIgh &ey can happen, this liow freqpenicy and slice we
hkwe rr3k sin a t ue dui stcwm h many ors are &e reascirks this hmard is ream flnnn
Hennepin C,aunty Multi-Junisdictilon � 0 2.5 s
Hazard IMiitiIgation Pin 2,0124 mews
tlI : 'iPi'dl mme mm r ire -ter 5
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GRAPHIC 5.1E Extreme Cold
Herd Rainking Jihu^stifi a n
(kicurrrence and limpacts were the primarf ids used to, rank ExbafmCodid an a hazard across Hennepin
County. ExItrame orM temperatures ooi ry year across.IHeannepin County, in ad&h an, 6wy can have extreme
conseqte-nices and impacts, especially & ektreme cdid comdii iioms persist far 113nig periods of tirne.,
Hennepin Couinty Multi-Judsdicfion 0 2.5 5
Hazard Mitigabion Plan 20124
lfld
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i
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GRAPHIC 5.1F Extreme Heat
'�ti,w
c:,",
77
u" ....... _......
......
kdqpendenoe ME
....._.__ ...
i
__........................ ___. �_.� ________ r
TA
efh
Owna
FAn rout _,.
0
F
ve" a �r
rho
_... ....,.,,..__...--... .._.____....�._ z aw
Hazard Ranking Char, *a
High Eden Praide
MilediIIu@'I N ..
m-�,.•
Hazard R;airtkiiin Jusitiificallj
C)oi urrprnce, and imparts, were the primary miefluxis used to iw k ExbPerne H'aat.E. a hazArd arram Hennepin
Couroty., ' *n County. tmically sees, arQauund one oftrerruum Ihu.1t wing rh year, that lasts behmeen one and
five o eras ranked mediuurn Rna Rine TFr� rnscsuns � rmr `�r�e r#, � N�u_arrrtn� rn y"
resk, us ffbese. dbes.amre indurar vAthin thearea of thie Urbw n Heat Bernd.
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Hazard IMitigation Dian 20,24 y
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N,4 tl�rl date: '11112324'�s ram,—
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GRAPHIC 5.1G Extreme Rainfall
Hazakll R;ankiling Jusfification
Occurrence, recent Ihuisturkdl disasters amnd fiAffe treads were the pniffnary rnebwds used to rank Extreme Rainfall
as a hkawd across Henneon CAxinty. Extreme rainfall events can ,gym ffirough mom drbd Iess ari'01&r however
tine Treat ofthem ors cbse to wry year witimn., Hennepin Cotes, IHennepin County has sin numerous
ext3sme rainfall events in the very shortr term past:,, in adidtion, bm nds: show dn,ad, them uev mts vd, OiL-4y continue,
w i mse. Hennepin a is P � tug much do rn a rr uurnt of raien , each and pa�s�ul�^ � � � g� � .
mmVounffi or year, but rather more rainfall at one bmiwe veTms.b6ng uncut, over lbirKjer Periods„
Hennepin C,auntyMulti-Jurisdictibn 0 2.5 5
Hazard Mitigation Plan 2,024 was
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Pub
�N,U:;3'II�mY � : ' �11 M'il' ra_�� FM .ttf r m h. lbwnv. k., W J .Kp, r 5^ �.
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GRAPHIC 5.1H Flooding, River
Champlin
Greenfield'
s..—...... ...-- __ . ..._ ..,.
_
a�
ai
Independenoe Medha
MY
r a Owna Po n
rm a
nk �Pam,
'. ��
�kho
r�
_. _._ .. _._ ___ ____ _ ........ __ _......_ , ___ _
_ - !tie
Hazard Ranking �Char, fossen . . .....
High Eden
Medium ab-77"o-
W°
Hazard Ranking Justification
Occurrence, iimpacts, the Hennepin n County FEMA FkxKWains Mai ac re the p unary methodts owed to u°ank.
River FloodhV as a ii kia awd acne 'iin County. Hennepin County has three major rhfET basins that b-avel alhrag
its bcwrdeers. Cyr Rhw, Mississippi River, and the IMiinnesota River. Every year at. lleask,nne of these rivers has
warnings issued,,, The Ckmw (River on, he. nGTffiwest side of : in Cmnty is ranked higher has hazardous
the Misssippi and Mi nnesa& Rivers ber-ause thne crow River droves not have as high of banks, cfiffs assmiated with
it.... The FEMA Fkmdoains Map, shiaws all ttroe rivers, being iiinmpwa�cded by' a 100 year food However, mere is more
infrasbucture that could be damaged a1ang the crow nvxe easily than akng, the Mississippi, or the' iinnesu:ta
Rivers.
Hennepin CauntyMulti-Jurisdictiran � D a.s 5
Hazard Mitigation Dian 20i24 w
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w,� �,
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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GRAPHIC 5.11 Flooding, Urban
irndgpend4 nw
Le
Nnne�
I D-
�a ,
__ ,
dak
Jlrecdure
�i
®�.
L14
Mar ..`; ..
�n
Hazard Ran rin
H-tir
Miediuirrw
Hazard Rankling Justification
Ldn d uuse,, and: the Hennepin Cgxmty FEt mans Map wwrere MJw prkrndry mebxxk used tD rand Urbdn
Flooding as a hazard s: He eon County�If citiies had greater than area aNeffed byaitfici.J serAres,
there vuere ranked ,as rnwediirurn wersuus bw (like 6'e rest of the county,. This is (because the more aitficiA surface, 'dv
more runoff wewiilll see versus inx%abo n. The FEMA Fbodplairts Kap was used to iguide the ;area around Lake
Minnetonka. Every cityaroLmid the LAm Ids to Ibe impactedby the k r flood.
Hennepin C,aunty Nlu tli-Judsdict[on ° 2.5 s
Hazaf 1 Mitigation Dian 2A ,024 MIS
HeM, gpIR CDLM EMe!WrCy WflageMeflt Nis :ur ®-A a; � Mwh r i'w
tl I :' iPi'd 1 54q �- H � mm ire -�r 5
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GRAPHIC 5.1J Hail
Reg—
iin
caroman
Greenfield'
........ _...... ...... �. � _. ,� .__ .
kdWendenoe
El n a
Medina
MY
......_.__.................................. _: �
Mdk
�
.__.__.. ..... n
Owna —ad
n .a
mufma -hfinne&nk P
ark
VL
- -__
._ _.. .. -
�u
RkhfieW
a
Hazard Ranking Char,
High Ede
M�ediwlrrlabonliblib-
LOW ..
HHazaiirdR,"a niung IListificaUon
OccurrerKe, limipkKts, and histmical evi&ence were thle pdrnary methods used to rimlk. Had as a haziad across.
Nennepdn 03unty. Hail' wwrithin the borders, of Hennepin err: ntyy occur wry year. Whik we Ihave seen, had in ex,ness
of three inks, !in dharneter, majority of hailBl ioccuning °nwid^iin H has been lessthan two inches,
in additicne the tyorAly its Less -If' a threat Im, pabCic inErastnicture dianovate residences with hamll stoirms, k4hife
hail JbTTm ican, cost a I �n damages, those hiigh cost! hall stD mns din not ocmr within Hennne n County on a
fr"uent
Hennepin C,aunty I Initli-Jurisdictiion D 2.s 5
Hazar Mitigation Plan 20,24 A y
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
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GRAPHIC 5.1K Ice Storm
Mzaird Ranking lustificaUon
i'emapads„ hiture. piechictions, and oEcturences were 'fe Ipri . q, methods used to irack Ica Stori as award, acres
Henneon County. ,f'Fri have not. ny, nepr bus. IH'a ewet, the
epui �F, kzn �, � ii� sl�rmrs � �-Ilartn, Ewa � ItN� � 1�
wide mark" arkimpacts are the driNing reasms bENM ranking t s hazard: as m ediurnacross the rountf. in
addition, mare. predidions are showing (Hennepin Cdunty. could lay at grewtmr risk dwn ever buefire
based ion a waning donate.,
Hennepin C,auinty M untli-a1urisdictiiian 0 a.s 5
Ha.za:rd IMiitiigation Plan 20124 m1"
�kbkMan : U11232A --ON ids pim e w .0.c 6. (1 ht _q d4i ijh^
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GRAPHIC 5.1L Lightning
Hazard Plan iiingi lu iificailioin
Occumaxe and kmpacts were the pminraryr meffials used to rank Lkjhkningas a hazard across Hennepin Coufty.
Litfitnihg accurs,wfth a ery 1e and Hennepin Counky recdoes an: abundance of hu!n norms
every year. VAde! Magnitude of lightming rs vialiable humri storms tlo, Mnm, inatiommide it posses greatgreatmsk every,
s gale year as well as millions of "Lmsm worth of darnage in Hennepin County, each year, lighting is also one dthe
c°arises of deak9is among the, natural' hazards each yeer across this mabon,
Hennepin n,
P'
j
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GRAPHIC 5.1M Tornado
Hazard Pldnkirg
High
E JyyI Medium
Hazard IRAnking Itisfification
cur h., . °ad data, impact and size of city, wor a the pnrnamyr rnetho& used tmr wank TbrmnadDes as a
hazard acmes Hennepin +buntyr. Tommadbes dim not! accur every year witfin HennefAn County. However
rneteordogicAlyr the doances & turnadoes aocurring nwrithimn, Hennepin CouF4 anre possible. every single yearr and
wm ffi high impacts annd a variety conseqLiences possible, the basefine ranking ryas medium for thins hiaad.
Histmircal data was used Im find cities that had nxwe thanme tcrrmadio occur within city hm#s;, with atleast, one. Cf
these hoeing greater than an FIEF 2e These cities viere ranked as high. Addbonally, iiF a city had mcwe than three Fj
FFi does in Ihisbw* there were Aso ranked as huh. Las4 city size was kxAed at WNfe metwwolixycally the:
chances are the same fbir any city to, beaffimted a ammadd cities that hive Mess surface arm are not as lily bD,
see a widiin, 6wir dty liffnits limp yr because there its less area fix, a tcrmmadD hit,
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GRAPHIC 5.11N Winds Non -Convective
Reg—
�«tiwa
Ih ...
s
..—........ ......._ .. _........ _
Independ4noe Medina R Ale
w .m............. ..-__
�._
03WEn
IRE
__._ _ _..._ ........ .�..��..-..__ _Edh7a,
Rxhfiem
Hazard Ranking Ch" _ . ......w.
High Ede Rairje
W
Milediuim
� d
..., G a ' a
Hazard R,art kffiq Jusfificabon
Y munr es d nd future predictions: vmre the pnirmdrry mmethads used to rank Hann •aye High Winds as, a
fin d across Hennepin Countv�y r w thin Hennepin
IP�nm- hii, ; sitamms , ram vror e+v�ea
County, in E the occurring typir .Jly an a 1-3 Amess, per yearwithin the SLke of IM"lin:. While these 'high
ids, can cause derma and in4kicts, they am typkAlly racrt as sevem as derechrscw, sftaiglrit-Iine s stonrrnns
erns' are nm ch nu re Tate„ In addiition, tine fim q rernry of dwse stwFm in, not di . stied Imi other aspects of systms
they we asssxcuetEd with, so one is ra(: eh'Ie to saywhether will increase a ar decre-ise in ffie hAi e,
Hennepin County M ntli-Jurisdie iion D 2.5 s
Hazard Mitigation Pion 20,24 N was
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GRAPHIC 5.10 Winds, Straight -Line
Hazard Raniking Justificabon
Occurreme, imipa and area of coverage were the p4imary rn ,used to rank erne' ai e Wimis as
a hazard cry jn Courdry. Hennepin Countydoes not see enhi-me sfxa g -lime winds or, derechosewry
single year, rm! conymon enouffi in,°the upper i ire possibility of tfuse mAndisdmirms occurring
°° ak^yr ie area %+dc �r can I �eaN ar, �n a�diiti�c�n;� 'tiM;� wwnde w�ari� irn� �1hs a��d nn�es� as wwse6l a,s ire
affected area reasons this i and its mandJmd) nnediiurrrn across #ewe entire county. Not one city or move or Mess
receiving s: tMes of steams.
Hennepin C,aunty Multi-Junisdictibni 0 2.s 5
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ws
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GRAPHIC 5.1P Winter Storm
Hazard Ranking JustWicaition
'")ccurrefxes, iff pares„ dnd historicaldata were the prinkay miedxNds used bo, rank Winter as ahazard across
Hennepin C+muint/,, Evoyr single yedr thwere� are winter sta cm wa mings with Renmepin County wherewe. see greater
than, six irKfies . of snow wwtthim t2 hwowuai^s and' nmanyr gimes greaterthdn Sincheswithiii 24 hnursu Thy dries not just
oocuir ionice a yearlseason, IbtA rrwuffiMie times wwiutinin time winter seem. Ewen thioumgh Kinneseta has adapted Wi he
nee& of rrmo%ing snc ww', the impacts amd c°ceding consequences of winter stmrmns alongwith their year occurrence
r es uk in a hw h wking for, this herd.
(Hennepin Ccaunty Multi-Jurtsdictinn 0 2.5 S
Haz :rd IMitiigiation Phan 20124 maw
F1��1Mm drtni ilm" woau kur wm�°�4� -w9� n P mUw � 6yfwwwmway„
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I Stf0"'N Cultural Resources Inventory
6.1. Inventories
The effects of a disaster can be wide-ranging from human casualty to property damage to the disruption
of governmental, social, and economic activity. Often not considered, is the potential devastating effects
of disasters on historic properties and cultural resources. Historic buildings and structures, artwork,
monuments, family heirlooms, and historic documents are often irreplaceable, and may be lost forever in
a disaster if not considered in the mitigation planning process. The loss of these resources is more painful
and ironic considering how often residents rely on their presence after a disaster to reinforce connections
with neighbors and the larger community, and to seek comfort in the aftermath of a disaster.
To inventory the county's cultural resources, the Steering Committee collected information from the
following sources:
• National Register of Historic Places
• Minnesota's National Historic Landmarks
6.2. National Register of Historic Places - Hennepin County
It should be noted that these lists may not be complete, as they may not include those currently in the
nomination process and note yet listed. TABLE 9.2A provides registered historical sites, please go to the
National Register of Historic Places website for additional information.
TABLE 6.2A Registered Historical Sites
Advanced Thresher /Emerson — Newton
Ames -Florida House
Implement Company
City: Rockford
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1856
Period of Significance: 1900-1924
Anoka -Champlin Mississippi River Bridge
Architects and Engineers Building
City: Champlin
City: Minneapolis
Historic Significance: Commerce/Engineering
Historic Significance: Commerce/Engineering
Period of Significance: 1925-1949
Period of Significance: 1900-1924
Atwater, Isaac, House
Baird, George W., House
City: Minneapolis
City: Edina
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924, 1875-1899,
Period of Significance: 1900-1924, 1875-1899
1850-1874
Bardwell-Ferrant House
Barry, Margaret, Settlement House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Education/Social History
Period of Significance: 1875-1899
Period of Significance: 1900-1924
Bartholomew, Riley Lucas, House
Basilica of St. Mary Catholic
City: Richfield
City: Minneapolis
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Historic Significance: Person
His
Historic Significance: Architecture /Engineering
Period of Significance: 1875-1899, 1850-1874
Period of Significance: 1925-1949, 1900-1924
Bennett -McBride House
Bovey, Charles Cranston & Kate Koon, House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1900-1924
Bremer, Frederika, Intermediate School
Burwell, Charles H., House
City: Minneapolis
City: Minnetonka
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1875-1899, 1850-1874
Butler Brothers Company
Cahill School
City: Minneapolis
City: Edina
Historic Significance: Architecture
Historic Significance: Person
Period of Significance: 1900-1924
Period of Significance: 1925-1949, 1900-1924,
1875-1899, 1850-1874
Calhoun Beach Club
Cappelen Memorial Bridge
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949
Period of Significance: 1900-1924
Carpenter, Elbert L., House
Carpenter, Eugene J., House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1900-1924
Cedar Avenue Bridge
Chadwick, Loren L., Cottages
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949
Period of Significance: 1900-1924
Chamber of Commerce
Chamber of Commerce Building
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1900-1924
Chicago, Milwaukee & St. Paul Railroad Grade
Chicago, Milwaukee, St. Paul & Pacific Depot
Separation
City: Saint Louis Park
City: Minneapolis
Historic Significance: Event
Historic Significance: Event
Period of Significance: 1925-1949, 1900-1924,
Period of Significance: 1900-1924
1875-1899
Chicago, Milwaukee, St. Paul & Pacific Depot,
Christ Church Lutheran
Freight House & Train Shed
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949
Period of Significance: 1875-1899
Church of St. Stephen (Catholic)
Coe, Amos B., House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949
Period of Significance: 1875-1899
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Como -Harriet Streetcar Line & Trolley
Country Club Historic District
City: Minneapolis
City: Minneapolis
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924,
1875-1899
Period of Significance: 1925-1949, 1900-1924
Crane Island Historic District
Cummins, John R., Farmhouse
City: Minnestrista
City: Eden Prairie
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1900-1924, 1875-1899
Cutter, B.O., House
Dania Hall
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1850-1874
Period of Significance: 1875-1899
East Lake Branch Library
Edina Mills
City: Minneapolis
City: Edina
Historic Significance: Architecture/Engineering
Historic Significance: NA
Period of Significance: 1925-1949, 1900-1924
Period of Significance: NA
Eitel Hospital
Excelsior Fruit Growers Association Building
City: Minneapolis
City: Excelsior
Historic Significance: Event, Person
Historic Significance: Agriculture, Commerce
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1925-1949, 1900-1924
Excelsior Public School
Farmers & Mechanics Savings Bank
City: Excelsior
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1950-1974, 1925, 1949
Farmers & Mechanics Savings Bank
Fire Station No. 19
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1900-1924, 1875-1899
First Church of Christ Scientist
First Congregational Church
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899
Period of Significance: 1875-1899
First National Bank — Soo Line Building
Fisk, Woodbury, House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1950-1974, 1925-1949,
Period of Significance: 1850-1874
1900-1924
Flour Exchange Building
Fort Snelling
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Event
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1900-1924, 1875-1899,
1850-1874, 1825-1849, 1800-1824
Fort Snelling — Mendota Bridge
Forum Cafeteria
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
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Period of Significance: 1925-1949
Period of Significance: 1925-1949
Foshay Tower
Fournier, Lawrence A. & Mary, House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949
Period of Significance: 1900-1924
Fowler Methodist Episcopal Church
Franklin Branch Library
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Social History
Historic Significance: Event/Person
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1900-1924
Gethsemane Episcopal Church
Gideon, Peter, Farmhouse
City: Minneapolis
City: Shorewood
Historic Significance: Architecture/Engineering
Historic Significance: Person
Period of Significance: 1900-1924
Period of Significance: 1875-1899, 1850-1874
Glen Lake Children's Camp
Gluek, John G, & Minnie, House & Carriage
City: Eden Prairie
House
Historic Significance: Health/Medicine
City: Shorewood
Period of Significance: 1925-1949
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924
Grace Evangelical Lutheran Church
Great Northern Implement Company
City: Minneapolis
City: Wayzata
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1925-1950, 1900-1924
Grimes, Jonathan Taylor, house
Hagel Family Farm
City: Edina
City: Rogers
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899, 1850-1874
Period of Significance: 1950-1974, 1925-1949,
1900-1924, 1875-1899, 1850, 1874
Handicraft Guild Building
Hanover Bridge
City: Minneapolis
City: Rogers
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1875-1899
Healy Block Residential Historic District
Hennepin County Library
City: Minneapolis
City: Robbinsdale
Historic Significance: Event
Historic Significance: Event
Period of Significance: 1875-1899
Period of Significance: 1925-1949
Hennepin Theater
Hewitt, Edwin, H., House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1925-1949, 1900-1924
Hinkle -Murphy House
Holmes, Henry E., House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899
Period of Significance: 1875-1899
Intercity Bridge
Interlachen Bridge (Ford Bridge)
City: Minneapolis
City: Minneapolis
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Historic Significance: Architecture/Engineering
His
Historic Significance: Architecture /Engineering
Period of Significance: 1925-1949
Period of Significance: 1900-1924
Interlachen Bridge (Cottage City Bridge)
Jones, Harry W., House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924
Period of Significance: 1925-2949, 1900-1924,
1875-1899
Lakewood Cemetery Memorial Chapel
Legg, Harry F., House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924
Period of Significance: 1875-1899
Linden Hills Branch Library
Little Sister of the Poor Home for Aged
City: Minneapolis
City: Minneapolis
Historic Significance: Event/Person
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949
Period of Significance: 1900-1924, 1875-1899
Lock and Dam No. 2
Lohmar, John, House
City: Minneapolis
City: Minneapolis
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1875-1899
Lumber Exchange Building
Madison School
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: NA
Period of Significance: 1875-1899
Period of Significance: NA
Martin, Charles J., House
Masonic Temple
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924
Period of Significance: 1875-1899
Maternity Hospital
Milwaukee Ave Historic District
City: Minneapolis
City: Minneapolis
Historic Significance: Person
Historic Significance: Architecture/Engineering
Period of Significance: 190-1924
Period of Significance: 1875-1899
Minneapolis Armory
Minneapolis Brewing Company
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949
Period of Significance: 1925-1949, 1900-1924,
1875-1899
Minneapolis City Hall -Hennepin County
Minneapolis Fire Department Repair Shop
Courthouse
City: Minneapolis
City: Minneapolis
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1900-1924, 1875-1899
Minneapolis Pioneers & Soldiers Memorial
Minneapolis Public Library, North Branch
Cemetery
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Event
Period of Significance: 1875-1899
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Period of Significance: 1925-1949
Minneapolis Warehouse Historic District
Minneapolis YMCA Central Building
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924,
Period of Significance: 1900-1924
1875-1899, 1850-1874
Minnehaha Grange Hall
Minnehaha Historic District
City: Edina
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899. 1850-1874
Period of Significance: 1900-1924, 1875-1899,
1850-1874, 1825-1849
Minnesota Soldiers' Home Historic District
Minnetonka Town Hall
City: Minneapolis
City: Minnetonka
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 192-1949, 1900-1924.
Period of Significance: 1925-1949, 1900-1924
1875-1899
Moline, Milburn & Stoddard Company
Morse Jr., Elisha & Lizzie, House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899
Period of Significance: 1850-1874
Neils, Frieda & Henry J., House
New Century Mill (Boundary Increase)
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1950-1974
Period of Significance: 1875-1899
New Century Mill (Boundary Decrease)
New Century Mill (Boundary Increase)
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1900-1924, 1875-1899
New Main — Augsburg Seminary
Newell, George R., House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924
Period of Significance: 1900-1924, 1875-1899
Nicollet Hotel
Nokomis Knoll Residential Historic District
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1925-1949, 1900-1924
North East Neighborhood House
Northwestern Bell Telephone Company Building
City: Minneapolis
City: Minneapolis
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1950-1974, 1925-1949,
Period of Significance: 1925-1949
1900-1924
Northwestern Knitting Company Factory
Ogden Apartment Hotel
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Event
Period of Significance: 1900-1924
Period of Significance: 1925-1949, 1900-1924
Old Log Theater
Owre, Dr. Oscar, house
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City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1900-1924
Parker, Charles & Grace, House
Peavey-Haglin experimental Concrete Grain
City: Minneapolis
Elevator
Historic Significance: Architecture/Engineering
City: Saint Louis Park
Period of Significance: 1900-1924
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899
Pence Automobile Company Building
Phi Gamma Delta Fraternity House
City: Minneapolis
City: Minneapolis
Historic Significance: Event/Person
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1925-1949, 1900-1924
Pillsbury Mill
Pioneer Steel Elevator
City: Minneapolis
City: Minneapolis
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899
Period of Significance: 1900-1924, 1875-1899
Pond, Gideon H., House
Prescott House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Person
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1850-1874
Prospect Park Water Tower & Tower Hill Park
Purcell, William Gray, House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924
Period of Significance: 1900-1924
Queene Avenue Bridge
Rand Tower
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924
Period of Significance: 1925-1949
Roosevelt Branch Library
Sanford, Maria, House
City: Minneapolis
City: Minneapolis
Historic Significance: Person
Historic Significance: Person
Period of Significance: 1924-1949
Period of Significance: 1900-1924
Sears, Roebuck & Company Mail -Order
Second Church of Christ, Scientist,
Warehouse & Retail Store
Administration Building
City: Minneapolis
City: Minneapolis
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1950-1974, 1925-1949
Period of Significance: 1925-1949
Semple, Anne C & Brank B., House
Shubert, Same S., Theater
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924
Period of Significance: 1925-1949, 1900-1924
Smith, H. Alden, House
Smith, Leno O., House
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Person
Period of Significance: 1875-1899
Period of Significance: 1925-1949
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South Ninth Street Historic District
St. Anthony Falls Historic District
City: Minneapolis
City: Minneapolis
Historic Significance: NA
Historic Significance: Architecture/Engineering
Period of Significance: NA
Period of Significance: 1925-1949, 1900-1924,
1875-1899, 1850-1874, 1825-1849
State Theater
Station 13 Minneapolis Fire Department
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Event
Period of Significance: 1900-1924
Period of Significance: 1900-1924
Station 28 Minneapolis Fire Department
Stevens Square Historic District
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Event
Period of Significance: 1925-1949, 1900-1924
Period of Significance: 1925-1949, 1900-1924
Stewart Memorial Presbyterian Church
Summer Branch Library
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Person/Event
Period of Significance: 1925-1949, 1900-1925
Period of Significance: 1925-1949, 1900-1924
Swinford Townhouses & Apartments
Thirty -Sixth Street Branch Library
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Event/Person
Period of Significance: 1875-1899
Period of Significance: 1925-1949, 1900-1924
Thompson Summer House
Turnblad, Sawn, House
City: Minnetonka Beach
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924,
Period of Significance: 1925-1949, 1900-1924
1875-1899
Twin City Rapid Transit Company Steam Power
United States Post Office
Plant
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Event
Period of Significance: 1900-1924
Period of Significance: 1925-1949, 1900-1924
University of Minnesota Old Campus Historic
Van Cleve, Horatio P., House
District
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899, 1850-1874
Period of Significance: 1900-1924, 1875-1899
Van Dusen, George W & Nancy B., House
Walker Branch Library
City: Minneapolis
City: Minneapolis
Historic Significance: Architecture/Engineering
Historic Significance: Event/Person
Period of Significance: 1875-1899
Period of Significance: 1925-1949, 1900-1924
Washburn A Mill Complex
Washburn Park Water Tower
City: Minneapolis
City: Minneapolis
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1925-1949
Washburn — Fair Oaks Mansion District
Wesley Methodist Episcopal Church
City: Minneapolis
City: Minneapolis
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His
Historic Significance: Architecture/Engineering
Historic Significance: Architecture /Engineering
Period of Significance: 1900-1924, 1875-1899
Period of Significance: 1875-1899
Westminster Presbyterian Church
White Castle Building No. 8
City: Minneapolis
City: Minneapolis
Historic Significance: Event
Historic Significance: Architecture/Engineering
Period of Significance: 1925-1949, 1900-1924,
Period of Significance: 1925-1949
1875-1899
Wiley, Malcolm., House
Wirth, Theodore, House —Administration
City: Minneapolis
Building
Historic Significance: Architecture/Engineering
City: Minneapolis
Period of Significance: 1925-1949
Historic Significance: Person
Period of Significance: 1925-1949, 1900-1925
Wyer, Allemarinda & James, House
City: Excelsior
Historic Significance: Architecture/Engineering
Period of Significance: 1875-1899
6.3. Hennepin County Historic Landmark Maps
National Historic Landmarks (NHLs) are historic places that possess exceptional value in commemorating
or illustrating the history of the United States. The National Park Service's National Historic Landmarks
Program oversees the designation of such sites. The following Hennepin County sites were designated by
the United States Secretary of the Interior because they met one of the criteria below
• Sites where events of national historic significance occurred.
• Places where prominent persons lived or worked.
• Icons of ideas that shaped the nation.
• Outstanding examples of design or construction.
• Places characterizing a way of life or.
• Archeological sites able to yield information.
TABLE 6.3A Minnesota's National Historic Landmarks- Hennepin County
Mrtinescta'sttc Fist La�nd�% E (r� "r� �Gcur�t
Landmark
Year
Christ Church Lutheran, Minneapolis
1/16/09
Fort Snelling,
12/19/60
Peavey-Haglin Experimental Concrete Grain Elevator, Saint Louis Park
12/21/81
Pillsbury A Mill, Minneapolis
11/13/66
Washburn A Mill Complex, Minneapolis
5/4/83
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Volume 2 — Hazard Inventory
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
THIS PAGE WAS INTENTIONALLY LEFT BLANK
248
2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
CRITICAL INFRASTRUCTURE & CRITICAL FACILITY INDEX (CFI) RANKING
Critical facilities and infrastructure are those that are essential to the health and welfare of the population.
These become especially important after a hazard event. Critical facilities typically include police and fire
stations, schools, and emergency operation centers. Critical infrastructure can also include roads and
bridges that provide ingress and egress and allow emergency vehicles access to those in need, and the
utilities that provide water, electricity, and communication services to the community.
7.1. Critical Facilities Index (CFI) Numbering Scoring System
For this update to the mitigation plan, Hennepin County Emergency Management (HCEM) ranked the
restoration priority of a facility using a score index of 1 to 5, 1 being the most critical to the overall health
of the community. Jurisdiction understand this as those critical facilities within their community that must
operate during times of disaster. The score is identified as an "all -hazards" CFI, which applies to private
and public critical facilities and is directly related to business continuity and continuity of government.
The following are definitions of each score index:
• CFI Priority 1: facility is identified as "critical" to public health, safety. These include Hospitals and
emergency medical facilities, emergency shelters, fire stations, police stations, prisons/jails, fire
rescue facilities, water pumping and wastewater facilities, major communication facilities, major
flood control structures, financial institutions, military installations, and critical electric utility
facilities. If possible, must be operational within 2 hours.
• CFI Priority 2: facility may include some of the same types of facilities described for CFI Priority 1.
These facilities provide significant public services but are deemed to be somewhat less critical by
government agencies. These include Nursing homes, major water and sewer facilities, fire and
police stations, minor flood control structures, fuel transfer/loading facilities (ports), airports,
schools and park facilities used to support other critical government purposes. If possible, must
be operational within 8 hours.
• CFI Priority 3: facility may include some of the same types of facilities described for CFI Priority 2
above. These facilities provide public services but are deemed to be somewhat less critical by
government agencies. These include apartment complexes for the elderly, assisted living
facilities, grocery distribution/large cold storage facilities, local water and sewer facilities, local
fire and police stations, medical service facilities (such as dialysis centers) and facilities having
critical impact on the environment. If possible, must be operational within 48 hours.
• CFI Priority 4: These facilities provide public services but are deemed to be somewhat less critical
by government agencies, and include: supermarkets, banks, gas stations, hotels/motels, and
lodging. If possible, must be operational within 72 hours.
• CFI Priority 5: These facilities provide a public service but are deemed to be less critical that the
other priority tiers.
CFI is used by HCEM with the intent for the coordination of restoration and post disaster economic
re -development and in coordination with infrastructure service providers. This information is intended to
improve communication with local EOCs and other coordination centers during any type of emergency
249
2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan
Volume 2 — Hazard Inventory
event. This scoring system, as well as planning during normal operations, will ensure that community
services are restored in a flexible and coordinated manner.
The following communities participated in the Critical Facilities Index 1-5 priorities risk assessment. Each
community used the 19 hazards in this plan and determined if the hazard affects their pre -identified
priority 1 facilities.
• Bloomington
•
Hopkins
•
Osseo
• Brooklyn Center
•
Independence
•
Plymouth
• Brooklyn Park
•
Long Lake
•
Richfield
• Champlin
•
Loretto
•
Robbinsdale
• Corcoran
•
Maple Grove
•
Rockford
• Crystal
•
Maple Plain
•
Rogers
• Dayton
•
Medicine Lake
•
Saint Anthony
• Deephaven
•
Medina
•
Saint Bonifacius
• Eden Prairie
•
Minneapolis
•
Saint Louis Park
• Edina
•
Minnetonka
•
Shorewood
• Excelsior
•
Minnetonka Beach
•
Spring Park
• Golden Valley
•
Minnetrista
•
Tonka Bay
• Greenfield
•
Mound
•
Wayzata
• Greenwood
•
New Hope
•
Woodland
• Hanover
•
Orono
Each city has two documents in this section.
1. The CFI 1 Facilities Hazard Vulnerability Assessment.
2. The Critical Infrastructure and Key Resources Overview
250
CITY OF ORONO
RESOLUTION OF THE CITY COUNCIL
No. 7478
CITY OF ORONO
b
Dennis Walsh, Mayor
1
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