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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />Thunderstorm and Tornado warnings, in that they are not issued in advance of the parent thunderstorm(s), <br />but instead after the storm has begun, ideally in advance of the flash -flooding itself. The behavior of <br />approaching storms is erratic enough that pre -storm lead time for flash -flood warnings would lead to high <br />false alarm rates. <br />Flash Flood Warnings are issued as polygons that attempt to match the spatial extent of the true threat <br />(as opposed to covering entire counties). Like Severe Thunderstorm warnings, they may cover slivers of <br />counties, or multi -county swaths. The warning period depends on the duration of the event itself, but <br />Flash Flood Warnings may continue for several hours after the precipitation has subsided. <br />4.3.6.10. Detection & Warning <br />The Chanhassen NWS Office and North Central River Forecast Center (adjoining the Chanhassen office) <br />monitor local flood conditions using a combination of manual and remotely sensed information. Key <br />warning detection and decision sources include but are not limited to: <br />• Radar -estimated precipitation, which can be used in conjunction with flash flood guidance values <br />to determine flood potential. <br />• Automated, real-time stream gaging, which indicates the level and flow of critical streams. <br />• Real-time, manual, or automated rainfall reports <br />• Radar and local meteorological trends, indicating potential for storms to continue and/or <br />redevelop in or near affected areas. <br />• Reports from spotters, emergency managers, first responders, the media, and the public <br />• Images or videos shared via social media or other means. <br />The Chanhassen NWS Office will issue a Flash Flood Warning if the forecasters determine that information <br />from the above and other detection sources indicate that flash flooding is occurring or is imminent in each <br />area. <br />4.3.6.11. Critical values and thresholds <br />Unlike other weather hazards, Watch and Warning thresholds for flash floods vary with the pre-existing <br />meteorological conditions. Conditions with saturated soils and high or overtopped streams require <br />substantially less precipitation to generate flash -flooding than conditions with low soil moisture and low <br />stream levels. Although some anticipated precipitation amounts may suggest to forecasters that flash <br />flooding is possible, irrespective of soil conditions, the Watch and Warning thresholds are generally <br />determined on a case -by -case basis, by considering the Flash Flood Guidance for the area(s) of concern. <br />Flash Flood Guidance (FFG) values estimate the average amount of rainfall (in inches) for given a duration <br />required to produce flash flooding in the indicated county or area. These values are based on a <br />combination on current soil moisture conditions and land cover considerations, and therefore change in <br />response to the local hydro -climatic situation. Throughout much of Hennepin County, and especially in <br />urban areas, less rainfall is required to produce flash flooding than in many neighboring areas, because of <br />the county's high concentration of impervious surfaces. <br />128 <br />