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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />4.1.3.6. Chronologic Patterns <br />Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored soil frost <br />hazard assessment in 2020. <br />4.1.3.7. Historical Data Bld <br />4.1.3.7.1. Comprehensive. Hennepin County Emergency Management (HCEM) has not yet <br />systematically investigated historical records of local frost depth. Precise frost measurements <br />using frost tubes or other sensors are unlikely to have been conducted anywhere in Hennepin <br />County prior to the HCEM program which started in 2015. The nearest historic soil frost records <br />are probably measurements taken at the University of Minnesota, Saint Paul campus. These St. <br />Paul records are for frost under sod. It is possible that written historical accounts of frost depth <br />and their effects might be found in records of municipal utility providers. These records, if <br />discovered, would probably be for frost under pavement which impacted water lines and other <br />utilities. <br />4.1.3.7.2. Winter of 2013-2014. The coldest Hennepin County winter since 1978-1979 occurred <br />in 2013-2014 with a sustained three-month cold snap. The mean temperature for the months of <br />December, January and February was 9.81F degrees at MSP airport. The normal for this time period <br />is 18.71F degrees. More snow fell than average during the period as well (57.2 inches three-month <br />total). Most of it fell late in the period. Frost was pushed much deeper than average. Anecdotal <br />reports by public work crews working on frozen water service lines reported frost as deep as 7 to <br />8 feet in Plymouth. Twelve cities, not including Minneapolis, provided information regarding <br />service interruptions. In these cities were a total of 324 water freeze up incidents, mostly service <br />lines. In addition, 1 hydrant froze, 2 water mains, and 4 sewer lines also became frozen. The <br />longest outages were over one week. Residences, businesses, care facilities, and government <br />buildings were impacted. In several instances, cities had to distribute bottled water to affected <br />residences. <br />There have been no other naturally caused incidents that are within the scope of this plan. <br />4.1.3.7.3. Pre -Historic Evidence: <br />Unknown. HCEM has not found any research regarding pre -historic frost depth in Hennepin <br />County. <br />4.1.3.8. Future Trends Ble <br />Undetermined. Climate change is having a significant impact on Minnesota and Hennepin County. Forces <br />generated by climate change are sometimes at odds over the net effect experienced in this area during <br />any winter. For instance, there has been an overall warming trend in Minnesota winters, including a <br />shorter winter season and higher average temperatures. More recently, prolonged outbreaks of extreme <br />cold air have impacted Minnesota and Hennepin County. These include the winter of 2013-2014 and the <br />winters of 2016-2017 and 2017-2018. These cold outbreaks appear to be related to warming in the Arctic <br />that has weakened the Polar Jet Stream. The weakened jet stream is less able to contain cold Arctic air in <br />high latitudes and block it from streaming south. Some scientists theorize that prolonged outbreaks of <br />extreme cold polar air may be a recurring feature of future winters in Minnesota. When coupled with low <br />or no -snow cover conditions, outbreaks of extreme cold may push frost deeper into the soil. <br />25 <br />