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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />Moreover, it is not uncommon for a major ice storm to be followed by or transition to a heavy snowfall <br />event or blizzard. In these cases, the ice produces the initial critical loading, but then the snow and/or <br />wind acts as the "final straw," resulting in severe and widespread power outages. In these situations, the <br />snowstorm or blizzard is just another link in a chain of cascading hazards already in progress. <br />Flooding <br />Depending on hydrological and meteorological conditions, ice storms may prime areas for both flash - <br />flooding, and river flooding. Flash -flood scenarios unfold when the glaze of ice is especially thick, <br />temperatures rise to slightly above freezing, and a period of heavy thunderstorms or heavy rain occurs <br />before the ice can melt. Because of ice restricting flow into storm sewers, falling rain can lead to rapid <br />ponding on roads and low-lying areas. If the storm water infrastructure is not obstructed, a heavy glaze on <br />the land will prevent absorption by soils, and will direct falling rain directly into area streams, which may <br />rise rapidly. It should be noted that these scenarios to date are extremely rare, and reports in Minnesota <br />have been highly localized. <br />River flooding can occur after a major ice storm if a large snowpack had been present and/or additional <br />rain falls over a large area. The melted snow would be the initial cause of rising river levels, which would <br />then be exacerbated by rain falling over ice, and to a lesser extent by the melting ice itself. Like flash - <br />flooding, these situations are not common and would require a convergence of many factors. The main <br />risks would occur during the late winter snowmelt period. <br />Severe weather <br />In rare situations, it is possible for ice storms to follow or be followed by a significant severe weather event. <br />November, March, and April are currently the most likely months. Power outages and compromised <br />communications from ice storms may limit situational awareness needed to heed severe weather <br />warnings. A direct hit by a major severe weather event on an area recently affected by an ice storm would <br />further complicate damages and compound clean-up efforts. Similarly, an ice storm following a damaging <br />severe weather event would threaten to worsen the impacts significantly, with additional tree, power, <br />structural, and interior damage possible. <br />4.3.13.5. Geographic scope of hazard Bic <br />Most major ice storms in Minnesota affect thousands to tens of thousands of square miles --generally an <br />area the size of 10-20 southern Minnesota counties. There have been larger events, and ice storms in the <br />central and southern US often cover 50-100 thousand square miles at a time, with total footprint of up to <br />250 thousand square miles in some cases. <br />The State Climatology Office has noted that historically, ice storms have tended to favor higher terrain <br />locations just inland from the north shore of Lake Superior, and along the Buffalo Ridge in southwestern <br />Minnesota. While ice storms have affected every part of Minnesota, these areas have elevated <br />frequencies. <br />4.3.13.6. Chronologic patterns (seasons, cycles, rhythm) <br />GRAPHIC 4.3.13A shows the peak months, historically, for ice storms in Minnesota are January and April, <br />but the main season should be considered November through April. Rare ice storms have occurred in <br />Minnesota in October and May. <br />207 <br />