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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />Consultation of all available research suggests that extreme non -convective winds have a frequency like <br />high -end tornado events, with recurrence intervals on the order of multiple decades within Hennepin <br />County. <br />4.3.12.9. Indications and Forecasting <br />Forecasting authority for non -convective high wind events rests with local National Weather Service <br />forecast offices. High -intensity mid -latitude cyclones are usually well anticipated by the numerical <br />weather prediction models. As a result, forecasters tend to have high awareness of potentially strong <br />winds 2 days or more before they develop. In ideal situations, progression of NWS products used will <br />include a Hazardous Weather Outlook, High Wind Watch, and High Wind Warning. In some cases, <br />damaging and even deadly winds have arisen within Wind Advisories. <br />Despite high awareness of strong regional wind potential, most non -convective high wind events in the <br />region, and all extreme events, have been under -forecast. As a result, the impacts have come as surprises. <br />An after -action report from the disastrous 1949 event concluded that forecasters had "little evidence by <br />which the severity might have been forecast." Although forecasting techniques have improved <br />dramatically since that time, underestimation is still a concern. The November 10, 1998, event forecast <br />products made no mention of winds exceeding 65 mph, yet there were dozens of separate instances of <br />winds exceeding 80 mph throughout the region. Even the lower -impact, October 2010 event had dozens <br />of gusts exceeding the maximum thresholds named in forecast products. The forecasting challenges arise <br />from a combination of low event frequency, low priority (when compared with other hazards), and limited <br />understanding of the latest research. <br />Recently, mechanisms contributing to cyclone -related, non -convective extreme winds have become <br />better understood. Events with extreme winds share the following commonalities: <br />Intense cyclone. The strongest 5% of cyclones in the Upper Midwest have minimum sea -level <br />pressure of 980 millibars or lower and produce strong regional winds. Both the likelihood and <br />coverage of high and extreme winds increase as the minimum pressure drops, with 972 millibars <br />serving as a threshold below which both are almost guaranteed. <br />The first indicator that extreme winds are possible is the forecast of a sub-980 millibar <br />cyclone within the region. The lower the forecast minimum pressure, the greater the <br />potential for impacts. Potential can be ascertained several days in advance. <br />Cyclone passes north or northwest of area. Although non -convective strong and high winds can <br />be distributed widely throughout the cool side of any intense cyclone, the most extreme winds <br />tend to be found to the south of the center of low pressure, especially in cyclones whose <br />minimum pressure is below 972 millibars. This is most likely within 300 miles of the cyclone, but <br />distances vary depending on the circulation structure. For example, the October 1949 event had <br />its maximum impact area 150-300 miles southeast of the low, versus 25-150 miles to the south <br />of the low in the November 1998 event. <br />The second indicator that extreme winds are possible is if the sub-980 millibar cyclone is <br />forecast to pass northwest or north of the area. The nearer the cyclone (to the <br />north/northwest), the greater the potential for impacts, especially if the minimum <br />pressure is forecast below 972millibars. Potential can be ascertained 1-3 days in advance. <br />199 <br />