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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />Hazard Assessment: WINDS, NON -CONVECTIVE HIGH <br />4.3.12.1. Definition <br />Non -convective high winds are rare, long-lasting, <br />sustained events that can pose significant life <br />safety risks and produce widespread damage over <br />a large area, while originating from sources <br />unrelated to thunderstorms (i.e., not related to <br />tornadoes or thunderstorm downbursts). In the <br />Upper Midwest and most of the US, they form in <br />association with intense and/or rapidly intensifying <br />mid -latitude cyclones (low pressure systems). <br />"Wake lows" developing behind thunderstorms <br />have been observed to produce relatively <br />prolonged bouts of non -convective strong winds in <br />Minnesota --sometimes resulting in damage-- but <br />these events are best considered within the <br />spectrum of consequences and cascading effects <br />resulting from derechos and other severe <br />thunderstorms events. <br />The most common scenario in Minnesota, occurring 1-3 times per year on a statewide basis, is for a <br />prolonged (multi -hour) period of sustained 30-45 mph winds, with frequent gusts to 60 mph, and isolated <br />gusts as high as 70 mph. These events tend to result in sporadic minor structural damage, and occasionally <br />cause isolated injuries or even deaths. <br />A more dangerous class of events occurs roughly once or twice per decade in Minnesota, and produces a <br />pocket of enhanced wind speeds, often sustained above 45 mph for several hours, with gusts exceeding <br />hurricane force. These events produce massive wind loadings that can result in significant infrastructural <br />and property damage, and the most extreme among them yield death and injury rates that resemble <br />those of tornado outbreaks. <br />Unfortunately, the meteorological differences between these two classes of events are quite subtle, and <br />identifying the potential for the higher -impact extreme cases remains a forecasting challenge. In fact, <br />every instance of them on record in the Upper Midwest has been under -forecast, in some cases <br />significantly. Like derechos, there is no specific National Weather Service warning product for them. Most <br />events in Minnesota have occurred during High Wind Warnings, within lower -priority Wind Advisories, <br />and even during Blizzards Warnings. Those latter cases will be considered under Blizzards and will be <br />discussed only briefly here. <br />Further complicating matters, no standardized database or method for cataloging non -convective <br />extreme winds exists. Therefore, precise statistics on areal extent, duration, and total impact are lacking. <br />4.3.12.2. Range of magnitude <br />Maximum event (Hennepin): measured gust 89 mph at MSP on October 10, 1949 <br />Maximum event (non -Hennepin): measured 100 mph at Rochester on October 10, 1949 <br />Maximum duration: 36 hours, Wisconsin, October 26-27, 2010 <br />185 <br />