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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />�Hazard Assessment: WIND, EXTREME STRAIGHT-LINE <br />4.3.3.1. Definition <br />Extreme straight-line winds are thunderstorm winds <br />that exceed 70 mph and can reach or exceed 100 <br />mph. Along with damage potential to trees, power <br />lines, vehicles and structures, these winds pose risks <br />to life and safety. <br />Most thunderstorms produce gusty winds from <br />downdrafts of air flowing from the tops of the storm. <br />Some thunderstorms produce winds of 58 mph or <br />stronger, officially making them "severe" by National <br />Weather Service standards. <br />Occasionally, severe thunderstorms will produce <br />destructive winds that far exceed the 58-mph <br />threshold. These winds are often referred to as "straight-line winds," to differentiate them from the <br />cyclonic, turning winds of a tornado. Extreme straight-line winds can indeed produce tornado-like <br />damage. <br />Extreme thunderstorm winds can be highly localized, or widespread along an arc of storms extending <br />dozens of miles or concentrated locally in numerous individual cells within a line or cluster of storms. The <br />duration of straight-line winds at a given location can be as brief as 30 seconds or can last upwards of 30 <br />minutes. The storms producing the extreme winds may cover just 30 miles, or they may track for hours <br />and cover hundreds of miles. <br />The latter case represents an important class of extreme thunderstorm winds called "derechos." A <br />Derecho is an extreme, widespread, and long-lived windstorm, usually associated with bands of rapidly <br />moving showers or thunderstorms variously known as bow echoes, squall lines, or quasi -linear convective <br />systems. If the swath of wind damage extends for more than 240 miles, includes wind gusts of at least 58 <br />mph along most of its length, and several, well -separated 75 mph or greater gusts, then the event may be <br />classified as a derecho. <br />In general, derechos follow two basic types: Progressive Derechos tend to form on the northern edge of a <br />steamy air mass, and the derecho is usually associated with one primary, very intense thunderstorm cell <br />that follows the boundary of the hot air. These derechos have the greatest potential for catastrophic <br />damage, and given enough instability, there is almost no limit to the intensity of their thunderstorm winds. <br />Serial Derechos, by contrast, tend to form to the west of warm and unstable air masses, often along cold <br />fronts, and often in the presence of very fast winds aloft. These instances lead to long, arcing, fast-moving <br />lines of storms with many different cells, any of which can harness the strong winds aloft and produce <br />damaging winds. These derechos can produce widespread damage because of all the "candidate" storm <br />cells, but they generally lack the destructive potential of progressive derechos. <br />Hennepin County has been affected by numerous extreme straight-line windstorms, including derechos. <br />Every decade from the 1950s through the 2010s had multiple extreme thunderstorm wind events within <br />81 <br />