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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />2020 derecho in Iowa revealed that while the storm would not necessarily have produced stronger winds <br />in a warmer world, the likelihood of a stronger nearby heat wave would have allowed the damaging winds <br />to cover more area and last longer. <br />Another investigation of extreme straight-line wind occurrences showed an observed increase both their <br />intensity and their areal coverage in the United States as the climate has warmed and theorized a 7.5% <br />increase in intensity for each additional degree C (1.8 degrees F) of warming. <br />Similarly, a study of a lethal 2022 Mediterranean derecho showed that the marine heat wave in its vicinity <br />that helped fuel it was itself made substantially more likely and more intense by rising global <br />temperatures. This marine heatwave contributed substantial intensity and wind energy to the <br />thunderstorm complex, which simulations showed would have been of "ordinary" strength in the absence <br />of climate change. <br />Taken together, these studies suggest that the changing climate can make extreme straight-line <br />thunderstorm winds and derechos larger, longer lasting, and in some cases, more intense. As the climate <br />continues warming, therefore, a given extreme straight-line wind event may be more likely to affect <br />Hennepin County and neighboring areas. <br />4.3.3.9. Indications and Forecasting <br />National responsibility for developing tornado indications and forecasts rests with the National Weather <br />Service's Storm Prediction Center (SPC) in Norman, Oklahoma, and the local National Weather Service <br />office in Chanhassen. <br />4.3.3.10. Critical Values & Thresholds <br />Winds in a derecho must meet the National Weather Service criterion for severe wind gusts (greater than <br />57 mph) at most points along the derecho path. Most other extreme straight-line wind events are well <br />above this threshold as well. In stronger derechos, winds may exceed 100 mph. <br />Based on current warning criteria and analysis of local and regional storm events, the following thresholds <br />apply: <br />• 58+ mph: Entry level for "severe." Some damage to trees and powerlines. <br />• 70+ mph: outdoor warning sirens activated in Hennepin County; significant tree and electrical <br />infrastructure damage, with structural damage possible. <br />• 80+ mph: Wireless Emergency Alerts (WEAs) triggered; structural and vehicular damage likely; <br />risks from airborne debris. <br />• 100+ mph: tornado-like damage expected, with secondary damage from debris -bombardment. <br />4.3.3.11. Preparedness <br />Hennepin County Emergency Management employs meteorologists who monitor the potential for <br />extreme straight-line winds and communicate with an array of county personnel as conditions warrant. <br />Those planning to be outdoors for a significant length of time must be aware of the weather forecasts, <br />especially if well -removed from sturdy shelter. Preparation means staying "connected" via television, <br />radio, NOAA Weather Radio, or social media. Extreme straight-line winds rarely occur without warning, <br />92 <br />