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<br /> <br />2021 Multijurisdictional Local Hazard Mitigation Plan <br /> <br />14.1.7 Windstorms <br />Windstorms are generally short-duration events involving straight-line winds or gusts of over 50–60 mph, strong <br />enough to cause property damage. Damage from such winds accounts for half of all severe weather reports in the <br />lower 48 states. Wind speeds can reach up to 100 mph and can produce a damage path extending for hundreds of <br />miles. The Beaufort Wind Chart (Table 14-2) provides terminology and a description of potential wind impacts at <br />different levels (National Severe Storms Laboratory, 2018). <br /> <br />Table 14-2. Beaufort Wind Chart <br />Beaufort <br />Number <br />Range <br />(mph) <br /> <br />Terminology <br /> <br />Description <br />0 0 Calm Calm. Smoke rises vertically. <br />1 1-3 Light air Wind motion visible in smoke. <br />2 4-7 Light breeze Wind felt on exposed skin. Leaves rustle. <br />3 8-12 Gentle breeze Leaves and smaller twigs in constant motion. <br />4 13-18 Moderate breeze Dust and loose paper is raised. Small branches begin to move. <br />5 19-24 Fresh breeze Smaller trees sway <br />6 25-31 Strong breeze Large branches in motion. Whistling heard in overhead wires. Umbrella use is difficult. <br />7 32-38 Near gale Whole trees in motion. Some difficulty when walking into the wind. <br />8 39-46 Gale Twigs broken from trees. Cars veer on road. <br />9 47-54 Sever gale Light structure damage. <br />10 55-63 Storm Trees uprooted. Considerable structural damage. <br />11 64-73 Violent storm Widespread structural damage. <br />12 74-95 Hurricane Considerable and widespread damage to structures. <br />Source: Lewis, 2018 <br />There are seven types of damaging winds: <br /> Straight-line winds—Any thunderstorm wind that is not associated with rotation; this term is used <br />mainly to differentiate from tornado winds. Most thunderstorms produce some straight-line winds as a <br />result of outflow generated by the thunderstorm downdraft. <br /> Downdraft—A small-scale column of air that rapidly sinks toward the ground. <br /> Downburst—A strong downdraft with horizontal dimensions larger than 2.5 miles resulting in an <br />outward burst or damaging winds on or near the ground. Downburst winds may begin as a microburst and <br />spread out over a wider area, sometimes producing damage similar to a strong tornado. Although usually <br />associated with thunderstorms, downbursts can occur with showers too weak to produce thunder. <br /> Microbursts—Microbursts are small concentrated downbursts that produce an outward burst of <br />damaging winds at the surface. Microbursts are generally less than 2.5 miles across and short-lived, <br />lasting only 5 to 10 minutes, with maximum wind speeds up to 168 mph. There are two kinds of <br />microbursts: wet and dry. A wet microburst is accompanied by heavy rain at the surface. Dry microbursts, <br />common in places like the high plains and the intermountain west, occur with little or no precipitation <br />reaching the ground. <br /> Gust front—The leading edge of rain-cooled air that clashes with warmer thunderstorm inflow. Gust <br />fronts are characterized by a wind shift, temperature drop, and gusty winds out ahead of a thunderstorm. <br />Sometimes the winds push air above them, forming a shelf cloud or detached roll cloud. <br /> <br /> <br /> <br /> <br />14-6