A derecho is a long-lived straight-line windstorm that is part of a line of powerful thunderstorms.
Derechos can produce winds that are equivalent or exceed hurricane force winds. The complex often looks like a bow, or backward shaped C, on weather radars.
The power in a derecho come from downbursts of air over a large area, according to the National Weather Service. When the wet air in a thunderstorm meets drier air, the water in the air evaporates. When the evaporation occurs, it cools the air around it. Since cool air is more dense, it rapidly sinks to the ground causing strong winds.
The Aug. 10 storms traveled hundreds of miles, developing in northern Nebraska and southeast South Dakota during the morning hours and quickly grew as they moved into Iowa. The storms reached severe status in western Iowa and damaging winds developed as they reached the Des Moines area. The line of storms tapped into a very unstable atmosphere over Eastern Iowa and began producing a much wider swath of destructive winds as it passed through Cedar Rapids around 12:30 p.m.
An official wind gust of 99 mph was recorded at the Marshalltown Airport, while a weather observer reported an unofficial wind gust of 106 mph at LeGrand, the National Weather Service’s Des Moines office reported. Linn County Emergency Management reported a 112 mph gust near Hiawatha.
07:12PM | Tue, September 22, 2020
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