Justin Glisan | January 18, 2021

Aug. 10, 2020 will go down as a significant weather date in state history. A derecho, which is a convectively (thunderstorm) initiated straight-line windstorm, propagated through Iowa’s central west-to-east corridor. The term “derecho” was coined by Dr. Gustavus Hinrichs at the University of Iowa in the late 1800s and is derived from a Spanish word that can be interpreted as “direct” or “straight-ahead.” Formed in the early morning hours in southeast South Dakota, the line of thunderstorms moved across the Nebraska border into Iowa where it significantly strengthened east of Carroll, Iowa, as downbursts formed. Downbursts are key for the formation of low-level, strong straight-line winds; moist air high up in a thunderstorm interacts with surrounding drier air, forcing atmospheric water vapor to evaporate fast. Rapid evaporation cools the air producing a relatively large volume of cold, dense air. These bubbles of dense air drop rapidly, hit the surface and spread out, creating straight-line winds that can produce widespread damage. As the derecho entered central Iowa, the center of the line pushed out creating a bow echo; this feature indicated rapid strengthening as downburst clusters became more numerous. The system expanded north and south as it moved through east-central Iowa where a broadening swath of damage was found in satellite images. The derecho held together for 770 miles and over 14 hours before losing strength as it entered western Ohio.

Damage to crops, grain bins and structures was catastrophic. The derecho also moved over the D3 (Extreme Drought) region in west-central Iowa, producing agricultural damage to already stressed corn and soybeans. USDA Risk Management Agency (RMA) data indicated that around 8.2 million acres of corn and 5.6 million acres of soybeans across 57 counties may have been impacted by the derecho. Urban areas from Des Moines, Cedar Rapids and the Quad Cities reported substantial and long-lasting power outages along with severe damage to trees and structures from extremely strong, sustained winds. Recorded wind gusts along the derecho’s path ranged from 58 mph to well over 100 mph; according to the National Weather Service, “maximum recorded wind speeds were around 110 mph over portions of Benton and Linn Counties in eastern Iowa.” A personal weather station in Atkins (Benton County) reported a gust of 126 mph.