Initial analysis shows Wednesday severe weather meets derecho criteria

Early information shows the storm system this week in Iowa was a derecho.
Published: Dec. 16, 2021 at 7:21 PM CST|Updated: Dec. 16, 2021 at 7:35 PM CST
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CEDAR RAPIDS, Iowa (KCRG) - Meteorologists analyzing data from Wednesday’s severe weather outbreak said preliminary information shows the system that moved across the Midwest was a derecho.

Many eastern Iowans associate the weather phenomenon with the August 10, 2020 derecho that tore across several states leaving billions of dollars damage in its wake.

“In the most basic sense, a derecho is a widespread and long-lived wind storm associated with a line of thunderstorms,” said First Alert Storm Team Meteorologist Jan Ryherd. “To officially be categorized as a derecho, winds must be 58 m.p.h. or greater, extending for 240 miles or more.”

Ryherd said that early data shows Wednesday’s line of severe weather met both the wind and distance criteria but adds the storms were far different than the August 2020 derecho.

“Wednesday’s storms are being initially classified as a serial derecho. Whereas the August 2020 event was a progressive derecho,” Ryherd explained.

Meteorologists define serial derechos as an extensive line of storms with multiple bow echoes - or curved line of storms with strong winds - which give it a scallop-shaped appearance. Serial derechos are a line of storms that move based on the direction of the winds produced by the larger storm system, a low-pressure system in Wednesday’s case.

Ryherd said the environment in serial derechos is more favorable for the development of tornadoes. At least 12 tornadoes touched down in Iowa Wednesday.

The August 2020 derecho is categorized as a progressive derecho; narrow in path but travel a long distance. They advanced as additional storms develop ahead of the initial cluster of storms.

“Progressive derechos are generally seen in the summer,” Ryherd said.

Climatology data shows that the state of Iowa is impacted by a derecho every one to two years.

“Derechos are nothing new to hit Iowa but the term really wasn’t commonly known until the horrible experience the state went through in August 2020,” Ryherd explained. “Climatologically speaking, they do happen regularly. But the 2020 event was unique in intensity and was unlike anything experienced before.”

Ryherd said more analysis will be done on this week’s system to help experts better understand and predict the phenomena.

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