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WEATHER LAB | What are derechos?

Four derechos have impacted Iowa in the last two years, including the most recent on July 5, 2022.

DES MOINES, Iowa — Ever since a powerful derecho tore across Iowa on August 10, 2020, the term seems to pop up more frequently than in the past.

The word "derecho" is not new. It was first used by Dr. Gustavus Hinrichs, a University of Iowa physics professor, in 1888. 

Since 2020, though, more people are aware of derechos. 

According to the Storm Prediction Center, central Iowa averages one derecho every two years. They are more common south and east of the state. 

Although the 2020 derecho is the most memorable, three other derechos have impacted Iowa since then.

On December 15, 2021, a rare late fall severe weather outbreak brought at least 19 tornadoes to the state, and caused extensive wind damage and led to more than 100,000 power outages. 

An evaluation from the Storm Prediction Center and National Weather Service determined this storm matched the criteria for a derecho.

Another derecho hit northwest Iowa on May 12, 2022, which many recall from the images and videos of a rare 'haboob' moving through the region.

RELATED: WEATHER LAB | What are haboobs?

Most recently, a derecho affected parts of northern, central and eastern Iowa on July 5, 2022. 

RELATED: Yes, Tuesday's storm was a derecho

What is a derecho?

A thunderstorm complex must reach specific criteria in order to be labeled a derecho:

  • A swath of wind damage extending at least 250 miles
  • 58 mph or greater wind gusts along most of its length
  • Include several, well-separated 75 mph or greater wind gusts
  • The damage path also needs to be greater than 50 miles wide

In general terms, a derecho is a long-lived line of thunderstorms that produces intense wind gusts over a large area.

RELATED: $1.4M granted to help 2020 derecho survivors find legal help

Progressive derechos, like the ones we saw on August 10, 2020, and July 5, 2022 usually begin as one thunderstorm or as a small cluster of thunderstorms.

They typically strengthen when they move through areas of high instability, making them most common in the summer.

Unlike tornadoes, the National Weather Service does not officially confirm derechos.

Data shows 22% of all derechos nationwide occur in May, 20% occur in June, and 21% occur in July. Less than 10% of all derechos occur in the other months of the year. 

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