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Severe weather awareness week: What is a derecho?

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(WAOW) — We are all familiar with tornadoes, but what is a derecho and what threat does it pose?

Many of you might remember the derecho that struck Iowa last Summer. It ended up as one of the costliest weather disasters of 2020, destroying some infrastructure and a lot of the corn crop in central and eastern Iowa. Normal thunderstorms with high winds can do this as well, so what makes a derecho different?

A derecho is a more expansive and long-lived storm. Whereas a typical severe thunderstorm might produce strong wind over a several mile area for a few minutes, a derecho usually lasts for a few hours, travels several hundred miles, and creates a damage path up to a hundred miles wide.

The official definition classifies a storm as a derecho if it travels at least 250 miles and has periodic wind gusts as high as 75 mph or greater which can cause damage similar to that of tornadoes.

A derecho forms when a line of thunderstorms, often called a squall line, develops a more permanent structure. One significant feature that helps in this development is a “rear inflow jet”. If there are persistent strong winds in the mid level of the atmosphere (jet stream-type winds), these can be drawn into the line of storms and cause the damaging winds at ground level often associated with derechos.

Derechos most often happen in the Midwest of the U.S. during warm season months of May, June, and July, and we average one significant derecho every other year in Northcentral Wisconsin.

All thunderstorms can be dangerous, but when you hear the word derecho, you will want to pay extra close attention. Read more about derechos here.

Justin Loew

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