What is a derecho?

KELOLAND.com Original

SIOUX FALLS, S.D. (KELO) — In KELOLAND, there doesn’t need to be a tornado for wind damage. 

Damaging straight-line winds are from any storm that doesn’t have any associated rotation, according to KELOLAND Meteorologist Adam Rutt. Tornados rely on rotating air, from up and down drafts. 

For severe wind, the wind speeds need to reach 58 mph or higher for severe thunderstorms. The 58 mph threshold was set in 1970 when it combined previously separated public (75 mph) and aviation (50 mph) wind thresholds. 

KELOLAND Meteorologist Brian Karstens said within severe thunderstorms, “intense belts of wind can create a swath a few miles wide to cause property damage in concentrated areas.”

For some thunderstorms, heavy rain can be the main threat. In others, hail or strong winds are the biggest threat. In more rare instances, thunderstorms can develop into derechos. 

Rutt said derechos form from long-lasting thunderstorms with multiple micro bursts and down busts. The damage swath needs to be at least 250 miles in length to be considered a derecho. 

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In Aug. 2020, the state of Iowa was hit by a derecho, causing billions of dollars in damage. Karstens said the term derecho was a term first used by an Iowa weather professor to describe a storm from July 31, 1877. 

Derecho events are not common events, happening only once every two or four years for some areas of KELOLAND. 

Karstens said the most recent derecho in South Dakota was June 11, 2017 in the Brookings and Watertown areas and a similar event occurred in Garretson on June 22, 2015. The damage looks like a tornado, but Karstens stressed it’s just straight-line wind doing the damage. 

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