What classifies a storm as a derecho?

DES MOINES, Iowa – Monday night’s storms brought strong winds, hail, and even a few tornadoes to central Iowa. The Storm Predication Center later classified that complex of storms as a derecho. But what makes a storm a derecho?

A complex of thunderstorms (or a singular, powerful thunderstorm) can be classified as a derecho if the storm has:

• A track of 240 to 400+ miles
• A width of 60+ miles
• Wind gusts of at least 58 mph along the entire path of the storm
• Several 75 mph wind gusts or greater

However, classifying a storm as a derecho isn’t that easy. A paper published in 2016 proposed changing the definition of a derecho from a path of at least 240 miles to 400 miles or more.

At the moment, the Storm Prediction Center makes the decision as to whether a storm meets derecho criteria, not local National Weather Service offices, according to National Weather Service meteorologist Brooke Hagenhoff in Des Moines.

It’s also important to point out that a storm is classified as a derecho after it has happened. Since derecho classification is based on storm reports, it’s not possible to consider a storm a derecho as it’s happening or before the storm has occurred.

This criteria notes that not every derecho must bring significant winds like the infamous 2020 derecho in Iowa. Most derechos bring winds around regular severe thunderstorm criteria — 58 mph or more. Typically, there are at least a few significant wind reports.

At the end of the day, defining a derecho is more about path length than extreme winds. So if a powerful thunderstorm hits your house, it doesn’t really matter if it was classified as a derecho or not. A storm classified as a derecho will just end up impacting more people.

Whether a storm ends up as a derecho, it’s always important to heed weather warnings and to stay weather aware.

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