What is a derecho?

A derecho is a widespread, fast-moving windstorm produced by a line of severe thunderstorms. To qualify, the wind damage swath must extend more than about 400 miles with wind gusts of at least 58 miles per hour along most of its length and several gusts above 75. The damage comes from straight-line winds descending out of the storm and spreading outward, not from rotation, which is the key difference from a tornado. The word is Spanish for straight, chosen in 1888 by an Iowa scientist precisely to contrast with tornado.

The practical distinction matters for what the damage looks like and how you respond. Tornado damage is narrow, concentrated, and often shows debris scattered in converging directions from the rotation. Derecho damage runs in a broad corridor with trees and structures pushed consistently one way, and it can knock out power to millions across several states in a single afternoon. The August 2020 Midwest derecho crossed roughly 770 miles in 14 hours, flattened an estimated 10 million acres of corn in Iowa alone, and caused damage in the billions. Derechos are most common in the late spring and summer across the corn belt and the Ohio Valley, and because they arrive with the speed of a squall line rather than the visible funnel of a tornado, people frequently underestimate them until the wind arrives.