Definition

A derecho is a widespread, long lived, straight line wind storm that is associated with a fast moving group of severe thunderstorms known as a mesoscale convective system. 

Although a derecho can cause destruction similar to the strength of tornados, the damage typically is directed in one direction along a relatively straight swathe. As a result, the term "straight-line wind damage" sometimes is used to describe derecho damage. By definition, if the wind damage swathe extends more than 240 miles (about 400 km) and includes gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho.