A derecho (from the Spanish word for "straight") is a widespread, long-lived straight-line windstorm associated with a fast-moving band of thunderstorms (typically a bow echo or squall line). To be classified as a derecho, wind damage must extend over a path at least 400 km long with wind gusts of 93 km/h (58 mph) or more along most of its length. Derechos combine the wind power of a hurricane with the speed of a fast-moving cold front.
The driving mechanism is the rear-inflow jet: strong winds at mid-levels behind the squall line accelerate downward via microburst-like downdrafts, hitting the surface and spreading outward. The outflow boundary acts as a miniature cold front, lifting warm, moist air ahead of the line and sustaining new thunderstorm development — this self-propagating mechanism allows derechos to travel hundreds to over 1,000 km in 12–24 hours at speeds of 60–130 km/h.
Derechos are most common in the central and eastern United States during summer, but they occur in Europe as well — including notable events in France, Germany, Poland, and occasionally Spain. The June 2022 derecho in France and Spain caused widespread damage across Corsica and the Balearic Islands. Unlike tornadoes, derecho damage is characterised by trees and structures blown down in a consistent direction (hence "straight"). Forecasting derechos is challenging because they develop rapidly from seemingly ordinary squall lines; key ingredients include a hot, humid air mass, moderate-to-strong mid-level winds, and a triggering mechanism. Radar reveals the characteristic bow echo pattern — a crescent-shaped line of strong reflectivity indicating the accelerating outflow.