Definition
A widespread, long-lived windstorm associated with a fast-moving line of severe thunderstorms. To be classified as a derecho, the storm must produce damaging straight-line winds along a path of at least 240 miles, with wind gusts of 58 mph or greater along most of its length, and several gusts of 75 mph or more.
Plain English
A huge, fast-moving line of thunderstorms that produces destructive straight-line winds (not tornadoes) over a long, continuous path.
Context Anchor
Pilots may see this term in preflight weather briefings, severe weather forecasts, radar summaries, and forecast discussions.
Derivation
From the Spanish word 'derecho,' meaning 'straight' or 'direct.' The name was chosen to contrast with 'tornado' (which involves rotating winds) -- a derecho's damage runs in a straight line along the storm's track.
Why Pilots Care
Produces damaging surface winds and possible embedded microbursts that can affect aircraft on the ground, during takeoff or landing, and at low altitudes.
Grounding Statement
Picture a fast-moving wall of thunderstorms pushing a broad burst of damaging wind across many miles.
Intuition Check
A derecho is not a tornado and not a hurricane. The key idea is widespread, damaging, mostly straight-line wind from thunderstorms.
Example Sentence 1
Convective SIGMETs warned of a developing derecho moving east across Iowa, prompting the flight crew to divert south of the storm line.
Example Sentence 2
Derecho winds can reach 80 to 100 mph and cause widespread damage to airport facilities and parked aircraft.