Storm science

What is a derecho

A derecho is a fast-moving band of extreme straight-line winds that can travel over 1,000 miles. In damage it rivals a tornado — but the winds go one direction.

The technical definition

A derecho (deh-RAY-cho, Spanish for 'straight') meets four criteria:

Wind damage swath at least 400 miles long.
Multiple gust reports of 58+ mph within the swath.
At least 3 gusts of 75+ mph or F1+ damage.
No more than 3 hours between successive wind reports.

The name was coined in 1888 by University of Iowa meteorologist Gustavus Hinrichs.

How derechos form

A cluster of thunderstorms organizes into a Mesoscale Convective System (MCS).
The MCS develops a bow-shaped leading edge — a "bow echo."
The bow accelerates as cold downdraft air surges outward at the surface.
A rear-inflow jet (upper-level winds punching in behind the storm) accelerates the winds further.
The bow may develop a "book-end" vortex, with rotation at each end.
Damage swath grows as the bow races across states.

The types

Serial derecho

Multiple bow echoes along a squall line. Widespread damage over multiple states.

Progressive derecho

Single bow echo racing along. Concentrated damage swath.

Warm-season derecho

May-August. Most common. Fueled by high instability.

Cool-season derecho

Rare. Winter/early spring. Fueled by strong shear.

Notorious derechos

1980 More Trees Down derecho — 3,000+ miles of damage.
1995 Right Turn derecho — one of the strongest ever.
2009 Super Derecho — May 2009. Kansas to Kentucky. 22 dead.
2012 June derecho (Ohio-DC) — 22 dead, millions without power for days.
2020 Iowa derecho — Aug 10, 2020. 140 mph gusts. $12.5B damage. Cornfield swathed 400 mi wide.
2022 Dust Bowl derecho — May 2022. Kansas/Nebraska. High-plains dust storm variant.

How derechos differ from tornadoes

Direction

Derecho: straight, one direction. Tornado: rotating.

Path

Derecho: 400+ miles. Tornado: usually under 30 miles.

Width

Derecho: 40-200 miles wide. Tornado: usually under 1 mile.

Duration

Derecho: hours across a swath. Tornado: seconds to minutes at any location.

Peak winds

Derecho: 100-140 mph. Tornado: up to 300+ mph.

Detection

Derecho: bow echo visible on radar. Tornado: rotation signature.

Warning and safety

Derechos are typically warned as Severe Thunderstorm Warnings, often with a 'DESTRUCTIVE' tag. Since 2021, these tagged warnings trigger Wireless Emergency Alerts, so your phone will alarm.

Safety is the same as any severe thunderstorm: get inside, stay away from windows, know where to go if a tornado warning is upgraded.