Derecho Facts, Formation and Forecasting

Derecho clouds show windstorm near Indiana.
An ominous line of clouds portends the approaching derecho in La Porte, Ind., on the afternoon of June 29.
Credit: NOAA/Courtesy of Kevin Gould.

A derecho is an impressively far-reaching and long-lived windstorm that is quite rare and difficult to predict.

Derecho (pronounced deh-REY-cho) means "straight" in Spanish and as its name suggests, this type of storm packs powerful straight-line winds, in contrast to the spinning winds of a tornado.

To qualify as a derecho, wind gusts throughout the storm must be at least 58 mph (93 kph), but particularly strong storms have had wind speeds exceeding 100 mph (160 kph). The storm also must cause wind damage across an area at least 240 miles (400 kilometers) wide to count as a derecho.

The storms are associated with rapidly moving bow echoes, or bow-shaped bands of severe thunderstorms. Barreling across the landscape at speeds typically 50 mph (80 kph) or greater, the gust front of a derecho is often marked by an ominous-looking arcus cloud, also called a shelf cloud.

While thunderstorms are relatively well-forecast, derechos are extremely difficult to predict. Meteorologists don't fully understand all the subtle environmental factors that need to coalesce in order for a derecho to form.

In the United States, derechos are most common in the late spring and summer and there are typically one to three events each year.

The storms can be highly destructive. A June 29, 2012, derecho traveled an astonishing 450 miles (724 km) in six hours from the Midwest to the East Coast. Along the way it left millions without power and 22 dead.

A particularly severe storm on May 8, 2009, earned a new classification, super derecho, as it carved a path of destruction 100 miles (62 kilometers) wide in Kansas. It spawned 18 tornadoes and hopped the Mississippi River into Illinois with 90- to 100-mph (145- to 160-kph) wind gusts.

More facts about derechos and significant derecho events can be found on the National Weather Service website.

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