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What is a derecho? Explaining the unusual storm that battered Houston.

Janett Martinez Avalos
Houston Public Media

Last week, thousands of residents received tornado warnings to take cover as a twister was headed their way. While that was true for Cypress and parts of Waller County, the Houston area actually experienced a derecho.

If you’re not familiar with that word, don’t worry. Most of the time the term “straight-line wind” is used to describe a derecho, according to the National Weather Service.

So what is it? According to the NWS, a derecho is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms.

“It’s a very self-sustaining system and can last a long time,” said NWS meteorologist Cameron Self. “It went well into Louisiana.”

Although a derecho can produce destruction similar to tornadoes, the damage typically is in one direction along a relatively straight path. That’s often why the term “straight-line wind damage” is used to describe derecho damage, NWS said.

“By definition, if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph or greater along most of its length, then the event may be classified as a derecho,” NWS said.

Winds reached between 90-100 mph in Cypress on Thursday and two confirmed tornadoes did touch down in the area. And tens of thousands were without power nearly a week later as powerlines and trees have been blown down throughout the Houston area. At least eight people were killed.

When it comes to tree damage, Self said one way to tell if the storm is a tornado or derecho is by seeing how they fall.

“When you’re looking for whether it’s a tornado or not, the trees will often fall in different directions,” Self said. “With straight-line winds, things will blow in a straight line.”

Many people were caught outside as the storm rapidly approached, and Self had advice if people ever find themselves near a quickly approaching storm: get inside, stay away from windows, and go in an interior room if possible.

“Some people were caught outside, and some of the deaths were people going out and moving their car,” Self said. “Think of yourself, not your car.”