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What is a Derecho?

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From our sister company, TWC, on the definition of a Derecho:


What is a Derecho?

Ever hear us throw around the term "derecho" on The Weather Channel and weather.com and wonder what it means?  Step into our classroom as we give you a little dose of Meteorology 101 below.

Derecho

Derechoes are large clusters of thunderstorms that produce widespread wind damage, usually as a result of one or more curved lines of thunderstorms known as bow echoes. The word in the Spanish language means "straight" and these windstorms leave wide, long swaths of straight-line wind damage. These winds can be as strong as 50 to 100 mph (or higher)!

According to our Severe Weather Expert Dr. Greg Forbes, (Find him on Facebook), an ordinary thunderstorm produces a swath of damaging winds usually only a mile or two wide and a few miles long, but derechoes can produce damage swaths tens of miles wide and several hundred miles long.

Derecho averages

Source: Walker Ashley

Unlike tornadoes, which are more common in the spring, derechoes are the most active during the late spring and summer. The graph to the right shows this with a ramp up in the annual number of derechos per month in May (over 4 per year), June (3-4 per year) and July (over 4 per year).

According to Dr. Forbes, because of the widespread nature of the winds in a derecho, the impact is somewhat like that of a landfalling hurricane, and affects a much greater area than most tornadoes. The extensive swath of downed trees and power lines causes a major cleanup and restoration effort that takes days to weeks and often requires relief workers to come in from other states to aid in these efforts.

(MORE: Thunderstorm safety)

To give you an idea of what kind of impacts these damaging thunderstorms complexes can have, here are some examples of significant derechoes in history.

Historic Damaging Derechoes

In July 2006, two separate derechoes hit the St. Louis metro area in a span of a few days.

  • The first one on July 19 packed wind gusts up to 92 mph. Thirty people were injured at the baseball stadium where the Cardinals were preparing to play the Atlanta Braves.
  • Power was knocked out to 560,000 customers.
  • Service had been restored to about 270,000 power customers when the second derecho hit the St. Louis area on July 21, driving power outages back up to around 560,000!

On July 22, 2003 a long-lived derecho struck a swath from near Memphis, Tenn. to east of Atlanta, Ga.

  • Over 338,000 lost power in the Memphis area.
  • Some people were without power for up to 15 days!

A swath from far northern Michigan to Southern New England was struck by a derecho on July 15, 1995,

  • This derecho moved at speeds of up to 75 mph.
  • Wind gusts up to 92 mph were measured along its path.
  • The massive damaging windstorm resulted in 7 fatalities.
  • The total damage estimate was placed at nearly half a billion dollars.