Derecho windstorm explained: As Midwest prepares for severe weather, many ask, 'what is a derecho?'

CINCINNATI -- The National Weather Service is tracking a so-called derecho weather pattern in the Midwest that could spawn severe windstorms in major metropolitan areas with gusts as strong as 100 mph.

But what is a derecho windstorm?

According to the National Weather Service, a derecho is a widespread, long-lived windstorm that is associated with a band of rapidly moving showers or thunderstorms. Although a derecho can produce destruction similar to that of tornadoes, the damage typically is directed in one direction along a relatively straight path.

Derecho windstorms occur once every year or two across the central and northeastern U.S. in a band from Texas to New England. They pack hazardous winds of at least 75 mph or more and maintain their intensity for hours as they sweep across vast distances.

In some cases, a derecho will spawn tornadoes and accompany storms that produce hail the size of golf balls.

"Derecho" is a Spanish word that can be defined as "direct" or "straight ahead." The word was coined for windstorms by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa.

In order to be categorized as a derecho, wind speeds must be greater than 57 mph at most points along the storm's path -- meeting the National Weather Service criterion for severe wind gusts. In stronger derechos, winds may exceed 100 mph.

Some stronger derechos in the past include one that roared through northern Wisconsin on July 4, 1977 with winds of 115 mph. A 128 mph derecho also swept across Wisconsin and Lower Michigan during the early morning of May 31, 1998.

Most recently, the Ohio Valley/Mid-Atlantic derecho of June 29, 2012, which clocked in around 65 mph, caused widespread power outages in cities from Cincinnati, Columbus and Dayton to Atlantic City, Baltimore and Washington, D.C.

The current weather pattern being analyzed by meteorologists Wednesday is expected to affect larger metropolitan areas like Milwaukee, Chicago, Detroit, Cleveland and Pittsburgh, according to Bill Bunting, a meteorologist in the agency's storm prediction center in Norman, Okla.

"We tend to be careful using the D word, but yes, a derecho is possible," Bunting told the Associated Press late Tuesday night.

Meteorologists predict a 45 percent chance Wednesday that thunderstorms, powerful wind gusts and possible tornadoes develop in an oval stretching from eastern Illinois across most of Indiana to the western half of Ohio.

For more information about derechos, go to: http://1.usa.gov/dFbiA

The Associated Press contributed to this report.

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