Local

What kind of storm was it that hit us?

You might be looking for a word – a G-rated one – for whatever it was Monday that knocked out your power and knocked down the tree in your front yard.

The thunderstorm line that included McHenry County in its 1,400-mile damage trail Monday was what is called a derecho. From the Spanish word for “straight” or “direct,” a derecho is a widespread and long-lived windstorm that can come with a band of fast-moving showers or thunderstorms.

McHenry County Emergency Management Agency Director David Christensen had a more succinct definition as his office spent Tuesday going through area damage reports.

“We basically had a wall of 70 mph wind roll right across the county,” Christensen said.

A derecho is a long-lived collection of downburst clusters at least 240 miles in length, most of which generate wind gusts of at least 57 mph, according to the Norman, Okla.-based Storm Prediction Center. They often are associated with bow echoes, or bands of showers and thunderstorms that assume a bow shape.

A trained spotter at 7:30 a.m. Monday reported 70 mph winds in Crystal Lake for two to three minutes, according to the National Weather Service.

The derecho started Sunday in Kansas and Nebraska, plowed across McHenry County and the Chicago area early Monday, and petered out near the nation’s capital, weather forecaster Richard Castro said.

A derecho also struck the county on June 18, 2010, Castro said. That event also packed 70 mph winds, and knocked out power to more than 100,000 ComEd customers in the northern suburbs.

Castro and Christensen said it is unlikely that the storm included a tornado because it lacked rotation.

The term “derecho” was coined in 1888 by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa. Meteorologists resurrected the term in the late 1980s to describe the phenomenon, according to the Storm Prediction Center.

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