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Sunday July 1, 2012
Type of storm a rare event for region
by The Washington Post


It's a hurricane. It's a tornado. No, it's a derecho.

Only a meteorologist was likely to have made the right guess about the violent storm system that hit the Washington area Friday night. Derechos occur only about once every four years in the District of Columbia area, according to the National Weather Service. They are more likely in the Midwest and Great Lakes, between May and July.

The worst news from the National Weather Service's website description: "Derecho damage to overhead electric lines sometimes results in massive, long-lasting power outages. . . . In the worst events, power may not be restored for many days."

A derecho is a fast-moving, long-lived, large, violent thunderstorm complex. By definition, it creates wind damage along a swath of more than 240 miles and produces wind gusts of at least 58 miles per hour.

Friday night's derecho raced along at speeds of more than 60 mph, with gusts clocked at 65 mph in Rockville, Md., and at 79 mph in Reston, Va. It formed west of Chicago about 11 a.m. and by midnight approached the Atlantic Ocean.

The trail of destruction more than fulfilled the definition of a derecho — it spanned from northern Illinois to the Delmarva Peninsula. To the north and west, 91 mph and 72 mph gusts were measured in Fort Wayne, Ind., and Columbus, Ohio.

Although the damage it can do mimics that of a hurricane or tornado, a derecho moves in one more or less straight direction. "Derecho" is a Spanish word that can be defined as "straight ahead," and was chosen as a counterpoint to "tornado," the whirling wind storm whose name is thought to derive from the Spanish word "tornar," which means "to turn."

Weather experts generally cannot predict a derecho as far in advance as they can other storm systems. Laymen who are not listening to weather reports can be left without time to take cover because the visual clues of impending danger, such as darkening skies, come at the last moment.

Derechos often form along the northern boundary of a hot air mass, right along or just south of the jet stream, where upper-level winds zip along at high speeds.

In summer, the jet stream atop a sprawling heat dome is sometimes called a ring of fire because of the tendency for explosive thunderstorms to form along the weather front separating hot, humid air to the south and cooler, drier air to the north.

On Friday, a historic, record-setting heat wave covered a sprawling region from the Midwest to the Southeast. Record high temperatures of 109 were established in Nashville and Columbia, S.C.

In Washington, the mercury climbed to 104 degrees — the hottest June day in the 142 years that records have been kept. The temperature broke previous records set in 1874 and 2011 by two degrees.

As stifling air bubbled northward, clashing with the weather front draped from near Chicago to just north of Washington, storms erupted. They grew in coverage and intensity as they raced southeast, powered by the roaring upper-level winds and fueled by the record-setting heat and oppressive humidity in their path.

The coverage and availability of this heat energy was vast, sustaining the storms on their 600-mile northwest-to-southeast traverse. The storms continually ingested the hot, humid air and expelled it in violent downdrafts - crashing into the ground at high speeds and spreading out, sometimes accelerating further.

This derecho event is likely to go down as not only one of the worst on record in Washington, but also along its entire path, stretching back to northern Indiana.

The intensity of the heat wave, without reservation, was a key factor in the destructiveness of this derecho event. It raises the question about the possible role of man-made climate warming from elevated greenhouse concentrations — a question that scientists will surely grapple with in case studies of this rare event.