A derecho -- fierce wind storms -- threatens a fifth of the U.S.

A line of fierce storms known as a derecho is threatening to move east from the upper Midwest through the mid-Atlantic states, bringing large hail and tree-breaking winds to a wide swath of the nation.

As many as one in five Americans live in the zone that is threatened by a line of squalls, a similar meteorological event to the one that that tore trough roughly the same region a year ago. A derecho can be as deadly as any outbreak of tornadoes, causing wind damage to structures and power lines.

Technically, a derecho is a wind storm associated with a band of rapidly moving thunderstorms. Unlike a tornado, which spins like a top, a derecho is a straight-line event. The line of storm carries winds in excess of 58 miles per hour along a front of at least 240 miles.

According to the National Weather Service, there is risk of severe weather hitting as many as 64 million people in 10 states including such major cities as Chicago, Indianapolis, Cincinnati and Columbus, Ohio. The band of storms will eventually travel through Detroit, Baltimore, Washington, Milwaukee, Pittsburgh and Louisville, Ky.

The immediate concern is in the Upper Midwest, the weather service noted in a posting on its website.

“The NWS Storm Prediction Center is forecasting a Moderate Risk of severe weather Wednesday afternoon and evening, including the development of widespread damaging winds, large hail and a few tornadoes, over parts of the Upper Midwest & Ohio Valley,” the service said.

As the front moves through, the area around the nation’s capital of Washington will be in the moderate risk zone, the second most deadly, by Thursday. That region has more than 18 million people. But another 60 million people from Raleigh, N.C., to New York will face at least a slight risk from winds, according to the weather service’s projection.

A derecho event last year caused about $1 billion in damage across much of the same region from Chicago to Washington. At least 13 deaths were attributed to the storm, which carried winds of more than 100 miles an hour. Power outages hit perhaps 4 million people and another 324 deaths were blamed on the heat wave that followed.

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