July 2, 2012

Some 3 million residents in the mid-Atlantic region of the United States were left without electric power after a series of extremely violent storms downed hundreds of trees and power lines late on June 29. At least 17 people were killed, most of them by falling trees. The National Weather Service described the storm system as a derecho, a fast-moving band that can produce hurricane-force winds. The crescent of storms swept down from Indiana and Ohio, across the Appalachians, and into the mid-Atlantic states, causing particually widespread damage in Virginia, the District of Columbia, and Maryland.

On June 29, a huge region of the United States from the Midwest to the Southeast had record-setting high temperatures: 104 °F (40 °C) in Raleigh, North Carolina, and Washington, D.C.; and 109 °F (42.7 °C) in Nashville, Tennessee, and Columbia, South, Carolina. The 104 °F temperature was the hottest June day in the nation’s capital in 142 years of record keeping.

A derecho is a widespread, long-lived windstorm associated with a line of thunderstorms. A derecho can sometimes last for more than a day and blow across a distance of several hundred miles. (© Jim Reed, Corbis)

As the hot air rose, it clashed with a cold front, giving birth to violent thunderstorms. Fueled by the heat and powered by roaring upper level winds, the storms grew in intensity as they moved southeast. Winds of 72 miles (115 kilometers) per hour were clocked in Fredericksburg, Virginia, and 80 miles (128 kilometers) per hour in Ft. Wayne, Indiana. Meteorologists believe that this derecho is likely to go down as one of the worst on record along its entire path, from northwest Indiana to the Jersey shore. Derechos are most common in the Midwest and Great Lakes region between May and July.

Additional World Book articles:

• Weather
• The Forecast: Better Weather Prediction Ahead (a special report)