Rare 'Derecho' Storm Ravaged Washington Area

July 2, 2012— -- The system of thunderstorms that ravaged the Washington, D.C., area over the weekend, leaving 1.2 million homes without power and killing at least five people, was not only destructive — it was also rare.

Known as a derecho, the string of storms combined intense lightning and rain with hurricane-force gusts as it swept from the Midwest into the mid-Atlantic Friday night. Meteorologists blamed the violent weather on the prolonged 100-plus temperatures that blanketed the eastern United States last week.

Derechos typically form when an atmospheric disturbance lifts the warm air in regions experiencing intense heat, causing thunderstorms and hurricane-force winds to develop, AccuWeather meteorologist Brian Edwards said. Traveling at an average speed of 60 miles per hour, Friday's storm took 12 hours to cover more than 700 miles before reaching the Atlantic Ocean.

By Monday, power had been restored in more than half of the homes affected by the storms in the Washington metropolitan area. But utilities predict that the roughly 660,000 homes still without power may remain so for the next several days as the heat wave continues unabated.

Ken Barber, vice president for customer solutions at Dominion Virginia Power, told ABC's affiliate WJLA that the utility is facing the "biggest non-hurricane outage in our 100 year history."

Derechos are more common in the Midwest, but the mid-Atlantic gets one derecho about every four years. University of Iowa physicist Gustavus Hinrichs coined their name in 1888, borrowing the Spanish word for "straight" because of the linear path the storms generally follow.

Derechos are harder to predict than other severe weather events, Edwards said, because forecasting tools used by most meteorologists are often unable to identify where the precise combination of factors needed to trigger a derecho will emerge.

As global temperatures rise, derechos, which can occur anywhere in the world, may become more common, said Tom Kines, a senior AccuWeather meteorologist, though he added that evidence on changes in their frequency is not yet available.

The derecho's impact was concentrated in Washington's Virginia and Maryland suburbs, but the weekend's storms rampaged from Kentucky to New Jersey and wracked the Midwest as it accumulated energy on its journey eastward.

All told, about 3 million homes lost power, 12 people lost their lives, and another 20 people were injured. The governors of Ohio, West Virginia, Maryland and Virginia, and the mayor of Washington, D.C., declared states of emergency.

Popular Internet services such as Netflix, Pinterest and Instagram were also temporary casualties of the severe weather after power went out at an Amazon data server in Virginia. Outages idled trains and caused delays at airports, frustrating travelers and commuters. In some places, emergency response systems were not working properly.

Forecasts for Monday and Tuesday predicted cooler temperatures across the Eastern Seaboard, with highs in the upper 80s and low 90s.