The massive wind-and-lightning storm that slammed the Midwest and Mid-Atlantic was a derecho, an unusual weather phenomenon consisting of long-lived gusty-type thunderstorms. Derechos are relatively common across the Plains, but are a fairly rare event in the East.

The name "derecho" comes from the Spanish word for "direct," indicative of the storm`s ability to produce very powerful straight line winds. This is comparable to the word "tornado", which comes from the Spanish word "tornar," or "to turn." The long length of a derecho is one of its hallmarks. By definition, a derecho is a line of thunderstorms that contain winds of 58 mph or higher, lasting for 240 miles or longer.

A derecho typically forms in the way that Friday`s storm formed - -a cluster of thunderstorms congeals into a band of activity. Rain-cooled air around the thunderstorm reaches the ground and spreads horizontally outward from the storm. This forces warmer air to spread above the ground ahead of the storm, feeding the storm`s development even further.

Over time, this development causes strong thunderstorms to build away from the initial band of storms, expanding its profile along the ground. As this growing activity advances, the process continues, with warm and humid air feeding storm development and allowing the thunderstorms to expand outward to the north and south of the initial line of thunderstorms.

Derechos have been compared to hurricanes, in the sense that they are self-supporting. In the case of a hurricane, the warm ocean water feeds and maintains the storm`s development. With derechos, the constant overturning of cold air at the surface and warm air aloft feeds the storm and keeps it going for hours at a time.

As it moves along, the derecho accelerates, reaching forward speeds of 50 to 75 mph. By itself, the thunderstorms are capable of producing 60 to 70 mph winds, and coupled with the forward motion, wind gusts can reach 70 to 90 mph, with occasional gusts in excess of 100 mph.

Friday`s storm system was a typical derecho event. A line of thunderstorms developed on Chicago`s west side during the early afternoon hours. As it advanced into Indiana and Ohio, it encountered temperatures in the lower 100s and high humidity levels. Similarly hot air was present across the Ohio Valley and into the Mid-Atlantic, where highs on Friday topped the century mark nearly everywhere.

Thus as the storm raced eastward through the evening hours, it was able to quickly feed and expand, producing wind gusts of 84 mph in Findlay, Ohio, and 91 mph in Fort Wayne, Ind. Even though it didn`t cross the Appalachians until after sunset, the storm system still found temperatures in the lower 90s with dew points in the middle 70s, producing plenty of energy to unleash powerful thunderstorms across the Mid-Atlantic from New Jersey to southern Virginia. Widespread 60 to 70 mph gusts were recorded in the Washington, D.C., area, with top gusts of 87 mph in Nelson, Va., and 81 mph in Tuckerton, N.J. The only thing that finally brought this monster storm to an end was the chilly Atlantic Ocean. After the derecho advanced offshore, it ran out of the warm air that was feeding its development and it finally died around 1 a.m. Saturday.

Typically, the strong, hurricane-force winds will knock over everything in their path, snapping trees like twigs, and tearing apart power lines and power poles. Not surprisingly, massive power outages can be expected along a derecho`s path, and that was what was seen on Friday night. At the peak, nearly 1.5 million customers in the Baltimore, Washington and Richmond, Va., area were without power, with another one million in the dark along the storm`s route from Illinois to West Virginia.

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Story Image: An American beech tree lies on Capitol Hill grounds in Washington. (Manuel Balce Ceneta, The Associated Press)

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