A violent electrical storm, heralded by an ominous darkness, thundered through the Washington metropolitan area yesterday afternoon and reduced the rush hour to a bedraggled tangle.

Service on both lines of the Metro subway was flooded out for more than a half hour, motorists were temporarily forced off the road, and widespread power failures were reported throughout the area.

The storm, which dropped almost two inches of rain in less than an hour in parts of the metropolitan area, flooded dozens of streets and major intersections. Police and fire department switchboards lit up with callers reporting roads blocked by trees, downed power lines, and numerous storm-related accidents.

The intensity of the storm was such that it greatly slowed movement in the area for a brief oeriod and created flash-flooding conditions, but the waters dissipated almost as quickly as they had risen.

Alexandria police, for example, said water ran four feet deep at the intersection of king and Union Street but quickly receded. A flood watch was put in effect in the Alexandria are a but was quickly canceled as the storm moved on.

Operation on both the Red and Blue lines of the Subway system was halted at 5:45 p.m. according to train superintendent Joe Taylor. Water was over the rails between the Federal Triangle and the Smothsonian stations on the Blue line and between Metro Center and the Farragut North station on the Red line, Taylor said.

Service was restored on the Blue Line at about 6:30 P.m. and several minutes later on the Red Line. Taylor reported that buses made a few trips to haul stranded passengers on the Red Line.

Taylor said water had come through the ceiling at the Metro Center station "like a busted pipe" during the hardest part of the storm at about 6 p.m.

Metro officials moved the trains to the closest stations, where riders were given the option of remaining on the trains or going out into the storm. About two-thirds of the passengers left.

The storm rose in the Shenandoah Valley area of Western Virginia just east of the Blue Ridge Mountains when northeastern winds from the south caused the hot humid air to rise and then turn into rain, according to Chet Hendrickson, lead forecaster of the National Weather Service in Marlow Heights.

"The storm fed on the contrast between hot, humid air and the rain-cooled air generated by the thunderstorm," Hendrickson said, adding. "lt was like a self-perpetuating machine: as long as there's hot air around the rain-cooled air, it gets more intense."

Hendrickson said the heavy electrical storm seemed to run from Dulles-Reston area to Rockville and then switched southeastward through the District toward lndian Head.

"lt was a northwest to southeast swath where most of the intense electrical activity was generated, and also the heaviest rain area," Hendrickson said.

"We really don't know what makes one storm more intense than another," he said. "High winds don't necessarily accompany high electrical activity and vice-versa. This was basically a heavy electrical storm, with frequent lightning and heavy rains."

Today's forecast by the National Weather Service calls for mostly sunny, hot and humid weather with highs in the mid-90s and a 20 per cent chance of rain during the day. Tonight's weather is expected to be fair with temperatures in the low to mid 70s.

Details about incidents caused by the storm differed but reports were similar throughout the area - sudden darkness, bright lightening and loud thunder, break accidents and late dinners.

D.C. ploice reported flooding at more than two dozen locations. ln some places, traffic stopped. Such spots included the Southeast Freeway under the 11th Street Bridge, the Duke Ellington Bridge and the intersection of New York and Montana Avenues NE.

D.C. Fire Department spokesmen reported more than 150 calls for help during a 20 - minute period at the height of the storm. A house at 2232 40th St. NW was struck by lightning, but no one was hurt; according to preliminary reports.

ln Mclean, rain fell so hard that Fairfax County police were ordered not to move their cars during a 10-minute period and to answer only extreme emergency calls during that time.

Police in Airlington and Alexandria reported power lines and trees down in many years, and Arlington police said much of the county was without power early in the evening.

Reports of fallen trees and electrical lines also were numerous in Montgomery County, a police spokesman said.

The thunder cracked so loudly over the (Rockville) Mall that it sounded like a bomb going off in the garage," the police spokesman for Arthur P. Brigham, spokesman for the Washington Suburban Sanitary Commission, reported brief power outages at WSSC's beleaguered Polomacand Patuxent water stations. Brigham said that emergency generators took over while power was gone and that no disruption of service occured.