A severe thunderstorm with winds of up to 40 miles an hour dumped more than a half-inch of rain as it swept through Washington yesterday, sparking at least two serious house fires and causing several accidents, including one in which two persons were critically injured.

The storm, which entered the area shortly after 4 p.m., also disrupted rush hour traffic and knocked out power to thousands of homes in the District, Maryland and Virginia, authorities said.

Fairfax County police said it was "likely" that rain-slickened roads contributed to a two-car collision in the Covington area of the county in which two women were critically injured and a man was seriously injured.

They said Jo Walker, 32, of 3563 Belfry La., Woodbridge, was driving east on Rte. 50 near Covington Street about 4:30 p.m. when her car crossed the median and collided with a westbound car being driven by Paul Harmon, 36, of 9539 Blake La., Fairfax.

Walker was listed in critical condition last night at Fairfax Hospital, while Harmon was in serious condition, police said. They said a passenger in Harmon's car, Rebecca Harmon, 24, also of the Blake Lane address, was in critical condition. Their injuries were not reported.

The most serious fire was reported at 4740 Quebec St. NW, in which five District firefighters suffered minor injuries.

Officials said the fire, at a house near American University, was discovered about 4:40 p.m., shortly after neighbors saw a bolt of lightning, followed by what they said sounded like an explosion.

The fire, which took about an hour to control, was confined to two bedrooms in the third floor attic, a firefighter said. No damage estimate was available.

Five firefighters were taken by ambulance to the Washington Hospital Center, where they were treated for smoke inhalation and released, a hospital spokeswoman said.

In Springfield, lightning struck the first floor of a house at 6001 Brandon Ave., which sparked a blaze that spread to the second floor and attic, causing about $75,000 damage, fire officials said.

Lightning was also cited as the cause of a fire that did $5,000 damage at a model house at 12600 Still Pond La. in Franklin Farms.

For about 30 minutes during the height of the storm, which was most intense between 5 and 6 p.m., no flights landed or took off from National Airport, a spokesman in the airport's operation office said.

"We had severe weather over the place, and pilots were saying, 'Hey, I don't want to stick my nose in that,' " he said.

Pea-sized hail was reported in some areas of Northern Virginia and in Charles and Calvert counties in Maryland, said National Weather Service forecaster Harold Hess.

Nancy Moses, spokeswoman for Potomac Electric Power Co., said the heavy rains caused power outages to about 4,500 customers, primarily in Bladensburg, Hyattsville and far Southeast Washington. Scattered outages were also reported in Prince George's and Montgomery counties, she said.

Vyron Howell, an operations director for Virginia Electric Power Co., said nearly 13,200 homes in Virginia were without power in Fredericksburg and as far west as Fauquier County as a result of high winds and lightning.

"We had about 2,000 down in Alexandria and another 2,500 homes out in Springfield and Herndon," Hess said. Power was returned to many homes by mid-evening.

Clearing skies were forecast for this morning, with highs expected in the low 70s and a 30 percent chance of showers.