A powerful, slow-moving thunderstorm dumped more than two inches of rain on the Washington area last night, flooding roads throughout the area, stranding motorists in quickly rising water, interrupting electrical service and apparently causing the partial collapse of a house in Forestville.

U.S. Park Police said early today that an undetermined number of motorists were trapped briefly in their cars in Rock Creek Park when Beach Drive was inundated with waist-deep water as the storm moved through Northwest Washington shortly after 10 p.m. The most severe flooding in the park was reported along Beach Drive between Broad Branch Road and Joyce Drive in the District, according to Park Police, who said all of the stranded motorists had reached safety by 1 a.m.

One of the motorists caught by the surging water was Adaku Ahaghotu of Washington, who said she was driving along Beach Drive near Broad Branch Road about 10:30 p.m. when suddenly the water rose to the windows of her compact car. "I thought I was going to die," she said later.

Ahaghotu said it took her about 20 minutes to walk and swim to safety through the swirling waters. All along the road were lines of other cars deep in water, she said.

Flooded roads that were closed last night by water ranging up to four feet deep included portions of Sligo Parkway in Montgomery County, Suitland Parkway in the District, Rte. 110 (Boundary Channel Drive) near Arlington Cemetery, Canal Road north of Georgetown and Glebe Road at Commonwealth Avenue in Alexandria. Washington Boulevard between Interstate 395 and Rte. 50 was still under about two feet of water at midnight, Arlington officials said. Fairfax County officials said the heaviest flooding in the county was along Braddock Road between Ravensworth and Backlick roads, where some roads were impassable, and along U.S. Rte. 1 at Fort Hunt Road.

Spout Run Parkway in Arlington was closed because a tree was blocking the roadway, Park Police said.

Officials said, however, that the high water began receding rapidly after the rain stopped and predicted that only the flooded portions of Beach Drive and Sligo Parkway would remain closed through this morning's rush hour.

In Forestville, the rear wall of a house in the 8500 block of Corona Road collapsed, apparently because of a mudslide, neighbors said. One nearby resident, Ricky Moyers, said that the first-floor rear wall of the house backed up to a hill. "The wall is now at the front door," Moyers said.

A number of residents thoughout the Washington area reported severe flooding in the basements of their homes.

Harold Hess, a forecaster with the National Weather Service, said the storm moved from north to south and was first sighted on radar in Howard County shortly before 9 p.m. As the storm moved southward, power outages followed in its wake.

Potomac Electric Power Co. spokeswoman Nancy Moses said a total of about 6,600 customers in Montgomery County lost power for up to 1 1/2 hours, including about 1,000 in the Norbeck area, 1,200 in the Aspen Hill area and about 4,400 in the Silver Spring area along Georgia Avenue between Colesville Road and the Capital Beltway.

There were other scattered power failures throughout the area as the torrential storm toppled tree limbs onto power lines.

Although rainfall up to four inches was measured in the Washington area Sunday night, forecaster Hess said that the greater flooding and damage reported last night apparently occurred because the ground was saturated by the earlier downpour.

By midnight, the storm had reached National Airport, where six-tenths of an inch of rain was recorded in less than half an hour, Hess said.

A spokesman for Virginia Electric & Power Co. said its only major outage reported was in Crystal City near Army-Navy Drive and Arlington Ridge Road where power to about 1,100 customers was out for less than an hour. Hess said that storms in this area normally move at a speed of about 20 to 30 miles an hour, but that the "steering current" of last night's storm was "very, very weak," so that it drifted along "in a very erratic fashion."

The intensity of the storm and the heaviness of the downpour along its path was due to the fact that it was moving so slowly, he said. An urban and small stream flooding alert was issued by the weather service last night.

Hess said the Washington area could expect some continuing thundershowers in the early morning hours, decreasing around noon and that skies should clear by this afternoon.