A violent thunderstorm, fueled in part by the record-breaking heat of recent days, ripped across Fairfax and Montgomery counties yesterday afternoon, snapping tree trunks, splintering branches, and cutting off electricity to thousands of homes and to scores of traffic signals.

In McLean, where the storm's intensity apparently reached a peak and golfball-sized hailstones covered the ground in spots, the lack of traffic lights snarled and delayed homebound motorists on and around such arteries as Rte. 123, and Westmoreland and Chain Bridge roads.

Winds were clocked up to 60 miles an hour in the Tysons Corner area, the National Weather Service said, and sent nine huge trees crashing to the ground within seconds on a single block in Kensington, according to a neighborhood resident.

Fallen leaves filled the rain-slick streets of McLean and were plastered against the sides of autos and buildings as residents emerged from their houses to pick their way past fallen tree limbs and branches.

"It was pretty terrifying there for awhile," said Dr. T.D. Boaz, a retired McLean physician.

"It was incredibly intense," said Marianne Lydecker of Kensington. "Trees went down like matchsticks."

Lightning accompanying the storm, which began moving through the area about 4 p.m., was blamed by authorities for a fire in the attic of a house on Overlook Drive in Silver Spring and for a blaze in a tire store on Fairfax Circle.

At the height of the storm, about 12,000 Northern Virginia homes, mostly in the McLean, Vienna and Tysons Corner areas, were without electricity, according to a spokesman for Virginia Electric & Power Co. By late last night, power had been restored to all but a handful of homes, the spokesman said.

In Montgomery County, the storm knocked out power to about 2,500 homes, according to a spokeswoman for the Potomac Electric Power Co.

Windows were reported broken in homes and in cars as a result of high winds, hail and falling limbs. One man was reported to have been trapped briefly in a car on Churchill Road in Fairfax County, and a higher than normal number of minor traffic accidents was reported there. No serious injuries were reported.

Forecasters said today is expected to be cooler with a high about 80 and a 50 percent chance of light rain.