Fierce but brief thunderstorms, bringing sudden torrents of rain and golf-ball size hailstones, swept through the Washington area yesterday evening, causing little damage and presaging the arrival of cool weather today.

As the storms rumbled in from the northwest, heralded by darkening skies and rising winds, the temperature plunged 19 degrees in two hours, from a sultry, sticky 94 at 5 p.m. to a mild and pleasant 75 at 7.

According to an initial report from the Potomac Electric Power Co., the storms knocked out 10 feeder lines in Montgomery and prince George's counties, affecting service to about to about 2,000 households.

The heaviest rain, strongest winds and largest hailstones apparently were associated with a storm center that formed over Montgomery County in advance of the main body of thunderstorm activity.

After taking shape over Rockville about 4:30 p.m., this storm thundered east toward the Laurel area, where hailstones of 1.5 inches were reported.

Then, according to the National Weather Service, the storm veered southeast toward Largo, and pelted the area with stones 1.75 inches around, denting car tops and cracking some windows.

Rain cascaded down along the storm's track - an inch and a quarter in Rockville and Largo, and an inch in the Brandywine area of Prince George's.

Winds gusted up to 50 miles an hour in Largo, and a few miles away in Upper Marlboro strong gusts flung sheets of rain against the windows of the county administration building, while the lights went out inside.

At 6:30 p.m. the main body of the storm burst over the heart of downtown Washington, sending torrents surging through the streets, and putting frowns on the faces of late-leaving commuters.

It was over in 15 minutes. Behind the line of storms, the weather service said, is a cold front that should clear the skies and cool the temperatures here for at least the next two days.