A line of turbulent thunderstorms swept eastward across the Washington area yesterday evening, blackening skies, bringing hail and high winds and knocking out power to thousands of homes.

At least one fire -- a blaze at a house in McLean -- was blamed on the storm, which struck the downtown area of Washington about 5:45. Boats were capsized in the Potomac River, a Washington-bound transatlantic flight was forced to land in New York, and hundreds of visitors to the Mall scurried for cover in museums. $ heralded by rumbles of thunder and tongues of lightning, the storm brought about half an inch of rain to many spots in less than an hour.

Arriving on the first Sunday in July, it also continued what has been a tradition here so far this year of some rain on most weekends.

With only a small chance -- 20 percent -- of more thunderstorms today, forecasters said they expected the next few days, including the Fourth of July holiday, to be fair and pleasant.

After forming over the Blue Ridge Mountains of Virginia in advance of a cold front, yesterday afternoon's storms headed swiftly toward Washington, growing more intense as they approached.

At Dulles Airport, the peak wind gust was about 37 miles an hour, at 5:13 p.m. Thirty-eight minutes later, the storm had reached the banks of the Potomac, where a gust of 43 miles an hour was measured at the official recording station at National Airport.

A British Airways jetliner, bound from London to Washington, abondoned plands to land here and went to New York, airport authorities reported. A second jetliner bound for dulles chose to land at Baltimore-Washington International Airport.

Meanwhile, dozens of electrical lines, ranging from small wires to major feeder cables, were snapping under the effects of wind, lightning and falling branches.

Almost all of Vienna, with a population of about 12,000, was without electricity yesterday evening, according to Vienna police.

A total of about 2,000 homes in Montgomery and Prince George's counties lost power at least briefly, according to a spokesman for the Potomac Electric Power Co. He said the storm damaged four heavy feeder cables.

A lightning strike ignited a fire that caused about $25,000 damage to a house at 8417 Martingale Dr., in MrLean, Fairfax County fire officials said. They said the fire, which began about 5:50 p.m., was largely confined to the roof area.

In Washington, harbor police went to the aid of what officers described as "quite a few" sailboats flipped over in the Potomac by the storm. No injuries were reported -- "just a lot of people wet," one harbor police officer said.

Created by the push of a cold front into a region of warmth and moisture, the storms drenched the Washington area with brief but torrential downpours.

In a period of no more than an hour National Airport recorded .43 inches of rain, and Dulles measured .60 inches.

The National Weather Service said 0.07 inches was measured at one location in Gaithersburg in about 12 minutes.

The deluge proved too much for the drainage systems of many streets and roads. Throughout the area pools of water, ranging in size from large puddles to small lakes, appeared and vanished, often in a matter of minutes.

On the Mall, where the Smithsonian Institution was staging part of its several-day-long celebration of the Fourth of July, hundreds of persons raced into museums to flee the sudden on slaught of wind and rain.

About 400 or 500 sought shelter in the Constitution Avenue NW entrance of the Museum of History and Technology, according to Lt. Francis E. Thomas of the museum's security force. About 1,500 pressed into the Mall entrance, he said.

Many parts of the metropolitan area were also peppered and pelted with hail. Much of it was described as about a quarter to a half inch in diameter.

Yesterday's rain followed a rainy Saturday, the last day of June.

Despite the rainy close to June and the month's memorably rainy earlier weekends, the total rainfall for that month was 2.99 inches, about half an inch below normal. CAPTION: Picture, Two drivers find themselves in a carwash-like spray as they encounter flodding on the George Washington Parkway near underpass of Memorial Bridge extension in yesterday's storm. By Gerald Martineau -- The Washington Post