An avalanche of hailstones and torrents of wind-driven rain burst over the Washington area late yesterday afternoon as brief but severe thunderstorms shut down outdoor celebrations, toppled trees, flooded roads and knocked out power to 12,000 area customers.

In little more than an hour, a pleasant late-summer afternoon became an atmospheric cauldron that sent thousands of people at Adams-Morgan Day in the District and the Ethnic Heritage Festival in Wheaton fleeing from lightning, rain and a cold north wind that gusted to more than 50 miles an hour.

The storms, signaling the advance of a cold front, formed this side of the Blue Ridge mountains in Northern Virginia and central Maryland, then headed east and "kind of blew up in our back yard," said meteorologist Brian Smith of the National Weather Service office here.

Golf-ball-sized hail was reported by a motorist on the American Legion Bridge. Hail also fell at many spots in the Washington area, including the Adams-Morgan Day festival, where witnesses said the sky seemed to show a greenish tinge before the swiftly rising wind ripped the tops from food vendors' tents.

Trees fell everywhere. One landed atop a car on the Ellipse. Two dozen other trees were reported uprooted in the District, according to the city's Office of Emergency Preparedness. In Virginia, a tree lay sprawled across the southbound lanes of the George Washington Parkway near Fort Marcy and another crashed down across the roadway in the 2500 block of Monroe Street in Herndon, authorities said.

In Montgomery County, police reported that a tree had fallen on Massachusetts Avenue, and another was blocking traffic on Old Georgetown Road at McKinley Street.

The Potomac Electric Power Co. said the storm knocked out power to about 3,500 customers in the District and Montgomery and Prince George's counties. The largest number of affected homes was in the District, where about 1,400 lost power, mostly in Northwest and Northeast.

Airline operations at National Airport were delayed by about a half-hour, a Federal Aviation Administration spokesman said.

Virginia Power said about 8,500 customers lost power temporarily, mostly as a result of lightning. Spokeswoman Robin Engh said the outages were scattered across Fauquier, Loudoun and Fairfax counties.

The National Weather Service said wind gusts of more than 50 miles an hour were reported at several places, and a gust of 61 miles an hour was reported in upper Northwest Washington. In less than an hour and a half, the temperature plunged 18 degrees at National Airport, from a high of 77 at 4:37 p.m. to 59 degrees at 6 p.m.

More than four-tenths of an inch of rain poured down at National Airport in about an hour. It was the most rain in any day this month and more than all the other days in September combined.

Slow traffic, automobile accidents and temporary road flooding were reported in many places.

Today is expected to be cool and windy, with high temperatures in the upper sixties.

One forecaster said that tonight's temperatures could plunge into the upper thirties in outlying suburbs.

Staff writer Jeff Rowland contributed to this report.