A bolt of lightning traveled down the chimney of a house in Poolesville yesterday afternoon, and a tongue of flame leapt out of a bedroom outlet, setting fire to a nearby bed, Montgomery County fire officials reported.

No one was injured and damage was relatively minor as a result of the lightning strike at the house in the 19100 block of Hempstone Avenue, which occurred just as a teenage occupant of the home was about to plug a hair dryer into the outlet, according to Capt. Jim Resnick, a county fire and rescue spokesman.

The incident was one of the most dramatic to occur as powerful thunderstorms, some with drenching rain and high wind, swept across the Washington region in advance of a mass of cooler air. Trees toppled and automobiles collided on suddenly slippery roads.

Power was cut at least temporarily to more than 15,000 homes, from Loudoun County in Virginia to Prince George's and Montgomery counties in Maryland. Hundreds of houses in the District also were without power for a time.

Although it was brief in most places, rainfall was substantial in some spots. The National Weather Service said that more than three quarters of an inch of rain was measured at Baltimore-Washington International Airport.

At Reagan National Airport, the rainfall figure was about one-third as much, but the shower rapidly lowered the temperature of what had been a humid and--like most of the summer--hot afternoon. At National, the mercury plunged 20 degrees between 3 p.m. and 4 p.m., from 93 degrees to 73 degrees.

The day's high at National was 95, and at Dulles International Airport it was 97, breaking the record of 95. Temperatures may remain below 90 degrees in much of the region today, the weather service said.

CAPTION: A vehicle sprays water at K and 15th streets in Northwest Washington during one of several area downpours.