Fueled by the heat and humidity of the warmest morning of the year thunderstorms swept in angry waves across the area yesterday, touching off fires, knocking out power and deluging some spots with as much as three inches of rain.

The three-inch measurement was reported in Nokesville in Prince William County. Manassas, also in Prince William, reported 2.7 inches, of which 1.7 fell during one torrential hour. Washington's official measuring station at National Airport recorded .54 inches, while the figure at Dulles Airport was three hundredths of an inch more.

As skies grew black, lightning flashed and flickered in washington skies for hours in the afternoon and evening. It was believed to have started fires in three barns in rural Maryland and Virginia.

A barn was reported heavily damaged in Prince William after a lightning strike, and a barn in Washington Grove in northern Montgomery County was destroyed after being struck, according to county fire officials.

Damage to a barn in Fairfax County hit by lightning and destroyed by fire was estimated at more than $40,000. The three-storey structure was located at 6713 Clifton Rd., authorities said.

Lightning struck a Virginia Electric and Power Co. substation in West Springfield about 6 p.m., knocking out power to homes in the Old Keene Mill Road area.

Utility company spokesmen said the lightning burned open a connector linking cables at the substation at 8430-A Old Keene Mill Road.

There was no immediate estimate of the number of homes affected in that incident, but wind, lightning and falling trees limbs were reported earlier to have interrupted power at more than 3,500 homes in the Virginia and Maryland suburbs.

Reports of minor flooding came from around the area. High water delayed traffic in Montgomery County on sections of Muddy Branch Road, Rte. 108 and Rockville Pike. Two roads in Prince William were reported shut briefly.

Weather forecasters said the storms were triggered by the arrival from the west of a weakening cold front, which ignited the atmospherically volatile mixture of heat and humidity that has plagued the area for the last few days.

In explaining just how sticky it has been, district forecaster Steven Pond noted that humidity generally keeps nighttime temperatures up. Thursday night and Friday morning the mercury never fell below 79 degrees, making it the warmest night of the year.

Today and tomorrow are expected to be very hot again but a little less humid.