Thunderstorms set fire to an attic in Potomac, knocked out electricity to hundreds of Northern Virginia homes and helped delay airline flights for hours here yesterday evening as an advancing cold front transformed high humidity into torrents of rain.

Failure of a computer about 4 p.m. at the Air Route Traffic Control Center in Leesburg combined with "very severe thunderstorms" to snarl schedules into and out of National and Dulles International airports, a Federal Aviation Administration spokesman said.

While the traffic control system was "able to operate in a safe manner" for the approximately 50 minutes the computer was out, delays ranged "up to three hours," said spokesman Bob Buckhorn. The cause of the computer failure could not be learned.

Two persons who said they expected to leave National at 5:55 p.m. said they were told at 10 p.m. that their flight was canceled.

Lightning that flashed from leaden skies throughout the evening struck a house at 18 Holly Leaf Ct. in Potomac about 8 p.m.

The strike ignited the attic of the house, which was unoccupied at the time, Montgomery County fire officials reported. They said the blaze was controlled in about 30 minutes and the extent of damage was not immediately known.

The storm cut off electricity to about 3,000 homes in the Northern Virginia suburbs, many of them in Falls Church and western Fairfax County, according to an official at Virginia Power. A scattering of homes in Hyattsville and Germantown also lost power for a time, a spokeswoman for Potomac Electric Power Co. reported.

The dense gray clouds that covered the sky throughout the day shielded the area from the sun and held the high temperature to 81 degrees, coolest in two weeks, according to the National Weather Service.

The blanket of humidity that covered the area was converted last night into more than three-quarters of an inch of rain at Dulles, more than eight-tenths at National, and more than two inches at Waldorf, Md.

The rainfall at National pushed the total for the month to 3.81 inches, well above the 3.35 that is normal for June.