On the eve of the Labor Day weekend, the traditional end of the summer vacation season, the Washington area suffered its hottest day in more than three weeks, with the mercury climbing to 94 before thunderstorms began driving it down.

The storms, which brought showers and lightning to the western and southwestern outskirts of the area about 7 p.m., lowered the temperature at Dulles International Airport from 89 degrees at 6 p.m. to 78 degrees a few minutes after 7 p.m.

The high for the day at Dulles was 95, 1 degree above the high of 94 degrees reached at National Airport for the first time since Aug. 8.

Slight relief may be available today and Sunday, not so much from the heat, which could raise temperatures to the 90-degree mark again, but from the humidity, which may be slightly less insufferable, according to National Weather Service forecaster Harold Hess.

He said a week front, which is bringing drier air from the north and which "just barely has enough oomph to get down here," should begin arriving today.

"It's not a big deal," Hess said, "not a big relief, but at this point, any change will be an improvement."

In addition to being the sixth consecutive day of a least 90-degree temperature yesterday was the fourth day of teh area's sixth air pollution alert of the year.

An air quality index of 115, in the 'very unhealthy' range on the Metropolitan Council of Government's scale, was recorded at measuring stations in Washington, Alexandria and Bethesda. The alert will remain in effect at least through today.

"Once the gunk is here, it's hard to get rid of," said Lucile van Omering a OOG pollution specialist.

The traditional holiday exodus from the city was under way last night. Between 5 p.m. and 6 p.m., 2244 vehicles crossed the Chesapeake Bay Bridge, toward the Eastern Shore. Greyhound was adding two or three extra buses on each scheduled run toward the New York area.

When vacationers return they may not be happy. Forecaster Hess said the humidity that plastered shirts to backs this week should return on Labor Day and remain at least until the middle of next week.