Thunderstorms, which knocked out power to thousands of customers and brought torrential rain in some spots, swept across the Washington area last night, signaling the expected arrival today and tomorrow of the hottest and most uncomfortable days of the year.

The storms, which appeared to strike most severly in Fairfax County, brought more than 2.05 inches of rain to one section in less than an hour, and also cut off power to the Tysons Corner shopping center, closing stores and sending some movie patrons home without even learning how the films came out.

According to National Weather Service forecaster Jim Travers, the storms were produced when a mass of hot, humid air surging north from the Carolinas bumped against the edge of a mass of cooler air characteristic of northern regions.

Winds of up to 40 mph were reported, and according to a spokesman for the Virginia Electric and Power Co., a tree branch evidently was hurled into one of the company's substations at Tysons Corner. The short circuit that resulted about 5:30 p.m. left about 9,200 customers, including both homes and businesses, without electricity.

Guests at the Tysons Corner Marriott Hotel were brought to the lobby after the hotel's emergency power system failed to function properly, authorities said. The dramatic blackout at the center occurred just in time to thwart the efforts of patrons at one of its movie theaters to learn the outcome of "Raiders of The Lost Ark," a highly acclaimed new Melodrama.

Elsewhere, minor flooding was reported in such locations as Baileys Crossroads and the stretch of the George Washington Memorial Parkway in the Stratford area of southeastern Fairfax.

Forecaster Travers said that hot air that precipitated the storms would provide hot, hazy and humid weather at least through Tuesday, and send temperatures up as high as a possible 99 degrees by Monday.

Although yesterday was hot, the high reading was only 88 degrees. Today's high could reach 95, Travers said.