The season's first storm deposited 3 to 5 inches of snow on the Washington area yesterday, slowing commuter traffic, closing some schools and bringing the long, glorious autumn of 1978 to an abrupt end.

With temperatures hovering just above freezing, snow blanketed the area Sunday night and early yesterday, then turned to cold, intermittent rain the rest of the day.

The storm, which was predicted by the National Weather Service about five hours before it hit the service's official observation post at Washington National Airport just after 9 p.m. Sunday, forced schools to close in Loudoun and Prince William counties and open late in most closer-in suburbs. Only Alexandria and the District of Columbia opened their schools at the normal time.

Metro trains and buses carried unusually heavy passenger loads as many commuters left cars at home. Metrobuses ran 15 to 30 minutes late in some areas. Passengers on the Metro train's Ted Line were delayed a minumum of 15 minutes at about 8:30 a.m., but the Blue Line reportedly ran on schedule.

At the Blue Line's Rosslyn station, the four escalators leading to the surface suddenly stopped at 9:15 a.m., forcing commuters 211-foot long stairway. (Work crews had to cut off the power for 12 to 15 minutes to repair a water leak in a high voltage electrical locker.)

"I can't talk," gasped Loretta Helm of Seat Pleasant as she arrived at the top of the escalator.

"It's good exercise," said Bob Vuolo. "I think I'm out of shape."

The snow was the first to hit Washington since a string of storms buffeted the area last February and March. The weather service measured three inches of snow at National Airport yesterday, but accumulations of up to five inches were reported west of the city.

Weather service forecasters said today should be variably cloudy with only a 10 percent chance of precipitation. Temperatures should run about normal for the rest of the week with highs in the upper 40s at night, but increasing cloudiness may bring more rain by Thursday or Friday, forecasters said.

Yesterday's snow was unusually early for the season but not a record. Just last year, .1 inches of snow fell Nov. 27, and the year before, .6 inches fell on Nov. 12. Traces of snow have occurred as early as Oct. 19 (1972) and Oct. 5 (1892).

The snow brings the official total precipitation for November at National Airport to almost two inches, still below the normal of 2.90 inches.

Unusually dry conditions in October and the first weeks of November sent the Potomac River and surrounding water table to the lowest levels in a year and caused the National Parks Service to issue a ban on campfires in park areas.

Drought conditions also left the town of Round Hill in western Loudoun County with an almost empty reservoir. Water rationing began in the town yesterday despite the arrival of rain and five inches of snow.

Town Mayor Jeffrey Wolford said it was too early to determine how much relief the precipitation may bring.