The Washington area braced early today for a possible major winter storm as the U.S. Weather Service predicted that as much as 4 to 6 inches of snow may fall by late afternoon.

The storm, which forecasters predicted would start about sunrise, was not expected to hamper morning rush-hour traffic. However, if the forecast is accurate, officials said, evening traffic may face serious obstacles.

Officials of area school systems said they would decide by 6:30 a.m. today whether schools would open on time. Area radio and television stations are to report these decisions.

"It has to be a very heavy snow to bother us," said a Metro spokesman, whose agency has purchased $500,000 worth of snow removal equipment to prevent its trains from becoming snowbound as they did during last February's storm.

Anticipating the snow's arrival, transportation officials from Montgomery County to Alexandria prepared snow plows, checked spreading trucks to make sure they all started, examined headlights and even replaced some windshield wiper blades yesterday while keeping an ear out for updated forecasts.

A rainstorm that was centered over east central Louisiana late yesterday and moving in an easterly direction triggered the area's first snow alert of the winter.

Forecasters for the Weather Service said as the rain storm moved northeasterly and brushed against cold air swirling over the mid-Atlantic states from Canada, the rain would change to snow.

Yesterday's high of 45 was expected to fall off to 35 today with temperatures dipping to the low and mid-20s tonight, forecasters said. And when the snow is expected to begin early today, temperatures will be around freezing.

Washington has been spared extreme winter temperatures and snow squalls for a month because wind patterns brought warm Pacific Ocean breezes straight across the country -- departing from their usual detour through the Canadian icebox, the weatherman said.

These wind patterns kept mid-afternoon temperatures above 50 degree during most of December and even caused several 60-degree days, according to the weather service.

But now the winds have shifted and the area is receiving its usual Arctic winds.

That wind change and the approaching rain storm sent Metro officials checking their stockpile of 500 tons of salt and 500 snow shovels.

If the snow comes, the District will detail 20 of its 87 salt spreaders to Metro to clear major bus routes, while city workers will use the remaining 67 to spread part of the city's own supply of 6,000 tons of sand and 30,000 tons of salt.

Some area residents, apparently recalling last February's sudden snow storm that shut down the area for almost three days, also took preparations yesterday.

By 5 p.m., the Little Falls Mall in Bethesda was sold out of milk and white bread.