Area transportation officials braced last night for road and runway clearing efforts as the first fine flakes began to fall in what weather forecasters said could be one of the heaviest snowstorms here in a long time.

National Weather Service forecaster Chet Henricksen foresaw a "high likelihood that Washington would be covered by four inches of snow with a "pretty good chance" that the accumulation would amount to six inches.

Either amount is substantial for the Washington area and, combined with the high winds expected to move in out of the Northeast today, likely would prove extremely disruptive.

With winds that could reach up to 30 miles an hour likely to furstrate efforts to sweep mounting accumulations of snow off roads and highways, morning rush hour "is liable to be a mess," Henricksen said. "It's going to be very difficult traveling . . ."

While thousands of Washingtonians listened to weather reports yesterday evening and brooded about the gloomy forecasts, area transportation officials began gearing up for the crucial contest with the snow.

By about 6 p.m. Arlington County authorities reported about 35 trucks fitted with chains and plows and waiting in the Shirlington property yard, "ready to go."

Normally, said Dennis R. Johnson, operations chief in the county Public Works Department, the first line of attack would have involved the spreading of salt.

But, he said, expecting a major storm, the county fitted its vehicles at once with chains and plows.

"It looks like an all-night operation," he said, adding that it might take "a couple of days" to clear all 375 miles of roads and streets under county jurisdiction.

At District of Columbia snow removal headquarters, 20 trucks were reported ready to begin salt-spreading operations, while scouts for the city Transportation Department roamed the streets in autos, watching for spots that required immediate attention.

Sixty more trucks and crew would be sent into action as soon as heavy snow - as opposed to last night's fine flakes and flurries - became imminents, a spokesman said.

A fine snowfall began as early as 2:26 p.m. at Dulles International Airport. It fell from a gray overcast out of which the sun at times seemed tantalizingly close to emerging. But the trend was toward stormy weather.

At 5 p.m. it was snowing in Loudoun and Fauquier counties in Virginia and in frederick County, Maryland, as well. By 6:40 p.m. snow was falling at National Airport here.

Henricksen, the weather service forecaster, said snow would become heavier after midnight this morning, as a low pressure system developing along the coast of the Carolinas moved very slowly northward.

He said the snow is expected to taper off into flurries by tonight and end entirely about midnight.

While predicting a "pretty good chance" of six inches of snow here, he said that even four would be "pretty significant," noting that the record snowfall for today's date is 4.3 inches, set in 1911. The greatest single snowfall measured at National Airport last month was 4.6 inches, on Jan. 13.