Snow and sleet slickened roads across the Washington region by last night. But forecasters predicted that the season's heaviest snows were still to come and would blanket the Washington area today and tomorrow, accumulating to more than a foot in many places.

The National Weather Service predicted the possibility of "a large-scale snow event," and almost all other available forecasts spoke with increasing assurance of snow depths greater than anything seen here in several years.

The storm that began here Friday brought a total of five inches of snow to Dulles International Airport by last evening. Much of the snow that fell over the past two days melted on roads, but surfaces began to freeze as daylight ended and about 30 collisions in which people were injured were reported in Montgomery County between 4 and 8 p.m., officials said.

The snow halted briefly last night, with about three inches measured in much of Montgomery and Fairfax counties. But huge amounts of moisture were expected to be funneled into the teeth of arctic air here today, setting the stage for a large storm, the weather service said.

According to the weather service, the snowfall could persist until tomorrow, leaving many parts of the region under as much as two feet of snow. AccuWeather, a private forecasting service, also foresaw the possibility of more than 12 inches of snow across the region, with wind and drifts making travel even more difficult.

The giant storm was expected to spread snow over much of the mid-Atlantic region, from southwest Virginia north and east into New York and Boston.

However, as of late last night, the heaviest snowfall was expected in Virginia west of the Blue Ridge mountains, where forecasts predicted that depths in the two-foot range could be common by the end of the storm.

Weather forecasters said the slow pace of the storm would account for the unusually high precipitation totals. It had been predicted previously that the storm would have blown out to sea by early tomorrow, but later forecasts suggested that it would stick around.

Yesterday's light but persistent precipitation, linked to the same weather system that poured rain on Los Angeles last week, marked the 11th day since December that snowfall has been recorded at Reagan National Airport, a Weather Service spokesman said.

John Newkirk, a program manager at the Weather Service office in Sterling, said today's storm was coming from the Lower Mississippi Valley and could gain much intensity once it hit the North Carolina coast.

"If this turns out to be the storm of the year, we will already have had a lot of practice," said Ryan Hall, spokesman for the Virginia Department of Transportation. The snowy winter has pushed Virginia's snow removal costs to $50 million, $2 million more than budgeted. Hall said 1,600 vehicles were ready to go in Northern Virginia for today's expected storm.

Instead of the bright white frosting of several recent snowstorms, yesterday's weather gave hardy shoppers gray skies and boots soaked by slush. The rapid melting meant "the worst problem in the last 24 hours has been boredom," Hall said.

In the District, 176 trucks cleared streets and prepared for today. A D.C. snow emergency was scheduled to go into effect at 3 p.m., meaning no parking on routes marked by red-and-white signs.

Some media commentators noted the absence of D.C. Mayor Anthony A. Williams, who was spending the weekend in Puerto Rico -- where the weather was in the 80s but cloudy yesterday afternoon. Former mayor Marion Barry suffered politically for being in California attending the Super Bowl during a January 1987 snowstorm when cleanup efforts faltered. But officials said they were confident they would get the job done while Williams got some needed rest.

"We consider it a compliment that he is comfortable enough to take the weekend away with his wife," said Mary Myers, spokeswoman for the city public works department. "We're good to go."

Maryland highway officials had 1,600 crews out statewide, including 400 in Montgomery and Prince George's counties. Yesterday, they were clearing what accumulated and preparing roads and crews for a bigger job today.

So far this year, Maryland has gotten 23 inches of snow, compared to 4.5 inches all of last year. Its $21 million snow removal budget is blown, with $35 million spent before this weekend.

"The prime focus is keeping the roads safe for motorists. We'll deal with the budget in the spring," state highway spokeswoman Lora Rakowski said.

Amtrak officials said they were prepared for a busy, snowy holiday weekend, staging diesel locomotives in case of electrical problems, putting emergency crews up in hotels along the Northeast corridor and issuing each train an official broom and shovel.

"This particular storm is getting the largest response of the season so far," said Dan Stessel, a spokesman for the rail system.

As of yesterday afternoon, all Amtrak trains were on schedule, and there were no reported problems at area airports.

On the roads, standing water turned to ice after dark, and as it did, calls began to flood into police and fire and rescue switchboards in Prince George's and Montgomery counties.

One notable pileup in Montgomery occurred near Montgomery Mall and involved a dozen vehicles, county fire and rescue spokesman Pete Piringer said.

Near the scene, on Westlake Drive near Democracy Boulevard, he said he saw several pedestrians slip and fall on the icy pavements.

Despite "some pretty slippery roads," there were no immediate reports of serious injuries, Piringer said.

By late yesterday afternoon, about 3 1/2 inches of snow had been measured in Brookeville and three in Olney, Gaithersburg, Glenmont and Montgomery Village. Five inches fell in Winchester and four at Gore in Frederick County. The 3.2 inches that fell yesterday at Dulles was a record for the date.

Staff writers Martin Weil and Clarence Williams contributed to this report.

A horse scratches the heavily covered ground in search of grass along Centennial Lane in Columbia.Jodi Jellie and her Labs tromp through the first bit of weekend snow in Myersville.A Virginia Department of Transportation snowplow, one of many on the road, clears Streamview Lane near Vienna. Snow removal costs are over budget.