A fast-moving snowstorm took the Washington area by surprise yesterday, causing hundreds of traffic accidents and delaying flights for holiday travelers as it cast a thick white blanket over much of the region and sent road crews scrambling to keep up.

With the possibility of up to an inch more snow today -- and with this morning's forecast calling for temperatures in the upper 20s and some icy travel -- officials announced last night that public schools in Montgomery, Prince George's and Loudoun counties would be closed today. Schools in Fairfax County and Falls Church will open two hours late.

"It's clear from the snow that's already fallen that there's no way we can safely transport children" or have them walk, "and ensure their safety for the rest of the day," Montgomery schools spokesman Brian J. Porter said early last night.

Porter noted that today is the last snow day allotted this year for Montgomery schools. If forced to shut down again this winter, he said, the school year might have to be extended or spring break reduced.

Yesterday's storm, which arrived more quickly and dropped far more than forecast, left nearly half a foot of snow on some communities in the Washington suburbs, rendering highways and neighborhood streets hazardous and catching road crews off guard before precipitation started to taper off at dusk.

Highway officials were vocal in their complaints about the forecasters, who had predicted only a light dusting and a much later start to the snowfall.

"They were just so awfully wrong," Dave Buck, a spokesman for the Maryland State Highway Administration, said of the forecasts. "So it did take us a little time to mobilize our folks. . . . There was definitely a little catching up going on for us."

Christopher Strong, a meteorologist with the National Weather Service, said the type of storm that swept over the area is notoriously difficult to forecast. Called an Alberta clipper, it is a fast-moving area of low pressure that dives south out of Canada, he said.

"Most clippers are pretty weak. Most don't do anything at all. . . . A smaller percentage give an inch or less of snow." Yesterday's storm, he said, was "pretty strong for a clipper system."

Another clipper is expected to come through today, Strong said, but it should be weaker than yesterday's.

The Weather Service reported accumulations of three to five inches north and west of the District, about three inches in the city and one to three inches to the south and east. The heaviest snow -- 5 1/2 inches -- was recorded in Vienna, with five inches reported in Rockville, Frederick and the area around Germantown.

With this and the December storms, most of the area already has seen more than a foot of snow.

The unexpectedly heavy snowfall delighted some residents but left others fuming over what they regarded as another blown call by weather forecasters.

"It's gorgeous," Sarah Agnew, 38, said as she arrived at a Fairfax City supermarket with her husband and two young children. The steady snowfall, with its winter-wonderland dusting of the area's trees, came as a pleasant surprise, she said, given that she was expecting only flurries.

Her husband, Jeff Agnew, 40, agreed that the thick, wet flakes made for a pretty scene. But, he noted: "They need to get the salt trucks out. I haven't seen any."

Highway department and airport officials said their preparations for the snowfall were initially based on weather forecasters' predictions of light snow or flurries starting in late afternoon, with an accumulation of about an inch.

"It surprised a lot of the weather people," said Steve St. Martin, duty operations officer at Reagan National Airport. "The timing was a little bit off. We had prepped for it last night in anticipation of weather arriving later on this afternoon."

The airport had to close its runways temporarily to treat for ice, causing some delays.

At Dulles International Airport, an operations officer said, "We're in the process of mobilizing our snow-removal equipment.

"Of course the weathermen didn't predict it," he said. "If you did your job that poorly, would you still have a job?"

On the last day of the busy holiday travel season, the Federal Aviation Administration reported a "traffic management program" in effect for Dulles, delaying arriving flights by an average of an hour and 20 minutes, with some as much as 3 1/2 hours late.

On the roads, conditions in the District were especially treacherous on untreated bridges, fire department spokesman Alan Etter said. Between 11 a.m. and 1 p.m., he said, fire crews were at "an accident on virtually every bridge in town." He said no one was seriously hurt in any of the collisions.

Joan Morris, a spokeswoman for the Virginia Department of Transportation, said 200 trucks were dispatched to spread salt on Northern Virginia roads and that 900 would be in action overnight to try to prevent icing during the morning rush hour. She said the trucks initially were not plowing because not enough snow had accumulated.

If the forecast had been more accurate, she said, "we would have started with more trucks."

Virginia State Police said troopers had responded to more than 100 accidents yesterday.

In Maryland, Montgomery and Frederick counties appeared to be the hardest hit. Maryland State Police reported more than a dozen vehicles spinning into ditches and colliding with each other on Interstate 270 alone.

By 5 p.m., Montgomery County police and rescue workers said they had responded to nearly 200 accidents, with about one-quarter of those resulting in injuries, few of them serious.

In Baltimore County, a driver lost control on Interstate 695 triggering a 19-car pileup.

Buck said the State Highway Administration had deployed 1,000 crews across Maryland, including 230 in Montgomery and Prince George's counties. But it took until noon to get all the trucks on the road, he said.

In Montgomery, the Division of Highway Services dispatched 200 plows and salt trucks onto the streets, but there, too, officials said they were caught by surprise. "The conditions deteriorated pretty quickly," said John Thompson, chief of the county's highway division.

Many area congregations canceled church services because of the snow. At Forcey Memorial Church in Silver Spring, the Rev. Philip Powers saw a Bible lesson in the blown forecast.

"The weather forecasters had said . . . it was going to be a light snow, but this shows that we don't know what God's will is," the pastor said. "But God certainly knows what he has planned for us."

Staff writers David S. Fallis, David A. Fahrenthold, Hamil R. Harris and Martin Weil contributed to this report.

A little snow isn't slowing down 7-year-old Noah Howard as he plays in the Bishop's Garden at Washington National Cathedral.Shoreham Drive NW slices a clear path through the blanket of snow in Rock Creek Park as pedestrians take in the wintry wonderland. At far left, Krista Kasprenski and Frank play on the grounds of the Washington Monument.About noon, an abandoned car sits in the median at Route 28 and Gude Drive. The weather didn't deter the crowds at Great Falls Park, where visitors could get a look at the flooding Potomac River. Between yesterday's storm and December's snowfall, most of the area has already seen more than a foot of snow.Near Lafayette Square, drivers weren't the only ones stuck in the snow.