A surprise four-hour snow shower caught commuters and removal crews flatfooted yesterday, creeping up to glaze the evening rush hour to a standstill in many places and cancel nighttime programs throughout the area.

The snow, which started as the flurries that forecasters had predicted, began to spread over the area about 1:30 and quickly blossomed into more than a dusting.

It accumulated quickly on roads chilled from the biting cold and soon was pressed into a slick, powdery icing that glazed the highways and within an hour had traffic jammed on major thoroughfares across the region.

Lingering icy conditions were possible on the roads this morning, although salt trucks had several hours to assail the remaining snow last night. And the week's worst weather may be yet to come, as a threat of more serious snowfall looms tonight and tomorrow.

Nineteen Prince George's County school buses carrying children homeward were still on the roads at 11:20 p.m., said county schools spokeswoman Jocelyn Harris, who estimated that each bus may have had 30 students aboard.

Thirty school buses also were stuck in traffic in Montgomery County at 9 p.m. but had delivered their passengers by 9:30, schools spokesman Brian J. Porter said. Some of the buses in in both counties were equipped with radios, and school officials were able to relay messages by phone to reassure parents of students on board.

Schools in Montgomery and Prince George's will open two hours late today. There will be no morning kindergarten or head-start classes in Montgomery; Prince George's morning and afternoon kindergarten, pre-kindergarten and head-start classes have been canceled.

Traffic was still creeping or not moving at all on some major arteries in the city and the Maryland suburbs after 11 last night.

Snow emergencies were declared in Montgomery, Prince George's and St. Mary's counties, but authorities appeared to be late in ordering out the salt and sand trucks. Some trucks were not seen making their passes until the snow was almost over.

"It caught us off-guard," said John W. Thompson, chief of Montgomery County's Division of Highway Services. "Our forecasters talked about a few flurries this afternoon, but by 2:15 we had quite a bit of snow coming into the county. It's just something we had to react to."

Weather forecasters said their predictions were not far off the mark.

"We had a heavy dusting of snow, and it immediately stuck to the roads because it wasn't anywhere near freezing today," National Weather Service meteorologist Dewey Walston said. "The high was only 22 degrees."

Officially, 0.4 inches of snow fell at Reagan National Airport, he said, and just 0.2 inches fell at Dulles International Airport.

But it was enough to sow traffic chaos.

In Montgomery, police temporarily closed winding East West Highway between Connecticut and Wisconsin avenues at the height of the crunch. In the District, 20-minute commutes degenerated into multi-hour ordeals. Traffic along Georgia Avenue NW, one of the main routes in and out of the city, was at a standstill for most of the afternoon and evening.

Wesley N. Gordon, a dentist whose offices are at Van Buren Street and Georgia Avenue NW, said he noticed a problem about 3 p.m., when he tried to leave. "All of a sudden, I saw everything getting serious," he said.

Almost three hours later, he still had not left his office. "I've never seen something like this, and I've been here 30 years," Gordon said. "I watched a bus for 35 minutes, and it didn't move."

It took Mark Dettelbach, a 35-year-old physician, four hours to get from the Washington Hospital Center to his home in Chevy Chase--a rush-hour trip that usually takes 30 minutes. "I could have been to New York in that time," he said.

Hundreds of trapped motorists vented over their cellular phones.

"I'm at Washington Circle, at Pennsylvania and New Hampshire avenues," one frustrated Virginia-bound driver, who declined to give his name, said via phone at 6 p.m. "I've been here for 35 minutes, and nothing's moving."

The man said he was upset with D.C. Mayor Anthony A. Williams (D). "He should have had the salt trucks out here three hours ago when the snow first started."

In Virginia, traffic also stalled to extraordinary levels.

Fairfax County police reported that traveling on the Capital Beltway's inner loop from Leesburg Pike (Route 7) to Georgetown Pike (Route 193), a three-mile stretch, was taking about 50 minutes.

With the tie-ups and fender benders, Fairfax police implemented their snow accident policy, in which they ask motorists in minor crashes to exchange information and keep driving if possible, because officers will be too busy to investigate crashes and take reports at the scene.

Off the roadways, Metrorail was a refuge for a while. But then a Yellow Line train got stuck between the Pentagon and L'Enfant Plaza after a water pipe cracked and fell onto the third rail, said Metro spokeswoman Cheryl Y. Johnson. She said the stalled train resulted in delays of at least a half-hour on the Yellow, Green and Blue lines and generated crowding on some Orange Line trains.

Johnson said that buses were avoiding hills and holding their own.

By 6 p.m. the snow had stopped most places, but the damage was done.

In Prince George's County, six county school buses were involved in minor crashes, although no students were hurt, said Capt. Mark Brady, spokesman for the Prince George's fire department.

Evening traffic was just as bad in College Park, a major bottleneck where Interstate 95 branches north from the Beltway. "It's pretty much a standstill," said Sgt. R.W. Riggin of the state police.

By 10 last night, public works employees in the District were beginning to make headway on the icy streets by spreading salt, Public Works Director Vanessa Dale Burns said.

Burns said that while major arteries were clearing, the biggest problem was on less-traveled streets such as residential roads. "In the main corridors, the traffic is allowing the salt to move around," Burns said. "In the side streets, the ice is becoming packed."

The precipitation was part of the abrupt arrival of winter weather to the area over the weekend, after a season of unusually high temperatures.

But yesterday may have been only the beginning, as forecasters warned of even lower, harsher temperatures later in the week, and a storm laden with several inches of snow due to arrive tonight.

Staff writers Katherine Shaver, Annie Gowen, Brigid Schulte, Tom Jackman, Josh White, Cheryl W. Thompson, Craig Whitlock, Steven Gray, Allan Lengel, Raja Mishra, Maria Glod, Arthur Santana, Angela Paik, Phuong Ly and Lindsey Layton contributed to this report.

CAPTION: Esaul Ramirez, 4, of the District, takes advantage of the weather and catches snowflakes on his tongue. For children in Montgomery and Prince George's counties, bus rides home became hours-long ordeals.

CAPTION: A pedestrian crosses 16th Street at Columbia Road NW. Yesterday's snow stifled the evening rush hour and caused fender benders across the region.

CAPTION: Esaul Ramirez, 4, delights in the accumulation and writes his name in snow along 16th Street.