Snowplows took to the roads and District officials declared a snow emergency last night as area residents braced for the season's second significant snowfall, a storm that was expected to drop up to four inches by this morning.

Under the emergency, which began in the District at midnight, motorists were forbidden to park, or drive without snow tires or chains, on the city's snow emergency routes. District officials said they would ticket and tow cars on snow emergency routes throughout the night.

Forecasters said the snow, which began falling shortly after 5 p.m. yesterday, was likely to end by 9 a.m. Temperatures are expected to rise to about 40 degrees today, forecasters said, adding that the snowfall probably will be melted by afternoon sunshine. The low temperature tonight is expected to be 20 degrees.

"The snow should end by daybreak," said forecaster Marty McKewon of Accu-Weather service, "but it will suddenly get much, much colder later in the day. So whatever snow melts will refreeze quickly Monday night, and there could be a lot of iced roads."

Metro transit officials said last night that they expected trains on all rail lines to be operating this morning. Rails were being cleared of snow through the night, officials said. All Metrobuses were scheduled to operate, but officials said some bus lines could be affected by road conditions.

Federal officials said last night that they would decide this morning whether to close government offices because of the weather.

Area school districts had not announced their plans by 11 p.m. yesterday. William Henry, a spokesman for Montgomery County public schools, said a decision on whether to close schools or open them late was expected to be made about 5 a.m. today.

Donovan Gay, spokesman for the D.C. Department of Public Works, said city snowplows began clearing District streets at 6 p.m.

The city's entire fleet of 84 snow removal trucks was dispatched to spread salt and sand on major arteries, said Kirk Johnson, the chief snow emergency coordinator for the city.

"We ought to be in pretty good shape for rush hour," Johnson said. "All of crews are continuing to work through the night, but we're still not sure what's in store for the morning. If we get another sudden blast of snow, though, that could cause problems."

Johnson said city snow crews worked to clear bridges and major access roads in the District and would begin plowing secondary roads only if snowfall stopped and major avenues were safe. He said at 11:30 p.m. it was too early to tell whether crews could prevent roads from icing during the night.

A snow emergency also was declared last night in Washington's Maryland suburbs, where about 140 sand trucks took to the streets, said Donna Eldridge, a spokeswoman for the Maryland Highway Administration. She said no major accidents were reported.

The snow was the result of a storm that raced up the East Coast late last week after forming in the Gulf of Mexico, forecasters said.

Another winter storm, this one arriving from the Northwest plains, is expected to reach the area by midweek and bring strong winds and temperatures in the teens, but not snow, forecasters said.

On Nov. 11, in the season's first big test of area snow removal efforts, a foot of snow fell. Roads were cleared with relative speed in the District, where a federal holiday eased traffic, but massive traffic jams snarled the Woodrow Wilson Bridge and hundreds of students were stranded at schools in Prince George's and Fairfax counties.