Butler County and the rest of Western Pennsylvania are bracing for another major winter storm that could bring a foot of snow to some areas.

The National Weather Service has issued winter storm warnings for the southern half of the state stretching from this afternoon into Saturday night.

In Western Pennsylvania, the snow was expected to start flying about noon. About an inch should blow in with winds this afternoon. Tonight another 4 to 8 inches may accumulate.

Six to 12 inches are expected in the Pittsburgh area.

Southwest Airlines has already canceled flights out of Philadelphia International Airport from this afternoon into Saturday because of the storm.

The state Department of Transportation asked people to stay off the roads tonight and Saturday and reminded drivers who must venture out to pack an emergency kit in their vehicle.

The powerful winter storm bore down on the Mid-Atlantic with as much as two feet of snow in store for the nation's capital, where the federal government prepared to shut early.

Wet snow was forecast from Virginia and West Virginia across Maryland into southern New Jersey and Pennsylvania.

The federal government, the region's largest employer, told workers they could take today off as unplanned leave and prepared to shut offices four hours early.

Those who can work from home should do so, said Joan Morris, a spokeswoman for the Virginia Department of Transportation.

"This is not a good mix," Morris said. "Heavy, wet snow with gusting winds is going to make it a very tough storm for us. I expect visibility will be very poor in spots, and we'll have to deal with drifting snow."

The National Weather Service issued a warning stretching from Baltimore to Northern Virginia and parts of West Virginia. Total accumulations of 16 inches to 24 inches are expected.

The weather service warns that the mix of heavy snow and strong winds would make travel tonight "very hazardous or nearly impossible."

Virginia's General Assembly canceled today's floor sessions and committee meetings, the first time anyone could remember that the threat of snow had sent the whole legislature home.

Amtrak canceled most trains heading south from Washington, D.C.

Virginia Gov. Bob McDonnell, who has been in office less than a month, declared his second snow emergency, authorizing state agencies to assist local governments.

A mid-December storm brought about 20 inches of snow to many areas in the region.

Between that and several smaller snowfalls, the region's road crews have had plenty of practice in the past two months.