The coldest weather of the season has settled into Western Pennsylvania and more snow is on our doorstep.

The National Weather Service issued a winter weather advisory through 7 a.m. tomorrow for Allegheny, Beaver, Greene, Washington, Fayette, Westmoreland and Indiana counties. A low-pressure system crossed the Ohio Valley today, bringing as much as 2 to 4 inches to the area. The advisory means periods of snow will cause travel difficulties.

Parts of Downtown were nearly gridlocked during rush-hour this evening. Much of that traffic has lifted, city dispatchers said, but some areas, including parts of Oakland, remained clogged tonight.

Port Authority spokesman David Whipkey said some busses are delayed due to road conditions, though he did not know specifically which ones. He said the T is running on schedule.

Traffic caused by slick and slushy roads was likely compounded by fans flocking to the Penguins' and Pitt basketball games, dispatchers and police said. No major accidents have been reported in the city.

Temperatures also will plummet over the next few days as arctic air takes over.

In fact, the West Mifflin Area School District has already canceled Friday classes because of the expected frigid temperatures. The Pittsburgh School District canceled night classes tonight. Duquesne University canceled all clases scheduled for 4 p.m. or later tonight.

The Pittsburgh Public Schools' early-childhood classes at the Kingsley Center in East Liberty are canceled for Thursday because of a heating problem. Teachers and aides are to report to the Pittsburgh Lincoln K-8 primary campus, 328 Lincoln Ave.

Check out the latest conditions and forecast for the frigid details.

A lake effect snow watch also is in effect from tonight through Friday evening for McKean, Warren, Crawford and Erie counties. Some locations could receive a foot of snow by Friday evening.

Commuters also can access the updated traffic conditions.

See information about school closings and delays on KDKA.com, the Post-Gazette's news partner.