Temperatures in Western Pennsylvania will approach record highs during what forecasters expect to be a warm and wet weekend before Christmas.

A low-pressure system pushing its way up the Tennessee Valley will move into the area Thursday, bringing temperatures predicted to hit a high of 52 on Friday, 55 on Saturday and 64 on Sunday in Pittsburgh, said John Barnley, a meteorologist with the National Weather Service in Moon.

As much as three quarters of an inch of rain is expected to fall in Western Pennsylvania over the weekend, and while that might swell smaller streams and increase current speeds on bigger rivers, forecasters don't expect a flooding risk, Mr. Barnley said.

"There will be some higher velocities. People who operate commercial vessels will have to be more vigilant. Water will be moving faster," he said.

As the system pushes out of the area late Sunday and into Monday morning, expect frigid air to fill the gap.

"The stronger the warm air ahead of the system usually means the stronger the cold air behind the system once it goes through," Mr. Barnley said, adding that the projected high for Monday was expected to fall to 33 after Sunday's balmy weather.

Sunday's high will approach the Dec. 22 record of 67, set in 1949.

The warm, wet weather is part of what AccuWeather calls a "brief, but major shift" in weather patterns that will send warm air north and produce heavy rain for a large part of the Eastern United States on the first weekend of Winter.

"Highs will be in the 70s over much of the South with a few spots flirting with 80 degrees Saturday and Sunday," AccuWeather said. "Temperatures will reach near 60 degrees along the Ohio River and the Mason-Dixon Line bordering Pennsylvania and Maryland."

AccuWeather warned that the 1 to 2 inches of rain expected over now snow-covered areas could pose risks for flat roofs.

"The snow may act like a sponge, absorbing the rain and gaining weight in the process," said Joseph Sobel, AccuWeather.com senior vice president and forensics weather expert. "Prior drifting on flat roofs can be major problem, by causing uneven weight distribution."

Robert Zullo: rzullo@post-gazette.com or 412-263-3909.

First Published: December 17, 2013, 7:59 p.m.
Updated: December 18, 2013, 5:34 a.m.