The National Weather Service has placed the Philadelphia region under a flood watch, forecasting up to 2.5 inches of rain from Thursday night into Friday, the first day of winter.

“Heavy rain will result in flooding of roadways and areas of poor drainage,” The weather service says in it updated advisory. "There is also the potential for significant river flooding. The flood risk will increase where heavy rain and embedded thunderstorms move over the same area, even potentially leading to localized flash flooding.“

The area in green is under a flood watch. The area in beige is under a flood watch and a wind advisory.
National Weathet Service
The area in green is under a flood watch. The area in beige is under a flood watch and a wind advisory.

The watch is in effect from 7 p.m. Thursday through 1 a.m. Saturday, with a rain total of 1.5. inches to 2.5 inches, and possibly more in some place, expected.

Winter, for the record, begins at 5:23 p.m. Friday.

Along with the flood watch, a wind advisory is in effect at the Shore for southeast winds 20 to 30 mph with gusts up to 50 mph. A gale watch for possible seas of seven to 10 feet also means pounding surf along the coast.

“In addition to the freshwater flooding, the combination of heavy rain and increased tide levels during the times of high tide Friday morning will result in minor to locally moderate coastal flooding along the Atlantic, Delaware Bay, the tidal Delaware River, and the eastern shores of the Chesapeake Bay,” the weather service said.

The cause is an expected convergence of an area of low pressure slowly drifting across the Southeastern states with a surface trough extending northward along the East Coast.

The forecast for more rain comes as 2018 is already at No. 2 as the wettest year on record dating back to 1885. Measurable rain has been reported an average of four out of 10 days so far this year.

After a frosty start Thursday morning, the weather system bringing the rain will be accompanied by warmer temperatures, with highs of 50 degrees Thursday and 61 on Friday predicted for Philadelphia.

Once the storm leaves, there is no precipitation in the forecast for the weekend and the beginning of next week, with daytime highs in the 40s and overnight lows around freezing.

The Christmas Day forecast calls for mostly sunny skies with a high around 41.