The forecast bears a remarkable similarity to what actually unfolded in mid-November: Snow, beginning by late morning, accumulating in a hurry, and threatening to put the evening commute on the skids.

“It certainly has those hallmarks,” sad Sarah Johnson, lead meteorologist at the National Weather Service, which has posted a winter-storm warning for parts of the area and advisories everywhere else in the region for 2 to 5 inches of snow, followed by ice, then rain.

The National Weather Service issued this snow projection map Tuesday morning for Wednesday's winter storm.
National Weather Service
The National Weather Service issued this snow projection map Tuesday morning for Wednesday's winter storm.

However, whatever happens on Wednesday will differ from what occurred three months ago in one fundamentally important aspect: This time, it won’t be an ambush.

The ferocity of that Nov. 15 storm — still officially the biggest snow of the season at 3.6 inches — caught almost everyone off guard. Hundreds of accidents were reported, traffic messes were exacerbated by early school closings, normally short commutes ended up taking hours.

Early school closings aren’t going to be a huge issue in Philadelphia on Wednesday. In light of the forecast, schools on Tuesday began announcing preemptive closings, including all Philadelphia public and archdiocesan schools, giving hundreds of thousands of pupils their second day off of the week.

Other school districts and private schools in the Pennsylvania suburbs that announced they would be closed Wednesday include the Abington, Springfield (both Montgomery and Delaware Counties), Central Bucks, Cheltenham, Lower Merion, Lower Moreland, Coatesville, Downingtown, Marple Newtown, and William Penn School Districts, and Abrams Hebrew Academy, New Hope Academy, Shipley Academy, and the Haverford School. In South Jersey, many of the schools and school district were closing early. There were a few exceptions, including the Camden City and Florence Township School Districts, that announced they would be closed.

Roads around the region have been generously layered with rows of anti-icing brine.

The forecasts Tuesday were confidently calling for snow at the start, and sleet and/or freezing rain could mix in during the peak afternoon commuting period, Johnson said. Temperatures are expected to hover near freezing, which means the snow is likely to be particularly juicy.

The precise timing of the arrival, and when the changeover would occur, remained critical questions.

Warmer air surging from an approaching storm will rise and condense into snow from southwest to northeast, forecasters said.

Areas well to the south and west of Philadelphia are likely to get higher accumulations, said Dave Dombek, a senior meteorologist with AccuWeather Inc., because the snow would arrive before the sun could have much of an effect.

In late February, even behind the clouds, the sun can be a major factor in accumulations, especially in urban areas.

This month, solar radiation in the mid-latitudes of the northern hemisphere is making significant gains daily, and paved surfaces have noticed: For evidence, check the car thermometer when you start up the engine after the car has been parked on sun-warmed asphalt.

Should the snow hold off until, say, noon, “you have to have it come down hard to get any accumulation,” said Dombek.

No matter what happens, this will be an evanescent event.

The temperatures will head toward 50 on Thursday, and the sun should go to town on whatever muck is left.

Appropriately, the snowfall is due to follow Tuesday’s evening’s so-called Super Snow Moon, which will live up to its name in a way in Philadelphia.

Staff writer Joseph A. Gambardello contributed to this article.