Snow, up to a foot, is forecast for Philly area Wednesday, with a big range of accumulation totals

»Update: Snow starts to fall in the Philadelphia region. Click here for the most recent updates.

A storm that could rival the coronavirus for its shutdown powers might layer a prodigious amount of snow on parts of the region Wednesday afternoon into Thursday, forecasters said, along with a disruptive mix of ice, cold rain, gale-force winds, power outages, and coastal flooding.

The National Weather Service has a winter storm warning posted for all the counties west of the Delaware River for up to a foot of snow in some areas, and a winter weather advisory for up to 5 inches for the New Jersey counties along the river. The weather service said that the region might even see lightning and hear thunder as the storm intensifies at night.

The first flakes were due to arrive in the region sometime between 1 and 3 p.m., the weather service said, and the latest trends in computer models suggested that the rain/snow line would be quite close to the I-95 corridor, and that could shave a few inches off the forecast amounts in and around the city.

About the only things that seemed certain were that snow totals would vary dramatically, even within the distance of a few miles, and that Philadelphia was about to end its 655-day snow drought. Just how emphatically would depend on what happens about 2 miles up in the atmosphere on Wednesday night.

Amounts in the city could vary from 4 inches in the lowlands of Philadelphia International Airport, where the stuff is measured officially, to 10 inches in the higher terrain of the city, the weather service said, with more in the counties to the north and west. The projected amounts drop off precipitously across the river.

“It’s pretty insane,” said Trent Davis, a meteorologist at the weather service in Mount Holly.

» READ MORE: How much snow for Philly this winter? After a bust year, TV meteorologists take their shots

Temperatures might be near freezing at the surface, so elevation could matter. Temperatures decrease with height, and even a rise of a few hundred feet can mean the difference between accumulating snow and rain. That’s one reason why amounts would likely be higher in, say, Roxborough, than at PHL.

But the big forecast conundrum is about 10,000 feet up, said Greg Postel, winter-storm specialist at the Weather Channel. The storm is expected to lure an upper-level nose of warm air, about 2,000 feet deep, from above the Atlantic, where water temperatures are in the upper 40s. That would cause falling snow to melt and become rain. If it refreezes, it will come down as sleet, cutting accumulations. Sleet accumulates at about a third of the rate of snowflakes.

The projected path of the storm makes the warming more likely, Postel added. Most big snows around here are generated by storms forming off the North Carolina or Virginia coast, but this is cutting across the Delmarva peninsula before it heads out to sea and makes a left.

Steve Decker, meteorology professor at Rutgers University, said it’s possible that the warm layer won’t be able to nose past the Delaware River. If that were the case, all of Philadelphia would have a major snowstorm. If even only 3 inches fell, that would be 10 times what fell all of last winter.

It looks as though everyone will have time to panic-shop Wednesday morning since the snow should hold off until midafternoon.

It is likely to become heavy quickly, and Davis said that areas that find themselves under snow “bands,” could see as much as 3 inches in an hour.

Dave Dombek, senior meteorologist at AccuWeather, said the bulk of the accumulation is likely to happen between 6 p.m. Wednesday and 1 a.m. Thursday. The Weather Channel’s Postel said an additional inch or two could be added Thursday morning as the storm pulls away.

» READ MORE: Feds’ updated winter forecast favors warmth here and in most of the nation, especially December

Yes, Peco has heard all about this, and so have the highway departments, officials assure and they are all taking requisite precautions.

With temperatures borderline, the flakes are likely to be water-laden and weighty, and that could lead to power outages, the weather service said.

They could be a major problem for SEPTA’s power lines, said Scott Sauer, assistant general manager.

They also might take a toll on trees, especially evergreens, which have shallow roots and needles that are efficient at capturing and holding snow, said Jason Parker, Horsham district manager for the Davey Tree Expert Co. tree service in Horsham.

Given the new moon, which actually has a stronger tidal pull than the full moon, and the potential for onshore wind gusts to 60 mph, moderate coastal flooding is expected with high tide Thursday morning at the Shore, and minor flooding late Wednesday, the weather service said.

Mixed precipitation is a seasonal standard in December. The winter chill hasn’t quite ripened in the upper atmosphere, and the ocean temperatures still are quite warm. On Tuesday, temperatures in the Atlantic Ocean off Atlantic City were in the upper 40s.

» READ MORE: Snow shocked Philadelphia in December 1960. The forecast whiffs these days tend to be quite different.

Air circulates counterclockwise around storms, and the Philadelphia region will be experiencing onshore winds as the storm passes to the south.

The circumstances were similar in one of the most historic December storms on record. During a powerful nor’easter on Christmas night in 1776, Gen. George Washington led his troops through a punishing mixture of snow and falling ice to surprise the Hessian soldiers allied with the British.

The mixed precipitation obviously held down accumulations. Well to the west in Monticello, Va., Thomas Jefferson reported 21 inches of snow, or about double what fell around Washington Crossing.

» READ MORE: Much has changed in 2020. The magic and mystery of snow persist.