AccuWeather Inc. forecast six inches for the city, with up to a foot in counties north and west. Although snow was expected to accumulate in a hurry overnight, computer forecast models late Wednesday still were bickering over when it might change to sleet and rain.
"There still seems to be an above-average model [variation] this close to the event," said Gary Szatkowski, meteorologist in charge of the National Weather Service office in Mount Holly.
But long before the arrival of the first flakes, the storm closed schools, courts and government offices across the region, canceled flights, and even nixed a Thursday circus performance in Philadelphia. Municipal emergency declarations swept across Pennsylvania and New Jersey.
In addition to snow and a challenge to the region's salt supplies, projected winds gusting up to 30 m.p.h. have the potential to generate more power outages, take down trees, and even cause road flooding. While snow usually isn't an issue for utilities, sleet and freezing rain can bring down trees and wires.
"I think Mother Nature said, 'We are going to give it to you all at once,' " said Nick Martino, the Pennsylvania Department of Transportation maintenance director in the Philadelphia region.
Unlike other, drier storms this winter, this system is laden with moisture from the Gulf of Mexico and Atlantic Ocean.
"This is going to be a heavy, wet snow," Mayor Nutter said at a Wednesday news conference, adding that it has the potential to bring down power lines on a scale similar to last week's ice storm.
SEPTA's Market-Frankford and Broad Street lines were to run through the night in anticipation of bus operations being disrupted.
PATCO said it would operate trains from 5 to 7 a.m. every 15 minutes, from 7 to 9 a.m. every 10 minutes, and from 9 a.m. to 3 p.m. every 15 minutes.
Inmate visits were canceled Thursday at all Philadelphia jails. Trash will not be collected Thursday and Friday but will be picked up Monday, even though it is a holiday.
School districts across the region canceled classes Thursday. Thousands of Thursday flights were canceled in Philadelphia, Washington, Baltimore, New York City, and Boston.
Peco, still recovering from restoring power to 715,000 customers who were knocked out by the Feb. 5 ice storm, was bracing for the third winter storm in the last two weeks. The company serves 1.6 million customers in Southeastern Pennsylvania.
"We're gearing up and getting ready for another round," said spokesman Greg T. Smore.
Peco's storm center has remained open since a Feb. 3 snowstorm caused 60,000 outages, days before the Feb. 5 ice storm knocked out more than half of the utility's suburban customers.
Peco began the last storm with 3,000 employees and contractors in place, and ended up with 6,800 at the peak, many supplied by other utilities from around the country and Canada. Even with such a big force of what are known as "foreign workers," it took nearly six days to fully restore power.
Atlantic City Electric Co. and Public Service Electric & Gas Co. said they were also preparing for the storm.
Across South Jersey, highway workers began salting roads overnight as multiple townships prepared emergency responders for heavy snowfall. Nonessential state offices will be closed Thursday.
In Camden County, 120 tons of road salt was delivered Wednesday morning, bringing the county's stockpile to around 5,000 tons amid an area-wide scramble that has left multiple counties across the area stuck without adequate supplies.
The Pennsylvania Turnpike Commission banned all empty and double tractor-trailers on the Blue Route and from the Mid-County to Clarks Summit interchanges, and officials urged residents to stay off the roads, especially during the early morning hours when the storm hits.
Meanwhile, state officials urged residents in rural communities to brace for the possibility of more dark, cold hours, and recommended stockpiling food, water, flashlights, and batteries.
Officially, Philadelphia has received 43.3 inches of snow this season, the 10th-highest total in the period of record dating to the winter of 1884-85.
Philadelphia has recorded three six-inch-plus snowfalls so far this winter; it never has had four in a single season.
Contributing to this article were Inquirer staff writers Ben Finley, Joseph A. Gambardello, Jerry Iannelli, Chris Palmer, Mari A. Schaefer, and Amy Worden.