Update 5:45 a.m.:
The National Weather Service lowered its snow forecast projections a bit this morning for areas near Philadelphia and south of the city. Colder air moved in and heavy rain turned to snow a little after 5 a.m. in areas west of Philadelphia.
However, forecasters warned conditions will worsen throughout the morning and roads will become hazardous.
Many school districts throughout the region announced closures.
In one of the more peculiar juxtapositions in local weather history, forecasters warn that region could get slammed with 6 to possibly 10 inches of snow just hours after setting a record for warmth.
At mid-afternoon, the official temperature at Philadelphia International Airport reached 66, surpassing the former record for the date, 63, set on Feb. 8, 1925.
The National Weather Service posted winter-storm warnings for the entire region, and the School District of Philadelphia said late Wednesday night that its schools and offices will be closed Thursday.
The Archdiocese of Philadelphia said all of its city schools will be closed, and the city announced a snow emergency, which means all parked cars must be moved off Snow Emergency routes for plowing.
The timing of the storm couldn't be worse: It is due to get cranking just before dawn, with rain yielding to heavy snow throughout what ordinarily would be the peak commuting period.
With temperatures near freezing, the snow likely will be as thick as paste that will stick to tree branches and wires, although widespread power outages were not expected.
At the height of the storm, the weather service said "thundersnow" — literally snow thunderstorms — is possible, along with snowfall rates up to 2 inches an hour.
In Philadelphia, forecasters are expecting about six or so inches, but noted a potential for up to 10 inches. Forecasters added that the snow is expected to end by midday; flurries could linger throughout the afternoon further north. But the snowfall is expected to be heavy, falling at a rate of up to two inches an hour, from 3 to 10 a.m. Thursday, with the potential for some thunder.
While the precise timing and amounts remain uncertain, "it's going to be a rough Thursday morning after this morning's near record warmth," a weather service forecast discussion said. Weather service records show just seven instances in Philadelphia history in which a 60-degree day was followed by measurable snowfall.
The precipitation will be accompanied by falling temperatures. Thursday's high, expected to only be in the low 30s, is likely to be set in the early-morning hours and the temperature will drop throughout the day, the weather service said.
That set-up could quickly lead to hazardous conditions during the morning.
"The storm will begin with a brief period of rain in most areas, but will quickly change over to snow as temperatures plummet," meteorologist Alex Sosnowski writes on AccuWeather. "Initially the snow will melt on paved surfaces. However, that can change in a matter of minutes as the rate of snow picks up."
Forecasters warned that the snow would be wet and heavy, and likely to stick to trees and power lines. Utility companies said they were prepared to have extra crews on standby to respond to any power outages.
The Philadelphia Streets Department said crews would be salting primary and secondary roadways in advance of the snow, with more than 50,000 tons of salt and 350 trucks available to respond to the storm's peak.
Trash collection in the city is suspended Thursday; residents with Thursday pick-up should hold their materials until next week.
Airlines were also preparing for the wintry weather. Many airlines, including American Airlines, the dominant carrier at Philadelphia International Airport, are allowing passengers traveling through Northeast airports to change their flights without paying a fee.
The band of snow is expected to disrupt much of the Northeast coast, with up to a foot possible in parts of New England, according to AccuWeather.
The cold will likely stick around at least another day: Overnight into Friday, the temperature in Philadelphia is expected to dip into the teens, ensuring that snow left on untreated surfaces will freeze. Friday's high is forecasted to also be in the low 30s.