In what already has been one of the most-eventful winters on record, what could become the most-disruptive storm of the season is on its way, with the first flakes starting to fall just before 10 p.m.

The National Weather Service has posted a winter-storm warning for the entire region for 6 to 14 inches of snow, falling heavily during the early-morning hours, mixing with sleet and rain, and then changing back to snow before finally ending sometime Thursday night. The warning is in effect from 10 tonight until 1 a.m. Friday.

The snow could mix with sleet and rain before daybreak in Philadelphia and areas to the south and east, said AccuWeather Inc., which still was calling for 6 inches in the city, with up to a foot north and west.

Well before the arrival of the first flakes around here, the storm closed schools, canceled flights, and even scrapped the Thursday circus performance in Philadelphia.

Municipal emergency declarations have gone viral, with the list including Philadelphia, Lower Merion and Radnor. All public and archdiocesan schools in Philadelphia will be closed Thursday. Gov. Christie declared a state of emergency for New Jersey and ordered all nonessential state workers to stay home Thursday.

Philadelphia's snow emergency went into effect at 8 p.m. For a list of snow emergency routes, go to: http://t.co/cMS7XtLImN

The city also announced that trash collections would be suspended for tomorrow and Friday. Residents are asked to hold onto trash and recycling until the scheduled pickup day next week.

In addition to heavy snow and a fresh challenge to the region's salt supplies, with winds gusting to 30 m.p.h. the storm has the potential to generate more power outages, take down more trees, and perhaps even cause some road flooding.

"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, which could see quite a variety of conditions from the storm now battering the South.

Thousands of flights were canceled for Thursday in Philadelphia, Charlotte, Atlanta, Washington, D.C., Baltimore, New York, and Boston.

The threat comes a week after back-to-back snow-and-ice storms took out trees and branches and conspired to cause knock out power to 785,000 Peco customers -- nearly half its customer base.

Last week was harder on the region's trees than superstorm Sandy in October 2012, said Mike Karkowski, Director of Horticulture at Tyler Arboretum, in Media, Delaware County. Comparatively, "Sandy was a cakewalk for us," he said.

Karkowski noted that some trees still are bearing snow that fell on Feb. 3, and that could mean more trouble tomorrow.

"We're gearing up and getting ready for another round," said Greg T. Smore, a Peco spokesman. He said Peco's storm center has stayed open constantly since that snowfall caused 60,000 outages before the Feb. 5 ice storm, blamed for knocking out power to 715,000 customers.

Peco ended up with 6,800 workers at the peak of the storm cleanup, but even with such a big force of "foreign workers," it took Peco nearly six days to fully restore power.

Unlike other, drier storms this winter, this will be a coastal "nor'easter," so named for the powerful winds from the northeast such storms produce, laden with moisture from the Gulf and Atlantic.

"This is going to be a heavy, wet snow," Nutter said, adding it has the potential to bring down trees and power lines on a scale similar to last week.

Nutter said that SEPTA's Market-Frankford El and Broad Street subway lines will run through the night in anticipation of bus operations being disrupted. The city even has canceled inmate visits for Thursday at all Philadelphia jails.

The city has begun treating streets and 400 plows manned by 700 employees will be operating overnight.

The precipitation is forecast to change to rain for awhile in the city, but a foot or more of snow is possible in outer Bucks, Chester, and Montgomery Counties. Lesser amounts are expected closer to the Shore.

Officially, Philadelphia has received 43.3 inches of snow this season, the 11th-highest total in the period of record dating to the winter of 1884-85.

Philadelphia has recorded three 6-inch-plus snowfalls so far this winter; it never has had four in a single season.

PennDot so far has spread 120,000 tons of salt on roads in the five counties, the third-most for any winter.

That total is about to go up.

Inquirer staff writers Bonnie L. Cook, Joseph A. Gambardello, Andrew Maykuth, Mari A. Schaefer, Allison Steele, and Anthony R. Wood contributed to this article.