Unprecedented: Record warmth, then record snow?

In what would be an unprecedented turnabout in records dating to the 19th century, the day after the region set a high-temperature record could be followed by a snowfall record Thursday.

The National Weather Service has issued winter-storm warnings for the entire region, calling for six to 10 inches of heavy, fast-falling snow in a compressed period starting before daybreak that would white-out the morning commute.

Flakes unseen, all Archdiocese of Philadelphia schools in the city along with other schools around the region announced Wednesday that they were closing Thursday, and the city declared a state of emergency. Later Wednesday, the School District of Philadelphia  announced that it, too, would close schools and administrative offices on Thursday.

Coming after Philadelphia set a Feb. 8 record with a high of 66 degrees at 3:24 p.m., the forecast was almost hallucinatory: paste-like snow at the rate of two inches an hour, temperatures tumbling below freezing, 20 mph winds out of the north, and perhaps "thundersnow."

The precipitation was forecast to start as rain in the early morning,  changing to snow from northwest to southeast.

The heavy snow should shut off by noon in most areas, but temperatures won't get out of the 20s through Friday. They should rebound into the 40s during the weekend.

Lastly, the weather service warned that widespread power outages were possible.

"It's hard to believe," said Dave Samuhel, a meteorologist with AccuWeather. "You'd think you have too much warm air to get rid of."

The atmosphere, obviously, is not paid to think.

So how can it behave like this?

Part of the answer is in Canada, which the latest Rutgers University Snow Lab analysis has being covered wall-to-wall with snow. So is New England.

Snow cover is an efficient refrigerant, and the cold air to the north of the region "just needs something to tap it," said Bruce Sullivan, a meteorologist at the Weather Prediction Center in College Park, Md.

Enter a strong coastal storm forecast to blow up off the Virginia coast. On Wednesday, its potential looked more impressive that it had the day before, said Sullivan.

Winds around the centers of low pressure blow counterclockwise, thus areas to the west experience winds from the north, and on Thursday they will import cold air at the surface and in the upper atmosphere, eventually changing rain to heavy snow.

While the winds will be brisk, extensive drifting is not expected, since the flakes will be moisture-laden, said Sarah Johnson, a weather service meteorologist in Mount Holly.

Warmth followed by snow is not that unusual, but 60-plus temperatures followed by a snowstorm this early in February could be unprecedented. Such a sequence has not happened at least since 1885, when official snow record-keeping began.

Thursday's snow could also wipe out the snowfall record for the date, 6.5 inches, set in 2010.

Accumulations likely will vary, depending on where the "bands" of the heaviest snow set up, and elevation. Temperatures drop with height, and a few hundred feet of elevation -- think Chestnut Hill and areas along the Main Line -- can make a huge difference in accumulation.

But in issuing its spring outlook on Wednesday, AccuWeather said the next several weeks could be adventurous, with that cold air to the north jousting with mild air from the Pacific. That could mean more storms and wild temperature swings.

This week, Samuhel said, "is a good example of what we'll be dealing with."