After strongly signaling its intentions on Tuesday, one of the wildest Marches in the 146 years that the U.S. government has been tracking the weather evidently is about to take a riotous turn with a meteorological rarity on the first full day of spring.

Meteorologists say the briskest coastal-storm traffic in more than three decades will culminate in a potentially historic spring snowstorm. By the time it stops, daily and monthly Philadelphia records might have fallen, along with up to a foot of snow.

It would be accompanied by what is now a familiar litany of disruption: coastal flooding; heavy, water-weighted snow; gale-force gusts; preemptive closings; and potential power outages. Utility crews were on alert, and Peco already had imported out-of-state help.

"Fortunately, and unfortunately, we've got a lot of experience," said Peco spokesman Doug Oliver.

"In March alone."

This one, an amalgam of several disturbances and the original coastal storm that began pestering the region on Tuesday, would be the fourth significant nor'easter to affect the region since March 2. The mid-Atlantic hasn't experienced a sequence like this in 31 years, according to meteorologists at the government's Weather Prediction Center.

Heavy snow is expected by late morning Wednesday and could continue well into the night. But even as the region was teased with a wintry mix of hors d'oeuvres on Tuesday, Wednesday was assuming the characteristics of a  classic snow day. Philadelphia public and archdiocesan schools announced they would be closed; NBC10 is keeping a running school list. SEPTA said Regional Rail would operate on a Saturday schedule. Philadelphia International Airport, where flight cancellations were accumulating, said to check with your airline.

"Take this storm seriously," the weather service said in its Tuesday afternoon discussion. "We continue to need to sound the siren for major or extreme impact."

A major issue for this one had been whether the snow would be a match for the potent equinoctial sun. That was answered Tuesday, said Daniel K. Petersen, a prediction center forecaster. The outbreak of snow and sleet was "a nice tip-off," he said. "We know we have cold air in place."

The supplier is cold high pressure over southeastern Canada, and the circulation of the storm, the source of the copious moisture, has lured it southward.

Also contributing to the cooling was the fact that when the precipitation started Tuesday, the atmosphere over the region was quite dry, he said. As precipitation falls into dry air, some of it evaporates; that gives off a cooling effect, similar to that of water evaporating from the skin after a hot shower.

Most of what falls in and around Philadelphia on Wednesday well into Wednesday night will be snow, he said, with the heaviest amounts along the I-95 corridor from Washington to New York.

The term heavy snow should be taken quite literally and seriously. With temperatures near freezing, the aggregated flakes will be waterlogged. A foot of snow could add 100 pounds or more of weight to a branch on a mature tree.

The trees do have a few things going for them – they are still mostly leafless, and it's at least possible that earlier storms pruned some of the weaker stock – said Tyler B. Altenburger, arborist at Longwood Gardens near Kennett Square. But the sheer weight of the snow would be a major concern: "If it's heavy and wet enough, then anything can happen."

Wind gusts close to 30 mph are expected Wednesday morning inland, and up to 50 mph at the Shore as the storm matures.

Nor'easters are so named for the winds from the northeast that they generate. They are a winter staple around here — but not four of them in three weeks. An air pressure pattern over the North Atlantic has been locked in place since the beginning of the month, delivering cold air that helps set off coastal storms, then blocks them from moving quickly.

The longer-term outlooks see a possible warm-up next week, said the Weather Prediction Center's Petersen. "We have to see whether this is the last storm of the season," he added.

"Luckily," said Peco's Oliver, "there's not many days left in March."

Staff writer Michael Boren contributed to this article.