Officially, winter begins at 12:47 p.m. tomorrow, but it will be an astronomical formality.

On the eve of the winter solstice, the most potent December snowstorm in 100 years - complete with thunder and lightning - has left the region under as much as 20 inches of snow.

And for the rest of the week, the faintest sunlight of the year will glance off a bright and dense cover, which may affect weather for several days.

Temperatures may not get out of the 30s until at least Thursday, when another storm threatens, although right now, that one looks to be wetter than whiter.

This nor'easter was at least as capricious as ferocious, layering the entire region with substantial snow, but in a reversal of the usual, it focused its greatest fury south and east of the city.

To the north for most of the day, snow was barely a rumor in the Lehigh Valley. Meanwhile, parts of South Jersey saw lightning from rare "thunder-snows" - the wintry equivalent of summer thunderstorms - that caused rapid accumulations.

In the immediate Philadelphia area, totals ranged from 10 to 20 inches. In South Jersey, Hi-Nella received about 20 inches and Southampton 17.5. Shore towns reported more than a foot. But in northern Bucks County, Perkasie got 7.

Officially, a whopping 16 inches was measured at Philadelphia International Airport, which weathered a brutal day of disruptions. Numerous flights were canceled.

Affecting areas from North Carolina to New England - blizzard conditions were reported in Washington - the snow was a transportation nightmare.

"We're doing the best we can with what Mother Nature's throwing at us," said Mark Gale, the airport's chief executive officer.

About 7:30 last night, a NJ Transit train struck one of its buses, which had got stuck on snow-covered tracks in Pennsauken.

NJ Transit spokesman Dan Stessel said that the bus, with 26 passengers, was heading from the Cherry Hill Mall to Philadelphia at 7:15 p.m. As soon as the driver determined snow and ice prevented the bus from getting off the tracks, the driver evacuated the bus.

"Obviously, it was a very good decision," Stessel said.

Passengers were off the bus and away from the tracks for about 10 minutes before the crash, he said.

No injuries were reported by the 38 passengers aboard the train, which was bound for Atlantic City. The train engineer received minor facial injuries.

Others were able to enjoy the snow.

Take Betty Miller, 26, who not only was unafraid of the white stuff, she was trying to eat it. The visitor from California was experiencing her first snowfall. She tried catching falling flakes on her tongue and ate some snow off a church wall in downtown Wayne, on the Main Line, where many shops bore "closed due to weather" signs.

"I've never seen it fall," she said. "I love it."

Nearby, more than 50 sledders were on the hills of Odorisio Park. "We are having a blast," said Wendy Norman, 52, who was there with her three children and husband.

In a move that is likely to boost the spirit of pregame tailgaters, if not Bud Light sales, the Eagles decided to delay the start of today's game with the San Francisco 49ers (they must be loving this trip to Philadelphia) until 4:15 p.m. By afternoon, major roads are expected to be relatively snow-free.

While yesterday held its share of challenges, one of the biggest snows on record was by no means a disaster.

Early on, Philadelphia and other towns declared a state of emergency, and Mayor Nutter, confronting his first major snowstorm at the city's helm, asked Philadelphia residents to stay home as crews cleaned the streets.

The first flakes arrived in the immediate Philadelphia area around 3 a.m., and with temperatures in the 20s, the snow stuck right away. It took longer for the snow to press northward, where it encountered cold, dry air.

That's one reason for the variations in accumulations. Another was that some areas were caught under bands of heavy snow, where it piled up in a hurry.

Capriciousness aside, the snow cover is extensive, from Virginia all the way to New England. Snow refrigerates the overlying air, so that will keep temperatures cold for a while. Meteorologists say it could create a cold-air "damming" effect. Thus, when the late-week storm arrives, it could result in more frozen precipitation than the computer models foresee.

For December snow, the official Philadelphia total was topped only by the Dec. 25-26 storm of 1909 at 21 inches.

But it surpassed the legendary storms of Dec. 24-25, 1966, 12.7 inches - another storm characterized by thunder-snow - and Dec. 11-12, 1960, when 14.6 inches fell Sunday into Monday. Unlike this year's, the 1960 storm was an ambush and disrupted the workweek.

Yesterday's storm was well-advertised, and because it occurred on a Saturday, the impacts were blunted - although not for the retail industry.

Nutter said he was sympathetic to shopping effects: "We're very concerned about all business retailers through the city. We understand that. That's why I'm encouraging people to take mass transit and SEPTA. You can still get to where you're going to get to."

Like crews on both sides of the river, the Philadelphia Streets Department was counterattacking nature's assault with salt trucks and plows.

Some people were just determined to get out of the house. Several showed up at 8:30 a.m. at the Main Line YMCA in Ardmore for their weekly "iron man" aerobics class.

Among them was Shelly Herndon, who walked nearly a mile from her Ardmore home to get to class.

"It was fine," she said. "I really wanted to get here, and I didn't want to drive."

Contributing to this article were Inquirer staff writers George Anastasia, Dorothy Brown, Kristen Graham, Maria Panaritis, and Mari Schaefer.