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.