Author: Anthony R. Wood

With a predictable precision that may forever elude meteorology, at 10:03 a.m. Philadelphia time Sunday, the sun will beam its most direct light of the year on the Tropic of Capricorn and the astronomical winter will begin in the Northern Hemisphere.

Sunday indeed is going to be the shortest day of 2025, with just over nine hours and 23 minutes between sunrise and sunset.

On the bright side for those who have about had their quotas of premature darkness, the day length would be a mere one second shorter than Saturday’s, and on Monday, we gain two more seconds. On the dark side, Sunday’s sunset is a full three minutes earlier than that of Dec. 12. (And don’t ask about sunrise.)

Plus, in two weeks the night skies will become considerably brighter with the rising of the last of four consecutive “super moons,” which will peak on Jan. 3.

Whether the brightness would be enhanced by snow cover is another matter: Meteorology has a long way to go to catch up to astronomy in terms of predictability.

In the early going, Philly is more than halfway to last winter — with 4.2 inches of snow, vs. 8.1 for the entire winter of 2024-25.

In the short term, this is a peak time for a perennial question.

Is it going to be a white Christmas?

“No” almost always is a safe answer in Philly, and all along the Northeast Corridor from Washington to Boston. And “no” it is this year, says Bob Larsen, senior meteorologist at AccuWeather Inc.

With a white Christmas defined as an inch of snow on the ground at Philadelphia International Airport on Dec. 25, the National Oceanic and Atmospheric Administration officially posts about a 1-in-10 chance that it will happen in any given year in Philly.

So why the fascination? Blame Irving Berlin, composer of “White Christmas,” and Bing Crosby, who crooned the most famous version, but probably a bigger impetus was the poem “A Visit from St. Nicholas,” published in 1823 and credited to Clement Moore.

The poem cast Santa Claus as personally delivering gifts via his sleigh. This predated Amazon Prime. That pretty well cemented the Christmas/snow relationship.

Philly gets most of its major bigger snowstorms from nor’easters, which tap the moisture of the Atlantic Ocean. The onshore winds can also import warm air from the ocean, and this time of year ocean temperatures still are well into the 40s. That’s why snow changes to rain so often around here early in the winter. It takes time for the ocean and the snow-making upper atmosphere to cool, and the snow season peaks in late January into February.

That doesn’t mean a storm can’t pop before then.

A very snowy anniversary

It so happens that next month is the 30th anniversary of Philly’s record 30.7-inch snowfall of Jan. 7-8.

It was so unbelievable that the record wasn’t verified officially until four years later, after NOAA commissioned a federal investigation. It turned out that the snow was not actually measured, but inferred from the liquid content of the melted snow and the air temperatures.

The investigators — David Robinson, the Rutgers University professor who is the longtime New Jersey state climatologist and an international snow authority, and Jon Nese, who then was the Franklin Institute meteorologist — affirmed the total.

They concluded that the snow reports in neighboring towns were close enough to support PHL’s.

Snow is a weighty matter

In the standard language used by the National Weather Service and commercial outfits, that certainly qualified as a “heavy” snowfall.

But it was the antithesis of “heavy,” at least in terms of relative weight. Snow comes not only in different shapes, but also in very different weights, depending on the snow-to-liquid ratios. On average around here, an inch of liquid yields about a foot of snow, a 12-1 ratio.

However, when temperatures are close to freezing as they were last Sunday, the snow has a higher liquid content and is thus heavier. On Sunday, 5 inches may have felt more like 8 to the average shovel. That’s heavy snow.

When it’s cold, as it was on Jan. 7, 1996 — temperatures were in the teens during the day — the flakes are way drier. The ratio for the storm was closer to 20-1, and overall the flakes were a whole lot lighter.

“Heavy” snow “applies to visibility ratios,” said Jim Eberwine, longtime meteorologist with the National Weather Service local offices, adding it might be time to reconsider the use of that adjective.

“Some things should be updated,” he said.

How about: Snowfall rates can be intense at times?

Snow: It’s a Northern Hemisphere thing

The solstice also marks the beginning of the astronomical summer in the Southern Hemisphere, so residents south of the equator probably won’t be using snowblowers for the next several months.

In fact, they won’t be seeing a whole lot of snow there during the winter. It snows robustly in the Andes and other mountain regions, but not in major population centers, the AccuWeather people note.

NOAA’s National Center for Environmental Information doesn’t bother to track snow cover in the Southern Hemisphere, for a couple of reasons, including that it’s 80% covered by climate-moderating water.

Plus its major cities are located at latitudes where snow is scarce.

How much for Philly this winter?

By contrast, the Northern Hemisphere has a plentiful supply of metropolitan areas that experience ruler-worthy snowfalls, including Philadelphia.

Making seasonal snow forecasts in this region isn’t quite like picking lottery numbers, but reasonably close. Seasonal totals have varied from 78.7 inches in 2009-10 to nothing in 1972-73. NOAA’s Climate Prediction Center doesn’t touch the stuff.

The guesses for this year are in, and here is a partial list with what’s out there: The “normal” season total is 23.1 inches.

Fox29 — 16 inches

AccuWeather/6abc — 14 to 18 inches

CBS3 — 18 to 24 inches

Arcfield Weather — 22 to 26 inches

WeatherBell Analytics — 22 inches

One prediction they all have in common: The winter of 2025-26 will out-snow that of 2024-25.

That won’t be hard.

What’s left of the season’s first snowfall is all but gone with the wind — and the heftiest rains since October. And having done its best to disrupt Friday’s morning commute, the weather evidently is executing an afternoon encore.

After backing off for a few hours, the winds came back with a vengeance Friday afternoon. A thunderstorm gust of 62 mph was recorded at Philadelphia International Airport at 2:35 p.m.

The weather service’s wind advisory for the entire region remains in effect until 1 a.m. Saturday, and conspiring with snow melt to saturate the soils, SEPTA is particularly concerned about the potential for uprooted trees along its Regional Rail lines, said media relations director Andrew Busch.

“The recipe for problems is there,” he said. “We will have crews stationed across the system to respond quickly.”

Peco heard the rumors, and while the utility is “not expecting impacts, we’re going to continue to monitor conditions,“ said Candice Womer, senior communications specialist.

The deciduous trees are mostly bare, so winds can sail through branches that are not weighed down with leaves, but the weather service advises that “some power outages” are possible.

About 16,500 outages were reported at midafternoon.

An additional concern was the predicted wind shift during the day, from southerly in the morning to westerly in the afternoon, and how that might stress vulnerable trees, Womer said.

The strongest winds will occur during the afternoon

The gusts are likely to be “widespread” during the afternoon, said Paul Fitzsimmons, a lead meteorologist at the weather service’s Mount Holly office. That would be after the rain-producing front crosses through the region and winds become westerly, peaking in time for the afternoon commute. Gusts to 50 mph are possible.

Rain totals in the Philly area are expected to be in the 1-to-1.5-inch range, Fitzsimmons said.

No stream flooding is anticipated, however, as levels are quite low. Despite the snow and a decent soaking on Dec. 2, precipitation the last two months is only about 75% of normal throughout the region, according to the National Oceanic and Atmospheric Administration’s Middle Atlantic River Forecast Center.

How dry we’ve been

The gusts aside — not a trivial omission — this system should produce a “beneficial rain,” the weather service notes.

A drought warning remains in effect for all of New Jersey.

Water levels are so low that computer models show only about a 10% chance of streams reaching even the preflood “action” phase.

In the interagency U.S. Drought Monitor map posted Thursday morning, most of Philadelphia, adjacent South Jersey, and Chester County were in the “severe drought” category, along with portions of Bucks, Delaware, and Montgomery Counties.

Most of Pennsylvania and New Jersey were in at least “abnormally dry” conditions.

The forecast for the weekend

After the winds die down Friday evening, the region should be in for a dry but chilly weekend.

Temperatures during the day Friday are forecast to dive from a high in the 50s in the morning to the 30s by nightfall, and they won’t get out of the 30s on Saturday.

Sunday’s forecast high in the mid-40s would be close to normal for the date.

Peeking ahead, the prospects of a white Christmas are not especially promising.

The region may have to settle for a wet one. Rain is possible Christmas Day with highs in the 40s. That’s not quite what Irving Berlin had in mind.

That icy glaze that has appeared on streets, sidewalks, and driveways this week reappeared Wednesday and will make an encore Thursday morning as water continues to ooze from the snowpack by day and freeze at night.

So-called black ice — ice that masquerades as harmless wetness — is especially dangerous, officials warn, and conditions are ripe for a harvest, given the generous snowpacks.

“Refreezing and black ice are definitely a concern,” said PennDOT spokesperson Brad Rudolph. “We will continue to treat areas as needed.”

SEPTA advised commuter to watch out for ice on platforms, walkways, and in parking lots.

Temperatures early Wednesday fell into the teens throughout the region — save for Philadelphia International Airport where the low was 26 — and on Thursday are due to drop into the 20s region-wide.

But along with that winter landscape, the atmosphere over the region is going to undergo radical changes by the end of the workweek, forecasters say, with a rapid but short-lived warmup, and something that’s been missing around here lately — a soaking rain.

As for more snow, the atmospheric scientists are saying it’s probably safe to put away the shovels for a while, and not just in the Philly region.

“In my opinion, cold looks hard to come by for most of the United States through the remainder of December,” said Amy Butler, research scientist at the NOAA Chemical Sciences Laboratory.

In short, prepare for a more typical Philadelphia December.

“With the pattern changing, we’re kind of getting closer to where we should be this time of year,” said Mike Lee, a lead meteorologist at the National Weather Service Office in Mount Holly.

The forecast for the rest of the week

Wednesday should be at least partially sunny with some substantial melting as temperatures are due to crack 40 degrees by early afternoon.

Then after one more overnight with readings dropping into the melt-freezing 20s, Thursday is forecast to become the first day with above-normal temperatures in Philadelphia since the Wednesday before Thanksgiving.

As temperatures rise through the 40s, several hours of soaking rain are forecast to begin in the early evening with the potential for up to an inch.

Throughout the region the snowpack contains about a half inch of water that will be heading to local storm sewers and local waterways, said the weather service’s Lee.

In addition to the rain, the high moisture content of the air is one of nature’s most-efficient snow-removal tools. When warm, moist air comes in contact with snow, it gives off a latent heating effect that accelerates melting.

“We’ll have to watch for the possibility of some localized flooding,” said Paul Dorian, a meteorologist with Arcfield Weather.

For now, Lee said, the weather service isn’t contemplating flood advisories. Precipitation the last 90 days has been substantially below normal throughout the region, and a drought warning remains in effect for New Jersey.

The outlook for the weekend and beyond

Friday’s forecast high in the mid-50s is likely to occur in the early morning hours, and then temperatures are forecast to crash, with wind gusts to 40 mph possible.

Saturday is expected to be dry but chilly, with readings in the low 40s, warmer than it’s been but still several degrees below normal, and in the mid-40s Sunday, closer to the long-term averages.

The polar vortex, the persistent low pressure system that spins around the Arctic and confines the cold air to the ice box of the planet, has been strengthening, said Butler.

It’s possible that it would undergo “stretching” and allow cold air to sink into parts of the contiguous United States, but “it’s not a signal that is very robust,” she said.

For now, she said no obvious dominant feature is emerging that would define the winter.

La Niña conditions continue in the tropical Pacific, where vast expanses of the sea surfaces are about a degree below normal. La Niña can have powerful effects on west-to-east winds that ferry weather to the United States.

In this case, however, La Niña is currently weak, meaning that its influence over global seasonal patterns is also weak, said Emily Becker, scientist with the University of Miami/Cooperative Institute for Marine & Atmospheric Studies.

For now, it would be prudent to savor what’s left of the snow cover, and it wouldn’t hurt to spread a little sand or gravel on the driveway or the front steps before going to bed.