Philly winter forecasts call for early snow, a cold December, and a 100% chance of uncertainty
Snow isn't out of the question for late November, AccuWeather says. But, yes, this winter forecast business is still a work in progress.
The first hard freeze is probably five weeks away, and the raking has barely begun. Yet among long-range forecasters, thoughts already have lightly turned to snowflakes.
They will come sooner rather than later, predicts Paul Pastelok, the long-range forecaster for AccuWeather Inc., perhaps as early as Thanksgiving weekend. The early-cold concept has support from at least one other commercial service.
For winter-phobes, the federal government’s winter outlook would be more palatable, and keep in mind that those who do this for a living acknowledge that long-range forecasting can be akin to driving on ice with bald tires. Past performance suggests the trepidation is merited.
That said, with a rather bold specificity, AccuWeather is calling for 20 to 26 inches of snow this winter for Philadelphia, similar to last year’s official total in the city, although amounts were double that in areas north and west of the city.
» READ MORE: What went wrong for Philly’s winter forecasts? ‘The polar vortex stole the show.’
Pastelok foresees the first measurable snows landing upon Philly in mid-December, but it might happen as early as “late November.” During those two periods he expects upper-atmosphere disturbances to force cold air to spill southward from the Arctic.
He also said some significant late-fall rainstorms are possible, and, overall, the temperatures for the winter are forecast to finish at slightly above-normal. Both results would be related to a phenomenon that might have given Ida’s remnants an extra kick.
WeatherBell analytics is on board with a cold December for the Philadelphia area. In the overall pattern it sees similarities to five other Decembers in the 21st century, four of which had significant snowfall, including last year, when snow was followed by a winter hiatus, followed by that memorable February.
For the season, WeatherBell is calling for things to even out, with near-normal winter temperatures and snowfall. That would be 22.7 inches at the official measuring station at Philadelphia International Airport.
» READ MORE: If you think the weather at your home is colder and snowier than Philly’s official measurements, you’re probably right
The government’s Climate Prediction Center — which eschews snow forecasts, citing the limits of the science — is calling for above-normal temperatures all the way through April, attributed largely to the developing La Niña in the tropical Pacific.
Weather tends to move west to east, and during La Niña, a periodic cooling of surface waters in the tropical Pacific, the ocean interacts with the overlying atmosphere to affect winters across North America. In Philadelphia’s case, that would favor generally warmer temperatures, by the climate center’s reasoning.
La Niña will be a player, Pastelok agrees, however, no two are identical, and they have coincided with snowless and quite snowy winters around here, including the winter of 1995-96, the third-snowiest on record.
» READ MORE: What ‘La Nina’ means for hurricane season and Philly’s winter snow prospects
And once again, a wild card will be the behavior of the polar vortex, the high atmospheric winds circling the Arctic. When the vortex weakens, very cold air can leak to the mid-latitudes. Pastelok is banking on that happening periodically.
The Atlantic factor
One other element to watch is the state of the western Atlantic, Pastelok said. Temperatures along the Mid-Atlantic and Northeast coasts have been running several degrees above normal.
That has made the ocean an abundant source of atmospheric moisture, and he thinks the warm Atlantic added extra juice to Ida’s remnants.
It also could make Philadelphia and areas along the coastal plain slightly warmer than inland areas, he said. That would mean that early-winter storms might favor rain or snow changing to rain as warm air is drawn landward.
“I do feel there can be a few big rain events in late fall and early winter,” he said. “That water is so warm, and as the atmosphere changes to colder air aloft, and stronger contrast from top to bottom, these storms can be strong.”
How’d forecasts fare last year?
Not so well.
For the record, Philadelphia ended with 23.9 inches of snow officially, 1.2 inches above normal, and a winter average temperature of 36.5, just a degree above.
AccuWeather Inc. had predicted 10 to 15 inches, with temperatures two to three degrees above normal.
The Climate Prediction Center had about 60% of the contiguous 48 states with above-normal temperatures including the entire East. Only about 25% of the country finished above normal.
Locally, save for Kate Bilo at CBS3, the TV meteorologists were well under on snow amounts.
NBC10′s Glenn Schwartz did not get it wrong: After 23 years, Schwartz decided to hang up his snowshoes and in 2020 stopped issuing his winter outlooks, saying that the climate change had made a wreck of the seasonal forecasting tools.
“I’m not going to disagree with him,” said Pastelok, “but I’m not going to give up.”