The snow rumors notwithstanding, the Philadelphia region and most of the rest of the Northeast can pretty well rule out a white Thanksgiving, nor will Black Friday turn white.
However, the upper atmosphere evidently is in a state of upheaval with a potentially rare event unfolding, and forecasters say something resembling winter may arrive around here before the holiday weekend ends.
NOAA’s Climate Prediction Center has chances favoring below-normal temperatures and above-normal precipitation for the Philly region from next Friday through Dec. 12.
However, the meteorologists who have grappled with longer-range outlooks are cautioning against taking social media snow forecasting too seriously.
“The observed snowfall is inversely proportional to the hype,” said Judah Cohen, a polar scientist and seasonal forecast specialist. He is among those who have noticed the snow mentions that have popped up on X accounts and popular websites.
The next week should generally be uneventful save for rain Tuesday night possibly into getaway Wednesday, when highs are forecast to reach the 60s.
Then a developing pattern change is predicted to import colder air into the Northeast. “I do believe it will get colder as the Thanksgiving week wears on,” said Bob Larson, senior meteorologist with AccuWeather Inc.
A rare event may chill December
What has the attention of Cohen and others in the meteorological world is the potential for a “major” stratospheric warming event in the upper atmosphere over the Arctic sometime in the next several days, a disruption that could allow significant cold air to pour into the United States.
Computer models have been debating over just what’s going to happen, but if a major warming occurred, it would be only the second time in records dating to the early 1950s that it has occurred this early, said NOAA meteorologist Laura M. Ciasto.
In fact, Cohen, seasonal forecast director for JANUS Research Group, said that given the timing he has been reluctant to forecast it will occur.
On occasion, upward-moving waves from the troposphere, 5 to 9 miles over the Arctic, crash into the stratosphere, 10 to 30 miles up. That has the effect of compromising the polar vortex, the west-to-east winds that lock cold air in the places where the sun disappears for the winter, said Ciasto.
When the winds slacken, the vortex can weaken and allow frigid air to spill southward. In some cases it might “stretch,” or spilt into pieces that deliver cold air to regions of the Northern Hemisphere.
A major disruption would have longer-lasting impacts, said Cohen.
The European forecast model has consistently predicted a major event, Ciasto said, while the U.S. model hasn’t been as impressed.
What is likely to happen if the warming event occurs?
A major warming in January 2021, when temperatures in the stratosphere suddenly jumped 65 degrees Fahrenheit, resulted in quite a snowy February in the Philadelphia region.
After a warming event, “there’s a greater chance that the jet stream will become more disrupted and dip down” over the continuous United States, Ciasto said, “bringing cold air with it.”
As for timing, the effects may show up anywhere from two to several weeks after the event.
In the meantime, she noted that “several other factors,” including patterns over the North Pacific, favor a chilling for the Northeast.
Don’t be surprised to see snow appear in an actual forecast, but not necessarily on the ground.