What is happening over the Arctic
The polar vortex, a large circular area of cold air high in the atmosphere that normally spins over the North Pole, is being disrupted by a process known as sudden stratospheric warming, USA Today reported. The disruption is unusual mainly because of its timing, which is so early in the season.
Laura Ciasto, a meteorologist with NOAA's National Weather Service, said forecasters are watching the vortex be "stretched out and splitting into two smaller vortex lobes." She wrote in an email to USA Today that it could become the earliest event of its kind since 1958, when reliable records began.
Ciasto said the stratospheric winds could reverse from westerly to easterly sometime between November 6 and 10. In the records since 1958, she said, only two other such warming events have happened in November. They typically occur between January and March.
How a stratospheric warming works
Sudden stratospheric warming is a dramatic warming of the stratosphere above the Arctic, typically 10 to 30 miles above Earth. It can weaken or even temporarily reverse the normal west-to-east winds of the vortex. Andy Hazelton, an associate scientist at the University of Miami, said the stratospheric vortex generally keeps the coldest air bottled up near the pole.
Large atmospheric waves rising from below can sometimes knock the vortex off balance, Hazelton wrote in a blog post on Brightband.com. Air over the pole then warms and the westerly winds slow or reverse. Very cold Arctic air can then spill farther south into North America, Asia or Europe several days or weeks later.
Experts also cautioned against treating "polar vortex" as another phrase for a cold snap. Climate.gov's Polar Vortex Blog makes the same point, and the vortex is not the only pattern behind blasts of cold air. "Weather can happen regardless of what the stratosphere is doing," said Isla Simpson, a scientist with the National Center for Atmospheric Research.
Why a November event may not bring a deep freeze
Ciasto said these events usually raise the odds of below-normal temperatures across the eastern U.S. and northern Europe. Those events typically come in January through March, though, when Arctic temperatures "have had time to get quite cold." She said, "In November, there hasn't been as much time so there's less cold to source a cold-air outbreak."
Hazelton said the Arctic as a whole was about 8 degrees warmer than the 1990–2019 normal at the end of October. Alaska and the Yukon were about 13 degrees warmer. The only real pool of cold air was over Eastern Siberia.
Ciasto added that the vortex is often not the only influence, especially this winter with a very strong El Niño. "How these two play together or compete will definitely influence what we feel at the surface in the next month," she said.
What forecasters expect next
Hazelton said an AI model ensemble from Brightband, a technology company that says it uses artificial intelligence to improve weather forecasts, suggests the event could be weak and short-lived. In that scenario the vortex would settle back toward the pole in late November with little effect on the United States.
Judah Cohen, a polar vortex expert at the Massachusetts Institute of Technology, previously told USA Today that the most certain impact is a period of milder weather in eastern North America, probably for much of November. He said the effects usually take about two weeks to reach the surface, though recently they seem to take longer. That puts the earliest impact in late November, more likely December, and possibly January.
Cohen said the downward influence is more likely to affect Northern Europe or Northern Asia than the eastern U.S. He noted that in recent years these events have skewed toward milder weather in the eastern U.S., contrary to expectations. The 2023-24 El Niño winter featured two such events and was very mild in the East, possibly record mild. "In short lots of uncertainty!" he said.
