Jens Asbjørn Bøgen
Regional impacts remain uncertain.
Every winter, a ring of fast-moving air high above the Arctic decides how brutal the cold will feel from Chicago to Berlin.
This year, that ring is already wobbling in a way nobody has seen before. Weather models have been flashing the same signal for almost two weeks, and a potentially historic shift in the stratosphere could tip the Northern Hemisphere toward an unusually cold run-up to December.
Forecasters expect a full breakdown of the stratospheric polar vortex within an early-November window, which would be the earliest major sudden stratospheric warming ever recorded in a dataset that goes back to 1958.
In plain terms, the band of westerly winds that normally locks Arctic air over the pole is weakening, with ensemble models showing stratospheric winds at 10 hPa reversing from westerly to easterly.
“This will be, I think almost undoubtedly, the largest, earliest disruption of the polar vortex on record,” Dr. Judah Cohen, an atmospheric scientist, told Fox Weather. He cautioned that “the timing, strength and eventual impacts remain highly uncertain.”
What a sudden stratospheric warming actually does
A sudden stratospheric warming, or SSW, is a rapid heating of the air about 30 kilometers above the Arctic, driven by large atmospheric ripples called Rossby waves surging up from the lower atmosphere.
According to analysis by World Climate Service, between 60.8% and 82.4% of ensemble members across seven forecast systems showed the zonal-mean wind at 10 hPa and 60°N reversing from westerly to easterly within a November 4-10 window.
The earliest major SSW in NOAA’s published ERA5 record, which covers 1958-2023, began on November 28, 1968. A separate, brief wind reversal on November 28, 2025, lasted just one day. A reversal in the first half of November would move the record forward by more than two weeks.
What it means for the winter ahead
A weakened vortex does not deliver a blast of cold on cue. It typically takes two to six weeks for the signal to filter down to the surface, and even then the effect varies by region.
For the United States, the strongest signal sits east of the Rockies, where cold outbreaks and snow chances rise from late November into December. Independent analysis by meteorologist Andrej Flis at Severe Weather Europe points to an increased snowfall signal from the Plains through the Ohio Valley into the Mid-Atlantic, with cold air spilling into the central and eastern United States by January.
Europe sees a similar fingerprint in the Severe Weather Europe analysis, with knock-on impacts across the Northern Hemisphere also expected to reach East Asia, though Flis stresses that the signal could be partly masked by the Super El Niño pattern in play this winter.
Why it is happening now
The cause is still being argued over. Strong El Niño conditions are in place in the Pacific, and researchers have not pinned the early timing on any single factor. Fox Weather notes that Cohen said the vortex was already showing signs of becoming unusually weak and stretched before the main disruption.
World Climate Service stresses an important caveat: a wind reversal 30 kilometers up is not itself a surface forecast. Some SSWs break through to ground level, others fizzle. Strong El Niño conditions generally favor a milder winter across North America, especially in the eastern US, and could blunt the impact.
For households watching heating bills and travel plans, the window to watch is the final week of November through December, when forecasters expect the first clear sign of whether the Arctic air locked over the pole is heading south.
