The Northern Hemisphere's polar vortex is breaking down weeks ahead of schedule, with stratospheric winds dropping to near zero and forecast to reverse in the coming days.
That would make it the earliest Sudden Stratospheric Warming on record, by about three weeks, according to an analysis by the meteorologist Andrej Flis for Severe Weather Europe.
The polar vortex is a broad winter circulation over the polar regions, rising from the surface into the stratosphere above 50km and trapping the coldest polar air inside. A strong, stable vortex acts as a barrier and tends to leave the United States and Europe milder; a disrupted one can release that cold.
Disruption usually begins with a rise in pressure and temperature in the stratosphere, which is what a Sudden Stratospheric Warming is. Forecasts for Friday show the vortex splitting into two cores at the 10mb level, around 30km up, with stratospheric temperatures rising more than 27C above normal for the time of year.
Mid-stratospheric winds are running well below normal, and below the previous record low strength for this point in the season. All ECMWF and GEFS ensemble members show a wind reversal at some point, which makes the event very likely. It also looks brief, with the winds recovering by mid-November.
The 2026/2027 season is dominated by a strong El Niño in the Pacific and by westerly winds in the tropical stratosphere at about 21km, known as the QBO.
Because no comparable event appears anywhere in the observed record, Flis searched model simulations for winters with the same combination. He found 21 cases in which the polar vortex collapsed in the same window during a strong El Niño and a westerly QBO, and compared them with similar global setups in which the vortex remained normal or strong.
Those simulated winters show a cold anomaly extending from western Canada into the United States in December, increased snowfall over the northern and eastern United States and parts of Canada, and below-normal temperatures with above-normal snowfall across much of Europe. Flis is careful to describe this as the event's own signal rather than a forecast for the winter.
He is equally careful about what the signal has to survive. "To oversimplify, if the Super El Niño wants to put a +5°F anomaly over Canada, and the SSW goes for -3°F, that can still end up +2°F above normal, despite the cold signal map," he writes.
It is at this point that the methodology achieves something quietly remarkable. The event is the first of its kind ever observed. Its typical consequences are already documented, across 21 winters, none of which occurred.
The logic holds together, which is the unsettling part. A thing that has never happened cannot be studied in the record, so it is studied where it does happen, which is inside the models. The sample size is adequate. The sample is simply not real.
Our analysts note that the resulting signal is also insulated from measurement. If the winter turns cold, the collapse did it. If the winter turns warm, the collapse was overwhelmed by the El Niño and the cold signal was present but suppressed. A minus three that comes out as a plus two has still been detected.
The Department of Unnecessary Research regards this as the most robust forecast in the field. The event has no precedent. Its effects are on file. Nothing December does can disturb either statement.