Will the polar vortex break this winter?
One of the biggest ways an El Niño winter goes wrong for the U.S. starts 30 km up. When the stratospheric polar vortex collapses in a sudden warming, cold air can spill south for weeks and override the mild El Niño north. Textbooks say El Niño makes that more likely. The record since 1959 is less sure, and this page shows both the odds and their uncertainty.
The live vortex, with the ECMWF forecast, is on polarvortex.earth →
The odds, and how sure they are
Share of winters with at least one sudden warming
Major warmings by the Charlton–Polvani rule (the 60°N, 10 hPa wind reverses between November and March), ERA5, 1959–60 to 2025–26. Bars show the 95% interval.
The model's many winters
Share of winters with a sudden warming: the real record against SEAS5's member-winters
The same test from the 1 October and 1 December starts, and all three pooled
Differences in percentage points of winters with a warming unless marked; 95% intervals below each number (whole real winters resampled). Observed: ERA5 over the same 36 winters and window. The pooled row puts all three starts together and still resamples the 36 real winters.
What a vortex breakdown does to an El Niño winter
January–March temperatureHow many of these winters fell in each division's cold third (trend removed)
How many of these winters fell in each division's cold third (trend removed)