Every major single-run weather model now shows the polar vortex weakening toward early November, and Severe Weather Europe — which tracks the stratosphere daily — reports the combined forecast puts it at record-weak levels for this time of year.

The European model goes furthest: it shows the west-to-east winds circling the pole at 10 hPa slowing and then reversing direction in early November, a sudden stratospheric warming that Severe Weather Europe says would be unprecedented for this point in the season.

A weak vortex lets frigid Arctic air spill toward the United States, Canada and Europe, as happened in late January 2019, when Chicago's O'Hare Airport hit minus 23 degrees with a wind chill of minus 52.

A reversal flagged in the forecasts for early November would be the first of its kind for this date in the observational record the site maintains going back to 1958.

The forecasts begin to split around October 29, and Severe Weather Europe notes the extended American ensemble 'doesn't see anything special,' with some single runs more conservative about a reversal — so the next several days of model runs will decide which version holds.

The evidence comes from 132 ensemble members across ECMWF ENS, AIFS ENS and GEFS, which Severe Weather Europe uses to compute the chance of a wind reversal within two weeks; above 30 percent is worth watching and above 60 percent makes an SSW likely.

Its Polar Vortex Explorer compares current vortex strength against every year since 1958 and tracks wave activity — heat flux at 100 hPa — where several days of strong pulses are the classic warning sign.

Meanwhile, NOAA's Climate Prediction Center outlook issued October 22 favors below-normal temperatures from October 28 to November 1 only in Washington, Oregon and Montana, and for October 30 through November 5 favors below-normal temperatures nowhere in the Lower 48.

The immediate impact is limited — there is little truly cold air available in late October for a disruption to work with, Severe Weather Europe explains.

But the longer-range stakes are large: across 34 major SSW events it analyzed, most of the U.S. ran colder than normal in the 30 days afterward, and the European seasonal forecast shows a weaker-than-normal vortex in the middle and back half of winter, with winds dropping well below normal from late December on.

All of this unfolds during a strong El Niño, with the Climate Prediction Center's October 8 update giving better than 83 percent odds of a strong-to-very-strong event lasting through January–March 2027.

Readers planning travel or ski trips should watch the CPC's daily 6-10 day and 8-14 day outlooks for the first official sign of cold heading south.