dropping in the stratosphere.

Latest ECMWF runs shows the Vortex holding relatively well into early winter before weakening sharply through January and February. UKMO is showing much the same thing: a pronounced slowdown of the stratospheric winds during mid-to-late winter.

A strong Polar Vortex generally helps keep the deepest Arctic air bottled up around the pole. When the vortex weakens or becomes disrupted, the circulation becomes less effective at containing that cold, increasing the opportunity for Arctic air to spill south into the mid-latitudes—including the U.S., Canada and Europe.
https://electroverse.substack.com/p/cold-descends-on-russias-far-east