Extended-range forecasts suggested December ~15th would be a key date for the stratospheric vortex evolution for some time. Now falling within the medium-range, GFS forecasts show an amplifying stratospheric Aleutian high displacing and weakening the vortex, right on cue... https://twitter.com/SimonLeeWx/status/1334077384400785409
This remains a fair way from an SSW, and it's perhaps easy to forget that in early December 2019 the vortex was weak. But it's a start.
December 2019 is a cautionary tale for early-season disturbances. This is how last winter's vortex evolved in ERA5. A weakened vortex developed in December and propagated downwards, before the abrupt transition to 'Beast Mode' from Jan onwards. [beast = zeer sterke poolwervel = vervelend = willen we niet]