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... pic.twitter.com/vplhHZHDki

— Dr Simon Lee (@SimonLeeWx) December 2, 2020