The critical period for the #PolarVortex evolution - and whether we get an #SSW - appears to be 2-4 Feb. Aleutian wave breaking & d[U]/dt -5 m/s/day. E-P flux turns quickly poleward during this time. pic.twitter.com/v18A8oPuME

— Dr Simon Lee (@SimonLeeWx) January 25, 2020