Polar night jet stream has already reversed at some places: this sounding from Bering Sea shows easterly zonal wind at 10 hPa.

As the #SSW progresses, we expect that the zonal wind turns easterly on the average on the whole latitude (which is 60°N). pic.twitter.com/OUdWCzai78

— Mika Rantanen (@mikarantane) December 26, 2018