~From Chicago USA: https://chicago.suntimes.com/news/2019/12/28/21039824/warm-christmas-chicago-climate-change-great-lakes
~To even Moscow: https://www.theguardian.com/world/2019/dec/29/moscow-resorts-to-fake-snow-in-warmest-december-since-1886
~The root cause, of course, was a warmer Arctic, with likewise much smaller size Polar Vortex
We go strait to the root cause:
The size of the Polar Vortex on December 25-26 may be defined at 248 K isotherm threshold at level 600 mb. As you can see the vortex is spectacularly small and covers even less ground over North America. It has 3 vortices, all quite cold, as per latest observations, the warmer the Arctic gets the more prone for small but very cold PV vortices. Obviously there is no or very little circulation originating from the entire sub Arctic towards North America and western Eurasia. How did it get this way?
This was from a build up of unusual Arctic circulation pattern covered here at EH2r since late November. Namely by cyclones heading North from the West side of Greenland.
Here was one of the last significant warm air blitz: