Best place to start is to show the upper-levels from the dynamic tropopause map for the next 12-days.

The coldest colors are mainly polar air with anomalies on the tropopause showing up as “white fuzzballs” that continue to rotate around cyclonically over the Arctic and dip into Canada. As Pacific short-wave disturbances race eastward with the jet stream, sometimes they can capture some of the “white fuzzball” and create a tail into the Lower 48 that serves as a pathway for extreme cold. Add in the formation of a long-lasting cold pool with high pressure at the surface, and you get what we just experienced as some of the coldest/latest winter air on record.