After a warm fall and focus of hemispheric cold over Eurasia, the tide turned in December as the weak, fluid polar vortex shifted into a favourable position to deliver one of North America’s strongest December cold outbreaks in the last 20 years.
What a difference a month makes!
December to date
A simple shift in the cold pools associated with the polar vortex made for a huge flip between North America’s record warmth and Western Europe’s cold.
But since the brutal mid December cold, the polar vortex has strengthened and the bitter cold pool has lifted back north presenting a mild Christmas and end to the year from Midwest to Atlantic Seaboard.
However, it’s a different story over the Rockies and particularly the Northern Plains.
With the PV gaining strength back over the pole, so the cold pool reloads up over NW North America but there is leakage into the Western US, helping fuel the Christmas blizzard.
Although there’s fluctuation, there’s a trend in the long range models suggesting the arctic cold reloads before resurging back south into the Lower 48.
I recon the first half of January will see moderate brands of cold in the East but more intense arctic cold may well drop into the WESTERN US but during the second half, ridging appears to return strongly across the north with bridging between Alaska and Greenland. The CFSv2 weeklies see this late January. The timing this far out is difficult but I think the overall idea of another near coast to coast cold outbreak is very much on the table between Jan 15 and 31.