With the exception of last winter, the dominant Gulf of Alaska warm pool drove a warm Western/cold Eastern North America winter pattern from 2013 to 2015. The El Nino overruled everything last year.
The lack of arctic sea ice those years on the North America/NE Asia side of the pole helped drive a strong blocking high from eastern equatorial Pacific all the way to Alaska’s Arctic coast.
However, this fall season has been different with origin of this large-scale change stemming back to the Arctic/Eurasia side of the Arctic where there’s record low sea ice and persistent blocking. A warm sub-continental Asia and frigid Siberia has enhanced the jet stream coming off Asia, forcing central Pacific warming upwelling has significantly cooled the northern Pacific.
This has fired a stronger-than-normal trans Pacific jet stream.
This powerhouse jet has, for the first time in 3-4 years, enhanced the Aleutian low and while it’s been a stormy October/November from Alaska to Pacific Northwest, this trough has led to unseasonable ridging and warmth from Rockies to Atlantic.
October temperature anomaly
November’s opening 21 days! Wow!
The lack of North American snow cover and Arctic Sea Ice this fall is on the other side of the pole, hence why North America is warm and Eurasia cold.
In the last 5 days or so, a major winter storm, the first of the season has crossed the country wiping out coast to coast record warmth and there are hints at the ridging that’s been so dominant over the central continent beginning to lift northward allowing more troughiness to establish underneath through late November and particularly early December.
Note in the below GFS 5-day mean 500mb height anomalies that the trough and cold core leans into the West towards day 15. A hint at a major cold outbreak?
GFS 10 day snow forecast.
Looking at the month of December overall, I think the cold will focus on the West, Great Basin with further Pacific lows sweeping in. The pattern within the Arctic looks favourable for transport of Siberian air over top and into Western Canada and the continuation of systems driving into British Columbia and Pacific Northwest could connect with building cold over Yukon/Northwest Territories and pull it down into Montana, Wyoming and Colorado in the wake of these storms.
While cold could well reach the East, significant moderation is likely with extreme cold unlikely to visit the Northeast and South through much of December.
CFSv2 for December 2016
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