Back on Oct. 12, I predicted winter would be getting off to a slow start, but that odds favor a more “typical” winter developing than last year’s unusually mild and less snowy winter, by around the end of December or early January. The slow start part was a good call and remains so as we head toward early December.
By “typical,” I mean more common ups and downs with more snow than we had last winter and temperatures finishing up closer to average. (In case you forgot, a smoothed average is pretty cold at times.) That outlook was appropriately vague due to the high degree of uncertainty so early in the autumn.
The problem I have now is I would have expected the picture to become less vague by now. For a plethora of reasons, it hasn’t cleared up as much as I would have hoped.
One important signal did develop. October snowfall in Siberia, especially below 60 degrees latitude, was much greater than average and advanced faster than average. The work of Dr. Judah Cohen and his colleagues has show a frequent relationship between such a setup and more frequent incursions of polar air into eastern North America, as well as more favorable conditions for coastal snowstorms to our east, and in parts of Eurasia.
So far, the incredible cold has been centered mainly in Siberia, with some very harsh cold in Alaska.
Some of that Siberian cold has pushed out over the north Pacific and cooled what had been an enormous blob of warm water present the last few years. That warm blob helped to pump up a warm ridge of high pressure over western North America. It also forced the polar jet stream to go up and over the ridge, crossing the polar region and tapping the coldest air to be delivered more often to the Midwest, Great Lakes and much of the east.
In the absence of the warm blob, however, the western ridge hasn’t developed this autumn, and the polar jet stream has actually been delivering much milder Pacific air to the lower 48 and southern Canada, following a west-to-east path. As of now, there is no reason to expect the western ridge to build up with any persistence. So, we’re not likely to see the pattern dominate in the near future:
This is not to say our part of the country won’t find its cold air masses in different ways. Occasionally, strong Pacific storms will plow into the west coast, pumping up a greater amplitude to the flow, with more tall ridges and deep troughs.
The relationship between the cold and the snow in Siberia should also help produce a colder phase of what’s called the arctic oscillation more often, and that can allow cold air from the north and northwest to filter into the region. We need a ridge to pump up over Greenland for that big dip in the polar jet stream to set up for eastern North America and amplify; a long trough over eastern North America butted up against a North Atlantic ridge can deliver cold and snow, with the best chance for more snow to our east, along the seaboard.
There is, at least, good agreement between three models’ ensembles (multiple runs of each model to get a better range and means than can be provided by a single model run) of a turn to pretty cold temperatures toward Dec. 9-10.
As we head into early winter, then midwinter, the most extended-range version of the European model (which offers trends out to 46 days) shows more frequent “shots” of wintry weather, which did not show up last year at this time due to the ongoing super El Niño. But there are still no signs of the kind of persistent cold showing that gripped us for a large part of the brutal winter of 2014-15. “Shots” are closer to typical winter, with persistence like that seen two years ago being more atypical.
For now, Cohen is much more confident about an exceptionally cold winter continuing in a large part of Asia and northern Europe. He leans toward a colder winter gradually developing for eastern North America, but his confidence – because of the conflicting indicators I’ve mentioned – is lower for our region.
That’s a hedge I have to go along with, for now. If I start to see the western ridge showing up more often in the models, that would cause me to change my view toward more persistent wintry outbreaks.
Story topics: By Don Paul