THIS POST IS OLD.
CLICK HERE FOR THE NEW FORECAST.
The fact of the matter is, the month of December will probably end up warmer than normal as a whole. Don't believe the CFS weeklies, they're bringing this cool-down too quickly.
As i've consistently been saying since October, Siberian snow cover anomalies for that same month can and will impact cold weather in the following winter months. Because of a deep below normal snow anomaly in the first half of October, I'm not surprised one bit by the warmth that has been experienced across the nation, and am not surprised by the incoming warmth that will push temperatures well past average values for this time of the year.
One thing I find encouraging is the Bering Sea ridge will be shifting north into the Arctic Circle. Good, that means the Polar Vortex will be faced with another challenge of holding itself together while the Bering Sea ridge has now moved into its territory. However (and this is the disgusting part), the ridge will set up in a manner that forces a persistent trough to set up near Greenland, which would mean the dreaded positive NAO.
As if matters couldn't get any worse. This is like trying to solve a puzzle, but you keep receiving some pieces at some times and losing other pieces at other times.
Don't think for a moment that this winter will be anything like last. Last winter, Siberia nailed the forecast with snowfall in October 2011 dry as a bone. Lo and behold, the winter of 2011-2012 ended up as one of the warmest in history.
Before we can even think about getting cold and snow to the States, we have to beat the 'obstacle course'. One of these obstacles is the Bering Sea Ridge / negative PNA issue. We should have the former cleared up in due time. However, the negative PNA obstacle could pose problems later on.
If we have the ridge moving north into the Arctic Circle, we're going to see an opposite reaction according to physics. East Asia will be observing this principle, with a deep troughing pattern setting up in response to this strong ridge now leaving the Bering Sea. However, the polar vortex has to go somewhere- it doesn't just leave because a big high pressure system is getting in the way. What we are likely to see is a shifting of the polar vortex east, either into Canada or Greenland. This track of the polar vortex will depend on the strength and placement of the ridge of high pressure. I find it very plausible for the polar vortex to want to move east into one or both of these areas in response to the high pressure development over north Asia.
THERE IS GOOD NEWS, HOWEVER: The ensembles are toying with the idea of this ridge vaulting into North Europe, which could then combine with a Greenland ridge and possibly strike up a negative NAO. Could this happen? I would like to see some more ensemble runs before I say yes, but if such a solution happens, cold could become more prevalent than I am thinking. Not a freeze-out, but on a general cooler tone.
We are indeed seeing a sudden stratospheric warming, but keep in mind this warming is coming from record low levels at the 70mb level of the stratosphere. This SSW puts the 70mb temperatures still well below normal, so don't expect the stratosphere to save the day and banish cold weather to the south for at least a couple weeks.
But before you sprint to the streets, proclaiming that the winter is coming on, full steam ahead, I must caution you to remain weary of the polar vortex. I do not trust the idea that the HP in North Asia will combine with a negative NAO to cut the polar vortex in half, as the MJO showed. No- I expect the HP anomalies in North Asia to overwhelm the Arctic. In response to the strong HP, a very strong LP (low pressure) anomaly will develop both in South Asia and potentially the Arctic Circle. If the latter idea occurs, Greenland would more than likely be affected. Hesitant ensemble forecasts of the NAO confirm my skepticism on the idea of the weather pattern suddenly favoring cold and snow in the East.
In summary, I'm not too optimistic for December. The storm pattern will get more active as the Bering Sea ridge moves north into North Asia, but this movement will provoke a stronger polar vortex (unless the ridge moves into Greenland), and thus better possibilities for warm weather. However, if all turns out alright, January and February will still be cold. Fingers crossed, folks.