After this week’s cold and snow there will be a significant warm-up starting this weekend, especially by Sunday.
This panel shows the mechanics of why we have not been able to sustain cold for weeks at a time, despite having the OPPOSITE North Atlantic/Arctic Oscillation pattern compared to last winter:
The strong blocking Bering Sea Ridge and Gulf of Alaska Vortex maintains a zonal (west to east) jet stream flow across southern Canada. This configuration makes it impossible for SUSTAINED cold air anywhere in the lower 48 states. Almost every time I have looked, that Gulf of Alaska Vortex has been sitting there. As long as we have a strong NAO-/AO- pattern, we’ll continue to get cold shots to skirt the northern United States from time to time. But in order to get cold and snow that lasts more than a week at a time, we need the Pacific pattern to change.
When will that happen?
To tell you the truth, I don’t know when. Omega blocks stick around for a while and the Pacific/North American Oscillation (PNA) has been stuck in its negative phase since October:
Not only does the Bering Sea Omega block teleconnects well with the negative PNA, but the sea surface temperature anomaly map show a vast cold pool over the east Pacific that strongly favors the negative PNA:
I was aware of the cold pool of east Pacific water when I wrote up my Winter 2012-13 Outlook. I was not gung-ho about severe cold this winter because of this feature, but didn’t explain that in adequate detail because I was building the case that we would probably not experience the amazing lack of snow that we had last winter.
A look into later next week:
Lo and behold! ECMWF is showing a powerful disturbance moving into southern Quebec by December 5th with another charge of cold air and perhaps a significant lake effect snow event:
But guess what? The Gulf of Alaska Vortex is STILL THERE, spinning away like a stuck pig! So we’re looking at another one-and-done cold shot, with no immediate reinforcement.
You can check out WXRISK for a look into why the Pacific pattern MAY change during the second half December. If you click onto the audio link, Mr. Tolleris will walk you through the maps that he is presenting. It’s a very interesting read, but I’m skeptical as to whether the Pacific pattern will change that soon.
Time to throw in the towel on Winter 2012-13?
Not yet! Assuming a persistant NAO-/AO- pattern, we’re going to have opportunities that we didn’t have last winter. I think we have a least one major snowstorm somewhere in the bag.
