Good evening, everyone!
As has been the case in previous attempts to re-start this blog, my stay here will be brief - I am blessed enough to be transitioning to a new job that will continue to allow me to forecast the weather, but for about the next month or so, the proprietary covers are off, if you will, allowing me to get back to where we first started!
Attached above is a look at the current state of the 10-millibar part of the atmosphere, corresponding to the upper levels of the stratosphere and - more importantly - the slice of the atmosphere most relevant to tracking the stratospheric polar vortex. Here, blues and greens represent below-normal geopotential height anomalies (in this case, indicating the polar vortex is stronger than normal) while oranges and reds represent above-normal geopotential height anomalies (here, meaning the polar vortex is weaker than normal).
As the graphic shows, the stratospheric polar vortex is currently pretty well consolidated, with broad below-normal anomalies across the Arctic Circle and minimal (if any) attempts at bringing warmer air towards the North Pole. This is a primary driver behind my expectations for a warmer than normal December across most of the United States, perhaps save for the Pacific Northwest in line with traditional La Nina climatology. But things begin to change as we move towards the middle of December:
The 10-day forecast from the latest ECMWF model shows a notable change by December 9th, as a broad ridge begins forcing its way poleward from western and central Canada. A secondary, weaker ridge is also evident in Eurasia, making for what looks on paper like a wave-2 vortex-splitting attempt but in reality ends up shoving the vortex into Siberia and not really splitting at all.
Temperatures also jump considerably over North America at this level of the stratosphere during this same time as this ridge formation...
All of this is good and well, and we do see some support from the GFS ensembles (not shown), but it doesn't look very promising to me. For one, this warming event appears to originate in North America; my preferred origination point for a polar vortex disruption is in the Bering Sea, as this area (in my experience) has tended to bring about the more powerful polar vortex disrupting events. Second, if this really is a vortex-split attempt instead of a vortex-displacement attempt (a la the weak secondary ridge in Eurasia), and if we take this forecast verbatim, there's no way we see this transpire.
Think of it this way: a vortex-disrupting SSW involves a single, massive ridge that brings blowtorch-level warmth into the Arctic Circle. A vortex-splitting SSW involves two somewhat-strong ridges that pinch the polar vortex somewhere around the North Pole, cutting the vortex into pieces. Here, we see a single moderately strong ridge. That doesn't inspire confidence to me, and as a result I think this is a flash in the pan: a good sign for future SSW opportunities, but not enough to result in the risk of a severe Arctic outbreak at the turn of the new year.
To Summarize:
- The stratospheric polar vortex is currently stronger than normal, raising the risk of above-normal temperatures across the U.S. through December.
- There are some indications of a warming event around the middle of the month, but from what I see, this doesn't appear likely at this time to significantly disrupt the polar vortex
- Consequentially, the chances of an SSW by mid-December appear low
