Tuesday, November 30, 2021

December 4-6 Potential Winter Storm

A disturbance is forecasted to track eastwards from the Pacific Northwest & Southwestern Canada across the northern Plains, bringing accumulating snow to the region before intensifying as an additional piece of energy interacts with this original system around and east of the Great Lakes.


This event begins during the day and evening on December 4th, with an upper-level disturbance riding east-southeast along a band of tight upper-level confluence and spawning a surface low-pressure system in the Dakotas. The above image shows forecasted precipitation types and sea-level pressure contours as of 12pm Central Time, December 1st. While the bulk of the precipitation is seen in the Rockies, the eastward progression of this system begins at this time as an arm of light snow pushes eastwards into the Dakotas.


By 6am Central Time on December 5th, the surface low has pushed east in accordance with the eastward progression of this upper-level system. Some moisture fetch from the South U.S., evidenced by weak rainfall in MO/AR/TX, adds to the system and helps gently boost snowfall totals in Minnesota and Wisconsin. Accumulations are expected to be in the 2-4" range across the Dakotas, northern Minnesota and northern Wisconsin over this December 4-5 period.


Things become much more interesting overnight December 5th into the morning of December 6th, as our surface low pushes eastwards and the upper-level piece of energy finally gets an opportunity to strengthen after being squeezed by a progressive and prohibitive jet stream back in the West and Central. As shown above, the latest European model run sees moderate to heavy snow in southeast Canada and into extreme northern parts of New England, turning into a mixed precipitation event as a surge of warm Southern air brings rain to most of the region.

The GFS model is a little better with snowfall chances in New England (1-3" in western and central New York), but in general we should see the best shot for over 4" of snow in southern Canada. The latest ECMWF snow accumulation chart for this event is shown below, and there is some question as to how strong this system could actually get. I don't think the ultra-aggressive totals shown by the ECMWF will come to fruition, but it seems plausible that some areas along or just north of the U.S. / Canada border in New England could hit 6".


To Summarize:

  • A winter weather event is expected to impact the northern Plains, Great Lakes and New England in the December 4-6 timeframe
  • Snow accumulations of 1-3" appear possible in the Dakotas, northern MN and northern WI during the December 4-5 window
  • Some heavier accumulations nearing 6" may be possible in extreme southeast Canada, but questions remain as to how notably this storm system will intensify as it nears the Atlantic
Andrew

Monday, November 29, 2021

Stratospheric Polar Vortex Showing Long-Range Rumblings

 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!

weathermodels.com

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:

weathermodels.com

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...

weathermodels.com


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
Andrew