Monday, December 9, 2013

December 13-16 Potential Winter Storm

There are hints that there may be another winter storm across the Midwest and Northeast through the December 13-16 timeframe.

The GFS model has a storm system moving north and east across the lower Midwest and towards the Northeast, as is shown by the elevated vorticity values over the Kentucky, Illinois, Indiana and into Tennessee. The GFS prefers to develop this storm system, and moves it northeast over the ridging in the Southeast, giving the Midwest, Lower Great Lakes, Ohio Valley and Northeast another round of snow. Given that the pattern does support this storm track, I do think that this is within the realm of possibility.

The ECMWF, at the same timeframe as the GFS image above, has this energy more elongated than the GFS, and it is not developed correctly as a result. Both model systems have ridging over the West Coast, as well as ridging in the Southeast, which is why I find this solution rather peculiar. I think the ECMWF may not be correct in its analysis of the system. Let's discuss why below.

For one, the ECMWF ensemble set (ECMWF EPS) follows the same track (drawn in yellow) that the 12z GFS put up earlier today. Both model guidance sets have the storm system traveling from northern Mississippi to West Virginia. Looking at the deviation on the image above, it does look like the ensemble members agree with the GFS, with dark blues rather than light blues (which would indicate low confidence in the solution) showing up for this potential storm. If we are to go by the idea that the ECMWF EPS and GFS are pairing up for this potential winter storm, we can take a look at the overall storm projection from the GFS and see what both guidance systems may be thinking:

Note, this storm would bring snows to the northern Northeast regions, not coastal states. Subtract an inch or two from western New York, VT, NH, ME, northwestern PA to get snow from only this storm. 
As the GFS projects above, this storm would bring plowable snows from much of Illinois to Indiana, Michigan, Ohio, Pennsylvania, New York, Vermont, New Hampshire, and Maine. I don't have the figures on the ECMWF ensemble snowfall (not sure if anyone has access to that), but based on the storm track from the GFS and ECMWF EPS matching up so well, I would think that snowfall orientation would be the same, stretching from the Midwest to the Northeast.

I'd like to wait for a bit more consistency before putting in more confidence to this system, which would include the GFS and ECMWF EPS matching up again on this system, and even the ECMWF operational model jumping on board.


Sudden Stratospheric Warming Incoming

It does appear likely that a stratospheric warming event is in the works for the next week or so.

An animation of 30 millibar temperature anomalies high up in the stratosphere suggest above normal anomalies in eastern Asia, combined with a shifting body of warmth to the east from Europe will provide a base for what is expected to be a sudden stratospheric warming event. As both masses of warmth in northern Asia and southeast Asia intensify, it is expected that they will combine, and possibly push northeast into the Bering Sea to provide a base for this SSW.

Mountain torque (MT) values around the 40N parallel are beginning to spike in coordination with this warmth in the upper stratosphere. Without getting too technical, it has been seen that above normal mountain torque anomalies come around when a sudden stratospheric warming event is immenint. I circled the spike in anomalies in black on the top panel to better highlight it. Taking a look back at the animation at the top of the post, we see this warming event is beginning around the 40N parallel, so it is no wonder we are seeing MT values on the rise in the same parallel region.

The ECMWF model projects this event to reach its peak in about five days, when the maximum strength of the Wave 2 event occurs. 

Now, there are two main types of stratospheric events that disturb or split the polar vortex. There is a Wave-1 response, which involves the polar vortex becoming elongated and/or weakened, however a split does not occur. This can result in displacement of the vortex out of the Arctic, but that prospect is not as likely as it is in a Wave-2 scenario. A Wave-2 stratospheric response involves the polar vortex being split into two main vortices, as the image from NASA shows above. While the temperature images show the Wave-1 and Wave-2 (top and bottom rows, respectfully) responses occur over Greenland and Eurasia, especially with the two split vortices where one vortex goes over Greenland and one goes into Eurasia, the split can lead the vortices into any land mass, not just those two regions. 

We can deduce a few things from that ECMWF forecast image. For one, it appears that the event will be enhanced mainly over the upper stratosphere, and with an unfavorable Quasi-Biennial Oscillation, I doubt the polar vortex will split from this anticipated SSW alone. Second, it looks like the event is projected to occur at the 60N parallel, which is located in Canada and northern Asia. This, if it were to come true, would indicate the sudden stratospheric warming event would have shifted north and impacted the upper latitudes. I don't see the warming event protruding into the 90N parallel, which is nearly the highest possible latitude parallel in the northern hemisphere. This means that the SSW shouldn't slice and dice the polar vortex, which is what we're expecting (not the polar vortex split, but a lack thereof).

As for what the future may hold for the stratosphere. 100 millibar eddy heat flux values have risen to normal levels after a nearly record-setting below normal stint in late November. Whether the rising heat flux continues remains in question, but such a trend would bode very well for the stratosphere, particularly later on in winter when the Quasi-Biennial Oscillation becomes more favorable for sudden stratospheric warmings.