Forecast models in the medium to long range,continue to show a pattern change throughout the northern hemisphere — characterized by a west coast ridge and a large, anomalous trough in the Eastern United States. This has been mentioned several times over the last week as our forecasters have suggested the pattern change by the end of January. Exactly how the pattern plays out remains a mystery at this juncture, especially as some continue to overanalyze and interpret operational model guidance — which is never smart. The individual nuances of the pattern are going to take days to iron out.
The first development of note is that the ECMWF has finally caved to the GFS and GEFS interpretation of tropical forcing which will increase later this week and help the development of some major pattern changes. The ECMWF had previously been very stubborn in insisting against this development and keeping the impulse quite week. This cave is very important for the eventual changes in the Pacific including the development of a +PNA regime (i.e the west coast ridge).
All forecast models are in very good agreement in the eventual development of a PNA ridge on the west coast — a beast of one, really — which will eventually push northward into British Columbia. In addition, a North Atlantic ridge is forecast to develop while the EPO dips down and eventually falls tremendously negative with the ridge building into the North Pacific. We can see in the below imagery that the EPO is forecast to fall tremendously by Days 5-7 and while there is some uncertainty amongst ensembles, all agree on a below -2 value and some are very literally off the charts with values well below – 5. As Don Sutherland noted a few days ago in his research, the last three cases of this type of EPO drop in January were 1994, 2003 and 2010. All three of those cases should bring back memories of winter weather in our area. For more information on what the EPO is, click here.
That being said, the pattern over the next few days will remain largely progressive prior to the development of these changes. A quick glance at all forecast operational and ensemble guidance shows several storm systems amidst an active flow of mid level distubrances, developing well to the east/northeast of our area. This is not surprising due to the gyre near the Great Lakes — and although the storms are developing well to our northeast we may still be able to squeeze out some snow at times which should come as a bonus to all of us.
This gyre eventually shifts north and east on all forecast models and becomes a centerpiece of the Polar Vortex over Eastern Canada. This becomes extremely important by Days 5-7 as its positioning will have a major say in the eventual track of forthcoming disturbances from the Pacific. The new trend on all guidance today is to elongate this vortex in a more west to east direction. Below, the ECMWF Ensemble mean (yes, this is a mean forecast) shows the setup on January 20th very well — with presumably decent agreement given the mean anomalies. There is a tremendous ridge in a favorable position on the West Coast, an elongated Polar Vortex over Southeast Canada, and importantly — some ridging and above normal height anomalies poking into Greenland and the Davis Straight (where have we seen that before, how about two weeks ago).
The elongation of this polar vortex is essential to the changing pattern– and it starts with the piece of the PV which is modeled to phase over or just north of New England this weekend. This elongated position allows the Polar Vortex to interact with any disturbance that drops into the trough over the MS Valley from the Pacific. This is a huge change on the forecast models from a few days ago which were showing this PV staying as a huge rotating gyre of cold air.
The guidance is beginning to show this potential as the elongated PV is now dropping in and phasing with the mid level disturbance over the MS Valley on Jan 22 or whereabouts, producing a potential event. This event may very well turn out to produce for our area — but it is the first in a line of several and the setup is still a bit too far east.
Still, by this time (Day 7) the pattern is in retrogression mode on all forecast models and ensembles. So the PNA ridge is pushing westward and the gyre over the Northeast is now forecast to phase in and then tug back to the north. The raging -EPO and +PNA pattern at this time open the door for this gyre to retrograde westward into Central Canada, while some additional ridging builds north of it in the higher latitudes. This brings the trough axis farther west towards the Mississippi River. Any disturbance that comes over the top of the Pacific Ridge at this time will amplify in a nearly ideal and classic spot for snowstorms in the Northeast US.
It is this point which we are trying to drive home in the upcoming pattern — this is not a one and done scenario. Although initially progressive, the anomalous nature and retrogressive developments in the pattern suggest not just one chance, but multiple chances for significant winter weather events in the Northeast United States. It may be a case of waiting out the changes, and pattern progression, before we see snowfall results.
That being said, nothing is set in stone. As we have all learned the hard way over the past several years, forecast models at this range can be wildly incorrect with their pattern evolutions. But this time, seeing tremendous ensemble support and having the backing of tropical forcing and changes which can be traced all the way up to the stratosphere — leads to enhanced confidence that we are about 5-7 days away from entering a very exciting time period.