As we are currently in the midst of a January thaw, many people are looking ahead to find out when this thaw will end and when snow chances will begin. Some rumors had been spreading about a snowfall event on Wednesday/Thursday, and again around Saturday, but both of these threats appear unlikely to produce much in the way of snow. It is not until next week and beyond when we expect the pattern to become more favorable for snow.
Taking look at the threat, or lack-there-of, for Wednesday into Thursday, the main problems stem from a progressive pattern and one that is still in flux.
The above image is the 500mb pattern and vorticity that is forecast by the European model for Wednesday night. There is a large area of vorticity in the southeast states, but notice how it is very elongated and not consolidated. Also notice how there is another disturbance right on its heels in the Northern Plains and Midwest. This leads to shortwave ridging in the Ohio and Tennessee Valley, which penetrates the trough and forces it to be very narrow and progressive. Also working against the potential for a bigger system is the lack Atlantic blocking to help the pattern to buckle. Instead, the entire country has a NW to SE axis to the heights, instead of a more meridional north to south flow. Thus, there is no room for the trough to grow and amplify. So this disturbance slides out to sea, and does not impact the area. There may not be any precipitation at all in the Wednesday night into Thursday time period.
The same general theme holds true for the threat on Saturday.
Moving forward to Friday night, we can notice a few important features. First of all, there is still a strong PNA ridge out west, which promotes a tough in the East, and could theoretically favor snowstorm chances. That being said, the Atlantic and Arctic blocking are still nowhere to be found, so the trough cannot buckle. It’s still a progressive pattern — one with a NW to SE height orientation throughout most of the country, which leaves no room for major trough amplification. We have a trough in the East, but once again, the vorticity is relatively disjointed and the trough is being pushed eastward. There is another disturbance right on its heels again in Minnesota, which forces weak shortwave ridging in Illinois, further preventing major amplification of any trough. By the time the trough is finally able to amplify and consolidate, it is already out to sea, with maybe a few passing snow showers. Eastern New England could potentially receive a couple of inches of snow, however.
The current weather pattern is still in a state of flux. It is changing and reloading to a pattern that will ultimately settle down and lock in for next week. Often times, however, when a pattern is in a state of flux, multiple disturbances will try to enter the country, but not one wave will really stand out. There also is the whole “two steps up, one step down” phenomenon, where the “one step down” is always on the pattern’s heels before the pattern can really reach the top of the flight of stairs — or in this case, its potential. The “one step down” is the remaining progressive regime.
Moving forward to next week and beyond, we will use the European Ensemble Mean guidance instead, since Ensemble forecasting tends to be better than Operational forecasting as one goes out in time.
The key feature is the strengthening PNA ridge, which eventually amplifies and retrogrades into Alaska. This ridging becomes so strong that it also penetrates into the Arctic Oscillation regions. This, combined with a bit of warming in the Stratosphere may not necessarily lead to strong Atlantic and Arctic blocking, but enough to that when combined with an incredibly strong PNA ridge, could provide several chances of a big snowstorm to close the month.
Let’s start with the January 22 time-frame on last night’s European Model ensembles.
What begins to happen here is that the aforementioned PNA ridge reloads and rebuilds even stronger than what it was this week. It’s so strong that it actually cuts itself off along the West Coast. This becomes important, especially in a pattern that still does not have blocking, because a cutoff ridge is much harder to move eastward — meaning, the pattern becomes much less progressive. This gives much more time for a disturbance to be able to dive southward from Canada, into the United States, and amplify. Although there is still not much blocking yet, which certainly still leaves a few question marks about the potential — especially regarding the ceiling of potential — but the fact that the PNA ridge becomes this strong certainly opens our eyes for this time period. Accordingly, the European Ensembles show a pretty potent trough in the East, which is a pretty strong signal considering how far out we are in time for a potential snowstorm.
Moving forward a bit more towards January 25, we can see more important changes.
The biggest change we notice is that the PNA ridge becomes even more amplified, more noticeably cut off, and also retrogrades towards Alaska. What this does is drop the EPO significantly, and as some of you know, when the EPO drops, this leads to the potential for major cold across the United States. This is because when ridging extends into Alaska, it helps to dislodge the Alaskan and Arctic cold towards the United States.
Also, this ridge becomes so strong that the heights begin to become much more meridional — meaning, a north to south fashion instead of a NW to SE fashion — downstream of this ridge; even more so than during the January 22 time-frame. This, combined with significant negative height anomalies across the East and in eastern Canada — which lead to strong cold thanks to the negative EPO — will help to provide an opportunity for a significant snowstorm chance. Any piece of energy that comes downstream from this ridge will have more time to amplify in a cold airmass.
Additionally, notice some positive height anomalies in Greenland! This is finally a hint of some Atlantic blocking, which further helps the pattern the buckle. With the January 3rd snowstorm, there was a temporary Greenland block that developed which played a big part in getting that snowstorm. This developed because a previous storm that took a trek towards the North Atlantic helped to pinch off some higher heights. Although sustained blocking is not expected, the upcoming pattern becomes much more favorable for these types of scenarios, and the European Ensembles show this quite well. And considering the PNA ridge is even better this time around than it was for January 3rd, the potential for a bigger snowstorm increases.
Let’s take a look at one last image, valid for the evening of January 26.
The PNA ridge is continually extremely strong and is now starting to connect with a bit of blocking in Greenland. This helps to generate a strong north to south flow extending from the Yukon straight down into the Desert Southwest! With continued very low heights in SE Canada and a trough in the East, cold air will be in place and will not leave. Also, any disturbance that travels through the downstream side of the ridge will be able to amplify significantly in the Rockies, and there is even a kink in the heights near the Texas Panhandle — a developing storm signal. Considering the temporary Greenland block and the PNA ridge, this potential storm is in a very favorable environment to become a big snowstorm in the January 27-28 period.
Of course, we are now getting pretty far out in time, so uncertainties still exist. That being said, the fact that the Ensemble guidance at a range this far out in time is showing signals this strong is pretty alarming. Usually, extremes get a bit muted in time with ensembles, which is why we are definitely not afraid to describe the potential in the pattern to produce cold and snow to close the month. Although the degree of Atlantic blocking is still a major question and is why we cannot majorly go gung-ho yet, the fact that temporary blocks could easily develop in a pattern that is otherwise very favorable for cold and snow, leads us to believe that we may indeed be tracking snowstorm threats to close the month.