As some of you may have heard, there is speculation going on about a potential winter storm next Wednesday and Thursday. Snowfall maps from various computer models, such as the European model, have been posted on numerous social media pages. Some of these maps have shown accumulating snow all the way down to the I-95 corridor, leading some to hype the potential for snow. Is this hype justified?
At the end of our previous post, we hinted that there could be a pattern-change coming towards mid-month as a cold air source builds in NW Canada and slides towards the United States via a strong cold front. It turns out this cold blast of air may be more significant than originally anticipated, as this source of cold air has little room to modify as it traverses through the Canadian snowpack. With this cold air push along with warm air still existing to the south, the temperature gradient is ripe for a potential storm system to form. Whether it forms out to sea, takes a track too far west and brings warm air and rain to our area, or takes a track off the coast and brings a snowstorm is yet to be determined.
What is causing this storm to be possible and where is the cold coming from?
It all begins with the Pacific and a unique blocking event near the Aleutian Islands. A very strong ridge is forming there, and what happens downstream of that ridge may become quite interesting.
The image above is today’s (11/7) 12z European Model valid for Sunday morning. An initially zonal pattern across the United States which lacks cold air will eventually become more amplified, allowing cold air to usher in. This is because that very strong ridge near the Aleutians will eventually become unstable and break, because of that area of vorticity just to the north of the red arrow drawn. When ridges break, they tend to break like a wave in the ocean, thus “crashing” in a downward direction. Of course, in this case the downward direction would be south instead of actually falling because of gravity.
This ridge breaking is important because once the ridge breaks, that forces a bunch of energy southward, instead of it allowing to progress eastward like most weather patterns do. This means that the trough and associated energy that is downstream of the ridge will also be pushed due southward and pinch off, instead of progressing due eastward. This helps to make the pattern more meridional as that trough becomes sharper. Once this trough dives further south and becomes sharper, it forces the initial weak ridging in the West to become much more amplified.
Fast-forward to Monday night, that initial ridge is broken and is entirely replaced by a large, sharp trough with consolidated and cut-off energy. Any ridging is now west of the Aleutians, which actually helps to further re-enforce this trough. Because the trough is now so sharp and oriented in a more north-south fashion, the downstream ridge in the Rockies thus becomes more amplified. This allows cold air to funnel on the downstream side of that ridge; allowing it to slide southeast towards the United States instead of traversing due east through Canada. This cold airmass will lead high temperatures to potentially struggle to get out of the low 40s on Wednesday!
Often times when troughs of cold airmasses head towards the United States, a lot of energy is associated with them, and it’s this energy that may turn into our storm system later next week. As the ridge amplifies, that energy will dive south and try to round the base of a trough somewhere between the Mississippi and Ohio Valley.
Now that we have the general pattern laid out, let’s go over some scenarios.
Scenario One: Storm cuts off too early, with the Appalachians getting snow, and coastal areas getting rain.
This scenario is illustrated well by today’s (11/7) 12z European Model. Just as we discussed before about the ridge in the Pacific breaking and forcing energy to pinch off underneath — the same type of scenario occurs in this case, only in the Tennessee Valley. The image below is valid for Tuesday night.
Today’s (11/7) European run showed a very impressive storm, but not a snowstorm for our area. Although the ridge becomes amplified, a very fast flow to the west and to the north of the ridge forces the top of the ridge to become tilted to the east — the flow is “pushing” the top of the ridge. When the top of the ridge gets pushed too much, it runs out ahead too far and thus “breaks” like a wave — just as we discussed earlier. Once it breaks like a wave, the energy trying to move east instead gets pushed to the south and pinched off from the flow, forcing it to cut off. Once the storm cuts off from the flow, the cold air that is rushing with the flow eventually runs out ahead of it.
Why is the flow so fast? This is because the NAO and AO are positive — the heights in Greenland and Arctic are very low. This creates an intense height gradient between the ridge (high heights) near the Aleutians and the ridge near the Rockies into southern Canada, with the very low heights near the Arctic and Greenland.
Fast-forward to Wednesday night, one can see that the ridge has successfully broken like a wave, as it is much weaker. Since the ridge wave crashed to the south, the storm system also crashed to the south and got pushed away from the main flow of the pattern — thus allowing it to cut off. Since this occurred so far to the south and west, only the Appalachians and Ohio Valley get hammered with snow. This is because the cold air is still running with the flow of the pattern, and thus outruns the storm by the time it can actually reach our area. Ironically, the fast flow actually caused the storm to be a warm solution, since the fast flow broke the ridge wave, forcing the storm to cut off to the south and west, pumping warm air into our region. Normally, fast flow sends storms out to sea, and this will be scenario two.
Scenario Two: The storm goes out to sea, giving our area very minimal impacts.
This scenario is illustrated by today’s (11/7) 12z GFS model.
The ridge spike in the Rockies is certainly still there, but the flow in Canada is still quite fast. This is where the lack of an NAO block and lack of a negative AO can really hurt for snow lovers. The fast flow in Canada forces the ridge to traverse eastward quickly and also forces the cold trough to move east quickly. Thus, by the time the storm rounds the base of the trough and actually tries to develop, the trough axis is way too far east and very positively tilted. This sends the storm out to sea.
Scenario Three: A strong storm system develops off the coast with a good cold air source, leading to accumulating snow.
This scenario is the least likely of the three, but is still a possibility. Last night’s 00z European model illustrated this potential.
The above image is valid for Tuesday afternoon. As one can see, the ridge spiking in Canada was much more impressive, as the ridge almost cut off from the flow of the pattern. This increased ridging helps to slow down the pattern somewhat, which prevents the ridge from becoming unstable and/or moving east too quickly. This would alleviate the concern of the storm cutting off too far south and west, or simply sliding out to sea.
Fast-forward to Wednesday night, and we see that the ridge is trying to break, but has yet to completely break. This allows the storm to cut off in the Ohio Valley instead of near the Tennessee Valley, and does not get pinched off as much. This means that heights can rise out ahead of the storm to turn it up the coast, but not too much to where the cold air source gets rotted away, like we saw with the 12z European model.
This complex pattern regarding ridge wave-breaking in the Pacific and even potentially in the United States will certainly wreak a lot of havoc on the models, so expect solutions to change in most cycles. Considering the general fast flow of the pattern and the time of year, the snowstorm scenario for our area remains unlikely, as a fast flow will generally mean either a breaking ridge wave, or a ridge/trough axis too far east.
That being said, if the ridge wave breaking is able to successfully amplify the ridge in the Rockies a bit more than anticipated, that could slow down the pattern just enough to yield a snowstorm here. Thus, the storm will of course have to be watched. But don’t get your hopes up, snow-lovers.