As a quick burst of moderate to heavy snow crosses the region, dropping 1-3″ area-wide, our attention begins to shift towards the latter half of next week, as some of the computer models have been showing a major snowstorm for the East Coast.
The setup is particularly interesting and unique, because when one first takes a look at the pattern, it does not appear to be all that impressive. There is no large-scale NAO blocking, the Polar Vortex is not in a position where it can truly provide confluence and prevent a storm from cutting inland, yet the pattern also has hints of being progressive as well, given the fast flow in the Pacific. This does not scream major storm at all — especially a major snowstorm. However, when one takes a closer look at every feature of the pattern, it becomes more evident that several little pieces to the pattern are taking the place and temporarily acting as favorable ingredients for a major snowstorm, at the exact time where moisture from the southern stream is streaming from the Gulf.
Normally, in evaluating the prospects for a major snowstorm, we like to take a look at the pattern 36-48 hours prior to the event. This way, we can view the setup, and if this setup is conducive for a major snowstorm in future panels. Let’s take a look at today’s 12z European Model, but for the entire Northern Hemisphere. This will show 500mb heights in contours, and anomalies in the shaded colors.
Let’s first elaborate a bit more on what appears unfavorable for snow. First of all, we notice quite the progressive pattern in the Pacific. This is because there is a large height gradient between a vortex south of the Gulf of Alaska (negative height anomalies), and the ridge at further south latitudes in the Pacific (positive height anomalies). This leads to a very fast west-to-east flow in the Pacific, which makes the pattern progressive. Additionally, there are also slightly negative height anomalies in Greenland, where we ideally want those to be positive. Those negative height anomalies are part of the reason why the large chunk of the Polar Vortex is not situated in SE Canada, like it had been all winter prior to our other storm threats; higher height anomalies near Greenland tend to force a piece of the Polar Vortex underneath it. This helps to make our cold air source a bit more moderated, and certainly brings the potential for any major storm system to bring rain instead of snow.
However, when we examine things a bit further, we can see there are several features that are counter-acting this. Several days ago, we alluded to the huge -EPO ridge retrograding a bit, and we can see this powerful ridge to the west of Alaska. This ridge has become so strong, however, that it has pulled in negative heights beneath it. This is what we call a Rex Block. This is quite important, because air downstream of Rex Blocks will have the tendency to move more due north to south, as opposed to west to east. Of course, downstream of the Rex Block is Western Canada and the Western United States — the PNA region. This means that we are going to have air flowing more due north to south in the PNA region — helping to lead to a +PNA spike. This will attempt to counter-act what initially looked like a progressive pattern. This greatly helps for major amplification downstream. To read a bit more about Rex Blocks, check out this link.
Moving a bit further east, we can see another favorable feature: the EPO ridge became so strong, that a lot of the higher heights got pinched off further east towards the Arctic and through Scandinavia. This is important because having higher heights in these regions prevents the entire country from being flooded with Pacific-oriented (milder) air, at least for a little while. It forces negative height anomalies beneath it in Canada, which helps to provide at least a modest cold air source. Additionally, this combines with the PNA spike to produce negative height anomalies in Central Canada. This will help to provide more energy to slide between the PNA ridge and the negative height anomalies in Canada, which may ultimately interact with our storm. It also provides the beginnings of an Omega Block in Eastern Canada, as well.
The Omega Block in Eastern Canada is an additional important feature for snow. Notice how we have negative height anomalies in Central Canada, a ridge of positive height anomalies downstream, and more negative height anomalies downstream of this. It’s shaped like an Omega, which is why it’s called an Omega block. The Omega block provides two important things: 1) It becomes hard for air to push eastward quickly, since the the ridging is in the way — so the pattern slows down and becomes more favorable for amplification. 2) It reinforces the negative height anomalies on both sides of it. We already talked about the one in Central Canada, so let’s talk about the one further east. This area of low heights is actually near classic 50/50 region (50 Degrees Latitude and 50 Degrees Longitude). Low heights in this region are generally favorable for East Coast snowstorms, since they prevent heights from rising too quickly along the East Coast, helping to prevent inland tracks and rain. Since air will not flow east as quickly due to the Omega Ridge to begin with, however, this may allow the 50/50 low to stay in place just long enough for a snowstorm. To read more about Omega Blocks, check out this link.
This leads us to believe in the potential for a significant snowstorm for Wednesday night into Thursday, and it has the chance to be the biggest snowstorm of the year. That being said, the fact the Omega Block is not exactly enormous in size, and there is not a whole lot to keep the low heights in the 50/50 region in-place since the blocking to the north of it is not all that strong. Additionally, we are relying on a Rex-Block-induced PNA ridge spike, as opposed to a strong PNA ridge already being in place. This adds a bit of uncertainty to the models and how exactly they forecast this PNA spike. Any small changes in this PNA spike could prevent the moisture in the Gulf of Mexico from cleanly phasing with energy diving down from the PNA ridge, which could lead to a sloppier, less organized storm, where a lot of the moisture misses out to sea, and/or the storm never truly consolidates. Additionally, if the 50/50 low moves out just a bit too quickly, the risk for snow turning to rain also exists.
We still have a few days to iron out the details in this storm, and confidence is not high just yet. Models will begin struggling with certain small features over the next few days, which will have a major impact on the final solution. However, there are enough ingredients to the pattern to indicate to us that the potential for a significant snowstorm is elevated for Wednesday night into Thursday.