As colder air begins to filter in over the next few days, the weather pattern will become more favorable to support a snowstorm for later this week. While details still remain a bit uncertain, the trends in all of the latest model guidance support the threat for a potentially high-impact snow event.
Although December was a pretty active month as far as winter storms were concerned, there were no major, widespread snowstorms for the entire area. The main reason for this was the lack of any North Atlantic blocking, which helped to keep the flow in the pattern too fast for any major, organized amplification. However, this time around, some North Atlantic blocking has developed. Although it is not tremendously strong, it is located in a perfect position to slow down the pattern and support a major storm. Let’s go over all the pieces in the latest model guidance.
The image above shows today’s 12z European Model at the 500mb level, valid for Tuesday evening. There are several key pieces labeled which indicate the potential for a big storm. Although the storm itself is still a few days away, the fact that this setup is in place within 36 hours gives us a bit higher confidence in a snowstorm of some kind.
The first feature is a -EPO block just east of Alaska. The primary mechanism for the cold periods we had last month was a large ridge in the general region of Alaska, which helps to dislodge Arctic cold southward towards Canada and the United States. After a brief relaxation, that pattern has returned, with a nice blocking (closed contour) ridge just east of Alaska. The fact that it’s located just east of Alaska puts it into the Arctic domain, where the Arctic Oscillation is calculated. This explains why readings for the Arctic Oscillation have dropped, which is more favorable for larger snowstorms.
The next feature is what was mentioned before — Greenland blocking. Although the ridge is not particularly anomalous, it is located in a perfect spot to lock in cold weather and a trough in the eastern half of the United States. Additionally, it is located in an area that is trying to connect with the -EPO block, and when that happens, polar vortexes are often forced to the south into southeastern Canada.
There is also a nice ridge that is developing along the west coast, which tries to connect with the -EPO block. This is arguably the most important feature, since when these ridges are amplified enough, multiple pieces of energy slide down the downstream side of the ridge, into the United States. And when there is a blocking pattern, this energy is forced to slow down and amplify, which we will see in later time stamps. The two pieces of energy that are associated with our storm are both circled.
The last feature we will talk about is the large polar vortex in southeast Canada. The previous rain storm and another disturbance that will slide out to sea will help to reinforce this vortex, and the blocking to the north will help to keep it in place. This is crucial, because this vortex is what is supplying the cold air. It also helps to prevent the storm from amplifying too quickly and flooding our area with warmth, since it pretty much acts as a brick wall. Without the blocking to the north, the polar vortex would have moved out of the way, decreasing our snowfall chances significantly.
Let’s fast forward a bit to Wednesday evening.
On Wednesday evening, all of the major ingredients are still there for our major storm. Also, the polar vortex has barely moved. This is a product of the blocking and why blocking can be so important. The fact that the polar vortex is so strong and has not moved leads to a strong height gradient between the ridge out west, and the vortex. This is why the height lines are so compressed in Saskatchewan and Manitoba. This compressed height field is what helps a strong piece of energy to develop. Also, the fact that the blocking exists forces the pattern to buckle somewhat, with the vortex being pressed against that ridge. When that happens, the ridge out west is forced to slow down, stay a bit further to the west, and the flow becomes more meridional (due north to south). If the vortex were further east (in a less significant blocking pattern), this gradient would have been a bit weaker, further east, and would not have been able to dive due north to south. It would have gone northwest to southeast, and not been able to amplify nearly in time for a significant storm.
What has been key is that the models have trended stronger with this secondary piece of energy, and have allowed it to dive from north to south. This allows the energy to eventually phase with our storm in the Mississippi Valley, which is a perfect spot for prime trough amplification for a major east coast snowstorm. The end results at 500mb are shown below:
Notice how in the two 500mb images, the polar vortex gradually gets compressed and moves away, as our storm really begins to take shape. This is a classic indicator of a major winter storm, as the polar vortex is initially in place to supply cold air, and provides a brick wall that slows the pattern down, giving our storm more time to strengthen, but also preventing it from cutting to our west. But then as our storm strengthens, it slowly moves away, giving a path of least resistance just off our coastline, and the storm takes that track.
Also, the blocking in Greenland shifts a bit further to the south and west, and becomes a classic west-based NAO block. The trough is able to amplify underneath this block, as the pattern gets more buckled. Although the block is not that strong, it is in an ideal position for a snowstorm.
Thus, the European Model shows the potential for 10-15″ of snow for most of the region. The cold airmass may allow snow to liquid ratios to be higher than 10:1, further leading to higher snowfall amounts — temperatures may be in the teens at times during the heaviest snow. Additionally, the winds could be strong, too, leading to blizzard-conditions.
Of course, we are still a few days away, so some uncertainty still exists. Considering the block is not all that strong — if its position is just a bit further to the north and east, the storm may take a bit longer to amplify and thus spare our area of the heaviest snows, and only New England would get a major snow event. When comparing this setup to other major snowstorms in the past, some of those storms had stronger blocking, and the ridging out west got flattened a little less. This is the one thing that is preventing meteorologists from having 100% confidence in a major event, since these features can still change over the next few days.
However, considering the trends that are taking shape with that secondary piece of energy, there is room for this storm to trend even stronger. What often times happens is that once energy in the North Pacific moves into Canada, it gets more accurately sampled by our computer models, since there is more accurate observational data in Canada than in the North Pacific. Once models can more accurately sample this data, they often begin to either trend much stronger, or much weaker with the energy.
The fact that the models have begun to get stronger with this energy as it has become more accurately sampled increases confidence of a major snow event, since the phase with this piece of energy is so crucial to our storm to begin with. The stronger piece of energy is what also helps to allow the flow to become due north to south, helping the storm to dig towards the Gulf of Mexico, and obtain additional moisture and even some convective and positive feedback processes that increase heights downstream more. Additionally, if the models continue to show this piece of energy being stronger, consensus for a major snow event would increase, and the models would start truly showing some ridiculous solutions.
Stay tuned to our website and our social media outlets, as although confidence is increasing, there are still several things that could change over the next several days — nothing is set in stone.