Explaining the uncertainties with the upcoming snowstorm

Although yesterday saw pretty good agreement among most computer model guidance regarding a major snowstorm on Monday, we were still four days away from the storm, which is a pretty long time in the realm of computer modeling. In more plain terms, this means that despite this agreement, we were still susceptible to changes in modeling that would ultimately affect the outcome.

The current weather pattern is dealing with a strong storm system crashing into southwest California, as well as a strong Polar Vortex in Southern Canada, supplying cold air into much of the country. This storm system in southwest California is forecast to move eastward, and as it does, will be increasing the warmth and moisture out ahead of it as it interacts with the Gulf of Mexico. Additionally, the Polar Vortex is forecast to exert its muscles and intensity the cold around it and just to its south. This leads to a strong source of anomalously cold air in the north, and anomalously warm air to the south — creating a strong temperature gradient which should lead to widespread, heavy precipitation just north of this gradient. This part seems easy.

But it’s much more complicated than that. The Polar Vortex in a way is a double-edged sword. While it can supply the cold air to strengthen the gradient, it can also be so strong that it shears out the initial storm system. If the initial storm system gets sheared out, then its ability to generate warm air advection out ahead of it and spread northward is decreased, which means the thermal gradient weakens and gets pushed to the south.

Let’s take a look at some computer models. Today’s 18z NAM showed a major snowstorm of 10-15″, whereas today’s 18z GFS only shows a few inches. Below, we will see the 18z NAM valid for 10:00pm, Saturday night.

Today’s 18z NAM at 500mb valid for 10:00pm Saturday night shows a favorable setup for major snow, due to the Polar Vortex becoming elongated, as opposed to compressed.

The above image is taken at 500mb — roughly the middle of the atmosphere. We can see our strong storm system in the Southwest. In Canada, we see a large piece of the Polar Vortex, which has become somewhat elongated. Notice the strong piece of energy stretched out on the west side of the Vortex, which reinforces its elongation. Additionally, this allows for some height rises in SE Canada just beneath the Vortex, despite the relatively compressed flow. The strongest height gradient is outlined in red, which is streaming right towards our area.

Now, let’s take a look at today’s 18z GFS, also valid for 10:00pm Saturday night.

Today’s 18z GFS at 500mb valid for 10:00pm Saturday shows a slightly less favorable pattern for snow than the NAM, due to the Polar Vortex being pressed a bit more, which suppresses the flow.

Looking at the storm out west, we do not see any major differences, which seem to indicate that the storm’s initial strength may not be as important as one would think for the final outcome of the storm. What is more important is the orientation of the Polar Vortex. Although it appears subtle, if we look closely, we can see that the Vortex is slightly less elongated, and has a greater north-to-south orientation as opposed to an east-to-west orientation. This allows the heights in SE Canada to be a tad lower, which leads to suppression. Additionally, notice how the energy rotating on the back side of the Polar Vortex is further east on the GFS than it is on the NAM. This will not reinforce elongation, and leads to lower heights further east — where the storm is supposed to track — which also helps to suppress the storm system. The initial gradient, however, is in generally the same location, which proves how subtle the changes really are and the fine details involved regarding the differences between a few inches of snow, and over a foot.

Let’s move forward a bit on today’s snowier NAM model to 10:00pm on Sunday night. The differences become a bit more pronounced.

Today’s 18z NAM at 500mb shows a favorable setup for major snowfall, as the strongest thermal gradient is oriented in a SW to NE fashion, which allows heavy precipitation to creep towards out area.

We can see that the storm system is still relatively strong in the Southern Plains, and the heights ahead of it have risen. This leads to warm air streaming northward. The Polar Vortex to the north has become quite elongated, in a southwest to northeast fashion. This allows for heights to increase a bit in that same fashion in the northeast as well. This means that the gradient can also align in that same exact fashion, which keeps precipitation pumping northward. Furthermore, note how that initial energy that was rotating on the back side of the Polar Vortex has still hung back pretty far west, and is attempting to interact with our storm system. This decreases heights within the longitude of the storm system itself, creating a “troughy” look, which also reinforces height increases out ahead of the storm, further helping precipitation to go northward.

Today’s GFS valid for 10:00pm on Sunday night is quite different, however.

Today’s 18z GFS shows the Polar Vortex compressing the pattern, weakening our storm, and keeping the heaviest precipitation to our south.

The storm system has become much weaker on this run, for a couple of reasons. The Polar Vortex is not nearly as elongated — in fact, it has become even more compressed, and it is suppressing the heights greatly in the East. Thus, our storm system is running into a hostile environment. Additionally, that piece of energy which was rotating around the back side of the Polar Vortex is much further east. This means that it is not interacting with our storm system, and instead races ahead of it. This means that the dip in the heights along the longitude of our storm is not nearly as pronounced, creating a flatter look, and the height rises ahead of our storm actually become a small area of height falls. Our storm system is forced to travel in an area where heights are flattening, which leads it to weaken. A weaker storm system will not be able to generate as much warm air advection out ahead of it, which means warm air cannot go as far northward. This means that the temperature gradient weakens, and becomes oriented to the south.

Today’s 18z NAM shows heavy snow with precipitation oriented from southwest to northeast.

From a more broad perspective, the Polar Vortex in general is a hair east on the GFS compared to the NAM as well. This means that in general, heights get suppressed more the further east you go — which in this case, is ahead of the storm. Thus, after 10:00pm on Sunday night, the strongest gradient gets pushed even more to the south, and the heavy precipitation ultimately misses our area.

Which model will be right? Modeling the exact orientation of the Polar Vortex is quite tricky, especially if the biggest differences really are as subtle as the exact location of a piece of energy on the back side of it — since this affects how elongated the Vortex becomes, as well as how much energy can interact with our system. It goes to show how delicate of a science meteorology is, and why forecasting these snowstorms can become such a nightmare.

Today’s 18z GFS shows lighter precipitation missing to our south, and it is oriented from west to east.

We are currently leaning towards a blend of the interaction with the Polar Vortex. The pattern in our opinion is anomalous and has enough blocking to prevent the Vortex and associated energy from moving east and compressing our heights. There is enough blocking to force some elongation, as well, which would argue that today’s GFS trends to the south are definitely an over-correction. Additionally, the storm should probably be a bit stronger than the GFS has it, due to strong tropical forcing and an active sub-tropical jet-stream. We do know the shortwave will weaken — but if the gradient initially becomes strong enough and oriented enough to the north, we can still get significant snow. That being said, the blocking in the North Atlantic in itself is not that anomalous, which means that the Vortex can only get pushed back and elongated to the southwest so much, before eventually sliding eastward. Thus, although a 6″+ snowstorm (or even 10″+) for our region is certainly possible, there are definitely enough variables which could keep this storm mainly to our south to prevent us from having more than 50% confidence in that.

What should we look for in future runs for a more significant storm?

1) An elongated Polar Vortex with more energy diving in on the west side of the vortex. This leads to more troughing in the longitude of our storm system.

2) Somewhat related to point #1, but a meridional flow (north to south) just west of the vortex, which is an indicator of a slower, more amplified pattern, where the Polar Vortex will not move east and suppress our heights as quickly.

3) A stronger shortwave in the West and Plains.

We will be sure to keep everyone updated on tonight’s and tomorrow’s computer model runs. Until then, enjoy your Friday night and the weekend!