In a pattern with a growing PNA ridge and a polar vortex sitting in SE Canada, many shortwaves travel down the downstream side of the ridge as they get squeezed between the ridge and the vortex and forced southward into the United States. Often times, computer models are not good at picking up the strength of these disturbances until we get closer to the event. This time around, that seems to be the case, as many computer models are slowly showing a strengthening system.
Let’s take a look at the GFS model:
Above is today’s 12z GFS 500mb and vorticity forecast, valid for Monday night. We see a strong PNA ridge to the west, providing some amplitude to the pattern and supporting a trough in the East. It also helps to displace a big chunk of the Polar Vortex in SE Canada, further providing cold and supplying the trough in the East. As a result, a shortwave disturbance can be seen in the Plains and Midwest, as it is getting squeezed between the ridge and vortex, and is forced to amplify somewhat. This all sounds good for a snow event so far, but there are still several caveats.
For one thing, the orientation of the ridge is a bit too much from NW to SE for major amplification in time for a bigger snow event. Usually, we like to see the ridge have a more North to South orientation, that way the disturbance can give due southward instead of southeastward — this allows it to amplify further west. Part of the reason why this is occurring is that we still do not have well-defined blocking in the Arctic and North Atlantic yet. There is a weak ridge nosing into Greenland, but not one that is overly strong. Thus, the pattern cannot buckle all that much, and the Polar Vortex cannot be forced westward. Instead, it gets stuck a bit east of where we want it, but also a bit south of where we want it as well — which compresses the height field along the East Coast. This makes it difficult for our disturbance to amplify and strengthen.
Let’s move forward a few time-stamps on today’s GFS:
The shortwave is now in the Tennessee Valley, and looks somewhat impressive. It’s carving out a decently nice trough, and some height rises out ahead of it as well. This would trigger a surface low somewhere along the East Coast, and it can head northeast.
However, look at the height orientation of the ridge out west. There is a “kicker” disturbance coming into the West from the Pacific, which helps to flatten the ridge just a tad. This further enforces the strong NW to SE flow with the downstream side of the ridge, which forces a bigger eastward component to our shortwave instead of one that digs further south, gathers more moisture, and amplifies more. Without it being able to dig further south and a further west longitude, a moisture presence from the Gulf of Mexico will be absent, which limits the ceiling of potential for this event. Ideally, you want the shortwave to big into the Mississippi Valley and become neutrally tilted — in this case, it is not neutrally tilted until it is much further northeast.
Of course, this is not a terrible look for snow. A nice little shortwave carving out a decent trough in a cold airmass can still produce anywhere from a coating, to several inches of snow. Plus, cold temperatures throughout the atmospheric profile favor good snow-to-liquid ratios, so even only a tenth of an inch of liquid could equate to nearly two inches of snow.
Let’s look at the final results for this system at both 500mb, and the surface:
Moving forward to Tuesday evening, we can see that the shortwave has made it to the East Coast, but has become quite elongated, ans there is no truly defined “center,” so to speak. Regardless, a decent amount of positive vorticity advection looks to take place, which favors lift and some pressure deepening off the East Coast. The trough axis is pretty far east, however, thanks to the lack of blocking leading to a progressive flow to the pattern — but a couple of fluffy inches of snow is certainly in the cards.
As a result, we have a 1000mb surface low off the east coast, throwing back some light snow to the coast. 850mb temperatures are below -10C, which is plenty cold for fluffy snow.
Although we have made all of the caveats well aware in this post, and have made it pretty clear that we do not expect this event to become significant (6″+), but there is still room for this to trend from the current coating to 2″ (with locally higher amounts) as of right now, to a more moderate hit of 2-4″ or 3-5″.
All winter-season long, the models have underestimated the strength of the shortwaves coming down from the northern stream — i.e. the North Pacific and Central Canada. Although there are several factors in the pattern that will prevent any shortwave from becoming significantly strong — if this shortwave can keep trending a bit stronger in its initial phases as it travels down towards the United States, it will be able to slow down the pattern a bit more and amplify the pattern on its own. If it does this, then the chances for 2″+ of snow increase.
We will keep you posted on this snow event. Regardless, however, it’s going to get very cold!