Multiple Wave-Breaking Events Support an early January Snowstorm

While the coast is experiencing yet another rain event to end 2016, there are many changes currently ongoing to the pattern that could open the door for a widespread snow event in the beginning of 2017. How long the pattern change lasts for is a different discussion and will be talked about in our long range update tomorrow, but for now we will be focusing on the changes over the next week.

We first place our eyes on the Pacific. A couple of days ago, there was a strong Pacific Jet and the flow was somewhat zonal in the Pacific, and to some extent that is still currently true. But looking closely, there was an active wave train of ridges and troughs, despite the relatively flat flow. It wasn’t just one straight buzz-saw — there were ripples to this pattern that could easily lead to more amplification in a short period of time. While this isn’t the sole cause of the eventual wave-break, we have a cut-off low in the Southwest that retrogrades, and that began to slow the pattern down upstream in the Pacific. And when there are already ridges and troughs in-place, sometimes it just takes one slow-down to truly shake things up.

The cut-off low slows the flow down in the Pacific just a bit, which forces that previously zonal ridge to build over the top. Once that ridge amplifies a bit, it forces more troughing upstream of that, which allows another true ridge to get going in the Central Pacific. While the flow upstream of that ridge is still fast, it also as a result of the slow-down downstream becomes a bit more amplified, so you have a gradually more amplified ridge-trough train on both sides of a developing large ridge. That gives a feedback effect to the amplitude of the Central Pacific ridge as it heads east, which allows it to truly strengthen, move poleward, and “break” the previous wave train and settle in as its own large entity. Because the pattern in the east Pacific slows down and is more amplified, the “train” slows down there as well, so the large ridge that forms has plenty of staying-power instead of just bowling east right into the US. Additionally, the pattern in the West Pacific is still somewhat fast and only somewhat amplified, so the train keeps coming pretty quickly on that side. Once that train “runs into” the slowed down, large ridge, the large ridge is reinforced by the smaller ridge in the big wave-train, and thus turns into one giant, cut-off entity in the NE Pacific. The entire pattern is slowed down with a cutoff low beneath the massive ridge, and the entire Pacific slows down. This gives the ridge plenty of staying power and a new regime settles in. If there were no wave-train in the Pacific, there would be no ridge to reinforce the ridge we have building.

This ridge cuts off near Alaska and allows plenty of Arctic air to get displaced southward. Initially, the ridge does not dominate the entire NE Pacific, so some troughing works its way into British Columbia and pumps a large SE ridge, which warms the East. But once the ridge gains a bit more longitude, the Pacific Northwest shifts from being in a trough to being on the downstream side of a ridge, so the cold is forced eastward. Initially, the SE ridge connects with a building North Atlantic ridge, which amplifies the SE ridge further. However, another smaller wave-breaking event in the Atlantic will eventually force a “thumb-ridge” towards Greenland at the same time that the trough swings eastward. This will allow more cold to lock into the whole US, rather than just the western two-thirds of the country. Now while there may still be some Southeast ridge influence, it will get shunted somewhat by this new pattern.

Anecdotally and meteorologically, the Pacific block idea makes sense given that we’ve had plenty of those over the past few years, and also have the active Pacific wave train. But it has been quite hard to get a true -NAO pattern, which leads to skepticism for one to come back. And while we still don’t buy the idea of a truly sustained -NAO or a cut-off Greenland block, there is supplementary evidence for a -NAO during this period, rather than just what is shown on model guidance.

First of all, we have a large cut-off ridge near Scandinavia which has helped form another nice wave-train in the Atlantic with that ridge, a trough in Greenland, and another ridge in SE Canada. When there already is a wave-train, the pattern is more vulnerable to a wave-breaking event. We have the late-developing storm that is currently giving New England snow which should help pump more latent heat into the North Atlantic, and that ridge will get reinforced by the Scandinavia ridge. It then becomes a cut-off ridge in the North Atlantic, which gets enhanced by the developing SE ridge. This forces a cut-off trough to form in the Central and Eastern Atlantic, which forces the SE ridge to go over the top when it’s scooted out by the large trough downstream of the Pacific ridge. This leads to a classic “squeeze-play” thumb-ridge that builds towards Greenland. The ingredients are there for this to happen. It cannot completely get into Greenland due to the Polar Vortex not truly getting dislodged from there, and then all of the troughing now in the rest of the Atlantic could make this -NAO transient. If the thumb-ridge though could eventually get reinforced by more latent heat pumping ahead of the January 6-10 storm(s), then perhaps the -NAO could last a bit longer — but that’s a different discussion.

There is also Tropical forcing. One big thing that the models have been and still are missing is the coherent signal of the MJO pulse. This is one reason why they have not shown enough amplification in the Pacific (until now) and why they may be breaking down the upcoming cold pattern a tad too quickly. While this isn’t the strongest MJO pulse on record or anything, it’s enough to constructively interfere with the above wave-breaking events to help the -EPO and -NAO pattern. The typical RMM plots have not accurately handled the MJO wave. When we look at 200mb Velocity Potential (essentially the 200mb divergence that results from convection), we can see that there is plenty of upper-level divergence being forecast between 90W and 60W in early January, and upper-level convergence to the west of that, which almost exactly resembles the MJO phases 8 and 1. Composites from these phases show poleward North Pacific Ridging as well as a -NAO. Thus, this MJO pulse should only help these features develop further.

So now we have shown evidence of the ingredients coming together on a -EPO and well-timed -NAO tandem for early January. Now let’s take a look at more direct modeling.

The 18z GFS shows a very nice setup for a snowstorm. You have ripples of energy that slid down the downstream side of the EPO ridge and into the Southwest US. There is a very strong temperature gradient which further fuels the development of lift and moisture, and plenty of cold air with which to work. 850mb winds in the Gulf are out of the south, which indicates Gulf of Mexico moisture getting worked into the system. There is plenty of confluent flow to the north thanks to the -EPO and -NAO, which prevents a storm from cutting inland and also provides a jet streak in the Northeast to lead to lift when a storm does arrive. There is also still a hint of a Southeast ridge, which means a storm cannot just innocently slide out to sea.

One caveat on this run is the fact that the flow is very slow out west, but gradually faster as one goes east — look at 300mb. When this happens, sometimes shortwaves can get a bit sheared out, because the slower flow out west may mean that energy wants to hang back there, but the faster flow to the east means that some of it wants to run eastward. Considering that initially the energy out west comes down in a north-to-south fashion, and the flow east is more east-to-west, you can thus get some bit of elongation or a sheared out nature to the storm, and the timing doesn’t necessarily work out that all of the pieces phase together into one big monster storm.

The result is still a nice snowstorm for the whole area — generally around 6″ or so — with a beautiful jet streak to our northeast thanks to the confluent flow and a low pressure off the coast. But notice how at 500mb, the vorticity is a bit sheared out and all of the pieces don’t interact — look at the separate areas of energy in the Plains, the Tennessee Valley, off the coast, and near Manitoba. Notice also how there are multiple areas of surface low pressure as a result. It is thus possible that we could instead get two moderate waves of snow — circa February 1994 — instead of one true large snowstorm. Also, there could be one moderate wave and a second wave misses the area due the barocolinic zone shifting away from the region from the first storm. But either way, most of the ingredients are certainly there, and if there can be more direct shortwave interaction between what’s our west and what’s in the Southern Plains, the ceiling of potential could be quite high with this storm. It’s still a long ways out, and lots can change — but it’s safe to say that this is a heightened period of interest for a snow event.