As you all know by now, over the last several days the talk of the weather world has been on a system that will bring the first substantial synoptic snowfall of the season to much of the east coast, primarily targeting the Mid Atlantic, with Washington DC currently in a blizzard watch and ground zero of the storm. All guidance is in near unanimous agreement on the development of a colossal system for this weekend, and someone, likely in DC or Virginia, is going to get a ton of snow.
However, the same cannot be said for the great state of Connecticut. Sometimes, these systems will continue up the coast, bringing their heavier snow further north and producing a true I95 blizzard, where everyone from DC to Boston gets buried under heavy snow. However, for one of those to happen, just about everything needs to go flawlessly, and in the weather world, it’s rare that that ever happens, hence the rarity of blockbuster blizzards in this part of the country, or anywhere, for that matter.
In this case, there are a couple of the proverbial “flies in the ointment” that will most likely stop this system from becoming a blockbuster for us. Each are relatively minor details in the grand scheme of things, but when put together, they serve as a deterrent to the system to move north along the coastline, and instead, cause it to stall along the Mid Atlantic coastline and then drift east without making it to a latitude to put us into the meat of the system.
The first issue is an area of strong confluence to our north. In simple terms, confluence is the joining of several streams in the upper air, which directly creates convergence, causing a stronger mass in the atmosphere and raising the surface pressure ahead of the storm. This creates a temporary “block” ahead of the system and suppresses it back to the south, resulting in precipitation not reaching our latitude.
The second issue is a lobe of vorticity that slides down from SE Canada and crosses east right around the NY/Canada border. This interacts with the storm, captures it, and pushes it east before the aforementioned confluence can retreat to the north.
The final issue is the high pressure to our north. We have a strong cold high to the north, which is usually an ingredient we look for when searching for a strong winter storm to impact our area. However, this time, when it is combined with the confluence and the vorticity, the end result is too much of a good thing, and the storm is suppressed past our latitude and escapes further south.
Here's a comparison of the 0z vs 18z GFS aloft at 7 AM Friday morning. Notice the energy over northern NY/southern Canada is further south on the 18z GFS, and that on the 0z GFS, the system is stronger and the energy to the north of us is weaker. Minor, but noticeable differences, that will have a large impact on the ground truth.
Scenario 1: The Miss -- 40% chance of verification.
This scenario was a long shot a couple of days ago, but as the confluence and vorticity have become better modeled and have strengthened, the storm has continuously been trending south over the past couple of days, even with slight shifts north every so often on a model. As a result, what previously appeared unlikely is now distinctively more likely, with the model consensus favoring either a close miss or a scrape. In this scenario, we have stronger confluence and a more well-defined vorticity lobe that is placed further south, and as a result, the system is squashed to the south of us and we end up with little or no snow. This is best shown by the GGEM and NAM models, and an image of the total snowfall of the latter is shown below. Notice the razor sharp cutoff to our south, however. NYC sees a foot or more of snow on the NAM, while Bridgeport sees only a couple of inches, and just north of there sees practically nothing. This scenario, while on the southern edge of most guidance, is becoming more likely as the general trend of the confluence and high pressure becoming stronger continues on the modeling. I caution you however, it is impossible to know if a shift or trend is an overreaction to additional data.
This scenario, which was essentially our consensus yesterday if you read our discussion, continues to represent the current consensus on modeling, and thus remains the most likely of the three scenarios despite the continued south trend. Under this scenario, the confluence and vorticity relax just enough to allow the storm to come a bit further north than in scenario 1, and the net result is a couple inches of snow across most of the state, with higher amounts on the south coast. This is well represented on the European and Canadian models, with the snow map of the Canadian being shown below.
I still think that this, as of now, is the most likely scenario. The consensus between the 12z GGEM and Euro as well as the 18z GFS for this scenario is fairly strong, and I think that the extreme southern scenario shown by the NAM is unlikely. I do expect a slight north trend with this system in the final days leading up to the event, as I think that the combination of our current strong El Nino promoting a strong jet stream, the tendency of large Miller A systems to have a deformation band on their far NW flank, and the fact that the lions share of past large storms have trended north as we came closer to gametime, makes me think that the extreme solution shown on the NAM is too far south. I currently expect this trend to be relatively minor, however, if it is more significant, we come to scenario 3.
This was the model consensus a couple of days ago when we were originally starting to talk about a large system potentially impacting the state, and formed the baseline for Greg’s discussion on Monday night. However, yesterday saw the shift towards a further south, more compressed solution as a result of the appearance of the shortwave to our north and stronger confluence. The end result was the shift of the system to the south that we saw on guidance yesterday, and that makes this scenario now the most unlikely of the three. However, should we see the lobe of vorticity shift to the north (or disappear entirely), the confluence weaken, or the upper level low accompanying the storm strengthens and keeps it closer to the coastline, we could see a significant snowstorm across the entire state. This is shown on last nights 0z GFS, which had a foot or more of snow across all of the state as a result of slightly weaker confluence and a stronger upper level low. It’s worth noting that the change in the overall storm track did not shift that much, nor did the upper air setup change all that much. Minor differences in the strength of the confluence and the timing of the upper level vort result in major changes on the surface, and as a result, this remains a very low confidence forecast.
- A significant storm is likely along the eastern seaboard this weekend. As of now, it appears that Connecticut will be right on the northern edge of it, with a small shift one way or the other making a substantial difference in verification.
- The best chance of heavier snows are along the south coast.
- The overall theme of guidance appears to be set, however, small shifts in the upper air features are likely, which will result in a substantial change to our sensible weather one way or the other.
- Snow is not the only possible impact from this system – high winds and coastal flooding are potential impacts as well.
- A first call map and detailed timing will be issued tomorrow evening – stay tuned to SCW for the most up to date information about this upcoming storm!
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