Following sunny and seasonably cool weather tomorrow and Saturday, all eyes then shift to an approaching system for Sunday. Unlike the other coastal storms we’ve seen so far this winter, there doesn’t seem to be as much uncertainty with how this one is going to develop, likely because the system is already on shore (currently bringing snow to the mountain west) and has a fairly straightforward evolution; the system tries to run into the lakes to our west, but is then shunted to the east by a strong high pressure to our north. Pretty classic setup for a winter storm in the area, and one that models tend to be more reliable than a more complex system involving phasing. That said, there is still of course plenty of variability, both in the strength of the system and the positioning of the block. Those, along with the airmass, will determine the mix of precipitation types that we see, and will determine if we’re in for a snowstorm, an icy mix, rain, or nothing at all. Generally, the stronger the high pressure to the north, the further south the system tracks, and the colder the airmass becomes, making it more likely that we will see a wintry solution. Concurrently, a stronger system can be more dynamic, and allow for better transfer of cold throughout the column, eliminating the nasty warm layers that can sometimes be found in a weaker setup. Of course, get things too strong up north, and it's a snowstorm for the mid Atlantic while we’re cold and dry here in Connecticut. Conversely, a weaker, more strung out system would introduce the chance of some icing, or if the high pressure were to trend further north or weaker we could see a scenario where the low tracked over or north of us and we ended up as mostly plain rain.
Here's a look at the overall scenario on the GFS. Notice the low pressure going up into the Midwest and the strong high to our north limiting how far north the primary low can go before it moves east and transfers.
As far as timing goes, this system looks to be relatively long duration. We’ll start off with overrunning precipitation from the parent low in the midwest on Sunday afternoon (the most recent GFS has snow just entering the state at 1 PM), with the parent low in the midwest transferring its energy to a new secondary low south of Long Island on Sunday evening and prolonging precipitation through Monday. Depending on how long the low stays stalled out, and where it ultimately tracks, there’s the potential for precipitation to continue right through Monday and into Monday night. Furthermore, as the low strengthens and moves east, we’ll see more cold air dump in from the NW behind it, so it’s certainly possible that we could see a flip back to snow later Monday or overnight into Tuesday for any areas that do change to rain from the primary.
Here's a look at temps on the GFS the last four runs of the GFS for 7 PM Sunday - you can clearly see the airmass ticking a bit colder on each run. I'd toss the thermal profiles on the GFS as it's notoriously bad for low-level cold, but the idea holds true across the other models.
The GFS has been the warmest model, flipping most of the state to rain after a quick burst of snow at the start. However, on the most recent run, we’ve seen the high to the north strengthen, pushing the whole system a little bit further south and allowing for a period of front-end snow. On this solution, however, most of the accumulations would come on the back side, as the low slowly strengthens and moves east, just cold enough air is able to work it’s way into the state to flip all areas back to snow Monday afternoon, with snow lingering through the night into Tuesday morning. On this scenario, we’d see noticeable accumulations in the hills, with lighter accumulations in the southern portion of the state. I would say that this scenario is the most likely, say 40-50%
Compare the GFS to the colder consensus (GGEM, Euro, NAM so far); on those models, the high is stronger and further south, allowing for better cold air to be in place and for more snow on the front-end. In fact, on last night's Canadian run, precipitation remained all frozen in the state, with all snow in the northern counties and snow to a mix to snow in the southern counties. While I think this scenario is overdone, it shows the bounds of what’s possible here if the system is able to stay on the colder side. Generally, the GFS keeps precipitation around longer than the rest of the model consensus, but all models agree that we’ll see continued precipitation into Monday at the earliest. Should a colder scenario like the GGEM verify, we could be looking at a significant snowfall for most of the state, especially away from the immediate shoreline. The Euro is a healthy compromise between the GFS and the GGEM, with a mostly snow scenario in the hills and snow-rain-snow for the rest of the state. This scenario is less likely than the first scenario, but becoming more realistic with each model that jumps onboard; I’d give it a 30-40% chance of occurring right now.
The third scenario, which the GFS was advertising until this morning’s run, is when the low passes to our north and puts us in the warm sector; a quick period of snow would flip to a cold rain for the state; an inch or two would be possible in the hills and no accumulation would be expected elsewhere. This is probably the least likely scenario given no guidance currently shows it, but I’d still give it a 20-30% chance of occurrence.
Here's an overview of the system on the most recent GFS and GGEM; I wouldn't pay too much attention to the actual outputs right now, but more get an idea of the track and evolution of the storm and the resulting impact to temps and precipitation type.
Here’s a view from the most recent Euro ensemble that shows the modeled probability of exceeding 6” of snow across the state.
The bottom line is that a significant coastal storm looks to impact the area from Sunday through Monday or Tuesday, and that the first significant snow accumulations of the season are possible. We’ll have more information and a first call map if needed tomorrow evening, until then, have a safe and enjoyable Thanksgiving!