As Don alluded to last night in his discussion, we're expecting an Alberta Clipper system to impact the region tomorrow afternoon and evening, which will bring a quick shot of wintry weather to the state. Models have been trending a bit stronger with the energy and a bit quicker to transfer the low to a coastal system that will go on to bring warning-level snows to Maine, and the result is a slightly more significant event for CT; enough to issue a snowmap and to give us something to talk about. As always, we'll divide this discussion into three parts; Models and Trends, Forecast and Timing, and Impacts. If you're just looking for the snowmap, scroll down to about the middle of the page.
Models and Trends
Generally, the trend on guidance over the last 24 hours or so has been for a slightly stronger system to impact the state. This trend was started with the 12z Euro yesterday bringing the system well west of previous runs, and over the next couple of cycles, most of the model guidance has followed. Differences remain, however, in several key variables that will determine what, if anything, we see tomorrow.
First, temperatures. The overall setup is essentially this: Temperatures in the northwest third or so of the state are cold enough for snow throughout the event, while areas to the southeast will start as rain before flipping to snow. When that flip happens, however, remains up in the air. The guidance is split into two camps; with the warmer NAM and SREFS arguing for primarily rain in this area, while the colder RGEM says that the entire state will flip quickly to snow as heavier preciptiation moves in and the column wetbulbs. The Euro and GFS are in between, with a slight cooling trend observed over the past 24 hours but no substantial shifts.
As of now, I am leaning towards the colder solutions. Models often underestimate the impact of heavy precipitation on the column, and often times, in a marginal scenario such as this one, when the entire column is below freezing except at the immediate surface, precipitation will allow the column to wetbulb to freezing the whole way down and allow for snow to the surface. I have based the map on roughly a 2:1 blend of the cold to warm solutions, which results in the northern tier of the state being all or almost entirely snow and the majority of the state starting as rain but changing to snow in time for a good portion of the QPF to fall as snow. In the far southeast corner, where ocean influence is at it's strongest and cold air will take the longest to arrive, I have left mainly rain in the forecast, but I may eliminate that zone in the morning after I get a look at the overnight guidance.
Here's a look at temperatures at 4, 7 and 10 PM on both the NAM(Left, warmer solution) and RGEM(Right, colder solution). Images courtesy Weatherbell Analytics.
Next, precipitation. Precipitation from this system will come in two parts; the first being from the clipper system as it passes to our north, and the second from the coastal storm forming to our east. Here's the 18z GFS valid for 7 PM Tuesday and 1 AM Wednesday, showing the precipitation that's fallen in the previous 6 hours. This illustrates that divide clearly.
Here are forecasted snow totals from most of the major models(All the ones which we can legally share for free). 1mm = ~0.04" of liquid = ~.4" of snow. Remember that these algorithms use the model data verbatim, and will ignore the wetbulbing and isothermal column which I describe earlier, and thus I believe that they may be slightly too warm on their borderlines. Regardless, they provide a good visual comparison.
Here's our forecast snowmap for this event. We apologize for the image quality of the town lettering; due to some technical difficulties, we had to use a different map software than usual and we prioritized getting you the information as quickly as possible rather than taking valuable time to try to troubleshoot those issues.
In the middle zone, I expect precipitation to start as rain before changing to snow from NW to SE during the later portions of the evening. I expect a coating to an inch of snow across this zone, with the inches more likely to be found in northern and western portions of the state as they will be the first to change over and will also be closer to the surface low -- meaning slightly higher QPF. I wouldn't be shocked to see some areas with up to 2" in this zone where banding develops, but don't think totals over 1" will be widespread enough to justify expanding the forecast range.
In the far SE zone, I expect mainly rain, with a changeover to snow occurring just before the precipitation wraps up, and thus, only a wet coating will be the net result for accumulations. Should a colder solution like the RGEM verify, I could see up to an inch in this zone, but as of now our blend of 2x RGEM to 1x NAM for temperatures still yields mainly rain in this zone.
Should the coastal storm develop a bit further west like as depicted on the GGEM shown further up, eastern areas could see up to an additional inch or so of snow. As of now, I have not included that in the forecast and expect that it will just miss us to the east, but it is none the less an interesting wrinkle in this system that's worth watching.
2-5 PM: Precipitation breaks out from west to east, starting as snow in the northern tier and rain elsewhere. Rain quickly changes to snow in the green zone.
5-7 PM: Rain begins to change to snow from northwest to southeast, likely having some impact on the evening commute for I84 and portions of I91.
7-9 PM: The remainder of the pink zone continues to change over from rain to snow from NW to SE. Should convective bands develop, this is the timeline where we would start to see them.
9-11 PM: Precipitation begins to move out from west to east, with the SE zone changing to snow as the precipitation comes to a close.
11 PM - 6 AM: Some lingering snow showers are possible, especially across eastern areas. Should steadier snow from the coastal impact the eastern portions of our forecast area, it would be during this time.
- Accumulating snowfall for the majority of the state, with a low-end plowable event possible in isolated areas in the northern tier
- At least some impact to the evening commute, especially on I84 and portions of I91
- Scattered flight delays at both Bradley and Tweed airports.
- Some school delays are possible Wednesday morning -- especially in eastern areas. No cancellations are expected.
- Blustery winds and some blowing snow possible Wednesday morning.
We'll keep an eye on the overnight guidance, and will update the snowmap in the morning if needed. Otherwise, we'll be updating rapid-fire on both our Facebook and Twitter pages throughout the storm, so make sure to like and follow us using the buttons below if you haven't already to stay up to date with this event!
As always, thanks for tracking this storm with us! -SA