Below, you’ll see our final call map for the upcoming system that will impact the state tomorrow and tomorrow night. Greg wrote a full discussion earlier this afternoon, which is included in an edited form in this post, but we wanted to give a quick and simple summary for those of you just looking to get the information you need to know now.
Model consensus has generally shifted north over the past several runs, and we have finally seen a global model(the 18z GFS) join the mesoscale models in predicting a statewide impact of larger scale. While we don’t expect it to verify, should a straight mesoscale/18z GFS solution prove to be correct, pretty much the entire state would see warning level(6”+) snows. As of now, we think that is about a one in four shot, but that’s not an insignificant number and so we mention the possibility here.
As far as our current forecast, model consensus is now quite strong on the coastline being just far enough north to make it into the heavier snow band for an extended period of time, and thus, we have upped our forecast snow range to six to twelve inches. We expect that most parts of this zone will remain in the 6-10” range, but have left the possibility for up to 12” in the forecast to account for localized heavy banding and the possibility of an even further north increase of precipitation.
In the next tier, we think that the heavier snow band will make an appearance for at least a short period of time, which will be enough to drop a few inches of snow, with lighter snow falling on either side. As such, we are forecasting a 3-6” snowfall for the central portion of the state.
In the northern tier, while the system has come far enough north to introduce light precipitation into the area, we still don’t have enough confidence in the heavier snow band making it that far north to go with a heavier forecast, and as such, are forecasting 1-3” for the northern tier of the state, with a small area of C-2” in the far NW corner where latitude and longitude together create a very unfavorable situation for any periods of heavy snow.
In addition, winds will be an issue with this system. It will become very windy along the south coast, regardless of how much snow falls. Winds could gust to 50 to 60 MPH right along the coast. This could cause power outages, especially if significant snow falls as well. Winds like that can cause power outages without any additional help, and if heavy snow falls in addition to those winds, it will make any power problems worse. Coastal flooding will certainly be an issue along the south coast, as well, as tides are already astronomically high.
Midnight-7 AM: Light snow develops over the southern portion of the state and gradually works its way north.
7 AM-1 PM: Heavier snow and stronger winds move into the southern portion of the state, while snow breaks out in the northern tier.
1 PM – 5 PM: Heavier snow continues over the southern portion of the state, while lighter snow continues up north, with brief periods of heavier snow in the northern zones.
5 PM-midnight: Snow tapers off from west to east across the area, with most precipitation being clear of the state by midnight Saturday night.
From here, I'll turn you all over to Greg for a more detailed technical discussion which also has more information on non-snow impacts that are expected. Thanks for reading SCW!
5:30 PM Update: The NAM has continued to be wet, and the latest GFS has continued to trend wetter, especially along the Northern fringe, namely the South coast! Therefore, after conferencing, we have decided to raise totals, especially in southern and central areas. The new totals are as follows: 6-12" along the south coast, basically the Merritt Parkway South, 3-6" in most of central Connecticut, 1-3" in most of Northern Connecticut, except a coating to two inches in the far Northwest corner of the state. It has become more likely that the south coast will see a significant snowfall. It is still likely that very little snow will fall much North of Hartford and parts of extreme Northern Litchfield County may struggle to see a flake. The most difficult zone will be basically between the Merritt Parkway and I-84 (basically interior central Connecticut). These areas are right on the line between significant snows and very little, and any slight shift in modeling will have drastic impacts on their snow totals.
Here's a look at the 12z and 18z GFS QPF totals, showing the significant jump north.
If the snow makes rapid progress northward, it means two things: One, the low pressure system is moving further north, and Two, the moisture is encountering less resistance as it moves into drier air. So that is definitely going to be something to watch for and we'll know by midnight tonight. So for now, the basic reasoning remains the same. Snow amounts will remain unchanged, but we will need to closely follow radar and observations to see whether the very snowy models are verifying or if the drier models are verifying, or if it's somewhere in between. No changes will be made for now, but for now, I would think small upward adjustments in accumulations would be more likely than downward, especially on the south coast. The NAM will not be used much in this package, but because of its consistency and other models getting wetter, we should at least consider it in the backs of our minds as a possibility and watch to see what real-time trends to. But of course, it's a lot easier to adjust upward as the event unfolds if necessary, rather than make a huge adjustment upward and then scale back again. There is going to be a huge cutoff to the snowfall amounts. It is entirely possible a place on the south coast gets 10", and just on the other side of the Merritt Parkway, only a couple inches fall. The locations are not absolute; I simply am using this as a reference for Connecticut residents to point out how close the line may be between big accumulations and not much at all. Of course, there will be a very small area that receives something in between, but that area is going to be so small it is impossible to fit on the maps we produce. As such, we have chosen to broad brush a bit more than the eventual result will likely wind up being to highlight the range of possible outcomes depicted on guidance.
In addition, winds will be an issue with this system. It will become very windy along the south coast, regardless of how much snow falls. Winds could gust to 50 to 60 MPH right along the coast. This could cause power outages, especially if significant snow falls as well. Winds like that can cause power outages without any additional help, and if heavy snow falls in addition to those winds, it will make any power problems worse. Coastal flooding will certainly be an issue along the south coast, as well, as tides are already astronomically high. For now, the vast, vast majority of any precipitation that falls will fall as snow. There is a small chance of a few sleet pellets mixing in at times along the immediate south coast, as with almost any nor'easter, but the percentage of sleet anyone can expect at any given location and the chancy nature of it happening would yield me to just go with a straight snow forecast. I may very well update this again later on as newer model data comes in. If we need to update the snow map again, that will also be done later.
There are a range of products out from the National Weather Service for this storm, including Winter Storm Warnings along the coastline, Winter Weather Advisories inland, and Blizzard warnings to our southwest. Here's a map showing the various products that have been issued(Blizzard warnings continue all the way down past PA, but a near term Special Weather Statement has been issued which is overlaying the Blizzard Warnings and making them not visible there).