Good evening to you from Southern Connecticut Weather!
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.
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.
Notice that the difference is just a couple of degrees! However, that small difference makes all the difference. Considering that a difference of a couple degrees is essentially model noise when it comes to verification, this is inherently a low confidence forecast in that regard.
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.
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.
On the GFS, the coastal has no impact to the state, and all of the precipitation that impacts us comes from the initial clipper system, and as such, snow totals are relatively light. However, on some of the higher res guidance, specifically the RGEM, GGEM, and Euro, the coastal is far enough west to clip eastern areas with the fringes of it's precipitation, and thus, add some additional snow accumulation to the area. For now, we have leaned towards the coastal being just a touch too far east to impact the state, however, should the western guidance verify(Such as the GGEM, shown below), some additional snow would fall in eastern sections. Given that temps would be colder, this would be snow to the shoreline, and could result in another inch or so across the eastern portions of the state. As of now, we haven't included this in our snowmap, but we're keeping an eye on it.
The final question left to answer is how much precipitation we get from the original clipper. On this, most model guidance is in solid consensus on around a tenth of an inch of QPF across the state, which would result in 1" of snow at 10:1 ratios. However, I am inclined to believe that some areas may verify slightly higher as these small clipper systems often have areas of strong lift and vorticity that are not well modeled, and when an upper level low is escaping out under us, as this one is, surprises often happen. As such, I expanded the upper end of the forecast range to 2" in the northern tier of the state where I expect all precipitation to fall as snow to account for these variances. Some models, chiefly the RGEM, form some very narrow but intense convective bands of heavy precipitation, which would deliver several inches of snow to the small area that ended up under them. I have not weighted that in the forecast as I expect those bands would develop further to the northeast if at all, but it's worth keeping the back of an eye on.
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 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.
Forecast and Timing
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.
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 northern zone, where temperatures will be cold enough to support all or mostly snow, we are expecting one to two inches of snow. Most likely, we will see most areas come closer to the 1" mark than the 2" mark, but I have left some wiggle room in the forecast to account for the possible development of convective bands or for a last minute uptick in QPF.
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.
Timeline
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.
Impacts
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
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.
Timeline
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.
Impacts
- 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
