A very good Christmas evening to you from Southern Connecticut Weather!
We have been experiencing what will almost certainly be the warmest December in recorded history, and given the record highs of the past several days, wintry weather seems to be the furthest thing from possible reality at the moment. However, lurking for several days on the model guidance, and now moving into the crosshairs of this forecaster, lays a complex system that will approach early next week and bring a wide variety of precipitation types to the area. In this first Storm Speculation of the season, we will break down the setup and the various model forecasts, and offer some first thoughts on possible outcomes, but due to with regard to the evolution of complex features, we will hold off on a definitive forecast map and timeline until Sunday morning, consistent with the ~48 hour timeline that we have used in the past.
The Setup
This system comes together as a result of two parts; a strong surface low forming in Texas, as well as a strong Canadian high-pressure system that passes to our north. The timing of these two features works out so that as the low is moving northward and advecting both precipitation and warm air into our region, the high is moving eastward and funneling colder air down into the Northeast. Here’s a look at the surface temperature departures from normal on the GFS on Monday evening, where you can see clearly the impacts of the high pressure on our corner of the country, as we are near normal while areas to our south-west are well above normal.
We have been experiencing what will almost certainly be the warmest December in recorded history, and given the record highs of the past several days, wintry weather seems to be the furthest thing from possible reality at the moment. However, lurking for several days on the model guidance, and now moving into the crosshairs of this forecaster, lays a complex system that will approach early next week and bring a wide variety of precipitation types to the area. In this first Storm Speculation of the season, we will break down the setup and the various model forecasts, and offer some first thoughts on possible outcomes, but due to with regard to the evolution of complex features, we will hold off on a definitive forecast map and timeline until Sunday morning, consistent with the ~48 hour timeline that we have used in the past.
The Setup
This system comes together as a result of two parts; a strong surface low forming in Texas, as well as a strong Canadian high-pressure system that passes to our north. The timing of these two features works out so that as the low is moving northward and advecting both precipitation and warm air into our region, the high is moving eastward and funneling colder air down into the Northeast. Here’s a look at the surface temperature departures from normal on the GFS on Monday evening, where you can see clearly the impacts of the high pressure on our corner of the country, as we are near normal while areas to our south-west are well above normal.
This high acts as a psuedoblock for the system, forcing it out under the high and eventually forming a secondary area of low pressure off the coastline, which turns into a coastal system. The strength and placement of the high as the low approaches will determine at what point the low has weakened enough that warm air will no longer continue to push north and secondary development will begin. This will have a substantial effect on our sensible weather, as it will determine both surface temperatures and temperatures aloft, which will determine whether we see snow, sleet/freezing rain, or just a cold rain.
Here’s a look at the initial setup aloft on the GFS at hour 60, or 1 AM Monday morning. Notice the low-pressure system moving north out of eastern Texas, and notice the high pressure system arcing over the Great Lakes.
Here’s a look at the initial setup aloft on the GFS at hour 60, or 1 AM Monday morning. Notice the low-pressure system moving north out of eastern Texas, and notice the high pressure system arcing over the Great Lakes.
The timing differences between the various model guidance with the ejection of the shortwave from the south and the placement of the Canadian high will be crucial in determining the overall setup. A faster ejection of the low should result in the low approaching the high while it is still anchored in a further westerly position, and as a result, will allow for colder air to persist and the system to be shoved out to our east before it approaches our latitude. This will keep our forecast area on the cold side of the system, and will result in a solution where most of the precipitation is frozen. A slower ejection, however, will result in the high beginning to slide to the east before the storm can reach it, allowing the storm to continue as a stronger primary low pressure system into the Great Lakes and flood the upper levels with warmer air, the end result being a brief period of snow before a flip to sleet, freezing rain, and rain. Models have not yet been able to nail down consistency on this evolution, and this kind of setup can be unusually difficult to project from this far out.
To show this, here are two sets of images from the day’s four GFS runs. The first set shows the placement of the low pressure system and the high pressure system at 7 PM Monday evening, and the second set shows precipitation type at 7 AM Tuesday morning. Notice the correlation between low placement/high position and the precipitation type!
To show this, here are two sets of images from the day’s four GFS runs. The first set shows the placement of the low pressure system and the high pressure system at 7 PM Monday evening, and the second set shows precipitation type at 7 AM Tuesday morning. Notice the correlation between low placement/high position and the precipitation type!
And here are the corresponding surface reflections.
Quite the difference! On the mid-day run(12z), the GFS has rain all the way up to the northern border of the state, while on the overnight run(0z), the GFS has the entire state in moderate to heavy snow! This difference can be traced back to the images above. On the 0z run, you can see that the high pressure is stronger and further southwest, squashing the storm and it’s associated warm air further south and a secondary is beginning to form(which can be seen in the precipitation type image), while on the 12z run, you can see that the high pressure is much weaker and further to the northeast, allowing the storm a slot to cut to the west of us and flood our area with warmth, since the high is no longer in a position to fight it with cold air. The 6z and 18z runs fall somewhere in the middle between those two extremes as is typical with the models.
In what is a somewhat rare occurrence in meteorology, all three major global models, along with their ensemble sets, have been in relative consensus with the overall synoptic, or large scale, concept of this system for several days now. However, what they have been waffling around on is the timing of the system and the placement of the high, and as you can see from the images above, even minor changes can make a big impact. As such, this remains a low-confidence forecast until we begin to see some more consensus on the guidance.
The Forecast – First Thoughts
Here’s a graphic that breaks down our first thoughts with how we see the system playing out. For a somewhat more detailed explanation of possible outcomes, read on, otherwise, this graphic gives you a good idea of where we stand at the moment.
There’s quite a bit of uncertainty with the forecast, most of which relates to the timing differences shown through the model images above. We had been seeing a trend towards an earlier ejection and thus a colder solution, but the mid-day runs reversed that trend in a rather dramatic fashion, although the evening GFS did cool back down somewhat. While it’s too early to get into specifics, I’m leaning slightly towards the warmer solutions at this time, taking into account the trends, the antecedent airmass being extremely warm, and the general tendencies of these SWFE’s(Southwest Flow Events) to always be a bit warmer in the mid-levels than expected and thus the sleet line is closer than you think. I’m also leaning towards a slightly faster scenario, given that SWFE’s often come in faster than modeled and that the general trend over the last couple of days has been to speed up the system. Finally, I’m thinking that we will see low-level cold hang on for longer than expected, considering that models frequently under-estimate the power of surface level cold, especially in the interior. As such, I think a period of sleet and freezing rain is likely, especially away from the immediate coastline.
Precipitation will begin to move in from the southwest on Monday evening. I expect all areas of the state to begin as snow, but areas along the coastline should quickly switch over to a combination of sleet and/or freezing rain as warm air moves in aloft. Further north, I expect this changeover to be more gradual, and thus we could see some light to moderate snow accumulations, especially in the northwest and northeast hills, before the warm air works in aloft and the interior also flips over to sleet. By daybreak, I think enough warm air will have worked in for the coastline and southern areas to change over to rain, but it’s likely that interior areas will still remain below freezing and some icing is definitely possible. Regardless, I think that Tuesday morning’s commute will be rather tricky, especially away from the immediate shoreline. Precipitation will move out Tuesday morning as all rain, and roads should be markedly improved by the evening commute as warmer air finally moves in at the surface and temps can move into the mid to upper 30s, melting any ice that has accreted on roads.
At this time, we’re holding off on releasing a snow/ice map due to the uncertainties that lie in both the arrival of the system and the changeover to sleet and then rain. Considering that precipitation rates will be on the order of .1” liquid per hour around the time of the changeover, that means that even a delay of two hours, which is nothing in the weather world, would be a 2” difference in the forecast. Considering the spread on model guidance and the fact that we are still 3 days out from the start of the event, we will hold off on releasing a first call map until most likely Sunday, with an outside shot at a map tomorrow evening if we find consensus on guidance within the next couple of model cycles. Either way, we will have another update tomorrow evening with more info and a look at the latest models and trends for you all.
Finally, on behalf of the SCW forecasting team, I’d like to wish all of our readers a very Merry Christmas! We hope you enjoyed this beautiful day with loved ones and were able to celebrate as you wished ;)
Until tomorrow, thanks for reading SCW!
-SA
Precipitation will begin to move in from the southwest on Monday evening. I expect all areas of the state to begin as snow, but areas along the coastline should quickly switch over to a combination of sleet and/or freezing rain as warm air moves in aloft. Further north, I expect this changeover to be more gradual, and thus we could see some light to moderate snow accumulations, especially in the northwest and northeast hills, before the warm air works in aloft and the interior also flips over to sleet. By daybreak, I think enough warm air will have worked in for the coastline and southern areas to change over to rain, but it’s likely that interior areas will still remain below freezing and some icing is definitely possible. Regardless, I think that Tuesday morning’s commute will be rather tricky, especially away from the immediate shoreline. Precipitation will move out Tuesday morning as all rain, and roads should be markedly improved by the evening commute as warmer air finally moves in at the surface and temps can move into the mid to upper 30s, melting any ice that has accreted on roads.
At this time, we’re holding off on releasing a snow/ice map due to the uncertainties that lie in both the arrival of the system and the changeover to sleet and then rain. Considering that precipitation rates will be on the order of .1” liquid per hour around the time of the changeover, that means that even a delay of two hours, which is nothing in the weather world, would be a 2” difference in the forecast. Considering the spread on model guidance and the fact that we are still 3 days out from the start of the event, we will hold off on releasing a first call map until most likely Sunday, with an outside shot at a map tomorrow evening if we find consensus on guidance within the next couple of model cycles. Either way, we will have another update tomorrow evening with more info and a look at the latest models and trends for you all.
Finally, on behalf of the SCW forecasting team, I’d like to wish all of our readers a very Merry Christmas! We hope you enjoyed this beautiful day with loved ones and were able to celebrate as you wished ;)
Until tomorrow, thanks for reading SCW!
-SA
