As you all have heard, confidence is increasing rapidly that the area will feel significant impacts from an early season winter storm. I will publish my first shot at a snowfall map but I encourage you to read the discussion as well which is where I explain the reasoning behind the snow map. This will be a little on the longer side just so you’re prepared. I have a timeline at the bottom should you want to skip to that part.
We have a complex pattern out there right now that will have significant implications for snowfall on Wednesday. Lets start not on Wednesday, not on Tuesday, but night now.
Lets take a look at the current observations across the area right now. Notice all the 50’s with some 40’s and some 60’s? These temps have been here for a while (since last night) and are expected to continue through tomorrow morning. What we have with this type of setup is a very warm boundary layer (boundary layer is the layer of the atmosphere right next to the ground). This prolonged period of warm weather is thawing out the ground and causing both the ground, and the air right next to the ground, to be quite warm. Both of these factors will have significant implications down the road with potential snowfall.
With a wind off the water, ocean temps are another important factor that many overlook. At bouy 44007 (~25mi off the coast near Portland), the ocean temperature this morning was 50.3 degrees. Other bouys in the area confirm ocean temps right around 50 degrees. If the wind turns more easterly (even NE winds could be a problem especially in MA) then the warm air would be pushed onshore and the whole snow operation would be shut down.
Let’s look now to the upper air pattern and step forward in time. All images come from Accuweather with text/lines/shapes added by me.
Notice the two pieces of energy marked off on the map. They will round the base of the trough and form a storm off the Carolinas. The key to this forecast is the exact track of the storm and that will be determined by two pieces of energy diving down across the plains.
They key to this forecast as I mentioned above will by the piece of energy pushing east across the Carolinas Wednesday. If this energy is faster, like the GFS, NAM, and CMC are saying, the storm tracks closer to the benchmark and a snowier solution ensues. Should this energy lag behind a little more, like the Euro says, the storm tracks farther west and a rainer solution ensues.
Right now I am leaning towards a compromise with a slight GFS bias due to the lack of blocking over the Atlantic. It is important to note that without strong high pressure in the western Atlantic, the door is open for the storm to slide a little farther east. This is the main reason why I favor the GFS solution over the Euro solution. However, with the Euro’s track record, it is very important to not discount it. A western track is still very possible.
Now that we’ve looked into the larger scale setup and some of the caveats to the forecast that only humans can pick up on, let’s get into what will really happen.
There are still differences between the models regarding the exact track and thus the westward progression of the rain/snow line. right now, I think it is safe to say that all coastal areas mix for at least some time. The greatest threat for mixing will be around 1 Am Thursday Morning and should come as sleet since the only nose of above freezing temps comes at 700mb. Everything above and below that is blow freezing.
There are a few things that are going in favor of coastal snow in this scenario. First off, this storm will be rapidly intensifying. The Euro has a pressure drop of 22mb in 24hours (7am Wed to 7AM Thu) so not quite an official ‘bomb’ but close enough (to be an official bomb, the pressure must drop 24mb in 24hrs). When storms intensify like this a few things happen. First, they tend to suck in cold air due to their rapidly expanding wind field. This could be a contributing factor in keeping coastal areas snowier for longer.
They also tend to manufacture their own cold air through a process known as dynamic cooling. This is a rather complicated process that can be boiled down to the cooling of the atmosphere in response to upward motion. We have a lot of upward motion associated with this storm due to its rapid intensification. This means that despite the fact that there will be little in the way of cold air at the onset of the storm, there will be a supply of cold air and that could potentially keep some coastal areas in the snow for longer.
I have opted to go conservative with the snowfall amounts especially at the coast to allow for the possibility of a westward track. I will continue to refine this and post revisions either here or on Twitter @Jacksillin.
Here is a timeline of how I expect things to unfold across the region. Highest impacts from heavy snow are expected to be felt Wednesday evening into Wednesday night.
Wednesday Morning: Snow approaches from the south, no travel impacts. Get travel and any storm prep done now.
Wednesday Afternoon: Snow overspreads the area and becomes heavy especially interior MA and CT. Snow changes to rain coastal SE MA. Travel impacts: Moderate
Wednesday Evening: Heavy snow moves into interior sections. Coastal areas south of Portland Maine inland to about 5 miles mix with sleet/rain. Midcoast Maine likely mixes as well. Travel impacts: High
Wednesday Night: Heavy snow continues especially interior NH/ME. Coastal Maine still stands the chance for mixing with greatest mixing potential still south of Portland. Rain/snow line starts collapsing in CT/SE MA. Travel impacts: High
Thursday Morning: Heavy snow moves into eastern Maine and snow starts to taper off. Mix line moves offshore of coastal Maine but still impacts SE MA. Snow ends CT/MA/NH. Travel impacts: Moderate, improving
Thursday Evening: Snow ends for everyone. Travel impacts: None
I will have more updates on Twitter this afternoon as well as all day tomorrow and throughout the storm itself. I’m @JackSillin. I will have a video out later today and will post the link to Twitter. I may or may not have another update on here tonight depending on how things play out. Next definite update will be tomorrow morning.