Good evening from Southern Connecticut Weather!
The week ahead is looking the most wintry we've seen in years, and it starts with this weekend. A wave developing along a boundary that will bring our initial press of Arctic air is poised to enter our third window for significant snowfall. However, today's trends have left a significant gap between two camps--one that sees the wave becoming more amplified and tracking closer to the coast, and the other, seeing the wave as weaker with a further southeast track. This is particularly glaring given how close we are to the start of the event, but as I said last night, this is where forecasters matter.
Given everything I've seen thus far, I think a compromise between the two camps makes the most sense for the first forecast. For this forecast and moving forward, we're going to add probabilities for "boom" and "bust" outcomes. This isn't much different from what that Capital Weather Gang have done for a while in DC, but I think it'll help enhance how we communicate what risks we see with the forecast.
With that in mind, let's dive in.
The week ahead is looking the most wintry we've seen in years, and it starts with this weekend. A wave developing along a boundary that will bring our initial press of Arctic air is poised to enter our third window for significant snowfall. However, today's trends have left a significant gap between two camps--one that sees the wave becoming more amplified and tracking closer to the coast, and the other, seeing the wave as weaker with a further southeast track. This is particularly glaring given how close we are to the start of the event, but as I said last night, this is where forecasters matter.
Given everything I've seen thus far, I think a compromise between the two camps makes the most sense for the first forecast. For this forecast and moving forward, we're going to add probabilities for "boom" and "bust" outcomes. This isn't much different from what that Capital Weather Gang have done for a while in DC, but I think it'll help enhance how we communicate what risks we see with the forecast.
With that in mind, let's dive in.
Above: our first call snow map for the weekend storm. We are utilizing a compromise here, weighing the Euro and its ensembles about 60% here, while weighing the GFS and its ensembles 30% and other guidance 10%. What this means is that while we currently expect a more southeast track, it won't be as flat and weak as the Euro/EPS. As a result, we anticipate a conservative 4-8" of snow for most of the state, with 3-6" for southeast CT. This map accounts for some mixing at the start, with a flip to snow for everyone relatively quickly.
Overall Setup
The overall evolution of this one has been straightforward for days now. There is unanimous agreement that pre-frontal rains arrive tomorrow as a boundary approaches the region. It will cross the region as we see energy diving down from the Midwest along the trough. That will begin to spawn a storm system Sunday.
The overall evolution of this one has been straightforward for days now. There is unanimous agreement that pre-frontal rains arrive tomorrow as a boundary approaches the region. It will cross the region as we see energy diving down from the Midwest along the trough. That will begin to spawn a storm system Sunday.
Above: the 500mb depiction of the 12z Euro from Saturday afternoon into Monday morning. Note the kink in the deep trough that's moving into the U.S., which will bring the cold. That energy on the right side is the vort that will become our storm. On the bottom we have the result at the surface. The Euro as it stands is the least robust, and this is still the surface solution, lending us confidence in the floor for the system.
Zoomed in. Note how weak the precipitation shield is. There's a solid track for mostly to all snow, but the system is weak, flat, and doesn't produce too much as a result.
This is one camp and the Euro/EPS are pretty much alone. We give enormous respect to the Euro, hence the weighing, but it seems unlikely at this stage that it's got this perfectly right this far out.
The other camp is the GFS/GEFS and most of the other guidance. You can see clear differences early at 500mb, and it leads to a stronger system with a closer to the coast track. This introduces mixing risk especially early in the storm.
The other camp is the GFS/GEFS and most of the other guidance. You can see clear differences early at 500mb, and it leads to a stronger system with a closer to the coast track. This introduces mixing risk especially early in the storm.
The surface result, zoomed in.
Here the stronger low and closer track brings heavy snow over the state, with the rain/snow line very close to SE CT until the temperatures crash on the backside. Longtime followers of our nor'easters know that you often need to be close to the razor's edge to get big snows, and that happens here, with the GFS toying with double digit snowfall totals for a significant part of CT.
This seems imperfect at this stage too, but I do think that the track will be less flat, which is significant.
So let's talk about the forecast.
This seems imperfect at this stage too, but I do think that the track will be less flat, which is significant.
So let's talk about the forecast.
First Call Forecast
Timing
This part should be straightforward. We're expecting the storm to take shape starting Saturday well to our southwest. The timing on the guidance had shifted a touch as I noted this morning, but has since stabilized. I think we see the onset of light snow and/or mixed precipitation between approximately 2pm-6pm on Sunday from SW to NE. The heaviest is likely to occur after sunset in a period between about 7pm and 1am, followed by tapering off between 3-5am.
Snow Accumulation
As you know by now, we weigh snowfall accumulation twice as much in grading because that's what everyone cares about. The first call here is a bit tricky but here is what we have after blending the guidance.
Most of CT: 4-8"
I think the least is right along the shoreline because of early mixing, but I can't rule out a higher result (in this range) because I think there is going to be banding in this storm that will create winners and losers. The more dynamic the storm is, the more track matters both for mixing and the greater banding. In addition, most of the state away from the shore is going to flip to higher ratio (more powdery) snow faster.
SE CT: 3-6"
Here, I'm not taking much off the overall accumulation. I just have less confidence that there will be widespread 4" as a floor in this zone. Again, we're watching out for banding, but that won't come into better focus until tomorrow.
Boom Scenario: Widespread 10-14" totals (20%)
Bust Scenario: Widespread 1-2" totals (10%)
Above are the boom/bust scenario probabilities. The boom scenario happens if the GFS turns out to be right and continues to amp up (though that would increase the mixing risk in SE CT). The bust scenario happens if either 1) the storm cuts well to the west, which is highly unlikely, or, 2) the storm whiffs to the southeast. There is less chance of a bust here than the opposite, but both are unlikely.
If I were to adjust a final forecast it would be up or down slightly.
Wind/Power Outages
This doesn't look like a very windy storm, even though cold will be advecting in. As a result, I do not expect anything more than isolated to scattered outages at this time. For those wondering about snow type, the snow should start out on the wetter side, but we will see temperatures drop through the afternoon Sunday as the Arctic cold behind the boundary begins to arrive and the storm itself reinforces the cold assuming a more offshore track.
Our wet snow or mix as a result will flip to cold powder statewide as temperatures drop into the low 20s and teens Sunday night and possibly single digits by sunrise Monday. As an aside, the latest Euro has the coldest stretch in likely a decade in its latest run. Fresh snowpack will help raise the odds that temperatures decouple at night, bringing below zero temperatures. We'll see if that holds as we get closer to next week.
Below are 18z Euro temperatures Monday morning. Frigid even before adding the wind chill.
Timing
This part should be straightforward. We're expecting the storm to take shape starting Saturday well to our southwest. The timing on the guidance had shifted a touch as I noted this morning, but has since stabilized. I think we see the onset of light snow and/or mixed precipitation between approximately 2pm-6pm on Sunday from SW to NE. The heaviest is likely to occur after sunset in a period between about 7pm and 1am, followed by tapering off between 3-5am.
Snow Accumulation
As you know by now, we weigh snowfall accumulation twice as much in grading because that's what everyone cares about. The first call here is a bit tricky but here is what we have after blending the guidance.
Most of CT: 4-8"
I think the least is right along the shoreline because of early mixing, but I can't rule out a higher result (in this range) because I think there is going to be banding in this storm that will create winners and losers. The more dynamic the storm is, the more track matters both for mixing and the greater banding. In addition, most of the state away from the shore is going to flip to higher ratio (more powdery) snow faster.
SE CT: 3-6"
Here, I'm not taking much off the overall accumulation. I just have less confidence that there will be widespread 4" as a floor in this zone. Again, we're watching out for banding, but that won't come into better focus until tomorrow.
Boom Scenario: Widespread 10-14" totals (20%)
Bust Scenario: Widespread 1-2" totals (10%)
Above are the boom/bust scenario probabilities. The boom scenario happens if the GFS turns out to be right and continues to amp up (though that would increase the mixing risk in SE CT). The bust scenario happens if either 1) the storm cuts well to the west, which is highly unlikely, or, 2) the storm whiffs to the southeast. There is less chance of a bust here than the opposite, but both are unlikely.
If I were to adjust a final forecast it would be up or down slightly.
Wind/Power Outages
This doesn't look like a very windy storm, even though cold will be advecting in. As a result, I do not expect anything more than isolated to scattered outages at this time. For those wondering about snow type, the snow should start out on the wetter side, but we will see temperatures drop through the afternoon Sunday as the Arctic cold behind the boundary begins to arrive and the storm itself reinforces the cold assuming a more offshore track.
Our wet snow or mix as a result will flip to cold powder statewide as temperatures drop into the low 20s and teens Sunday night and possibly single digits by sunrise Monday. As an aside, the latest Euro has the coldest stretch in likely a decade in its latest run. Fresh snowpack will help raise the odds that temperatures decouple at night, bringing below zero temperatures. We'll see if that holds as we get closer to next week.
Below are 18z Euro temperatures Monday morning. Frigid even before adding the wind chill.
Impact
Overall, this looks like a moderate impact event. I'm going with moderate rather than low impact because I do think that travel conditions Sunday evening will be pretty bad. Otherwise, the timing is actually great considering that Monday is a holiday and we don't expect powerful winds. That said, going from wet snow to frigid temperatures means that whatever falls could become a glacier. That could lead to issues if the snow is not cleaned up quickly.
As always, please like, share, and interact with us on our social media--on Facebook to share our discussions, and follow us on Twitter @southernctwx to retweet our posts. Hit the buttons below to join.
Thank you for reading SCW.
-DB
Overall, this looks like a moderate impact event. I'm going with moderate rather than low impact because I do think that travel conditions Sunday evening will be pretty bad. Otherwise, the timing is actually great considering that Monday is a holiday and we don't expect powerful winds. That said, going from wet snow to frigid temperatures means that whatever falls could become a glacier. That could lead to issues if the snow is not cleaned up quickly.
As always, please like, share, and interact with us on our social media--on Facebook to share our discussions, and follow us on Twitter @southernctwx to retweet our posts. Hit the buttons below to join.
Thank you for reading SCW.
-DB
