Good evening from Southern Connecticut Weather.
Whew. What a day!
When I woke up this morning, less than 24 hours before the start of our coming storm, I was prepared to make the argument for the SCW team to do an 8-16" snowfall map for all of CT. I thought that a forecast of 12-18" wouldn't have been unreasonable for parts of interior Connecticut.
Then came the extraordinary shift south on the guidance this afternoon.
While we still believe this will bring a significant snowfall to the state and be a high impact event, there is an incredible amount of uncertainty on the floor and ceiling of this storm.
If you just want to know what we think will happen in your backyard, that's totally fine and we have our section at the bottom of the discussion.
If you want to get a deep rundown of what the heck happened today and why we're placing our chips on the snowfall map below, get your favorite beverage and settle in because this is going to be quite a read.
Whew. What a day!
When I woke up this morning, less than 24 hours before the start of our coming storm, I was prepared to make the argument for the SCW team to do an 8-16" snowfall map for all of CT. I thought that a forecast of 12-18" wouldn't have been unreasonable for parts of interior Connecticut.
Then came the extraordinary shift south on the guidance this afternoon.
While we still believe this will bring a significant snowfall to the state and be a high impact event, there is an incredible amount of uncertainty on the floor and ceiling of this storm.
If you just want to know what we think will happen in your backyard, that's totally fine and we have our section at the bottom of the discussion.
If you want to get a deep rundown of what the heck happened today and why we're placing our chips on the snowfall map below, get your favorite beverage and settle in because this is going to be quite a read.
Above: our final call snowfall map. We think that Litchfield County has the greatest uncertainty as we closely follow the northwest edge of the coming precipitation shield, with a gradient from far NW CT down toward I-84 where a substantial snowfall is expected. For the rest of the state, we think a general 6-12" is most likely. Given the uncertainty, we should all be prepared for surprises.
What Happened?
If you recall, at the beginning of this journey about a week ago, we wrote about the track uncertainty and how a further north track could allow for rain for many, especially at the coastline. A few days after that, the storm track on the guidance started shifting south, but it was still too close for comfort for our southern Connecticut friends. Then we saw the track stabilize. This brought the most dynamic conditions and greatest amount of precipitation over CT, which led us to start raising the interest level for a major winter storm. Even yesterday afternoon, there was strong consensus among our most reliable operational and ensemble guidance that a major storm was coming.
Last night however, more models started gradually moving the storm south, before seemingly reversing this morning.
This afternoon's model runs however accelerated the trend in a dramatic way.
I loathe to do this, because we do not use or post model snow maps in our forecast, but I am making an exception here because it will be easiest for you to visualize. On the first model, the GFS, it is least pronounced. These are total snowfall algorithm outputs for each GFS operational model run between yesterday morning and this evening. There are two things I want you to notice. The first is the shift of heavier snow across southern CT. This is because the heaviest precipitation has trended further south. The second thing is Litchfield County. Notice how the amount of modeled snowfall collapses across much of New England before entering CT. It's not so much the specific numbers that are cause for high uncertainty. It's the trend.
If you recall, at the beginning of this journey about a week ago, we wrote about the track uncertainty and how a further north track could allow for rain for many, especially at the coastline. A few days after that, the storm track on the guidance started shifting south, but it was still too close for comfort for our southern Connecticut friends. Then we saw the track stabilize. This brought the most dynamic conditions and greatest amount of precipitation over CT, which led us to start raising the interest level for a major winter storm. Even yesterday afternoon, there was strong consensus among our most reliable operational and ensemble guidance that a major storm was coming.
Last night however, more models started gradually moving the storm south, before seemingly reversing this morning.
This afternoon's model runs however accelerated the trend in a dramatic way.
I loathe to do this, because we do not use or post model snow maps in our forecast, but I am making an exception here because it will be easiest for you to visualize. On the first model, the GFS, it is least pronounced. These are total snowfall algorithm outputs for each GFS operational model run between yesterday morning and this evening. There are two things I want you to notice. The first is the shift of heavier snow across southern CT. This is because the heaviest precipitation has trended further south. The second thing is Litchfield County. Notice how the amount of modeled snowfall collapses across much of New England before entering CT. It's not so much the specific numbers that are cause for high uncertainty. It's the trend.
On the once vaunted European model, the trend is even more extraordinary. Because of how often the model runs, I am showing a multi-day trend. The acceleration at the end, inside 24 hours of onset of a storm, is almost unbelievable. Adding to this are the ensemble shifts that cut precipitation by upwards of 50%.
Let's talk about the science of this.
There are a few key reasons why we think we've seen so much change.
First, the southern stream system has changed quite a bit in both its strength and orientation. It has become weaker due to the lack of convection generated in the south yesterday. The lack of convection also leads to lower heights ahead of the system, which feeds into a more southern track. Here's the Euro depiction of this happening the last few days. Notice how it's weaker with fewer closed isobars and further south than at the start of this gif.
In addition, the tilt of the trough here is more positive than neutral trending negative. Negatively tilted troughs are what bring us our explosive storms. This has always been a thread the needle storm. Too early and this cuts so far north we rain. Too late and we run the risk of what the Euro is depicting.
There are a few key reasons why we think we've seen so much change.
First, the southern stream system has changed quite a bit in both its strength and orientation. It has become weaker due to the lack of convection generated in the south yesterday. The lack of convection also leads to lower heights ahead of the system, which feeds into a more southern track. Here's the Euro depiction of this happening the last few days. Notice how it's weaker with fewer closed isobars and further south than at the start of this gif.
In addition, the tilt of the trough here is more positive than neutral trending negative. Negatively tilted troughs are what bring us our explosive storms. This has always been a thread the needle storm. Too early and this cuts so far north we rain. Too late and we run the risk of what the Euro is depicting.
The other critical piece that you see above is the Great Lakes region vorticity trending stronger. While enough of this was good to allow for colder air to press in via a southern colder track, too much of it makes it difficult for the southern stream to turn north and get close enough for a partial phase, which would more efficiently trigger bombogenesis.
Let's fast forward to what the Euro shows for Tuesday afternoon.
Let's fast forward to what the Euro shows for Tuesday afternoon.
The northern stream is overpowering the southern stream and pushing it south, and it also helps create a gradient in precipitation that introduces the risk of a whiff, particularly in northern CT.
In a vacuum, this would be a gigantic red flag to cut snowfall numbers across the board and prepare for a whiff. A more southern track from a weaker storm in a marginal temperature environment is the recipe for a bust. However, the guidance has been consistent on a vital element of the forecast even through the craziness of the last 12 hours: the upper level centers. Let's look specifically at 700mb.
In a vacuum, this would be a gigantic red flag to cut snowfall numbers across the board and prepare for a whiff. A more southern track from a weaker storm in a marginal temperature environment is the recipe for a bust. However, the guidance has been consistent on a vital element of the forecast even through the craziness of the last 12 hours: the upper level centers. Let's look specifically at 700mb.
Here is the trend on the European model. Note how the 700mb center has shifted south, but it has also become stronger (closed isobars/red shading) and more consolidated. That indicates that a band of heavy snow is likely to be northwest of that center. It is even more pronounced on the GFS below, which again, has been more consistent thus far.
These images tell us that there's a lot more going on here than the surface depiction, and when we look at the soundings, which are critical forecasts of every layer of the atmosphere we need for snowfall, we see signs that it wouldn't take much for accumulating snow and heavy snow would be on the for a period tomorrow if there are minor shifts in track northwest.
These are GFS soundings for tomorrow at Hartford, BDL, and Bridgeport (BDR) each show heavy snow and very efficient snow growth tomorrow morning.
These are GFS soundings for tomorrow at Hartford, BDL, and Bridgeport (BDR) each show heavy snow and very efficient snow growth tomorrow morning.
By afternoon, intensity gets tricky which makes temperatures and intensity of snow tricky. Any further moves back north would squarely put the rest of CT in line for a major snowfall.
Shifts south would create the risk of only a quick burst of heavy snow for interior CT, in particular NW CT.
Shifts south would create the risk of only a quick burst of heavy snow for interior CT, in particular NW CT.
Here's the 3km NAM just to show what a glancing blow would look like.
As a result, we have major mixed signals as the storm approaches tonight. It makes for an exceptionally difficult forecast.
The Forecast
This is what we're getting graded on. Snowfall accumulation as is customary practice will be weighted twice.
Timing
This is a fast mover and with the southern track the timing has shifted. While there is some precipitation on the front end of this on radar, it looks like the start time will be from south to north between 2-5am. While the onset timing has changed a bit, the end time looks fairly consistent. The heaviest snow looks to be during the daylight hours, particularly in the morning, with a gradual tapering off early to mid afternoon between perhaps 2-5pm. This is highly impacted by track. A further north track would bring a longer duration (but still fast moving) snowstorm.
Wind/Power Outages
Given the changes we've seen, it doesn't look as windy. This should help keep power outages isolated to scattered. Maximum gusts should be between 25-40mph with strongest gusts at the coast. With a further south track the snow does not look as pasty, but we will need to watch the track closely in case things bump northward tonight.
Snow Accumulation
As the map above shows, our final call is 4-8" in part of central and NW CT, with lowest amounts in far NW CT. We see 6-12" in the rest of the state. I cannot emphasize enough how dependent this is on the track and intensity of the low. We will almost certainly see high snowfall rates tomorrow with very efficient snow growth tomorrow morning. The longer we hold onto that the higher the snowfall totals will be. While it's not out of the realm of possibility that NW CT sees less than 4" and other parts of the state see 12+", it looks unlikely at this time.
Impact
While this may be a more moderate impact event in NW CT, this looks like a high impact event for most of the state. We already have closures, roads will deteriorate tomorrow during the heavier snow, and this will likely be a substantial snowfall for most. The latest European model run at 18z stopped its south trend, but that's not saying much. We should all be prepared for a period of hazardous driving conditions tomorrow, low visibility because of gusty winds, and potential minor to moderate coastal flooding.
It's time to let the chips fall and watch the hour by hour trend.
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
This is what we're getting graded on. Snowfall accumulation as is customary practice will be weighted twice.
Timing
This is a fast mover and with the southern track the timing has shifted. While there is some precipitation on the front end of this on radar, it looks like the start time will be from south to north between 2-5am. While the onset timing has changed a bit, the end time looks fairly consistent. The heaviest snow looks to be during the daylight hours, particularly in the morning, with a gradual tapering off early to mid afternoon between perhaps 2-5pm. This is highly impacted by track. A further north track would bring a longer duration (but still fast moving) snowstorm.
Wind/Power Outages
Given the changes we've seen, it doesn't look as windy. This should help keep power outages isolated to scattered. Maximum gusts should be between 25-40mph with strongest gusts at the coast. With a further south track the snow does not look as pasty, but we will need to watch the track closely in case things bump northward tonight.
Snow Accumulation
As the map above shows, our final call is 4-8" in part of central and NW CT, with lowest amounts in far NW CT. We see 6-12" in the rest of the state. I cannot emphasize enough how dependent this is on the track and intensity of the low. We will almost certainly see high snowfall rates tomorrow with very efficient snow growth tomorrow morning. The longer we hold onto that the higher the snowfall totals will be. While it's not out of the realm of possibility that NW CT sees less than 4" and other parts of the state see 12+", it looks unlikely at this time.
Impact
While this may be a more moderate impact event in NW CT, this looks like a high impact event for most of the state. We already have closures, roads will deteriorate tomorrow during the heavier snow, and this will likely be a substantial snowfall for most. The latest European model run at 18z stopped its south trend, but that's not saying much. We should all be prepared for a period of hazardous driving conditions tomorrow, low visibility because of gusty winds, and potential minor to moderate coastal flooding.
It's time to let the chips fall and watch the hour by hour trend.
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
