Good evening from Southern Connecticut Weather!
A mix of winter weather headlines are up for the state in advance of our first significant snow event of the winter season. Most of the details seem sorted out, but the most important one, snow accumulation, is not settled yet. That said, it's time for our final call. Let's dive in.
A mix of winter weather headlines are up for the state in advance of our first significant snow event of the winter season. Most of the details seem sorted out, but the most important one, snow accumulation, is not settled yet. That said, it's time for our final call. Let's dive in.
Above: our updated snowfall forecast, which slightly increased expected snow totals in most of CT from 4-8" to 5-10", and slightly raised the SE CT zone to 4-8", in line with the rest of coastal CT.
Below: the current winter weather headlines for the state.
Below: the current winter weather headlines for the state.
The Forecast
As a reminder, we get graded on the elements of this forecast. The overall setup has not changed. It was a warmer than normal and rainy Saturday as pre-frontal precipitation reached the area. A boundary will cross the region and the first press of Arctic air will arrive tomorrow, right as our storm system gets cranking to our south.
The expected track has shifted toward the state, increasing the chance we see more precipitation thrown back into the state. However, as we noted over the last few days, a closer track also introduces the risk of mixing at the start of the storm, especially along the shoreline.
Timing
While the individual models have bounced back an forth a little, there's not much change in my mind. I still 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.
Wind/Power Outages
No changes here. This doesn't look like a very windy storm. There's no big pressure gradient and the low itself isn't looking particularly powerful. Part of the reason for slightly increasing inland numbers is the likelihood of more powdery rather than wet snow. That reduces power outage risk. I think that'll be the case along the shoreline too, but it may take a little more time. I don't expect anything more than isolated power outages with gusts below 35mph.
Snow Accumulation
Last night I wrote that if I were to make an adjustment it would be up or down slightly. That ended up being the case. This gets weighed twice as much as the other factors, and it's a pretty close call here. There was never a real thought of taking numbers down, but there was a lot of consideration--which ended up winning out--of taking the numbers up slightly for interior CT. Let's briefly talk about why.
As a reminder, we get graded on the elements of this forecast. The overall setup has not changed. It was a warmer than normal and rainy Saturday as pre-frontal precipitation reached the area. A boundary will cross the region and the first press of Arctic air will arrive tomorrow, right as our storm system gets cranking to our south.
The expected track has shifted toward the state, increasing the chance we see more precipitation thrown back into the state. However, as we noted over the last few days, a closer track also introduces the risk of mixing at the start of the storm, especially along the shoreline.
Timing
While the individual models have bounced back an forth a little, there's not much change in my mind. I still 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.
Wind/Power Outages
No changes here. This doesn't look like a very windy storm. There's no big pressure gradient and the low itself isn't looking particularly powerful. Part of the reason for slightly increasing inland numbers is the likelihood of more powdery rather than wet snow. That reduces power outage risk. I think that'll be the case along the shoreline too, but it may take a little more time. I don't expect anything more than isolated power outages with gusts below 35mph.
Snow Accumulation
Last night I wrote that if I were to make an adjustment it would be up or down slightly. That ended up being the case. This gets weighed twice as much as the other factors, and it's a pretty close call here. There was never a real thought of taking numbers down, but there was a lot of consideration--which ended up winning out--of taking the numbers up slightly for interior CT. Let's briefly talk about why.
Above are the last five runs of the European model (not including the latest at 18z). There has been a clear trend toward more precipitation thrown back toward New England. This is in large part because of a more GFS like track closer to the coast. That increased confidence that even a more middling Euro depiction could bring widespread 5" snow amounts to most of the state, especially considering that ratios are likely to be somewhat greater than 10:1 as the storm intensifies.
Most will end up with 5-8" I think, but the ceiling is higher if strong banding can cross over CT tomorrow night. I want to account for that. I also want to account for the possibility that we trend more amped in the last 24 hours, which often happens.
On the other side of that however is coastal CT. It'll likely be a significant storm there too, but the closer track means that there could be some mixing issues at the start. This is something that really needs to be respected. Any mixing cuts down on potential snow totals, and the possibility of banding adds another wrinkle as it means there will likely be areas of subsidence (read: dry slotting). That's extremely hard to predict this far out, but using frontogenesis forecasts we can get a sense of where the banding could be.
Most will end up with 5-8" I think, but the ceiling is higher if strong banding can cross over CT tomorrow night. I want to account for that. I also want to account for the possibility that we trend more amped in the last 24 hours, which often happens.
On the other side of that however is coastal CT. It'll likely be a significant storm there too, but the closer track means that there could be some mixing issues at the start. This is something that really needs to be respected. Any mixing cuts down on potential snow totals, and the possibility of banding adds another wrinkle as it means there will likely be areas of subsidence (read: dry slotting). That's extremely hard to predict this far out, but using frontogenesis forecasts we can get a sense of where the banding could be.
Above is the GFS looking at 700mb. It's not perfect actually as you generally want the best forcing SE of you (as it puts the banding a bit NW), but as it stands that's probably a good signal for heavy snow especially in inland NW CT tomorrow night.
At the same time, the soundings tell the story of where things could be at the time. Let's quickly look at Hartford first. You have good lift through the DGZ--or dendritic growth zone (note on the left) with temperatures solidly below freezing. There's no mix risk here at this timeframe. If you get good enough snow growth inland, you can put up good numbers even in a quick hitter.
At the same time, the soundings tell the story of where things could be at the time. Let's quickly look at Hartford first. You have good lift through the DGZ--or dendritic growth zone (note on the left) with temperatures solidly below freezing. There's no mix risk here at this timeframe. If you get good enough snow growth inland, you can put up good numbers even in a quick hitter.
Now look at Groton below. Temperatures at the surface are marginal, as in just above freezing. However, it's likely snowing given the column above. The ratios are not great here as well because of the relatively poor lift that's mostly happening below the DGZ.
Finally, let's take a look at Bridgeport at the same time. It's very different from SE CT. Solidly below freezing through the entire column, but the lift is also not ideal here either. Plenty good for snow and maybe a lot of it if you can get banding over you for a time, but it's far from perfect--which is to be expected in a storm like this.
Boom Scenario: Widespread 10-14" totals (20%)
Bust Scenario: Widespread 1-2" totals (10%)
Above are the boom/bust scenario probabilities. No change from yesterday. Let's be clear here though. If you're in the 5-10" zone and you end up with 4", that's not ideal but it's not really a bust. The same goes if you end up with an isolated 11 or 12" report on the map. At the coast, 3" in a 4" floor isn't great but it's not terrible. Boom or bust scenarios require widespread errors.
The boom scenario happens if the models amp up at the last minute--which has happened fairly frequently in recent years once we've been able to bring a storm our way. The bust scenario happens if the storm cuts well to the west, which is unlikely, or it comes in even weaker at the last minute. That's something to watch but I think that's unlikely as well.
Impact
Overall, this still appears to be a moderate impact event. Travel conditions will deteriorate by late afternoon tomorrow with the worst happening Sunday night. Snow is likely to be moderate to heavy at times. Otherwise, the timing is actually great considering that Monday is a holiday and we don't expect powerful winds.
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
Bust Scenario: Widespread 1-2" totals (10%)
Above are the boom/bust scenario probabilities. No change from yesterday. Let's be clear here though. If you're in the 5-10" zone and you end up with 4", that's not ideal but it's not really a bust. The same goes if you end up with an isolated 11 or 12" report on the map. At the coast, 3" in a 4" floor isn't great but it's not terrible. Boom or bust scenarios require widespread errors.
The boom scenario happens if the models amp up at the last minute--which has happened fairly frequently in recent years once we've been able to bring a storm our way. The bust scenario happens if the storm cuts well to the west, which is unlikely, or it comes in even weaker at the last minute. That's something to watch but I think that's unlikely as well.
Impact
Overall, this still appears to be a moderate impact event. Travel conditions will deteriorate by late afternoon tomorrow with the worst happening Sunday night. Snow is likely to be moderate to heavy at times. Otherwise, the timing is actually great considering that Monday is a holiday and we don't expect powerful winds.
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
