Good afternoon to you from Southern Connecticut Weather!
It’s funny; when I was looking over the models this afternoon in preparation to make this forecast, it almost felt like we’d just had a storm like this one, modeled to be no big deal until right before the event when the models took a sudden jump westward. Oh wait. We did have one, two days ago. For better or worse, we’re right back at it tonight with another snowmap and discussion for tomorrow into Tuesday morning. This is a bit of a different setup in that instead of it being an anafrontal wave, it’s a strong Miller A style coastal storm that will primarily be offshore, but has come just far enough west with it’s broad precipitation shield to allow for a moderate impact to much of the state.
Until a few hours ago, we were pretty confident in a fairly high impact event. The 12z suite of guidance came in with pretty good consensus on the fact that the northwest trend was real, and that eastern portions of the state would likely see warning level snowfall. Visions of the last storm and a continued west trend until go time danced in our heads, and we were preparing to honk the horns on a significant event tonight with this forecast update. That all got shot to pieces when the 18z suite rolled in, with the NAM, then the RGEM, and finally the GFS, shifting significantly east of their 12z runs. The question that we now have to answer is; do we believe them?
To illustrate, let’s take a look at the RGEM. Here’s a look at it’s 12z and 18z forecast snow totals, 12z on the left, 18z on the right. Remember that 10mm = ~.4” and therefore ~4” of snow.
It’s funny; when I was looking over the models this afternoon in preparation to make this forecast, it almost felt like we’d just had a storm like this one, modeled to be no big deal until right before the event when the models took a sudden jump westward. Oh wait. We did have one, two days ago. For better or worse, we’re right back at it tonight with another snowmap and discussion for tomorrow into Tuesday morning. This is a bit of a different setup in that instead of it being an anafrontal wave, it’s a strong Miller A style coastal storm that will primarily be offshore, but has come just far enough west with it’s broad precipitation shield to allow for a moderate impact to much of the state.
Until a few hours ago, we were pretty confident in a fairly high impact event. The 12z suite of guidance came in with pretty good consensus on the fact that the northwest trend was real, and that eastern portions of the state would likely see warning level snowfall. Visions of the last storm and a continued west trend until go time danced in our heads, and we were preparing to honk the horns on a significant event tonight with this forecast update. That all got shot to pieces when the 18z suite rolled in, with the NAM, then the RGEM, and finally the GFS, shifting significantly east of their 12z runs. The question that we now have to answer is; do we believe them?
To illustrate, let’s take a look at the RGEM. Here’s a look at it’s 12z and 18z forecast snow totals, 12z on the left, 18z on the right. Remember that 10mm = ~.4” and therefore ~4” of snow.
The difference is clear. Across the board, the heavier snows and precipitation have moved east, cutting snow totals across Southern New England, not just in Connecticut. The model’s “rationalization” for this is that the system is modeled to be weaker on the 18z run than the 12z run, and as such, the precipitation field does not develop as much as it does on the 12z run. A perfectly reasonable adjustment, should it’s reasoning for adjusting be correct.
To find out if it is, we can look to real time data by comparing radar and observations on the ground to current model forecasts. At 21z, which is hour 3 on the 18z guidance and hour 9 on the 12z guidance, the surface low is progged to be further southeast than reality on both the 12z and 18z NAM runs. Additionally, the 18z NAM is verifying about four millibars too shallow compared to the system, which is deeper than the models anticipation. As a result, the model’s estimation of the precipitation shield being weaker is likely somewhat off, and we can see that in the reality as depicted on radar, with the northwest edge of the precipitation shield being about 50 miles northwest of where the 18z NAM had it at this time.
As such, I’m not completely buying the 18z guidance. While I think a further west trend into a blockbuster is off the table, I think we saw an overreaction in the 18z data, and that we will see a tick back west with the 0z models tonight and to verification. As such, the snowfall map below is slightly more aggressive than a current consensus of 18z guidance. Should the guidance maintain the correction from 18z at 0z, we will likely bust too high and will cut totals in the morning or later tonight, but for now, this is what we’re going with.
To find out if it is, we can look to real time data by comparing radar and observations on the ground to current model forecasts. At 21z, which is hour 3 on the 18z guidance and hour 9 on the 12z guidance, the surface low is progged to be further southeast than reality on both the 12z and 18z NAM runs. Additionally, the 18z NAM is verifying about four millibars too shallow compared to the system, which is deeper than the models anticipation. As a result, the model’s estimation of the precipitation shield being weaker is likely somewhat off, and we can see that in the reality as depicted on radar, with the northwest edge of the precipitation shield being about 50 miles northwest of where the 18z NAM had it at this time.
As such, I’m not completely buying the 18z guidance. While I think a further west trend into a blockbuster is off the table, I think we saw an overreaction in the 18z data, and that we will see a tick back west with the 0z models tonight and to verification. As such, the snowfall map below is slightly more aggressive than a current consensus of 18z guidance. Should the guidance maintain the correction from 18z at 0z, we will likely bust too high and will cut totals in the morning or later tonight, but for now, this is what we’re going with.
Forecast
Above is our forecast snowmap for this event. In the western portion of the state, we’re expecting one to three inches of snow, in the central portion of the state, along with the southwest corner, we’re expecting two to five inches, and in the eastern portion, we’re expecting three to six inches, with some isolated amounts of up to eight or nine inches where the best banding sets up. It should be noted that this forecast only includes snow from tomorrow’s coastal storm, and not from the possible inverted trough/followup system for Tuesday into Wednesday. We’ll talk about that one a bit towards the end of this discussion, and it’ll get it’s own snowmap and discussion tomorrow night if needed.
Here’s a look at our expected timing:
5-8 AM: Snow breaks out south to north across the state, with some minor to moderate impacts to the morning commute. Some closings and delays are likely, especially in the eastern portions of the state.
8-11 AM: Snow becomes heavier in eastern portions of the state, with generally light snow continuing in the western and central regions.
11 AM – 2 PM: Heavier snow begins to move out of the state, although some lingering bands could remain over eastern areas.
2 PM – Tuesday morning: Residual snow showers, flurries, and squalls are expected to hang back over the state, with some quick periods of heavier snow possible.
Tuesday PM – Wednesday morning: Inverted Trough potential – more on this below.
In general, I think bust potential is more towards the lower end, should the system continue to trend any further east, this will quickly turn into no big deal for the state. There remains a slim possibility of a strong west trend tonight and a major system being the end result, but after seeing the 18z runs go east, I think the odds of that are pretty low(~10%).
Impacts
But wait, there’s more!
Up to this point, we’ve been talking about a brush from a coastal storm to impact the area tomorrow. However, this is not the only possible system that could impact our state this week. Models have been consistent in showing an inverted trough developing to the southwest of us after this first system exits, and many show a narrow band of very heavy snow forming. As of now, most guidance keeps the trough well southwest of our region, however, considering that these troughs almost exclusively trend northeast as we approach verification, I continue to be intrigued by the potential that it holds for another snow event across the region. Regardless of the inverted trough, it looks like a clipper system will come down from Alberta and transfer to the coastline, bringing some more snow showers and squalls to the region Tuesday afternoon through most of Wednesday. That will likely combine to a light to moderate accumulation across most of the state, but as of yet, it’s too early to discuss amounts for the area with these features. Greg will have a full discussion tomorrow evening with the details on the inverted trough and clipper system along with a snowmap if necessary.
Until then, however, we’ll be keeping our eyes fully peeled to the system that is about to impact us tomorrow, with updates throughout the day tomorrow as needed. Remember to send us your pictures and observations via Twitter @SouthernCTWX or our Facebook page, and please share this discussion with friends and family. Thank you for reading SCW and enjoy the snow!
-SA
Above is our forecast snowmap for this event. In the western portion of the state, we’re expecting one to three inches of snow, in the central portion of the state, along with the southwest corner, we’re expecting two to five inches, and in the eastern portion, we’re expecting three to six inches, with some isolated amounts of up to eight or nine inches where the best banding sets up. It should be noted that this forecast only includes snow from tomorrow’s coastal storm, and not from the possible inverted trough/followup system for Tuesday into Wednesday. We’ll talk about that one a bit towards the end of this discussion, and it’ll get it’s own snowmap and discussion tomorrow night if needed.
Here’s a look at our expected timing:
5-8 AM: Snow breaks out south to north across the state, with some minor to moderate impacts to the morning commute. Some closings and delays are likely, especially in the eastern portions of the state.
8-11 AM: Snow becomes heavier in eastern portions of the state, with generally light snow continuing in the western and central regions.
11 AM – 2 PM: Heavier snow begins to move out of the state, although some lingering bands could remain over eastern areas.
2 PM – Tuesday morning: Residual snow showers, flurries, and squalls are expected to hang back over the state, with some quick periods of heavier snow possible.
Tuesday PM – Wednesday morning: Inverted Trough potential – more on this below.
In general, I think bust potential is more towards the lower end, should the system continue to trend any further east, this will quickly turn into no big deal for the state. There remains a slim possibility of a strong west trend tonight and a major system being the end result, but after seeing the 18z runs go east, I think the odds of that are pretty low(~10%).
Impacts
- A light to moderate snowfall for the state, with warning level amounts possible in eastern areas.
- Minor to moderate travel impacts, including both the morning and evening commutes. Significant travel impacts are possible in eastern CT
- Reduced visibility due to a combination of heavy snow and strong winds. Winds will not reach blizzard criteria levels, but they will be strong enough to create some blowing snow and quick “ground blizzards”
- Minor to moderate coastal flooding is possible, and a Coastal Flood Advisory has been issued for coastal Fairfield and New Haven counties.
- Given that this will be a fluffier snow, widespread power outages are not expected, but the combination of this snow, the previous snow, and winds could create some additional outages, especially in eastern CT.
But wait, there’s more!
Up to this point, we’ve been talking about a brush from a coastal storm to impact the area tomorrow. However, this is not the only possible system that could impact our state this week. Models have been consistent in showing an inverted trough developing to the southwest of us after this first system exits, and many show a narrow band of very heavy snow forming. As of now, most guidance keeps the trough well southwest of our region, however, considering that these troughs almost exclusively trend northeast as we approach verification, I continue to be intrigued by the potential that it holds for another snow event across the region. Regardless of the inverted trough, it looks like a clipper system will come down from Alberta and transfer to the coastline, bringing some more snow showers and squalls to the region Tuesday afternoon through most of Wednesday. That will likely combine to a light to moderate accumulation across most of the state, but as of yet, it’s too early to discuss amounts for the area with these features. Greg will have a full discussion tomorrow evening with the details on the inverted trough and clipper system along with a snowmap if necessary.
Until then, however, we’ll be keeping our eyes fully peeled to the system that is about to impact us tomorrow, with updates throughout the day tomorrow as needed. Remember to send us your pictures and observations via Twitter @SouthernCTWX or our Facebook page, and please share this discussion with friends and family. Thank you for reading SCW and enjoy the snow!
-SA
