First Call Snowmap and Discussion for 3/7/18

​Good evening to you from Southern Connecticut Weather!
 
A strong coastal storm is expected to impact the state on Wednesday and a significant winter weather event is expected as a result. This discussion provides both an overview of the system and our first call map – look for a final call tomorrow night!
 
Models and Trends
 
All guidance is in good agreement that low pressure moving into the lakes is then forced to redevelop to our south due to a strong –NAO block. The system then moves northeast and passes just south of Long Island, strengthening in the process and developing a classic comma head signature. Eventually, the system moves almost due north and moves into the Gulf of Maine, but by then the damage has been done and we’ve seen a significant amount of QPF. Here’s a look at the evolution of the system on the 3km NAM – note that this is one of the colder solutions.

​While the guidance is in good agreement on the large-scale synoptic setup, there are some important differences with regards to the track of the low and the ultimate result in Connecticut. The GFS is the most amplified model, and flips a large portion of the state to rain later in the day on Wednesday (although not before a warning level snowfall for all except the southeastern portion of the state), while the Euro is the most progressive and keeps most of the state as all snow, with slightly lower QPF totals as a result. The other models fall somewhere along that spectrum, with the mean of the group leaning a bit more towards the Euro than the GFS, but there aren’t any major outliers or clusters that would lead me to toss any one model over another. The trends over the last couple of cycles have been slightly more progressive, but overall guidance has been relatively stable. It’s also worth noting that this season, we’ve generally seen things become a bit more amplified in the short-range. Taking all of that into account, will go with a 2:1 blend of the Euro and the GFS, allowing for some wiggle room towards a more amplified solution but generally favoring the colder guidance.
 
Here’s a look at total QPF from the GFS, NAM, and GGEM; the Euro is generally very similar to the GGEM. 

The GFS and GGEM are in good agreement with approximately 1”-1.25” QPF across the state; the NAM is a bit more bullish but given its tendency to be too wet I am inclined to discount that in favor of the GFS/GGEM/Euro blend. At a 10:1 ratio, this would yield 10-12” of snow for the state. However, I believe that we will see at least some rain mixing in for most of the state, with the southeast corner seeing the most rain and the northwest corner likely remaining all snow. Given that, I’m inclined to trend snow amounts slightly lower to account for mixing.
 
Forecast & Timing
 
Here’s our forecast snow map for this event.

 Given that mixing in the southeast corner is a near certainty and that some guidance shows a substantial portion of the event as rain there, I’m not confident in a warning level snow event for that area. As such, have decided to go with a 3-6” forecast as a starting point, but I wouldn’t be surprised to see that adjusted in either direction tomorrow as we figure out exactly how much QPF we lose to rain in that area. Elsewhere, have decided to go with a broad brushed 6-12” for now; it’s likely that we will need to refine that somewhat tomorrow and likely break the state into a couple of zones, but given the differences in modeled QPF jackpots and temperature gradients we decided to keep it simple for now and highlight that a significant snow event appears likely.
 
As far as timing goes, expect the first flakes to start in the SW corner before the morning commute on Wednesday, with snow becoming steadier and heavier throughout the morning. The heaviest snowfall will be Wednesday afternoon into the evening commute, where we could see rates in excess of 1” per hour at times. At the same time, however, we will see the rain/snow line start to move inland, limiting snow totals in the southeastern portion of the state. As the changeover should occur during the heaviest period of snow, even a small change in the timing of the changeover could lead to a difference of several inches in the final totals. We’ll make sure to keep an eye on the timing of that line and will try to nail it down with more detail tomorrow. Otherwise, precipitation continues through the evening and well into the overnight, with the GFS even keeping some final flakes around for the morning commute on Thursday. While the morning commute on Wednesday will likely be tolerable, conditions will deteriorate rapidly as we head into Wednesday afternoon and I expect the evening commute to be a mess, to put it lightly. If you can work from home Wednesday, I’d recommend you do so.
 
Impacts
 
Here’s the SCW impact scale for this event. 

​I’d expect a number of school closings across the state, although I wouldn’t be completely surprised to see some districts try to sneak in a half day. While winds won’t be anywhere near the level of our last storm, it will still get gusty for a time on Wednesday afternoon, and the combination of the wind and the heavy wet snow means I wouldn’t be surprised to see some power outages.
 
All in all, we’re looking at a classic late-season Nor’easter, featuring heavy precipitation, a dynamic rain/snow line, and some gusty winds as well. Nothing we haven’t seen before, but still a high-impact event that demands attention. We’ll be back tomorrow with a final call and a more detailed timeline, until then, please ask us any and all questions you have here or on our social channels, and make sure to share this discussion with your family and friends. Thank you for trusting SCW!
-SA

Follow @SouthernCTWX