It’s hard to believe, but we’re already tracking our first snow event of Winter 2018-2019. We’re expecting a minor to moderate snowfall across the state tomorrow, and it’s time to put our first call map out.
The Big Picture
Low pressure moves northeast into the area Thursday afternoon and rams into a departing area of high pressure, creating some initial warm air advection snows followed by a warming of the mid levels and surface and eventually a flip to rain. A light to moderate snowfall is expected for most of the state, and there is the potential for moderate icing in the interior as well. Everything should flip to rain by Friday morning except for possibly the far interior, and the storm winds down by mid-day Friday.
Here’s an overview of the system on the GFS.
I thought I’d try something new with this discussion and instead of looking at the individual models, look at some of the key variables that will drive how much snow we’ll get. We’ll use models to illustrate those variables, but I think it provides a more informed picture to see the ingredients behind the forecast instead of just the model forecast itself. Let us know what you think!
1. Arrival Time and Intensity of Precipitation
The guidance has been going back and forth a bit on the arrival time of this system, with the consensus yesterday being through the afternoon and having snow by the evening rush hour. The later runs though have pushed that back a bit, with snow not fully overspreading the state until the early evening hours. Generally, we’re seeing that a later start means less precipitation falls as snow as the temperatures warm at about the same time on all of the guidance. Furthermore, the models that are showing a later start are doing so because of less warm air advection driven precipitation - this is initial QPF out ahead of the main system that has the benefit of falling in the colder air mass that we have in place to start before the main system pushes it out - hence it would be all snow for everywhere in the state. The trend to cut back on the amount of WAA and hence the amount of initial precipitation means generally less snow for the state, and of course if the trend were to reverse we’d see more wintry precipitation and have a better chance of significant impacts to the evening commute.
To show this, here’s a look at yesterday's 12z GFS vs todays 12z GFS for 7 PM tomorrow night. You can see that on yesterday's run, we’ve got snow across all of the state with the mix line just starting to work its way up into SWCT, while on today's run, the snow is much lighter in the north and the mix line is already up into central CT.
It’s pretty simple - colder surface temps = more snow. The guidance differs a bit on how well the cold air is able to remain in place. Some models, like the Euro, keep the cold air locked in right down to the coastline and as a result allow for several inches of snow to fall across the entire state, with warning criteria for interior areas. The warmer solutions however quickly warm up the boundary layer at the shoreline and eventually further inland as well, with only a quick burst of snow on the shore and a couple of inches further inland. With November climo, I feel that you need to favor the warmer solutions, but that said, strong Canadian high pressure like we’re seeing here provides a lot of opportunity for staying colder and often allows us to remain on the colder side of a storm for longer than you’d expect.
To show the point, here are surface temps on the GFS and the RGEM For 7 PM tomorrow, along with the forecast snow amounts from each model. Don’t take those snowmaps as the gospel here since they tend to over do amounts significantly in marginal events, plus they use a fixed 10-1 ratio where I expect snow ratios to be lower here (sorry - I don't think 11" of snow is going to happen with this event :) ). That said, they do illustrate the differences between the models well, although some of the difference can also be attributed to the RGEM bringing in heavier precipitation sooner.
Finally, we need to look at the mid-level warmth that this system will bring in. Even if we get strong cold air damming at the surface (which is common with high pressure like we’re going to see), it means nothing if the mid levels warm. The result in that case could be an icy mix for a time before things fully flip over to cold rain - we’ll have to watch for that in the typically colder hollows and valleys of the interior. Here’s a look at the NAM that shows the warm tongue and the surface temps - that area in NWCT where temps are above freezing aloft but below freezing at the surface would mean an icing event for them. The consensus that the warm air aloft will move northwards is fairly strong, but heavy precip rates could result in it pushing north slower than expected - we’ll have to keep an eye on that for a possible increase in snow amounts.
Here’s a look at our forecast snowmap for the event.
I do think there is bust potential on both sides, but I’m leaning more towards the possibility of busting low rather than busting high. If we get a solid burst of precipitation in the late afternoon and early evening, I think we’ll quickly be able to pick up a few inches of snow, and then it’ll be up to the changeover to see if we pass the high end of the forecast. The key reason for busting low would be weaker WAA meaning less precip before a changeover, but given the trend over the last few model cycles I think that’s less of a possibility.
Here’s the SCW impact scale for this event.
We’ll be back tomorrow morning with a final call and updated map if needed, until then, ask us any questions you have on our Facebook and Twitter pages (use the buttons below to follow and like us), and thank you for reading SCW!