It was certainly an active day in weather, with icing and a little snow this morning giving way to torrential rains across a majority of the state, with some areas reporting over three inches of rain as a result of a large deformation band which parked itself right over our area. That band has exited to the north, and much of the state is in a dry slot as the low has for the most part passed right over us. You can see on the radar image below the track the midlevel dryslot has taken(Over CT into NH), with areas of precipitation to it’s north, east, and west.
There are essentially two camps on the guidance right now, both of which would bring some wintry precipitation to the state. The first is a scenario which places the center of the closed off low in SE CT, essentially bringing precipitation to all areas north of there. This is depicted well by the 18z RGEM, shown below. The second, a scenario which has the low slightly further south, would bring higher totals to the SW coastline. This is shown well by the 12z RGEM is very similar to the 18z RGEM, but in general, places the low slightly(and I mean slightly, maybe only 10 to 15 miles) south of the 18z, but what that does is drag the entire setup of precipitation 10 to 15 miles south, meaning areas on the CT shoreline which saw little on the 18z RGEM now see a substantial event on the 12z RGEM. Here are the MSLP placements at 7 PM tomorrow night.
In terms of total QPF expected from this wraparound band, we’re thinking anywhere from 0.1” to 0.4” or so of QPF, generally on a gradient from SE to NW, with the highest totals being in the NW hills as they are closest to the center of the CCB over upstate NY. The foreign guidance, that being the UKMET, GGEM, RGEM, and EURO, is wetter than the American guidance, that being the GFS and NAM. We will discount the GFS for this forecast due to it’s notorious dry bias and general poor handling of mesoscale features and weight all other guidance equally, essentially meaning a 4:1 blend of the foreign guidance and NAM. This works out to the QPF forecast described above.
Now that we’ve got down how much precipitation is going to fall, the next question is what will it fall as? All guidance agrees on temps crashing from west to east as the low moves east and colder air rushes in, but the question is how quickly in comparison to the arrival of the precipitation will they do so, and as a result, how quickly will our areas change to snow? For this progression, temps will generally crash from west to east, meaning we will see a rain/snow line running north/south for the most part, with areas west of that line seeing primarily snow and areas east of it seeing primarily rain. Over the course of tomorrow evening and Thursday, that line slowly moves east, allowing just about everyone to finish as some snow, but by the time the line reaches eastern areas, most of the QPF is gone.
To show this transition, here are images of the estimated precipitation type of the 18z RGEM over the course of tomorrow afternoon and overnight. While these can be unreliable and as a result we don’t use them when forecasting, for a visual explanation, they are excellent. Notice in the early panels, when the rates are stronger(Darker colors), the rain/snow line is very slow to move east. Towards the end, as the low finally begins to pull away, the line begins to push east as temps crash, but by then, the heaviest of the precip has moved out and we have only light, showery precip remaining.
To sum it up, we’re expecting up to a few inches of snow in western areas away from the coastline, up to an inch or two on the western coastline due to lack of QPF, and up to an inch or so for the eastern 2/3 of the state after rains. You’ll notice we used the wording “up to” in our forecast, and that is because of the relative uncertainty, which exists with the placement of the banding and overall temp profile. As a result, we chose to mention the high end of what could realistically happen (around the 75th-80th percentile), but to prefix that with a warning that the low end of a realistic forecast(Say 20th-25th percentile) is 0” accumulation with perhaps some snow showers. As always, the truth is somewhere in the middle, and considering the impacts will be relatively mesoscale in nature, did not feel confident enough to go with specific ranges. Here’s a map depicting this forecast.
We’ll have an update tomorrow morning if any changes need to be made as a result of the 0z suite, until then, thanks for reading!