We have been briefing Premium subscribers the past few days on expected conditions Monday night into Tuesday as they also receive daily email updates on changing models, 15-day forecasts, and the latest expected impacts to travel, businesses, and institutions from the storm. To subscribe for our Premium service, click here. Below we list our most recent forecast for accumulations, timing, and precipitation type across the SWCT/NY region. Another free update will be published tomorrow, although delay/closure forecasts for institutions opening on Tuesday are available for Premium subscribers.
ACCUMULATIONS: The accumulation forecast has become must easier as it is no longer expected that there will be major snow/sleet accumulations at all across the region, and even minimal sleet accumulations look relatively minimal. Coastal regions are unlikely to see any sleet accumulations, though up to a half inch of sleet is possible. A trace or glaze of ice along the coast will also be possible, though as expected weather models are picking up on slightly warmer than average waters across the Long Island Sound and showing surface temperatures warm slightly quicker, which should limit how long temperatures are below freezing along the coast. Further inland, we could see a slightly longer period of sleet initially as warmer air takes an hour or two longer to extend inland and then maybe 2 or 3 more hours of freezing rain that could lead to light/moderate ice accumulations. Up to an inch of sleet will be possible, though it is unlikely to get more than that. No snow accumulations are expected anywhere across Fairfield, New Haven, or Westchester Counties. Inland up to .1 or .15 inches of ice are possible on top of any minimal sleet accumulations as well, which is what will primarily determine travel impacts regardless. None of these accumulations are major, but any ice is still enough to cause problems, especially if there are accumulations more than a glaze.
TIMING: A chance of sleet will begin sometime between 9 PM and 12 AM, with small pockets of sleet beginning to move through. There will not be one large precipitation shield but instead isolated to scattered small instability cells of sleet that will move in overnight and gradually become more widespread. Precipitation should start by midnight for any regions that do not see a sleet shower earlier. By 2 or 3 AM bursts of sleet will be more common with the main precipitation shield moving in by 4 or 5 AM at the latest. Heaviest precipitation is expected to move in between 4 AM and 12 PM Tuesday before it dies off from there. Scattered showers will remain possible through the afternoon and dies down in the evening but most heavy rain slides through late morning.
PRECIPITATION TYPE: This remains the lowest confidence portion of the forecast, as various models show different forecasts for precipitation type timing during the morning commute in the SWCT/NY region. There is high confidence that no snowflakes will be seen across the region. The warm layer from 700-800mb will be dominating and temperatures there will never fully drop below freezing, making it impossible for snow to fall. In earlier forecasts I had been monitoring this closely which was part of the reason I was skeptical of how much snow would actually accumulate with this storm. Now, it is clear that there will not be any sizable snow accumulations, and even sleet accumulations will be minimal. Precipitation should start as all sleet across SWCT/NY. The first mixing looks to occur around 4 or 5 AM after a couple hours of sleet. In coastal areas, we could see a brief burst of freezing rain sometime between 5 and 6 AM. However, most guidance agrees that at least coastal areas will be over to plain rain by 7 AM with travel conditions improving. Further inland more questions remain. Most model guidance continues to show that we flip from sleet over to freezing rain sometime in the 4-6 AM timeframe. We should see freezing rain through 8 or 9 AM, and some valleys/pockets could see it linger as late as 10 AM. However, the consensus has been trending gradually warmer at the surface, meaning even inland areas (especially south of Interstate 84) could see a transition to plain rain by as early as 7 AM. It is towns around and slightly north of Interstate 84 that could see freezing rain linger until 8 or 9 AM, but almost all guidance show the entire region having temperatures in the mid 30s by 10 AM with coastal areas already in the upper 30s. It is the 5-8 AM timeframe that we should see the most transitions from sleet to freezing rain to plain rain across SWCT/NY, and that three-hour window is what I will be watching most closely for potential impacts.
OTHER: Overall, the forecast and our expected model trends have been verifying well. The warmer trends both at the surface (due to warmer sea surface temperatures) and aloft (due to a Miller B setup and recent warm weather) have been seen across most weather models, though a few short-range models that have a bias to trap more cold weather at the surface continue to show such solutions. Confidence sits at around average for a storm with this type of changeover threat, with that main timeframe from 6 AM-10 AM being most in question and the focus of most hourly forecasts that will be published over the next few days. However, the fact that models have trended as expected makes me more confident in where they go from here, and it appears we are finally getting close to the final solution with a few slightly more mild trends still remaining possible on a couple of key models. Winter Weather Advisories inland cover expected impacts with minimal ice and sleet impacts. There is a chance that Winter Weather Advisories may get extended down to the coast simply because of the chance of freezing rain, which they are almost always issued for, but if not that will be addressed in a Hazardous Weather Outlook. Overall, travel from 2-6 AM is discouraged at the coast and 2-9 AM inland with future revisions likely tomorrow.
Premium posts will be available through the day tomorrow into early Tuesday morning covering the storm threat, with another free post coming tomorrow afternoon/evening as well. To get the email updates, percentage chances of impacts, and detailed travel analysis, sign up here. Please be sure to stay tuned as the first winter weather threat of the winter season approaches!