Here We Go Again: Snow, Wintry Mix, Slick Roads

Yet another storm is approaching the area, one which is quite likely to bring numerous types of precipitation.  This blog post will try and break down the various model solutions for all of the storms and distill them into a single forecast, even though the storm is far enough out that various model trends could result in significant changes to the forecast.  At the same time, a relatively strong model consensus with this storm makes forecasting it fairly easy compared to many other winter storms.  This post will be broken down into an overall discussion of the pattern creating this storm, precipitation types and timing, and total accumulations of each precipitation type.

Note: This will be the technical part of the forecast.  For forecasts on real conditions and expected timing, please look at the next paragraph.  To begin, the overall makeup of this storm is not that dissimilar of the makeup for the storm this past Sunday night, as there will be one primary low inland that will be weakening with another secondary low taking over by the coast and actually moving fairly close to the 40/70 benchmark.  In January or maybe even February, this scenario typically ends up with an all snow event, and there is the potential that it does here too.  However, as I will explain shortly, upper level dynamics with this storm do not look supportive of a large-scale snow storm that many in New England are used to.  While this may be a 3-6 inch or 4-8 inch snow storm, I do not think this is one of those storms where someone sees 10 or 12 inches of snow.  I could very well be wrong, but when I look at the upper level signals I just do not see it.  What is happening aloft is that a weak disturbance at 500mb is moving across the country.  This upper level low is currently over southern California, and will move almost due East after it is kicked out by another piece of energy swinging down.  The first piece of energy will move through on Saturday, resulting in scattered light to moderate snow showers throughout the day.  The second piece of energy is the one that has the potential to create a larger storm, but the first piece of energy almost strings it out and remains in the way.  What you get then is energy diving from Canada trying to meet up with this second piece of energy, and most models show it unable to because of that first piece of energy.  If it were, this would be the “phase” that allows low pressures of this nature to strengthen very rapidly as it would draw on very cold northern air and have the very warm southern air.  While there is some interaction, it is not as large as it would typically otherwise be with disturbances this time of year.  Accordingly, the northern energy diving down remains “progressive” longer with a positive tilt, meaning that the bottom of the trough in the jet stream, shaped like a U, points a little west of south.  When that trough finally turns east of south, baroclinic energy is pushed along through the jet stream and is infused into the low pressure, creating that mix of warm and cold air and thus allowing the low pressure to bomb out.  If the upper level energy is progressive longer, then that means that the low pressure is weaker for a longer amount of time.  Snow would be avoided if the secondary low pressure at the surface moving to our south is infused with energy as it moves by, allowing it to rapidly strengthen and pull in colder air.  At this time, though, most models have it remain relatively weak, so it would not quite be strong enough to pull in colder air to replace the flow of warm air that the primary low pressure center to our west will have.  This means that as the main circulation approaches, warm air aloft will be moving in, and this is what will likely end up turning the snow to sleet and eventually freezing rain and/or rain.  In summary, if the pieces of energy aloft meet up better and if that trough becomes neutral and then becomes negatively tilted, it can allow the low pressure to strengthen faster, bringing in more moisture and pulling in cold air to keep Southwestern Connecticut snow.  At this time, there is not much evidence for that, meaning that the low pressure will stay weaker and thus slightly less precipitation and warmer air aloft will be the main stories for our region in this storm.

So with the overall dynamics and meteorology explained with this storm, I want to focus on what that really means in terms of precipitation types and timing.  Snow looks to break out sometime Saturday morning, but it will likely be very scattered in nature at first.  We will look to see maybe an inch or two throughout the day on Saturday.  Saturday evening snow will become a little more steady and we could see a little more in terms of accumulations with possible an additional few inches, and then overnight Saturday is when the mixing will come to play.  The issue is that the warm air aloft moves in almost entirely overnight, when surface temperatures will be below freezing.  At time, mixing with sleet will likely happen between 10 PM and 1 AM and then right after that precipitation should go to freezing rain and then to rain (mainly at the coast).  There is a chance that inland areas stay freezing rain throughout the storm, as there will be a lot of cold air entrenched at the surface, but the coast is likely to turn to rain for a period of time overnight Saturday night.  Then there is a chance that as colder air comes back in Sunday morning that a few snow showers could take over, but don’t expect anything more in terms of accumulation after the turnover occurs.  Once again, if that secondary low pressure system strengthens quicker than expected and pulls in more cold air, or the cold air turns out to be stronger than expected (which is a very real possibility) then the turnover could be delayed, more precipitation would fall as snow, and we would be seeing even more in terms of accumulations.  I would say there is about a 30% chance this happens, and if it were snowfall amounts of 5-6+ inches would not be unheard of, but I will talk more about this in the next section.  For now, just realize that mixing will likely keep snowfall amounts from being too high with the upcoming storm.

Generally I am looking at accumulations of somewhere from 2-5 or 3-6 inches across the area, but I will narrow that down as we get closer.  A few tenths of an inch of ice could also be possible overnight Saturday from freezing rain.  With a stronger secondary ice would be less common, as precipitation would likely stay as snow or sleet longer, but right now very few models show all snow/sleet as they almost all have a turnover to rain at some point.  If I had to guess right now, I think a forecast of 3-6 inches of snow across the area before a turnover to sleet and freezing with a coating to a tenth of an inch of ice at the coast with up to 2 or 3 tenths of an inch of ice inland would be the best forecast, but as I have said that is likely to change a fair amount over the coming days.  I do think a slightly cold trend is likely because of just how cold the current air mass is across the area.  But the last few storms also underperformed across Southwestern Connecticut, and I have always said that each winter is its own beast and whether storms tend to over-perform or underperform always means something.  That’s why I am not yet ready to factor into a cold trend that very well could dominate.  As the shorter range higher definition models begin to come into range over the next few days we will see.  As of right now, the GGEM/CMC is definitely the coldest model with almost no changeover, and that is also the model I tend to weight most in systems like this.  However, the other models have been consistently warmer, and the ECMWF just shifted from one of the coldest models to warmer than the GGEM/CMC, though not as warm as the GFS or its ensemble mean, the GFS.  So it is very possible that a cold trend dominates and the storm stays generally snow with maybe a little sleet, and I am prepared for that possibility.  Both low pressure strength and placement matter a lot for this, two things that I can forecast more accurately as we approach the event, so as always keep it here for the latest on winter weather as it approaches the Southwestern Connecticut area.

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