24 Hour Turnaround: Moderate to Heavy Snow Coming for SWCT

I have been tracking this week disturbance for a little under a week now and this is now the first time since I began tracking it that I have been convinced that Southwestern Connecticut could feel major impacts from it.  As I have been outlining on Twitter over the last few days, we are in a pattern where numerous weak disturbances are swinging down from the North.  These weak disturbances are often called Alberta Clippers as they originate from the Alberta region of Canada.  I had said that each one would need to be watched closely as it was possible that they would rapidly strengthen near the region even though weather models were not picking up on them.  The one last Saturday did just that, throwing very heavy snow across the region, even though it did not stick across most of SWCT, and if it did then it was not on the ground for long at all because surface temperatures were still quite warm.  There I busted in northern regions because the snow was so heavy from the rapidly intensifying low pressure system that it did stick.  This storm is similar in that the low pressure center is now expected to rapidly strengthening much more than was thought yesterday or any day previous, but meteorologists this time are given a little more advanced warning from weather model guidance.  With that I am now going to break down the threat into the three main categories I often use: timing, accumulations, and school impacts.  Those will all come after a brief summary regarding the pattern and what has happened over the past 24 hours, it will be followed by a brief technical report on what weather models are now showing, and it will then end with a conclusion on exactly what could change in the forecast, what I’m watching closely currently, and what I think is most likely to change.  Each section will be labeled accordingly.  Enjoy!

SUMMARY: An Alberta Clipper is moving across the country currently.  Associated energy with the storm is moving through the Dakotas, and since last night has moved into a region with better data sampling and more weather stations resulting in weather models converging in on a solution and increased accuracy.  Weather models have trended towards a solution where energy is better bundled and an upper level trough turns “negative” earlier, basically meaning that a surface low pressure center will strengthen quicker closer to the coast.  In terms of track, weather models have also been slowing the low pressure center down and pulling it more northwest, resulting in a longer period of moderate to heavy snow for the region.  This is not a classic scenario of a low pressure moving more north than east, but instead the low pressure center will be moving more east of north as it moves off the North Carolina coast before it gets picked up and begins to move much more northerly.  It is during that turn from ENE to NNE that the precipitation from the low will move into the region, and that pivot will allow a longer period of snow across Southwestern Connecticut.  There is not a lot of stagnant high pressure centers to the north or east of the storm, meaning there is not a lot of blocking in the atmosphere to significantly slow down this storm, meaning instead of a historic snow storm we are dealing with a significant to memorable one, but regardless impacts are expected to be impressive as this low pressure moves much closer to Southwestern Connecticut at a much stronger level than previously thought.  More details on all of this can be found in the technical section of the blog post, but this is a basic summary of the setup for the upcoming storm.

TIMING: Models are beginning to come into better agreement with regards to timing, especially on the front end of the storm.  General model agreement now shows precipitation breaking out between 11 AM and 1 PM Tuesday from west to east.  In 24 hours I am predicting I will be following a wall of snow moving across Southwestern Connecticut.  It is a known bias of weather models to be too slow handling overrunning precipitation, where warmer air associated with precipitation rises up over the cold air at the surface spurring precipitation ahead of the main surface low pressure, meaning there is a chance that light snow breaks out as early as 10 AM across the region.  Either way, I don’t see steady snow setting up until 12 or 1 PM at the earliest, possible waiting until 2 or 3 PM further east.  By 4 PM the entire region will likely be in light steady snow, as though the wall of snow from the overrunning precipitation will have moved through we will still be awaiting the enhanced precipitation from the low pressure center itself.  It looks like the steadiest snow will be falling between 7 PM and 1 AM, as that is when the enhanced precipitation from the actual low pressure center and its associated precipitation shield will move through.  It is in this 6 hour time frame that all driving is highly discouraged as snowfall rates could hit or exceed an inch per hour and all snow will stick immediately on roads (more in this in the next section).  After 1 AM confidence drops on timing as models diverge.  Some models have light snow from 1 AM to 3 AM but then snow basically ends from there.  Other models have moderate snow through 4 AM with light snow continuing until maybe 6 or 7 AM.  As I’ll discuss in the school section, this could have major impacts relating to what schools decide to do on Wednesday, but in terms of accumulations this should not have a significant impact.  My current forecast is an average of most of these models, with snow likely ending between 4 and 5 AM in western SWCT and further east light snow potentially holding on until 5 or 6 AM.  By 7 AM the entire region should be snow free and the clean up from the next freak snow storm will begin.

ACCUMULATIONS: Accumulations with this storm, like the January 2nd/3rd storm, are extremely tricky.  I was burned on the last storm not because of the amount of liquid from the storm or the track of the low pressure, each of which were correctly forecasted, but instead because of snowfall ratios.  Winds shattered high-ratio snowflakes as they fell to the ground, especially inland where ratios were supposed to make up for a lack of liquid, meaning that snowfall accumulations inland did not get into the forecast range, though snow at the coast fell into the range nicely.  With this storm, temperatures aloft are going to be even colder, with temperatures at 850mb (~5000 feet in the atmosphere) generally between -12 and -15 degrees Celsius, some of the coldest that I have ever seen in a snow storm.  At the surface, temperatures will likely be in the mid to low teens, with the possible for low single digit temperatures inland by the end.  What this means is that the entire column (besides the surface) is slightly colder than the last storm.  Also, latest indications are that winds will not be as strong, which is why I do not think even Wind Advisories will be necessary for the coast unless models this evening jump further northwest yet again.  That should keep winds from shattering snowflakes and result in more impressive ratios than with the last storm.  Liquid amounts will likely be very similar to the January 2/3 storm, meaning that currently I am forecasting most locations (except a few lollipops at the coast in January 2/3rd) to get more snow than they got with the last storm.  At the same time, I don’t want to get burned my a forecast for impressive ratios, so I am going to play this storm more conservatively and not forecast based solely on impressive ratios of 15:1 or 20:1.  Again, when temperatures are colder, snowfall is fluffier and more crystalline, meaning that there is less liquid per crystal and that each crystal takes up more surface area, so that snow flakes stack up quicker.  Thus, those ratios mean that instead of the average 10 inches of snow per inch of liquid (or what would be an inch of rain), areas could see 15 or more inches of snow if they saw an inch or liquid.  As of right forecasting guidance indicates that the entire storm could have 20:1 ratios for most of Southwestern Connecticut, but this is similar to the guidance that missed the ratios on the last storm.  In terms of liquid amounts, the general model consensus is for anywhere from .4-.6 inches of liquid for the SWCT coastline with closer to .3-.5 inches of snow inland.  Inland I’m expecting ratios of 16:1 and at the coast I’m expecting average ratios of 14:1 or 15:1.  In the end this results in snowfall forecasts of around 5-10 inches at the SWCT coastline and 4-8 inches inland north of the Merritt Parkway.  This is on the slightly higher side of some forecasts I have seen, but it is close to what the National Weather Service is calling, just a tick higher as they tend to be conservative more than 24 hours before a storm.  I expect to see their forecasted snowfall amounts rise over their coming updates to take into account the guidance that I am seeing.  One thing that I will hit upon in the technical section is a tight gradient that may set up here with 8 inches in one town and the town next to it seeing only 4 or so inches of snow, and this is why the range is currently high with snowfall and confidence is relatively low with the snowfall forecast, but this will be updated as I have increased confidence.  I’ll deal with impacts in the next section, which will also deal with schools.

SNOW IMPACTS, INCLUDING SCHOOLS: With snow beginning between 10 PM and 1 AM or so tomorrow, I expect that there will be at least some impact on schools.  In a preliminary forecast update/school impact assessment to be released around 2 PM I will release the first chance of Early Dismissals for tomorrow (Tuesday).  I think it is likely that most schools dismiss early tomorrow as I see an inch of snow on the ground by 2 or 3 PM.  Surface temperatures when the snow begins tomorrow morning will be in the upper teens, meaning snow will have no trouble sticking immediately, even to treated roadways.  Again, very cold surface temperatures make salt or sand on roadways less effective, meaning that road conditions will go downhill quicker than with a normal snow storm.  I believe that the cold temperatures along with the earlier snowfall time and the impending large scale snow storm will scare superintendents into releasing schools early.  At this time I cannot say with high confidence that this would be the right call, but given the data I am looking at I can say it is reasonable to close schools early.  With steady snow by mid afternoon and heavier snow in the evening any schools that do not dismiss early will certainly cancel all after school activities.  As short range high resolution models pick up on the overrunning precipitation confidence regarding an Early Dismissal will increase.  There is a slight chance that some districts overreact and close for Tuesday, but at this time it does not seem necessary as the steadier snow should not arrive until 12 PM or 1 PM at the earliest.  This chance will be reflected in Early Dismissal forecasted percentages later too.  As for Wednesday, impacts rely less on timing and more on accumulations.  With accumulating snow ending by 5 or 6 AM, it is not snow falling that could close schools on Wednesday.  Instead, it would be snow on roadways that have not yet been cleared.  Each district has a different Department of Public Works that will clear roads at different rates, but with all snow sticking on even the treated roads and a very fluffy snow that could easily blow around it will be very difficult to have roads cleared in time for school on Wednesday.  A delayed opening for schools is almost a guarantee at this time for all of Southwestern Connecticut.  The bigger question is whether schools will end up closing because of the volume of snow that will fall.  At the coast, I think it is a very real possibility.  Inland, even though accumulations will not be as high, I think it may be just as real a possibility because of how easily schools there close and how hilly many of the roads are.  No percentages yet for Wednesday, as those will be released tomorrow (24 hours and in for school forecasts) but I can say that it is highly unlikely that schools open on time Wednesday, and it is very possible at the last that they do not open at all, especially if the higher range of these snowfall totals are reached.  Again, there just is not yet enough known about exact accumulations by the tail end of this storm to be sure whether schools will close or not Wednesday, so make sure to stay tuned here as I update the forecast and break down additional data as it comes in.

TECHNICAL: I briefly want to touch on this, but the blog post is late being released because of the length so I won’t spend too much time.  The next update coming around 2 PM will also be brief but will be almost all technical.  Basically, model guidance has trended further northwest because at 500mb in the atmosphere the trough tilts more negative quicker and digs more.  This pulls the low pressure center at the surface further northwest and greatly strengthens it.  I believe that this trend could continue a little more, but I do NOT expect major changes towards more snow than most models show.  Ensembles generally agree with weather models here, and in some cases ensemble guidance is a little bit drier.  This means that amount of moisture with the associated system is likely getting locked in, and while models will change back and forth I would be surprised to see another major trend to the northwest with even more precipitation.  If this were to happen (I’d give it around a 20-25% chance) some areas of SWCT could see up to a foot of snow, but again it is unlikely and I feel that models are honing in on the most likely scenario for the region.  Along with this, because it is a late-blooming Alberta Clipper that is rapidly strengthening just to our east, a very tight snowfall gradient will set up.  Snow will be moving from west to east at first before it tilts and orients itself more southwest to northeast, meaning that somewhere could get 2-3 inches of snow in a few hours where only 10 miles north only a flurry or dusting is seen.  Though the entire region should see moderate to heavy snow for a period of time, this original precipitation shield orientation will result in very sharp snowfall cutoffs that could make forecasting exact amounts for different locations extremely difficult.  This is something I will detail more as the storm approaches and high definition models attempt to pick up on the gradient, but I think my snowfall differences from inland to the coast do a good job of highlighting this for now.

CONCLUSION: A rapidly strengthening low pressure center will move to our south and throw back moderate to potentially heavy snow across the region Tuesday afternoon and evening into very early Wednesday morning.  The movement of the low pressure will result in a very sharp gradient, with up to 10 inches of snow possible at the coast and as little as 4 inches (or maybe even less!) of snow possible up by and north of Interstate 84.  Heaviest snow will likely be between 7 PM and 1 AM and at times could approach or exceed rates of an inch per hour.  Schools are likely to dismiss early on Tuesday and will either be delayed or closed Wednesday due to the amount of cleanup that the storm will require.  Accumulation forecasts stand at 5-10 inches for the coast and 4-8 inches inland (anywhere north of the Merritt Parkway).  Updates will come throughout the day, both on Twitter at @SWCTweather and here on the website.  The next update looks to come just past 2 PM and will have the first percentage chance for Early Dismissals on Tuesday.  Make sure to keep it here for the latest on this storm as I try to keep you one step ahead of its next move.

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