6:00 PM Update, Large Snow Storm Coming: 20% Delay Thursday, 15% Snow Day (mainly inland)

We have a major snow storm coming.  My goal here is to break down exactly how this will impact you and your life over the coming 48 hours.  I will start by focusing on timing, then accumulations, then winds, and then end with the cold.  After this I will discuss how I came up with the school forecasts for tomorrow, though that will also be interspersed throughout the blog post.  Then, similar to what I did in the last blog post, I am going to label a part of the blog as technical and break down yet again why I remain aggressive with my snowfall totals of 8-14 inches for the area based off of various weather models that have come out throughout the day.  I’ll then have a similar conclusion to last time too where I will wrap everything up.  I am unsure if there will be another blog post today; it will only be necessary in my opinion if there are major changes to what is expected tomorrow.  As of right now, most models appear locked in on the scenario and there is high confidence in only light snow tonight and tomorrow, so I doubt there will be significant changes.  I’ll be on Twitter all night, though, so if anything changes you will be the first that.  With that, here we go:

Snow showers break out tonight around 1 or 2 AM across the area and be mainly scattered in nature coming up as flurries from a southerly flow.  Overrunning precipitation will create a tongue of moisture to the north that might come close to Southwestern Connecticut sometime between 3 and 5 AM dropping maybe half an inch of snow.  Otherwise, all snowfall through this time looks to be negligible with maybe a dusting, coating, etc. through 6 or 7 AM.  Isolated areas could approach half an inch or an inch but I would be surprised if there were widespread areas approaching an inch by 7 AM.  Scattered snow showers will continue through the day on Thursday, though once again I don’t expect any sizable accumulations through noon or so.  After noon a more pronounced lull will move in between the snow showers that will continue until around 4 or 5 PM in the afternoon where the scattered snow showers will come back.  Then between 7 and 8 PM I expect a wall of moisture to begin to move in that will be associated with the main precipitation shield of the storm.  Models vary on exactly when the heaviest snowfall would be, but generally it looks like the heaviest snow will fall between 11 PM and 7 AM.  In that 8 hour period up to 6 inches of snow may be possible, especially as the snow gets more fluffy as time goes on.  Snow then looks to continue for maybe another few hours, but most models have snow ending by 9 or 10 AM at the latest for the cleanup to begin Friday afternoon.

This leads me to a discussion on accumulations.  The going forecast is 8-14 inches of snow.  The forecast I put out now over 48 hours ago was very similar at 8-16, though at the time I said I favored more of an 8-12 inch storm.  That seems to be exactly what will end up playing out.  In the technical discussion I will explain what the models came around to show and why it supports my forecast that many said yesterday was far too high with snow.  But for now I’ll just detail why I’m thinking 8-14 inches generally and when you can expect that snow to fall.  As I’ve said on Twitter, I do not expect any sizable accumulations through 7 AM, with maybe half an inch coating the region.  Another half inch is possible through noon.  Then there will be a lull through 6 or 7 PM, meaning I only expect an inch if that of snow to fall by 6 or 7 PM.  There’s a distinct possibility that some people only have a coating or have only seen flurries by then; that does not mean the storm is a bust.  It is the snow falling between 7 PM Thursday and 8-9 AM Friday that will really accumulate.  In this time I expect a general 7-10 inches of snow to fall.  Some banding and high ratios inland could result in up to a foot falling in this time period.  The reason there is such variability here, and why the range is 8-14 inches, is because of the extreme cold coming in behind the storm.  When snow falls in colder air, it tends to be fluffier and have less moisture in it.  There is more air than snow, thus it piles up on the ground faster and blows more.  A standard ratio of rain to snow is 10 inches of snow for every one inch of liquid equivalent, AKA an inch of rain.  When there is extreme cold like we have here, and surface temperatures are in the teens or low 20s, that ratio can turn into more like 15 inches of snow per an inch of liquid, and in extreme cases it can sometimes reach 20 inches of per snow per inch of liquid.  I explained this on Twitter earlier, but as the storm strengthens Thursday night and pulls in more cold air the ratios will gradually go up and the snow will get lighter.  While most of the day on Thursday ratios will be around 10:1, by Friday morning they will probably be around 15:1 at the coast and may approach 20:1 inland.  They also look to generally increase as snow intensity increases due to the backbuilding of the low pressure center.  What this means is that average ratios for the storm will probably be around 13:1/14:1 by the coastal CT plane and probably 15:1 inland.  So even though some weather models give us half an inch of liquid, that equates to 7 inches of fluff at the coast and 7.5-8 inches of fluff inland, ending up close to the lower end of my forecast.   Granted I expect the storm to strengthen a little more than some models currently show, which has been a trend in recent years that I will explain later, but this is why you may see lower liquid equivalent numbers matching up with higher snowfall totals.  Ratios could be even a little higher than I’m forecasting, but I also have seen forecasts bust by expecting ratios to be that high, and it is rare they really get that high, especially by the coast, so I am playing this conservatively.  With some models still showing up to an inch of QPF, I am happy capping off snowfall totals at 14 inches for the region.  I may reinstate the original range to 16 inches if the 0z models trend towards a stronger, more consolidated surface low pressure center, but as of right now I have growing confidence in the 8-14 inch range.

Along with snow, we will be dealing with wind issues from the storm, though I do not expect them to be that severe as the low pressure center is likely to pass near or just southeast of the 70/40 benchmark that meteorologists use to determine impacts of a coastal low pressure center.  There will be a very sharp pressure gradient setting up, which is why I do expect some strong winds.  However, winds won’t strengthen until the low pressure center is to our southeast, meaning winds will be coming from the north and significant friction from land to our north will keep the winds from being that bad, even at the coast.  I see sustained winds of 20-25mph with gusts to 35 mph at the peak of the storm, which is likely between 2 and 6 AM.  Inland winds won’t be quite as bad.  It is Long Island and Southeastern Connecticut that likely will see significant wind impacts as they are more exposed to this type of storm, and this is why Long Island has a Blizzard Warning.  I don’t believe one will be needed in Southwestern Connecticut unless the low pressure strengthens significantly more than most models expect it to and more than I am forecasting it to.

Finally, both during and after the storm we will have some very severe cold weather.  Temperatures tomorrow will likely not get out of the 20s, but as winds turn to the north temperatures will rapidly drop into the teens by late evening and into the single digits overnight as snow picks up and then begins to wind down.  On Friday temperatures will struggle to reach 15 degrees, even by the coast.  Friday night will likely be the coldest night of at least the last few years, with surface temperatures getting down below 0 for all inland areas of SWCT and maybe even hitting 0 at the coast.  As I’ve said, this extreme cold the likes we have not seen overlap with snow in a long time is what will boost snowfall ratios and add maybe a few fluffy inches to total accumulations, and the snow pack is also what will help temperatures stay cold on Friday and get even colder Friday night.  A moderation is expected on Saturday and then on Sunday before another storm tries to move into the area Sunday night into Monday.

SCHOOL: I want to briefly talk about how this impacts school.  As I have said, I believe that schools should be opening tomorrow without significant issue across all of Southwestern Connecticut.  There are two scenarios I am watching that could impact schools tomorrow but both are increasingly unlikely.  The first is that the mesoscale band of overrunning precipitation shifts to the south from where it is forecasted and SWCT gets more steady snow during the day tomorrow.  There is only a 5-10% chance of this, but it would close schools.  Hence the mention of snow days possible.  Another reason I have school closure percentages is schools may close simply because they know they are closing Friday and don’t see the point in opening one day and then closing for three more.  Unlikely, but I’ve heard of stranger things.  The other scenario is that the time when the band move through, from 3 AM to 5 AM, the mesoscale band is slightly stronger so SWCT gets maybe a couple of inches of snow by 6 AM.  Again, the chance of this is only 15-20%, so it is not likely, but if this were to happen then schools may have a delayed opening tomorrow.  That being said, the chances that schools close on Friday are definitely over 50%.  During the day tomorrow, unless I see significant forecast changes, it is likely that I will forecast snow days across Southwestern Connecticut.  In fact, many districts may choose to just close in advance and make the call Thursday night.  I will continue to keep track of this and keep you updated.  So that’s the storm and its impacts.

TECHNICAL NOTE: Because of the length of this blog and of timing, I have decided that I will write another blog that focuses solely on the technical aspects of the storm after I publish this one.  I have a lot to cover in the technical part of the bog, and I want to get this published ASAP so my new forecast update can be out.  The technical update can be expected by 7 PM or so.

CONCLUSION: A multi-faceted snowstorm is on the way with moderate to maybe heavy snow, frigid temperatures, and gusty winds.  Though not exactly a classic Nor’ Easter setup, this will essentially turn into a Nor’ Easter as we head into Thursday night.  Snow of 8-14 inches, falling mainly between 7 PM Thursday and 8 AM Friday, will be the main story before the potentially sub-zero temperatures move in.  I would expect travel to be very hazardous anytime between the aforementioned hours, and travel to be slow most of Friday as the cleanup begins.  With temperatures so cold, salt and sand on roads will struggle to melt the snow, so snow will pile up on roads fairly quickly, treated or not.  Do your best to avoid the roads Thursday night and Friday morning, and prepare for a nice snow storm.  While no blockbuster, this is still a strong snow storm.  Stay tuned for the technical discussion and additional updates throughout the next 36 hours!


One response to “6:00 PM Update, Large Snow Storm Coming: 20% Delay Thursday, 15% Snow Day (mainly inland)

  1. Great post, love this website

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