12:15 PM UPDATE: Storm Dynamic Shifts, Snow Day 75%

Well, as with any dynamic storm, 12 hours between updates can result in some fairly significant changes in how the storm will play out.  What I am going to outline here are not major changes to the accumulation forecasts, because I remain confident in 6-12 inches along the coast and 8-14 inches inland, but instead major forecast changes to some of the timing and how we get to those accumulations.  We are getting more precipitation but now I am also more confident that we will be mixing and turning to rain for a significant amount of time even into inland Southwestern Connecticut, which has forecast and travel ramifications and makes the forecast tricky enough that I cannot guarantee snow days, even though I am forecasting them as I believe they will be necessary.   Please note that it is expected the snow day percentage to rise throughout the day as confidence with this storm increases, but with still so many unknowns I cannot guarantee them just yet (though again, they are expected).

First, I want to go over some timing differences here.  The start of the storm MAY be pushed back a few hours.  Originally, one may be tempted to say schools could open if the start of the snow was pushed back until 6 AM and the heavier snow did not move in until 8 or 9 AM.  But all this means is the storm lasts as snow later into the day, which is shown well by the HD ARW/NMM model combo.  One model has heavy snow begin by 4 AM, completely disrupting the entire morning commute but mixing with freezing rain/rain by 1 PM.  In this case, it would be the morning commute that should cancel schools.  The other model has snow wait until 5/6 AM to start and does not get heavy until around 9 AM, but it stays very heavy snow and by 5 PM it remains all snow with over a foot of snow on the ground in areas.  In this case, it would be the afternoon conditions that should close schools.  Either way, with everywhere in Southwestern Connecticut expected to get at least 6 inches of snow during the morning/afternoon tomorrow, I find it unlikely schools will be able to open, hence the forecasted snow day and high snow day chance.  However, I want increased confidence during the day before I ensure snow days, so there is not the potential for schools to open because the storm somehow ends up much warmer.  Due to this lack of confidence some schools may wait until tomorrow morning to decide to close, but I still think a safe call in closing tonight may be the correct decision.

Again, I want to emphasize the huge model spread that remains.  Of those 2 HD models that I mentioned, one gives us almost 12 hours of rain and the other keeps the region entirely heavy, wet snow with enough snow to maybe beat my forecast ranges.  Now I’m going in between the two, and I have to update the timing of the changeover accordingly.  I do not expect a significant period of sleet/freezing rain from this storm as the entire boundary layer is very marginal; instead we will see snow mix with rain and then turn to all rain for a period of time.  Right now, it looks like snow will start around 3 or 4 AM, and still get heavy by 6 or 7 AM, like the forecast shows.  Rain could start mixing in at the immediate coast between 12/1 PM, and the immediate coast could turn to all rain as soon as 2 PM.  Further inland, the mixing happens between 1 and 2 PM, and most of Southwestern Connecticut is all rain by around 3 PM.  Further inland there is a chance of some freezing rain, but anywhere south of the Merritt Parkway will just be rain.  Please note that all of this will be after 6-8 inches of snow for the entire region, so though roads will not be as icy they will still be dangerous with slush and a large amount of snow.  This light rain/drizzle will continue for most of the afternoon and into the evening before cold air and additional precipitation swings through between 10 PM and 1 AM.  This will start as rain, but as the low pressure pulls out and the banding pulls through temperatures will rapidly drop resulting in a few hours in a burst of snow.  This could accumulate another 2-3 inches or so on top of what falls tomorrow morning, putting my expected range at around 8 inches of snow at the coast and closer to 10-11 inches of snow further inland.  Again, a last minute colder trend like some models show could end up giving the region more, and a warmer trend could give the region less, but total snowfall accumulations of 6-12 inches at the coast and 8-14 inches inland look like the way to go at this time.

So that is what I am looking at right now.  The track of the low pressure is set, but I STILL believe the dynamics may not be totally taken into account by some of the weather models here.  This leads me to worry that the last second cold trend, as seen on the NMM, could be a very real possibility, and I will continue to follow it throughout the day.  As confidence increases in exact conditions, that snow day percentage will rise and I will continue to update.  I remain very busy through 5:30 PM today, but will try and get an update on the site before them.  If not, continuous coverage from 5:30 PM until 12:30 AM Thursday will commence, and there will also be continuous coverage tomorrow throughout the majority of the day.  As always, keep it here for the latest potential weather impacts across Southwestern Connecticut.

One response to “12:15 PM UPDATE: Storm Dynamic Shifts, Snow Day 75%

  1. Brunswick School in Greenwich cancelled for tomorrow already. (Before 10 a.m. Today)

    Sent from my iPad


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