7 PM UPDATE: 30% Delay (coast) 15% Delay (inland)

No new data to really report on and nothing big in this update.  The latest RAP short range weather model guidance has trended slightly more offshore, giving the immediate coast close to inch with all of it falling between 3:30 AM and 6:30 AM and inland areas getting a half inch or less.  Most of guidance has trended the same way, though the GEFS (which are likely too low resolution for this forecast) still show up to 2 inches of snow in the highlighted areas that could get a little more.  Sadly short range guidance is experiencing some technical difficulties, so I do not have all the latest runs, but the HRRR had the entire region staying dry tonight, while the RUC was fairly aggressive.  The RPM also shows just flurries and snow showers with no real snow accumulations.  Surface observations generally support half an inch at the coast with not much inland.  Thus the inland delay chance was dropped but I didn’t change the one for the coast because of the timing, which I’ll get into in a second.  Basically, guidance continues to hone in on a range from half an inch to an inch of snow at the coast with less inland, and this convergence will likely continue through the evening as I follow the short range guidance.

The region I didn’t lower the chance at the coast is specific timing of the burst of snow from the precipitation shield.  Basically, model guidance is agreeing that the start time of precipitation aloft is different than when it will hit the surface.  With humidity percentages in the mid 30% range, it will take a lot of moisture from up in the atmosphere to moisten up lower levels before snow will actually reach all the way to the surface.  So though snow may start falling as early as 11 PM or so aloft in the atmosphere, it will likely be until 2 or 3 AM that the first flakes begin to reach the surface across Southwestern Connecticut as they will be evaporating before then.  At this point we can expect the snow intensity to be heaviest (I use that word very lightly) between 4 and 6 AM per the latest RAP guidance.  All accumulating snow will likely fall in that short range, and as I said at the coast we could see up to an inch.  Typically, that would be fine, but the snow, if it occurs, will be falling at basically the perfect time to cause a delay.  Granted, an inch of fluff likely won’t be worth a delay for many districts, but there remains a few at the coast that could choose to delay if roads are slick, especially as it will be so cold that roads will be icy regardless.  That’s really the only reason there is a 30% chance of a delay at the coast, because the timing with this storm could just be perfect enough to get a few districts to delay.  If the timing shifts a little bit the chance could very well go down like the inland one, but if precipitation amounts are expected to be a tad higher the chance could go back up.  At this point both the timing and the amount are equally important in determining whether any districts will delay due to this small amount of snow, and I’ll keep an eye on it and continue to report here.  Expect the next update around 9 PM.

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