Freezing Rain to Ice Roads Monday

Freezing rain will fall across the region tomorrow before it gradually turns over to rain.  Fairly significant impacts are expected, and for an impact analysis on schools, including school delay/closure percentages, along with detailed model analysis and images, you may subscribe to our premium service here or purchase the individual forecast update posted at 9:15 PM to premium members here.  If not, we will outline parts of the most recent official forecast, including some piece of travel analysis that Premium subscribers have received:

There is a wide range of model disagreement on this storm, as short-range models generally have precipitation starting earlier and bringing more impacts than medium or longer-range models.  First, as this is an overrunning event in a gradient pattern, most global models normally underestimate the speed of the precipitation.  With the ARW/NMM showing some agreement with the RAP/HRRR, I am forecasting light precipitation to break out now anywhere between 2 and 5 AM in one burst, as precipitation will begin in a patchy manner.  A lull is even possible at the coast between 5 and 7 or 8 AM before the steadier precipitation moves in as freezing rain.  Between 9 AM and 1 PM, most of the region begins to turn from freezing rain to rain at a varying rate, and I expect that by 2 PM or so the majority of the region should be above freezing.  Most of the region by 12PM should be above freezing, but patchy freezing rain still looks likely, especially across areas such as New Canaan and valleys in the Danbury area, etc. per latest guidance.  The immediate coast could turn over to rain as soon as 9 AM, but that really is only within a few hundred feet from the coast as warm ocean air heats the region up; the rest of even coastal areas could be spared that warmer weather.  In terms of accumulations, some snow remains possible around and north of I-84 at the onset of the storm, as models indicate that lift will be greater inland and thus it will be easier for precipitation to start across inland areas.  Up to an inch of snow is thus possible inland, though I see no more than maybe a dusting of snow possible at the coast before a turn to freezing rain.  Then, across the entire region I see up to or around a tenth of an inch of ice as the most likely scenario.  I do not see really steady, heavy rain freezing, especially as if the rain gets too heavy only part of it freezes and the rest just runs off.  However, I am concerned that the recent cold has the ground and outdoor surfaces so cold that even if air temperatures rise up to 33 degrees or so, some surfaces will remain below freezing.  This will allow additional ice accumulations, and so up to or around a tenth of an inch of ice turns into an official forecast of .05-.15+ inches of ice across the region.  The difference here is not too significant in that either one causes serious problems, but should we see a colder trend and ice amounts of .15 or more become widespread then that is when we run into problems.  That is why, as expected and said yesterday, the entire region is under Winter Weather Advisories from the National Weather Service.

Any time I see a threat for freezing rain, I become concerned about travel.  Temperatures today likely will not get above freezing, and tonight even coastal areas will get down into the mid 20s.  This means that roadways will be very cold, and it will be difficult to properly even treat for this freezing rain.  As a result, as soon as the freezing rain starts we could be seeing trouble.  I expect inland road conditions are going downhill as early as 4 AM, and coastal road conditions go downhill between 5 and 6 AM.  This is a variable estimate primarily because of the precipitation timing issues among the models; an earlier start to precipitation allows roads to deteriorate much sooner.  Across the entire area, roads do not look to start improving until 11 AM or so, and inland this could wait until as late as 2 or 3 PM depending on the exact timing of the warmer air.  The latest ECMWF guidance indicated that Danbury could still be at freezing at 1 PM, an indication to me that road conditions could still be poor in that region even later than that.  By 3 or 4 PM I do expect road conditions to improve, as the approaching cold front will allow southerly winds to strengthen and pull warmer air briefly into the region.  7 to 9 PM look to actually be the warmest as the cold front begins to swing through, so any residual ice should briefly melt in that timeframe as the rain comes to an end.  However, between 10 and 11 PM the region looks to drop below freezing again, meaning any standing water will freeze as well.  This could result in icy patches on roads through the night tomorrow night and into Tuesday morning.

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