Monday Ice Storm Update

Premium members recently received a detailed forecast outlining the potential for significant ice and snow accumulations with the storm tomorrow.  Due to the potentially dangerous nature of this storm, a large piece of the forecast has been posted below to make the public aware of numerous hazards.  To subscribe to our Premium service and receive detailed forecast updates emailed directly to you, please click here.  Otherwise, the relevant parts of the forecast are attached below:

First, with accumulations, the overall forecast remains on track with a few small tweaks.  We are looking at 5-10 inches of snow/sleet inland with 4-8 inches of snow/sleet at the coast.  Additionally, the entire region is looking at up to .1-.25+ inches of ice, with the larger ice accumulations at the coast than inland.  Inland looks to remain primarily sleet, especially up by I-84, after the turnover from the snow, while the coast will go from snow to sleet to freezing rain before turning back over to snow right at the end of the storm.  The “+” is in the ice forecast due to a number of variable factors with ice accumulation that I will get into a little later in this post; I do not expect widespread icing above a quarter of an inch, but there are a couple worst-case scenarios where that is possible, and I want to highlight them there, as I would say that there is around a 20-30% chance of a severe ice storm.  That is my largest concern overall with this storm.  The good news is that sleet looks to be the predominant precipitation type over freezing rain at the peak of the storm, especially inland, which means more likely than not we do miss the worst of the icing.  So thus the accumulation forecast remains relatively unchanged, and the snow/sleet accumulation forecast is actually fairly high confidence with the upcoming storm.  It is the amount of ice with the sleet/freezing rain combo that is lower confidence, and thus it is hard to know exact impacts as it comes down so much to amount of freezing rain vs. sleet.   Both cause travel impacts for sure, but freezing rain is undeniably worse.

Light snow breaks out between 6 and 9 PM across the area.  This is mainly just snow showers in western areas, and may not even accumulate.  It is not until midnight or 1 AM that the main, steady precipitation shield moves into the region.  By 6 AM coastal Westchester County is turning over to sleet/freezing rain, and by 7 AM the changeover is in coastal Fairfield County.  It takes until 8 AM for coastal New Haven, and then the changeover works inland from there.  By 9 AM, all of Westchester, Fairfield, and New Haven Counties look to have changed over from just snow, with a snow/sleet mix inland and a sleet/freezing rain mix at the coast.  The mix inland fairly quickly goes to all sleet, while some coastal areas could see all freezing rain.  Now, one big question is how cold surface temperatures remain at the coast through the storm.  Given the extensive cold air up to the north and the surface low pressure passing to our south, I expect surface temperatures to remain below freezing throughout.  Hence the icing concerns I have highlighted from the beginning.  By 10 or 11 AM, the heavy precipitation finishes moving through and we begin to get a lull.  However, around noon or 1 PM we get another round of precipitation to move in with surface temperatures even colder due to winds changing to the north now, and so additional icing will be possible until we turn back over to snow between 2 and 3 PM.  Then, snow ends sometime between 4 and 7 PM, though some light snow showers could last even longer. We are only looking at an additional inch of snow on the backside of the system to accompany a general 4-7 inches on the front-side for the region.

Thus, the two main variables for determining icing here are surface temperatures and intensity of precipitation.  We are confident on intense precipitation between 7 and 10 AM, with a brief lull having just light precipitation (all freezing rain will freeze there because it is lighter, creating no run-off) and then the early afternoon could have one more heavier burst of freezing rain before that ending burst of snow.  Inland areas are more likely to end as a burst of snow than the coast, but snow could wind down to the coast at the end.

The final impact assessment, detailed more to Premium subscribers, is for power outages, and this is the one that is hardest to nail down.  In worst case scenarios, we are talking about scattered to locally widespread power outages due to the icing nature.  Ice over .3 inches, especially approaching half an inch, can create numerous power outages.  Ice in the .1-.25 inch range, which I still believe is most likely, can cause isolated to widely scattered power outages, but not quite as bad.  Luckily, winds with this storm will not be too strong, mainly due to the close passage and elongation of the surface low pressure center, so this will keep power outages from being widespread.  This is a threat that could become a serious problem, but right now does not look nearly as bad as it could be in worst-case scenarios.

A final Free update will be available sometime late this evening, so please stay tuned.

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