Wicked Wednesday: Heavy Snow, Ice Coming; SNOW DAY CHANCE: 80%

Lots changing with a very dynamic storm.  Many forecast changes to cover, and sadly not enough time to go over everything I want to.  And a storm on Sunday that is causing forecast headaches of its own, looking like it may not significantly impact the region.  Here we go.

This blog post is going to start with an updated timeline of impacts for the region.  I’ll delve into accumulations by time then, and then in the next section will go into total accumulation changes based on specific weather models and QPF amounts.  I’ll go through a theory on “mixing out the warm layer” that models such as the RUC seem to support, though there is not very much other model support for it at this time.  I’ll finish by detailing why I am so certain school districts will close, and why the 80% chance could easily rise to 90-100% by the end of the night, and I’ll advise districts to just close tonight so that they do not need to go through the trouble of waking up and deciding tomorrow morning.  No matter which of the scenarios play out tomorrow, roads will be so treacherous that there is no available scenario for schools to open.  Of that, I am certain.

Let’s begin with timing.  Snow will break out across the region between 1 and 3 AM from west to east.  Within 2 hours of the snow starting it will be reaching rates of an inch plus per hour.  There is an extraordinary amount of moisture with this storm, and before sunup we will be seeing the snow pile up at rates similar to those in the last storm, if not even faster as surface temperatures will be beginning below freezing.  This means that snow will stick to almost all surfaces immediately, and whatever it doesn’t stick to immediately, like treated roads, it will cover within an hour or two as it will be falling at a rate that no salt or sand could melt it.  This is where things begin to get interesting.  Recent modeling has shifted more towards the NAM idea from yesterday, the one that I was not as sold on and did not think had as much credence, where snow begins to mix with sleet at the coast as soon as 6 AM.  I expected once the models sampled the data and snow pack it would reflect a slightly colder shift.  This does not appear to have been the case, with models converging onto a slightly warmer solution.  While I am not totally sold on this scenario, I feel it is prudent to update my forecast accordingly to reflect this new trend.  So the new going forecast is for sleet to begin mixing in with the snow between 6 and 8 AM at the coast and between 7 and 9 AM inland.  The further north you are, the longer you will be all snow, and the higher your accumulations will be.  Between 8 and 9 AM at the coast and around 9 or maybe 10 AM inland the snow/sleet mix will turn to more of a freezing rain/sleet mix with occasional bursts of snow in the heaviest convection.  Sleet accumulates much slower than snow because it is ice pellets instead of snow flakes piling on top of each other, and once we see the changeover to sleet/freezing rain between 8-10 AM across SWCT snowfall accumulations will likely be over for the storm.  I’ll get into accumulations in the next section.  From 10 AM on I expect the sleet/freezing rain will go over to plain freezing rain by 11 AM or 12 PM for most of the region except maybe the northernmost sections.  There is a chance that between 1 PM and 4 PM temperatures rise to 33 or 34 degrees at the immediate coast, meaning plain rain could mix in, but temperatures will be so close to freezing and will have been below freezing for so long that I doubt it will have much of an impact.  In general, I expect all of Southwestern Connecticut to get only frozen precipitation, whether it be snow, sleet, or freezing rain, for the entire storm.  Most precipitation should wind down between 1 and 3 PM with maybe some freezing drizzle continuing into the evening in areas.  Temperatures will then drop into the upper teens or low 20s overnight, meaning ice will still be an issue into tomorrow night and Thursday morning.  So, to summarize, snow breaks out between 1 and 3 AM tonight, mixing with sleet between 6 AM and 8/9 AM from south to north, then mixing with freezing rain from 8AM-10 AM from south to north, then going over to all or almost all freezing rain for a period of time after 10 AM before precipitation winds down.

Now I’m going to get into my adjusted snowfall forecasts for the region.  Due to the slightly earlier mixing with snowfall, I am not entirely confident that all of my snowfall forecasts will be met, especially at the immediate coast.  The old forecast was for 4-8 inches of snow at the coast and 5-10 inches of snow inland.  I doubt we will be seeing 10 inches of snow, even further inland, and I struggle to see a scenario in which coastal areas south of the Merritt get to 8 inches of snow from the storm.  I’ll be revising each down slightly, with coastal SWCT now forecasted to get 3-7 inches of snow, and inland SWCT getting 4-8 inches of snow.  If you go far enough north of Interstate 84 then amounts more than 8 inches are possible, but once you go that far north you are out of the zone that I forecast for so I am not going to focus on that very much.  Now, some models show as little as 2-3 inches of snow before sleet mixes in, but for a number of reasons I do not believe that.  The main reason is because of what I mentioned earlier with a “mixing out” of the warm layer.  What happens in storms like this with intense convection and a LOT of moisture is there is a lot of upwards and downwards motion in the atmosphere.  This is what creates the heavy precipitation we will see.  When air is moving up and down quickly, temperatures between layers in the atmosphere can sometimes mix together and moderate.  So, with very cold air up in snow growth regions, a warm layer between 800mb-850mb, and another cold layer down by 925mb in the atmosphere, I believe there could be enough motion to “mix out” the warm layer from 800mb-850mb for a little while.  This is why, while sleet may mix in by 6 or 7 AM, I think that snow could continue a little later into the morning.  The RUC short range weather model shows this well, saying that snow could continue until 10 or 11 AM in areas even though temperatures at regions of the atmosphere look like they could be slightly above freezing.  While sleet/freezing rain will be dominating, it is that time frame from 8 AM-11 AM where the really intense periods of precipitation could mix out the warm layer enough to support heavy bursts of snow.  This is not a guarantee, but it is a scenario like this that could help the higher range of my snowfall forecasts be realized, and that is why I still believe there is at least a chance that coastal areas see 6 or so inches of snow and inland areas could see more than that.  If short range weather models continue to pick up on this possibility or show increased cold air damming then I may revert back to the old forecast, but I wanted to at least update the snowfall forecast to show you the trends that are going on with recent weather models.

Even though snowfall amounts were slightly lowered, I would argue that in a storm like this that means that impacts will be even worse.  Freezing rain/sleet/ice accumulation amounts are being updated for the coast to go anywhere from .15-.35 inches.  This, along with the heavy, wet snow that will fall beforehand in the heavy convection, could be enough to bring down power lines and cause isolated to scattered power outages.  I will be following that throughout the morning tomorrow.  Along with that, roads will be extremely treacherous tomorrow, first covered by snow, and then covered by sleet, followed my a relatively thick layer of ice.  I cannot remember the last time coastal CT was at risk of seeing more than a quarter of an inch of ice, so branches on trees may come down and roads will be extremely slick.  This will continue into the early afternoon.  Further inland, I think freezing rain amounts will range more from .1-.3 inches, with the higher amounts a little further south, as areas further inland will be seeing more snow/sleet and not quite as much freezing rain.  Still, even a tenth of an inch of an ice is enough to make roads even more treacherous than snow, and that is why travel is not advised tomorrow unless ABSOLUTELY necessary.  There is a chance that the inland ice forecast will be adjusted if a few models, like the ECMWF, are correct and the cold air aloft puts up more of a fight inland, which could mean mainly sleet and snow inland.  That’s one of the many things that I am following in this highly dynamic forecast, and of course I will keep inland folk updated on that.

In terms of schools, at this point I am virtually certain that every school district in not just the county but the state will have to close.  Winter Storm Warnings are up for the entire region from midnight tonight to 6 PM tomorrow.  If there is one thing experience has taught me it is that there is no district in SWCT that will mess with ice/freezing rain, the most dangerous of all precipitation types for roadways.  I expect most school districts to close tonight unless there are extremely remarkable warming trends on the models showing that temperatures at the coast will rise above freezing by 8 or 9 AM (less than a 20% chance of happening, and will likely be 0% by 8 or 9 PM tonight).  If you live north of the Merritt Parkway, you have a 100% chance of a snow day.  I can guarantee you that your schools will close.  If you live south, I can virtually guarantee it as well, but with dynamic forecasts like this one a lot can change quickly so I want to wait for maybe one or two updates before I guarantee it.  It is forecasted, and at this point you can begin to plan not going to school tomorrow and get whatever errands you need done now.  Roads will be icy/snowy all day tomorrow and there really isn’t a point in the day where travel is recommended, though after 3 or 4 PM would likely be better than any earlier.  With freezing rain, sleet, and snow continuing region-wide through at least 12 PM, schools will be close en masse.

As a reminder, travel goes downhill tonight around 2 or 3 AM and does not get better until late tomorrow afternoon into the evening hours.  Honestly, do not travel tomorrow unless absolutely necessary, and begin making preparations now.  This is not a normal snow storm, where taking it slow will get you were you mean to go; ice storms of this nature are especially treacherous and that is why I am using such strong language to emphasize you to avoid the roads.  I am not trying to hype up this storm or scare anyone, I just can say from experience that ice storms like this one are the ones people underestimate the most.  It may look like it is raining outside, but if temperatures are below freezing (which they very likely will be) that rain is freezing on every surface it falls on.

CONCLUSION: Travel tomorrow is discouraged as snow starts between 1-3 AM from south to north, mixes with sleet between 6 AM and 9 AM south to north, then freezing rain mixed in somewhere between 8 AM and 10AM, taking over completely at some point after 10 AM except maybe pretty far inland.  At the coast, snowfall amounts of 3-7 inches (a wide range to account for potential snow bursts in the sleet/freezing rain and because of it being difficult to nail timing of the mix) and ice accumulations of .15-.35 inches is expected.  Inland, the snowfall range is 4-8 inches with .1-.3 inches of ice being expected, though there is a chance a few regions will stay mainly sleet and not see very significant ice accumulations.  A third an inch of ice is something coastal SWCT has not seen in a long time, and though at this time not likely (I expect most coastal areas to see .15-.2 inches of ice) there is the high-end potential if the atmosphere warms just a little faster than most models show.  I’ll have more throughout the day on this storm and will go more in depth on the Sunday storm this evening when I have more time, but I would not focus on it just yet as some models even have it going out to sea. I’m forecasting a hit, but we can deal with snow, it’s the ice tomorrow morning that I’m worried about.  Keep it here for the latest as I break down this storm.

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