Dreary December: Quick Succession 5-Day Storm Train

As has been very apparent on Twitter and on the side-bar updates that are updated daily (or more often, which has been the case recently), we have winter weather on the way for Southwestern Connecticut.  This is why the site is live again, why the Twitter account is tweeting about various acronyms you may or may not understand (they stand for weather models), and why this blog post is being written.  The pattern is setting itself up tonight, where the first wave of precipitation will move to the east of a large, extremely powerful cold front to our west.  This front essentially stretches all the way from Ontario to the panhandle of Texas, to put it in perspective, and the size and strength of this front are what will dominate the weather over the next few days.  This blog post will break down the 2-3 waves of low pressure that will move along the front as it stalls in the area over the next few days.

First, expect some light to moderate rain throughout the night tonight and into the day tomorrow.  Along with that, expect some dense fog.  With every strong cold front is a strong warm front out ahead of it, and that warm front has already moved through, bringing with it the mild surfaces you feel at the surface.  That warm air sped in and got trapped below some colder air aloft (which will also warm overnight, but more slowly) resulting in some very dense fog out there.  As surface temperatures fall tomorrow after the first wave, the fog will dissipate significantly.  The cooling surface temperatures will set the path for what I am calling the REAL first wave, or the first wave that could actually bring wintry precipitation to Southwestern Connecticut.

This first wave will begin tomorrow afternoon as rain.  All weather models agree on that, as the entire atmospheric column over SW CT will be above freezing.  Throughout the wave, though, temperatures aloft will cool fairly dramatically, with temperatures at the surface also dropping, but not quickly enough for them to get below freezing before precipitation ends.  Instead, we will see surface temperatures in the upper to mid 30s overnight tomorrow night with moderate to sometimes heavy rain.  But because temperatures aloft at different layers in the atmosphere will be cooling at different rates, I would not be surprised if sleet began to mix in, especially inland, around 2 or 3 AM.  Over the next few hours precipitation will end and temperatures will continue to drop, and there is a chance that a few remaining blotches of precipitation move through Southwestern Connecticut and bring a quick burst of snow and sleet.  As of right now, there is a chance of a coating or so inland from this band but no serious accumulations are expected, especially at the coast.  If there are last second cold trends tomorrow you will be seeing tweets and blog post updates about it I am sure, but at this time I do not see this causing many disturbances, and for the most part precipitation should be gone by Saturday sunrise except for maybe a few scattered snow showers.

What’s more important about this wave, though, is that it will help guide the cold front through, meaning Saturday it will be absolutely frigid aloft and noticeably colder at the surface (highs generally in the upper 30s).  Overnight Saturday temperatures will drop into the mid 20s, and they may go even lower inland, with temperatures only getting into the mid 30s on Sunday.  While this goes on, another wave of low pressure will try and succeed to break through the “frontal boundary” that will have stalled to the south keeping the region cold.  This will allow warmer air to slowly stream up north over the cold air in place, resulting in both warming surface/aloft temperatures and an increase in precipitation.  At this time, it appears most likely that precipitation starts between 7 and 10 PM across Southwestern Connecticut as light snow.  Temperatures will generally be below or right around freezing, so it should stick on contact.  We will likely see a couple hours of light to moderate snow, maybe accumulating up to an inch or two, before we see a changeover to sleet.  It is important to note that there is a LOT of model disagreement here.  The ECMWF and CMC international models have the region as snow for longer, and show the potential for up to 4-5 inches of snow.  The American models show a warm nose working in quickly aloft and thus a changeover to sleet earlier.  Past experience tells me this: the warm nose is more likely to be in line with what the American models say, but in terms of overall dynamics and storm progression, along with Cold Air Damming (what keeps cold air entrenched in the region even as warm air tries to kick it out) the international weather models have the upper hand.  This means that I do see a changeover to sleet within 2-5 hours of the snow starting, and that will likely cap snowfall amounts Sunday night at 2 to maybe 3 inches at most further inland.  I then see a fairly extended period of sleet for another period of 2-4 hours or so, and by this time we are approaching Monday.  Surface temperatures at the coast will likely be above freezing, and I think that the most likely changeover from mixed wintry precipitation to plain rain will happen between 5 AM and 7 AM.  It could happen sooner or later, this is just the sense I get from the strength of the cold air and the storm progression.  This means that some superintendents will have a hard decision to make as to whether open schools on time Monday morning or maybe issue a delay to avoid potential ice problems.  Further inland, I think the changeover could be delayed by another hour or so, with sleet ending between 5 and 7 AM similar to the coast but that there could be a brief period of freezing rain, as further inland the cold surface temperatures will be better entrenched.  The icy roads make it more likely that inland schools will have to have delayed openings.  After 8 AM or so, all of SW CT should turn over to all rain, and up to half an inch of rain is possible from this storm as the primary low moves to our north with a weaker secondary to our south.  Granted, if the secondary to our south turns out to be stronger than modeled, it could help keep the cold air in place longer, but I do not find that very likely at this time.

Of course none of this is guaranteed, and there are still huge model disagreements.  The ECMWF weather model tries to give SWCT up to 2 inches Saturday morning after rain and maybe up to another 3-4 inches of snow with the wave explained above, and the CMC sets up the rain/snow line right over the SW CT coastline for 3-4 hours on the latest run, which could be the difference between that 3-4 inches of snow in one town and a coating to a slushy inch in a town adjacent south.  As with any storm system where all four types of wintry precipitation (sleet, snow, freezing rain, and rain) are involved, there is a lot to try and figure out and the details will likely not be worked out until the day before.  If I can issue a high confidence forecast even within 12 hours of the precipitation falling, that is a win as that means I have a true handle on upper atmospheric and lower atmospheric dynamics and temperatures together.  It is too far out for specific details, but from my experience with storms of this nature I tried to outline what the most likely scenario is, and as the storm closes in and I will further refine what I outlined there.

I then come to the third wave that could bring wintry precipitation.  This wave will be Tuesday to Tuesday night, and is the lowest confidence wave as it is very possible that it completely misses the area.  This is also the wave that is most likely to be close or maybe completely all snow.  After the second wintry wave moves through Sunday night, some upper level energy will be left to the south  and it will try and make its way northeast along the stationary front, instead of breaking through it.  After that second wave and the strong primary low that caused it move through to our north, another weak cold front will move through restoring the colder air mass and allowing snowy conditions.  Now if this energy threads the needle properly along the front, we could be looking at a couple inches of snow on Tuesday, and the GFS weather model seems to show just that.  The issue is that the international models have that energy being whisked out to sea with no precipitation for Southwestern Connecticut.  What you can kind of see here is a “back up” from this wave, if the GFS ends up verifying and the second wave is also mainly rain for SW CT because it is warmer than expected, there is a chance of this third wave Tuesday to bring snow.  This is because the primary was stronger and thus the front moving through after will be more dominant for this wave to ride up along.  If the storm is colder like international models show and the secondary low pressure system to our south is stronger, then it is more likely the front won’t be as defined and the energy Tuesday will be sent out to sea.  It’s frankly too early to say which is more likely (even though I personally favor the out to sea scenario), those are the two scenarios that I am dealing with.

So that is the breakdown of weather over the next 5 days for Southwestern Connecticut.  This post is already far too long so I won’t expand on the long term much more than saying that I do see additional snow chances in the future past this (though there will be at least a 3-5 day break), but none of these look like blockbuster storms.  I see more of a pattern with smaller snow events, at least for the next couple of weeks.  But a pattern like that can mean that there are a lot of these small events and they can still cause numerous disruptions, so as always make sure to keep it here for the latest on weather throughout Southwestern Connecticut.  It’s great to be back!

One response to “Dreary December: Quick Succession 5-Day Storm Train

  1. Happy to see you back on the weather. We miss you and your reports.
    Keep them coming.
    Dee Hychko

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