Thursday Thrasher: Heavy Snow, Wind, Rain Expected

There is just so much to cover with this storm that I almost don’t know where to begin.  The orientation of this blog post will be fairly straightforward; I’ll be breaking down each aspect of the storm in its own elongated paragraph.  The sections will be as follow: Introduction/storm pattern, timing/precipitation type, accumulations, school impacts, winds, technical/involved discussion, future pattern (time permitting), conclusion.  As always, similar themes will result throughout to tie the forecast together, but in case there is any one aspect of the storm that you are most interested in I will divide it that way.  I will try and cover all aspects of this very dynamic storm and update this forecast as always, but if I happen to miss anything please Tweet/comment at me and I will do my best to cover any other material.  Let the fun begin…

INTRODUCTION/STORM PATTERN: While I typically don’t spend the introduction to a forecast laying out all the factors going into the forecast, I think it is especially important here because it is the current surface analysis that is really dictating this storm (as always).  This surface analysis is unusual for a couple of reasons I will outline, though.  The first is that it is very tightly packed with storms.  This has two consequences.  One: there is a chance for moderate snow right after this storm Friday night into early Saturday morning as an Alberta Clipper moves through.  Time permitting, I’ll get more into that in the end of the forecast.  Two: This does not allow a full phase of the polar jet stream and there is no properly established high pressure to the north, thus disallowing a fresh flow of cold air to be pulled into the storm.  This is the main reason why we are seeing most models show mixing problems for Southwestern Connecticut; there is no proper inflow of cold air.  The Clipper behind the main storm system hitting Thursday will prevent the cold polar air from moving downward, and instead we are left with stale cold air that is in the region currently.  This air is generally sub-freezing, but the storm system is also coming from the Gulf of Mexico and then the South Carolina coast, not exactly frigid regions.  It is this confluence of warm air with stale cold air that I think will result in very heavy precipitation, but at the same time it is why mixing is such an issue.  Another issue with this tightly packed storm system is that while the storm will rapidly intensify and slow down near the region, it will not stop or stall for long periods of time.  Yes, for reasons I will explain I think this will be the most significant winter event of the SWCT winter season, but it could be worse.  The track may end up being fairly similar to the Boxing Day Blizzard of 2010, but there is not quite the high pressure blocking there was with that storm to slow the storm system down and stall very heavy snow over the region for as long as happened in that storm.  In some ways, the storm has similarities to the Blizzard of 2013 (Nemo) last year with an originally less defined low pressure system but very impressive moisture transport (at least according to some short range models).  That is an analog that will be considered, but each storm is its own beast.  With the stage now set as to why we are dealing with warmer air than usual in such a large storm in mid February, I’m going to delve into the actual forecast.

TIMING/PRECIPITATION TYPE: With the start of the storm around 36 hours out, you would expect some more model agreement with regards to the timing of the storm.  Sadly, I would be lying if I were to say that I could pinpoint an exact hour that I expect snow to start across the region.  There are huge differences between models, with some having snow start near midnight, and others waiting until closer to 6 or 7 AM.  The current forecast is for snow to start between 2AM and 4 AM across the region, and I have moderate confidence in that starting range.  The ECMWF weather model wants it to start even earlier, but it is an outlier in its current data output.  Most weather models seem to agree 2-3AM is the best range, though a slightly more amplified storm could take a little longer to bring precipitation to the region.  Either way, I expect by 4 or 5 AM moderate snow will cover all of Southwestern Connecticut.  This moderate snow should then become heavy by 6 or 7 AM, and the morning rush on Thursday will be severely impacted.  The storm is virtually guaranteed to stay all snow through noon/1 PM, which is the first point that I could see a mixing with sleet.  Sleet and rain will mix in for the coast around 2 PM or so, and maybe wait until 3 PM inland.  Precipitation remains heavy through the evening with a mix of snow/rain/sleet depending on how heavy precipitation is and exactly how strong the low pressure is (in the technical section I’ll get into why I favor more snow than sleet/rain for most of SWCT here).  Back end banding from the rapidly strengthening storm will then move through around 6 or 7 PM as precipitation turns back to all snow, and then all snow will wind down fairly rapidly between 9 PM and 11 PM.  I expect that by midnight or 1 AM all snow will end across the area, and that skies will clear up by Friday AM.

ACCUMULATIONS: This is the lowest confidence part of the forecast because there are 3 different things to take into account here: amount of liquid equivalent that falls with the storm, amount that falls as snow vs. sleet/rain, and what the snowfall ratios are with that which falls as snow.  Here’s what I’m looking at now: we know the storm is going to be all snow through 12/1 PM.  Models generally look to show half an inch of liquid through that point.  Some a little bit more, some a little bit less, and some (the overzealous GGEM, for example) a lot more.  But what this means is that at this point I am confident in the region seeing at least 6 inches of snow through that point, though it could be significantly more.  Assuming sleet/rain mix in and limit accumulations there for the coast, that is 6 inches of snow.  However, I am favoring a slightly colder/snowier solution due to the intense dynamics of the storm, meaning I think that snow could be heavier/continue for a few more hours.  Plus, we have a few more inches that will fall with intense banding as cold air rushes in behind the storm (low pressure systems pull down the best cold air behind the system).  This means that even at the immediate coast a foot of snow remains very possible with what I am seeing.  Inland, I expect that conditions will stay snow at least for an extra hour or two, resulting in likely an additional 2 inches or so of snow compared to the coast.  The result: a snowfall forecast of 6-12 inches at the SWCT coast and 8-14 inches for areas a few miles or more north of the Merritt Parkway.  Again, this is a VERY low confidence accumulation forecast.  In many ways, this is a low end forecast of accumulations, because there is the potential (I’ll label it at 30%) that the storm stacks and strengthens to a level (I’ll explain those more in a bit) that it pulls in enough cold air that most of the storm says snow.  I’ll assume that average snowfall ratios are 10:1 for the storm (though they may vary from 9:1 to 11:1 depending on specific surface temperatures), and modeled liquid amounts range anywhere from .9-2.5 inches of liquid.  Realistically, I see 1.5 inches of liquid for the area with strong moisture transport from the south and intense convective banding.  Assuming that the storm was all snow, that is around 15-16 inches of snow or so, above both of the forecasted ranges.  Thus, if the storm ends up being all or almost all snow, I would have to fairly significantly raise the forecasted snowfall ranges, but currently I think that a sleet/rain mix due to the stale cold air will hold down snowfall accumulations, at least for a period of time.  The further inland you get, the higher amounts you get, and it is north of Interstate 84 that I think sees the best chance at getting more than a foot of snow.  So that’s where I stand with accumulations, as I have 80+% confidence that the entire region will see at least 6 inches, it is how much more that we see, and there is a chance it is a lot more.  I will be updating the accumulation amounts as necessary as I follow high resolution detailed model guidance.

SCHOOL IMPACTS: With 6 inches of snow guaranteed to fall between 2-4 AM and 12/1 PM on Thursday, it is forecasted and expected that all school districts that are currently in session will close across Southwestern Connecticut.  Snow will be very heavy during the morning rush and will still be heavy when schools would be letting out, meaning I do not see a viable solution in which schools open.  Questions begin to arise when we focus in on Friday, as snow could continue until 12 PM or 1 AM and many towns are running very low on sand/salt to treat roadways.  At this point, delayed openings for most districts, either inland or for the cities along the coast are expected.  Depending on the severity of the storm, there could also be closures along the coast.  If the high-end amounts of 10-12+ inches of snow are realized for most of SWCT, closures Friday could be seen as well.  This will be watched closely, so stay tuned.

WINDS: One good aspect of the storm is that Southwestern Connecticut looks to avoid the worst of the winds.  Those will be seen most in eastern New England.  That does not mean the storm won’t be gusty, though.  At the coast I see sustained winds of 20-25 mph, inland sustained at 15-20 mph at the peak of the storm.  Inland gusts to 30 mph are possible, and at the coast gusts could reach or exceed 35 mph.  I expect Blizzard Watches and then Blizzard Warnings to be issued for parts of Long Island and maybe parts of eastern New England.  At this time, I do NOT expect Blizzard Warnings for Southwestern Connecticut as I do not believe gusts to 35 mph will be common enough to warrant them.  A chance in track of the storm or an intensity to the level that the GGEM/CMC weather model shows, which is not quite expected, could warrant Blizzard Watches/Warnings, but I do not see that as necessary at this time, especially inland.  I would say inland areas have around a 25% chance of Blizzard Warnings, coastal SWCT is closer to 40%.  They may be issued, but with wind gusts only occasionally getting to 35 mph or so I am not yet expected them to be needed.  Still, isolated to scattered power outages associated with heavy, wet snow and then maybe rain are a possibility, and I bet this will be outlined in the Winter Storm Watches/Warnings that the National Weather Service will be issuing.

TECHNICAL/INVOLVED DISCUSSION: This blog post is already longer than I expected, so I won’t go TOO in depth here and will instead save that for another post that I may release tonight.  What we have with this storm is a rapidly strengthening low pressure center that will originally be elongated as part of it extends northward.  This is a prime signal for very heavy precipitation to take over from the beginning, and some models are picking up on that well.  An upper level low at 500mb closes off, after lows at 700mb and 850mb lows close off, all to accompany the defined low pressure center at the surface.  This is what we like to call a “vertically stacked” low pressure center, as there is a defined point of low pressure throughout all layers of the atmosphere.  This helps the storm at the surface to rapidly strengthen as air can rapidly rise through the entire column, resulting in very heavy precipitation and intense dynamics.  This rapidly rising air and intense dynamics are also what can often cool the atmosphere slightly more than models forecast.  We have seen this a number of times through the last few years.  That is why I am forecasting more snow than some, and why I see the potential for a mainly snow event in SWCT even though only one model (the highly inaccurate NAM) shows that scenario.  I will say, though, that the NAM is one of the best equipped models to pick up on dynamics, though this far out I don’t trust its placement of banding, etc.  Anyways, it is the vertically stacked low pressure movement that could induce dynamic cooling and thus result in more snow with slightly lower boundary layer temperatures.  A lot of this also depends on exactly where the 850mb low pressure moves.  If it stacks perfectly with the surface low pressure center, then expect to remain mainly snow.  The storm is going to rapidly intensify, meaning the temperature gradient around it should crash in.  Models seem to expect more warm air will be pulled into the storm because of the weak cold air onshore in our region, but storms of this nature can “manufacture” their cold air by the amount of rising air they produce, like I stated above.  When low pressure strengthens to the levels of this one as quickly as it will (sub-990mb in 12 hours or so) we often see cold air pulled into the center and warm air not able to enter as far on the east side.  The low pressure center will pass fairly south of Southwestern Connecticut, hence why I am inherently skeptical of so much mixing.  Yes, I do expect a period of sleet and maybe rain, especially at the coast, but I think many weather models are extending it too long because I think the temperature gradient and the freezing line sets up much closer to the extremely strong low pressure center.  It is rare that such a strong storm in February, even without a very strong cold air pool, allows warm air to flood itself, especially as sea surface temperatures off the coast are not extremely warm right now.  That limits dynamics a little bit but also should weaken the warm air trying to flood in from the south and the east, and these are things that only really short range weather models are able to pick up on.  That is why I am concerned about high end snowfall accumulations and why I could see a southeasterly jog in the rain/snow line during the storm that could have major ramifications.

FUTURE PATTERN: This update is already delayed half an hour due to the amount of information it contains, so I won’t go much in depth here.  Basically, after this storm we have two vigorous Alberta Clippers moving in.  One looks to bring light snow into the region Friday night into very early Saturday morning, and another could impact the region Saturday evening into Sunday (though with these storms, it all depends on which one gets amplified).  I do not see any major storm threats directly after these, but expect a few small threats this weekend.  These are what are keeping this storm from being all snow, though, so you win some and you lose some.

CONCLUSION: An extremely dynamic storm is moving into Southwestern Connecticut, one that will likely be the most intense of the year, but which is also proving incredibly difficult to forecast.  6-12 inches of snow/sleet are expected along the coast and 8-14 inches are expected inland, with higher amounts the further inland you go.  Schools are expected to close on Thursday with residual delays Friday likely and isolated residual closures on Friday also possible.  Winds will be gusty, but Blizzard Warnings are not quite expected.  Winter Storm Watches will be issued shortly, as snow starts between 2 and 4 AM on Thursday, and lasts throughout the day, mixing with sleet/rain in the afternoon, and then ending as all snow Thursday evening.  I will have continuous coverage of the storm throughout the rest of the day today, throughout Wednesday, and throughout the day on Thursday as well.  Make sure to keep it here for the latest forecast breakdowns of your local weather in Southwestern Connecticut as I do my best to keep you ahead of the storm.

5 responses to “Thursday Thrasher: Heavy Snow, Wind, Rain Expected

  1. Great blog. I loved reading every word., very informative. Thank you!!!

  2. Thank you for your detailed information — your reports are my best source for accurate weather in CT!

  3. Wondering when the winds will be at their peak, and how doe the winds look for Friday morning. I am trying to get out there for some sailing activities on the snow and I would like to know when I should head out.

    Thanks for the great storm info.

    Mark b

  4. Love this information, thank you so much for your hard work. It is great to understand the reason behind the forecast.

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