Largest Winter Storm Of Season Approaches

In my forecast from two days ago I began with a long, technical paragraph of why at the time I did not expect the storm to bring significantly amounts of snow.  The overall idea behind it was that energy would not phase in time, the trough would not tilt negatively, and in general there would not be enough energy to support the rapid strengthening of a low pressure center to our south.  Well, things have changed.  Almost all models show just that.  As energy moves across the southern half of the country it will link up almost just in time with energy from the north to create a more broad-scale trough that, when tilted negatively, will infuse the storm with baroclinic energy and allow it to strengthen rapidly.  Simplified version of what this means for Southwestern Connecticut: the storm will be stronger and bring more snow and wind than previously expected.  In this blog post I will first be outlining both the timing and types of precipitation that can be expected, and in there I will be detailing the overall dynamics of this storm and the pattern coming with it.  I’ll then go into a discussion of total accumulations.  I’ll end by detailing the wind threat, which at the coast is certainly more pronounced than I expected it to be a few years ago.  But that’s the beauty of these winter storms, they can change in a moment.

What we have with this storm is a double-barreled low pressure system that will be moving from the southwest to the northeast.  A weak primary low pressure center will move between the Ohio and Pennsylvania border, but will then quickly transfer all of its energy to a secondary low pressure center forming over Virginia and moving over Delaware before it continues on to the northeast.  Not a classic Nor’ Easter setup, but generally it is pretty close.  What is different about this than most setups, though, is that NAO and AO are both positive, and the PNA is negative.  What does this mean?  The pattern is quick, with few large-scale high pressures stalled out to slow it down.  This storm will zip right through, and a lack of blocking will keep the cold air from staying locked in over the region.  This is why even with a strengthening low pressure center and dynamic cooling it is likely that Southwestern Connecticut does not stay all snow, as mid levels of the atmosphere are going to still warm with the flow of air following the low from the southwest.  This is also why even though the heaviest precipitation waits to start until close to 7 PM, it will be on its way out by 5 AM or so, as this is not a long-term storm.  There is a chance that it snows for close to 18-20 hours, but the first 9-10 will just be a light, scattered powdery snow that is overrunning precipitation during the day tomorrow.  In terms of the real storm, it will likely only persist for 10-12 hours, not a very quick ordeal by any means, but there would certainly be a potential for this to be a true blockbuster if there was proper blocking and the pattern was better primed for a snow storm.  As it is, abundant arctic air and a lot of moisture will still combine to bring the region some very respectable snowfall amounts, especially as the two branches of the jet phase at a proper time to strengthen the low pressure center rapidly.

So what does this mean in terms of timing?  Well, light snow will likely break out between 7 AM and 10 AM tomorrow morning and fall on and off throughout the day.  Temperatures throughout the day will rise from the upper teens to the low 20s by sundown, and with upper level temperatures very cold as well any snow that will fall will be very high ratio, meaning up to 3-4 inches of snow are possible by 7 or 8 PM even though only around two tenths of an inch of moisture will have actually fallen.  Once the heavier precipitation moves in around 7 PM though, surface temperatures and atmospheric temperatures will have risen enough that ratios will be dropping closer to the standard 10:1 or so, and with sleet mixing in later they may drop even further.  So there will be up to 3-4 inches of snow on the ground, though the heaviest snow/sleet to fall later may compress that some so that it doesn’t really add up to that much by the end of the storm.  Heavy snow will continue across the entire from 7 PM until midnight for sure.  Then around midnight the mixing begins.  Sleet will begin to mix in from south to north between midnight and 3 AM, likely right as the heaviest precipitation begins to move on out, leaving behind moderate precipitation.  There is a chance coastal areas turn to freezing rain, and the latest NAM actually warmed up the surface just enough that the immediate coast could turn to rain for a period of time.  Again, this could compact snowfall so that accumulations are not quite as high as expected, but I will address this in the next part of the forecast.  If precipitation were to turn to freezing rain or rain, it would likely be south of the Merritt and occur between 2 AM and 5 AM.  North of the Merritt it is more likely to stay all sleet and if there is a last minute warmer trend there would be freezing rain, not plain rain, as temperatures north of the Merritt look to remain below freezing throughout the entirety of the storm.  By 6 AM or 7 AM at the latest precipitation will move out, likely ending again as sleet, and then the cleanup will begin during the day on Sunday.

Now we focus in on accumulations.  Models are coming into agreement with regards to total amounts of moisture with the storm.  General Quantitative Precipitation Forecast (QPF) amounts from weather model show anywhere from .9-1.3 inches of moisture falling across Southwestern Connecticut, with slightly more (maybe a tenth to two tenths of an inch or so) at the coast.  The GFS brought in too much QPF, and with the latest 18z NAM being at the lower end of QPF guidance I will weight it heavily because the model typically overdoes it in terms of moisture.  That leads me to believe that there will generally be about 1.1 inches of moisture at the coast and around exactly an inch inland.  Up to a quarter of an inch of moisture is possible in either direction around these totals, but that is based off of what I am seeing right now.  Still, this doesn’t tell us very much as what really matters is what the precipitation type is when this moisture falls from the sky.  First off, we are getting likely anywhere from 2-4 inches across Southwestern Connecticut before 7 PM when the heaviest precipitation comes in.  Snowfall ratios will be close to 20:1 at the onset of the storm, and the light fluffy snow will instantly stick and will pile up quicker than usual because of how cold the entire column is. Between 7 PM and midnight I expect an additional 3-5 inches of snow or so, as I see 5 hours with snow piling up at up to an inch per hour.  That’s a general total of 5-9 inches of snow.  Inland areas are likely to get another few inches before a turnover to sleet, while coastal areas also could see another inch or two of snow with sleet before sleet and maybe freezing rain/rain take over.  However, the much wetter snow at the end mixed with the sleet and maybe liquid precipitation could compact the snow as well, lowering total accumulations.  All of this was factored into the final forecast of 5-9 inches of snow at the coast and 6-11 inches of snow inland.  The snow compacting at the end with the sleet/rain is what could lower amounts at the coast down into the 5-6 inch range, though I still think that most areas get a little more than that.  Inland there will be slightly less compacting and a slightly longer period of snow, hence the higher amounts.  Some area could end up seeing a foot, so I have isolated areas up to a foot inland, but that likely will be north of Interstate 84 and on the border of the region that I forecast for.  Further north also has a better chance of staying all snow, which could make it easier to see a foot or more of snow.  Along with the snow, a period of freezing rain away from the immediate coast but still in the coastal area could result in up to a tenth of an inch of ice, with a glaze of ice also possible inland.  Again, even minor warming beyond what is currently expected could extend freezing rain inland, in which case more significant icing could occur which would bring much more severe travel problems.  At this time that is not expected, with snow and sleet expected to be the main precipitation types, but I will be watching that freezing rain threat closely.

The final threat I want to highlight is the wind.  With a stronger than expected low pressure center, the pressure gradient around it will be tightening fairly rapidly, resulting in potentially gusty winds.  As of right now I expect most wind gusts inland to only get into the 20 mph range and maybe approach 30 mph, but at the coast wind gusts could get into the mid to upper 30 mph range.  Winds look to peak between 10 PM Saturday and 4 AM Sunday, meaning that with the now conditions could approach blizzard-like.  If there is a turnover to freezing rain, the winds could enhance power outages cross the area, though again significant ice is not expected across the area.  Winds could cause scattered power outages, but even that is not too likely as they will not be dangerously strong, just noticeably strong enough to reduce visibility.  The coast will fare the worst with winds.  The main problems they will bring will only be noticed by motorists trying to drive through the storm, and they add just another reason why everyone should stay in if they can Saturday night.  Conditions will be quite dangerous overnight and there is no reason to risk it in the first major winter storm of the year.

In conclusion, this is a significantly storm that will be impacting Southwestern Connecticut, but it could have been worse.  A lack of upper level blocking will keep the storm moving and will ensure that there will be some mixing, especially of the coast, that will limit total snowfall amounts.  Still 5-9 inches at the coast and 6-11 inches inland is nothing to scoff at, and it will be falling in a relatively rapid period of time overnight, so it is advised that you prepare for the worst and expect to stay in Saturday night.  As always, I will have constant updates throughout the rest of the day today and throughout the day and night tomorrow both on the website here and on Twitter at @SWCTweather.  As the storm approaches, be sure to stay tuned to Southwestern Connecticut Weather Updates for the latest, most accurate regional weather forecasts!


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