Mid-Week Mess: Rain, Snow, or Combo?

We are following a very complex set of upper level energy moving across the country mid-week that has the potential to result in a moderate snow storm.  This blog post is going to detail the possibilities with the system, as there are currently 2 main scenarios; either the storm cuts inland/over us and the region sees mainly rain or the storm goes a little further south and the region sees a period of moderate to maybe briefly heavy snow.  This is not a “major” snow storm in that I don’t see the blocking in place for anything historic, but there is ample moisture so this could be a quick-hitting decent-sized snow storm for the region if everything comes together just right.  I’ll explain why it may be hard for everything to come together perfectly for big snows, but also why I think we are more likely to see at least some snow from this vs. an all-rain event like some weather models are showing.  This discussion will be fairly technical, so as always please consult the Weather Glossary should you have any questions about any of the weather terms used here.

One thing I’m putting in perspective here is the overall pattern of the past month.  There is a common phrase in weather that “snow begets snow.”   With the last storm, we saw a large suppression due to a polar vortex being stronger than expected and in a different position.  With this storm, there is no polar vortex interaction like in the previous one, and there are not too many similarities.  However, there is a trend for models to trend southeast in the medium range, and this supports the idea that areas that have received the most snow this winter will continue to see more.  Winters tend to be consistent; it’s how you can get winters with 8 or 80 inches of snow in the same area, and as this storm does not look to be so big that it “makes its own rules” or “reshapes the pattern” I think it will continue to follow in the overall pattern.  The winter has been fairly progressive; we have had a large number of storms but no real “blockbusters,” and this storm looks to only continue that trend.

Any expectation of storms should be based on more than just a theory of what prior storms of similar composure have done, though, and that’s why I’ve spent most of the day breaking down some of the most recent weather model runs.  To give a brief roundup: the CMC weather model shows an all rain event with maybe some snow showers on the back end, the ECMWF has almost all rain except maybe a few inches of back-end snow, the DGEX is similar to the CMC/ECMWF, the NAVGEM weather model has the storm almost entirely suppressed to our south with just light snow, the 12z GFS had almost all rain with some snow, and then the 18z GFS trended towards a heavy snow storm.  The ensemble means from each of the model runs seems to support the operational runs, though there is a massive spread in the GFS ensemble members (the ones we can see individually).  There seems to be a consensus thus for a more wet than wintry storm if you take these weather models verbatim, but as we learned with the last storm, taking the weather models verbatim can be very dangerous.  I’m going to go over why I believe a trend of the consensus closer to the 18z GFS solution (but likely not that extreme) is likely over the next 48-60 hours.  With the energy for the storm not even moving into a better data-region (the west coast of the United States) until Sunday night or Monday morning, there likely will be many models jumps and we won’t know a solution no matter what consensus seems to form.  Still, I’ve seen worse spreads with model solutions, and what I see here tells me that confidence is high in impacts of some sort from a storm, but the real question is whether it is going to be more rain or more snow.  That will come down almost entirely to low pressure track at the surface and interactions of upper level energy at 500mb that I’ll be outlining.

First, I want to go through some basic model understandings.  Every weather model has its own inherent biases: the GFS can be too progressive in an amplified pattern, the CMC is typically too amplified in any pattern, ECMWF can be too amplified in a progressive pattern, the NAVGEM is too progressive in almost every pattern, the DGEX is always too amplified as well, etc.  Applying a model’s biases to a scenario does not always give you the scenario, but you are more likely to find a more realistic forecast in the medium-term when accounting for them.  What this shows for me, though, is that the CMC solution and to a lesser extend the ECMWF are likely too extreme in how far north the low pressure is brought.  I’ll give a reasoning for this focusing on upper level interactions at 500mb too, but from model analysis we know they tend to over-amplify in progressive patterns like the one we are in and this likely results in the influx of warmer air in front of the low pressure and then the eventual track further northwest.  The DGEX is in the same camp as them.  Meanwhile, the NAVGEM is likely too progressive, having the storm be somewhat suppressed to the south.  That is not at all a realistic scenario I don’t believe, but the model showing it is support for a more standard snow event, vs. the solutions that the ECMWF and CMC have.  Those that have followed me for a little while know that I am not typically one to express much confidence in the pattern for a snow event and normally remain skeptical, but this storm I think definitely has the potential to be a moderate snow-maker based on what I’m seeing on the models and in the pattern.  The GFS, somewhere between the 12z warm solution and the 18z colder solution, I think is the most likely range of scenarios we are dealing with here.  I am not sure the storm will be all snow for SWCT, but I find an all-rain scenario highly unlikely, not just because of what models verbatim are showing when accounting for their biases, but also because of the upper level pattern, which I’m going to be getting into.  I think somewhere just southeast of the current ECMWF ensembles is the most likely track if I had to pick one, but with such a variety of models it is quite difficult.  Models do seem a little too warm on the front-end of the storm, at least based off of what I’m seeing aloft.

So what is this 500mb pattern I keep talking about?  Essentially, think of it like the jet stream aloft that dictates how disturbances throughout the atmosphere move.  Height differences at that pressure in the atmosphere create troughs and ridges throughout the atmosphere that can move disturbances down to the surface.  It is a piece of energy in that level of the atmosphere that will move onshore late Sunday into Monday morning in the western United States, and once we properly data sample then I expect model accuracy to increase, though it will be a slow process.  One encouraging sign I am looking at with regards to storm track has to do with the placement of a ridge in heights over Idaho.  This is standard placement of a ridge at 500mb for cyclogenesis of a low pressure system somewhere between Cape Cod and the 40/70 benchmark that we use in the Atlantic that is the “perfect” location for New England snows.  Recent GEFS runs, the GFS operational run at 18z, and even recent CMC ensemble mean runs have shown the ridge placed well come 8 AM on Wednesday.  Can this shift?  Certainly.  But as it is now I think that in a progressive pattern the placement of a ridge with higher heights over Idaho is a big signal for a trend southeast with the surface low pressure center.  Models will certainly struggle with the storm because of how complex it is at 500mb, though.  There are 3 pieces of energy, one over Texas and in the southern stream, another that will move through Colorado with a piece getting left behind, and a third being a trough of energy moving down through Minnesota into the United States.  If all these pieces of energy “phased” together, we would see a massive storm likely inland and an all rain scenario.  Right now, models do not show the southernmost piece of energy phasing at all.  This is expected; it is rare we see energy in that stream phase with the polar jet.  That leaves two other pieces of energy that will likely at least partially phase.  The sooner they do, the more they pull the low pressure center to the northwest, and the more likely it is that we get rain.  As has been the trend this winter in a more progressive upper level pattern, it is not expected for the phase to happen that early.  There will be at least a partial phase I think that will allow the low pressure to rapidly deepen, but I think it’s late enough that the low pressure center moves to our south.  There may be enough warm air at the lower levels of the atmosphere that the storm cannot pull in enough at first, resulting in rain, but back end snow is very likely, and there’s a chance that there will be front end snow as well as the cold air moving in earlier is not that weak.  This is why I am worried about a snowier scenario for Southwestern Connecticut than a few weather models are showing verbatim.

Overall, we have a complex storm setup here (haven’t heard that much this winter, have you?)  With many factors, the scenario is far from certain, but I wanted to break down what trends I think we may see in the future with the storm and what my current thoughts are without getting TOO technical (though I realize this blog post was about as technical as they come).  A storm is coming, and impacts are very likely to be felt.  I am leaning towards a snowier solution, but there could be more rain than snow with this storm as well.  Just makes sure to stay tuned for the latest as we continue to follow the latest model runs and trends to keep you ahead of the incoming storm.

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