Think Snow, Tweet Snow !!!

Saturday, December 6, 2014

Tuesday/Wednesday Nor'easter is happening but expected track shifts west

We're a couple inches closer thanks to some snow on Friday and Saturday and could get very close with a little help from some fortuitous "bomb-o-genesis" Tuesday and Wednesday. Every time a model shows an event like this I think of Bob Weir's solo effort "Bombs Away". The lyrics in the chorus are "Bombs Away (bombs away), I think I am back in love again". Nothing but love for Bob Weir, but it is not one of my favorite songs. I just can't keep it out of my head when a nor'easter shows up and why not. The lyrics are perfectly appropriate in regards to pre-storm anticipation and those lyrics will be a future tweet sometime but just not now. The uncertainty regarding development of this storm has vanished, it will happen and it will be big. Unfortunately, a big snow event at MRG is far from guaranteed. At the least though, the storm will bring some drama and if your a weather enthusiast get ready !

I referenced the great Nor'easter of December 1992 and there are some similarities with the upcoming storm. That storm also formed without much help from the polar jet and had limited amounts of cold to work with. The storm caused historic coastal flooding that in the last 30 years has probably only been surpassed by Hurricane Sandy. It also dumped 30-50 inches of snow on the Catskills and Berkshires (I can't quite remember how Vermont fared on that one). The lack energy from the polar jet can open the door for warm intrusions of air and a dreaded change to rain but it also means a very slow moving event. These are the very items that need to be discussed with the upcoming storm.

As of late Saturday we are starting to establish a consensus based on data from several medium range computer models. This consensus has actually shifted the track of this very strong storm farther west, particularly Tuesday evening. The European and Canadian model now suggest an inland track that allows the storm to track west of Boston. The limited cold would be at least partially flushed out and the region would have to endure a change to rain at least at low elevations late Feb 2010-style. The American model has been the last arriving guest to the nor'easter party but would suggest the most ideal solution - all snow, all the time at all elevations.

With the European/Canadian blend performing better with this event so far I am inclined to believe the "warmer" scenario which does allow for a period of rain at low elevations. The silver lining has to do with the aforementioned paragraph.  The storm will be a very slow mover and its final phase of maturation, the closing off of the low pressure system or occlusion, it will reposition itself along the coast and could allow for elongated period of light to moderate snow later in the upcoming week.

As for specific timing, precipitation will begin as all snow and become very heavy for a time later in the day Tuesday into Tuesday evening. By early Wednesday morning, warm ocean air would allow low elevations to change to plain rain while snow continues to fall near the summits of the Green Mountains. This type of storm will be incredibly elevation sensitive on Wednesday. By Wednesday night, once the storm begins to occlude, we could see a change back to snow. The snow intensity is unlikely to be heavy and will fall at varying intensities (we could also see a lull in all precipitation) but accumulations could be significant. Even with a warmer track, we still could see 6-12 inches of snow on the front end of this event and another 5-15 inches at the back. The rain at low elevations Wednesday is a "downer" but not an "outer" for this event. Total snowfall could still be over 20 inches, especially above 3000 feet.

Following the storm the region will have to endure a gradual warm-up and what could be some very warm days around the 14th to 17th of December. This is in response to the overall North American retreat of the cold thanks mostly to the severely tightened jet in the Pacific. Around the time of the Solstice the pattern might normalize, thanks mostly to a PNA ridge which is expected to form around that time. The southern branch of the jet though is clearly and very back in business this year.  This is a classic El Nino personality and this means lots of potential storms event with just a minimal amount of cold, very much like the one in front of us.

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