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Thursday, December 26, 2013

Weather in the pre - New Years Day period "evolving"

We have a bit of powder covering the frozen crust and can now look toward the end of the weekend for some more potentially good news. The weekend weather has been "evolving" and the medium range models have more or less failed to indicate with any consistency how it will all play out. We had an idea that temperatures would at some point make a run at the freezing mark and we knew that by Sunday, we had a chance for some snow from a clipper system marking the leading edge of very cold arctic air that is poised to send temperatures below zero on the mountain by New Years Eve. Things have gotten more interesting with the southern branch of the jet stream, which a few days ago appeared as if it would have a negligible impact on the weather through at least New Years. With each cycle of model data however, there is more and more evidence that a low center with Gulf of Mexico origins will indeed move up the Atlantic Coast and impact New England with its moisture.

We discussed the idea of the limited amount of available cold air this weekend over New England. The current air mass will modify and the bitterly cold air will advance to most of the east coast a day after this system exits. Across coastal sections of New England, precipitation will fall as plain rain through much of Sunday. Moisture from the Gulf/Atlantic southern branch system pushes into Vermont just as energy from the clipper system begins arriving from the west. If the two storms were to phase, a bit earlier it would open the door for some incredible upside. Models right now are indicating that this won't quite happen, but we do have barely enough cold to support some snow late Sunday into Sunday night. Remnants of the clipper system and some lingering instability will help support some additional terrain induced or Champlain induced snow showers Monday. I think there will be some better clarity on accumulation totals in the coming days but snow Sunday afternoon, Sunday night and snow showers Monday should total at least 4 inches over that 24 hour period and perhaps upwards of a foot. Because this situation has evolved so much in just 48 hours, we have to leave some room in the forecast for some additional changes so I would expect some modifications in the coming days or so.

Bitterly cold temperatures will make a grand entrance Monday with the aforementioned snow showers. The region, as mentioned in previous updates, will be in the firm grip of arctic air through much of the New Years week thanks to the emerging western North American upper ridge and the negative Arctic Oscillation (AO). Snow could fall from a weak clipper system around the time of January 2nd. By the first weekend of 2014 there have been hints of some southern branch activity combining with the Polar Jet to produce a storm of significance but those hints were mainly yesterday. Today's models/ensembles suggested some additional snows from a clipper along with what should be the continuation of very cold weather. There are also indications that the position of the western North American ridge will retrograde toward Alaska allowing arctic air to loosen its grip on the region around the 7th or 8th of the month. This would go against my idea of the cold weather persisting through January 10th. The AO though is expected to remain negative throughout much of the first half of January, an important ally to have in the prevention of any damaging thaws.

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