Think Snow, Tweet Snow !!!

Saturday, December 27, 2008

NAO turning sharply negative - more favorable pattern will be the result !!!

In the meantime though we are watching the final chickens as they return home to roost in the form of a stretch of mild weather. I am actually feeling a bit fortunate today as it appears rainfall amounts will be much less than feared a few days ago. This mild weather was foreseeable more than a week ago and there was a time when it appeared that some heavy rainfall could accompany this balmy binge. Instead it appears as if rainfall will be intermittent and light and for this I am thankful. No doubt we will lose some snow as temperatures soar to near 50 Sunday and new snow will be slow to arrive in the wake of all this warmth but come in will and ultimately it will come in significant quantities.

Transition back to winter is gradual but two clipper promise results by New Years Eve
The return to the seasonable chill that typically grips the Green Mountains this time of the year will also be somewhat slow to return. Above average temperatures will continue to grip the mountain Monday and it will be mainly dry but by Tuesday winter makes a full return. The catalyst is a clipper system which is a bit short on moisture but with an adequate upper air dynamic punch is likely to produce a few inches of powder prior to Tuesday's opening ceremony. Tuesday morning will be the first of two quick hitting clipper systems and the second on New Years Eve appears more organized and more capable of producing significant snowfall. Snowfall from the second clipper should begin in earnest in the pre-dawn hours Wednesday and continue well into the ski day Wednesday evening. Several inches of snow could be the result so long as this storms track doesn't drift too far to the south (some recent model guidance has been drifting south with the storm track and we will continue to watch it). Overall the period between Monday night and New Years eve could very well consist of snowfall at varying intensities through the entire period but accumulating snow is likely to be limited to the early hours of Tuesday and the early hours of Wednesday. At least one of these days will turn out to be the honest-to-god powder day that we are both hoping for and deserve.

Negative NAO and its results
All of the above is a result of a pattern transformation, largely the result of changes downstream in the jet stream. On the SCWB we refer to the NAO index as a measurement of the amount of jet stream blocking is occurring across the northern Atlantic Ocean stretching from the Davis Straits east across southern Greenland. The most important result of a downstream block in the jet stream is that it becomes increasingly difficult for storms tracking across the U.S. to take a northern track through the Great Lakes and into the St Lawrence Valley. The block, in essence, forces storms that are tempted by such a fate to often reform along the east coast. This in turn keeps mild weather and thaws very well contained. There are times when strong NAO regimes can drift west and cause an outbreak of mild weather across eastern Canada and interior New England. Such was the case in early February of 2005 although the mild weather was accompanied by dryness and sunshine. In this case the end result will not be mild weather at least through the first 5 days of January.

Major storm for the weekend 3rd and 4th
The possibility was in fact mentioned in the last post of a major storm for the weekend of the 3rd and 4th of the month and this is becoming increasingly likely. More specifically the storm appears to be product of a some very intense Pacific energy which may have the capability of reaching into the Gulf of Mexico or Atlantic Ocean for moisture. This could turn the weekend in question (the first in 2009) into an incredible period at MRG since it arrives on the heels of the two clippers in the days prior to New Years Day. It is somewhat disconcerting to see such intense activity in the Pacific and the long range ensembles do suggest that the strongest area of unsettled weather could be out in the Pacific Northwest for the first 10 days of January. If the downstream block does evolve into a major driving force in the pattern I suspect that we can avoid the outbreaks of mild weather in the first half of January which has plagued us even during a rather snowy December of 2008.

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