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Sunday, March 8, 2009

Some wet snow for Monday, rain on Wednesday and then winter regains control of Vermont

The National Weather Service has a headline today talking about the "weakening La Nina". We did see sea surface temperatures throughout the winter drift into the La Nina category for a time but it was about half the strength of the La Nina in 2007-2008. Not inferring anything about the National Weather Service but headlines can be subjective and I like to include the quantitative part as a way of providing the proper context. Vermont is losing snow pack but is managing relatively well in a very adverse longwave pattern. MRG will have to endure some rain Sunday night but it may not amount to much. Furthermore, dewpoint temperatures are falling as drier and cooler temperatures build across the region. Drier air in particular has the impact of lowering the wet bulb temperature or the temperature at which 100 % humidity can be achieved. By early in the day Monday, precipitation at most elevations will be snow. Valley locations up through the MRG base can expect mostly a wet snow Monday but higher elevations could see a bit of powder which is a pleasant and somewhat surprising development since we are still in the middle innings of a relative thaw. Accumulations will vary based on elevation ranging from 4-6 inches at the summits to a wet inch or two on the valley floors.

Rain and ice Tuesday night into Wednesday
Monday's snow will be followed by a dry and seasonable March day. Since temperatures will climb above the freezing mark and winds will be light, skiing or any other outdoor activities will actually be rather pleasant. The tranquil conditions will not last however. The NAO is turning sharply negative and a strong cold front stretching southward from an advancing storm system will approach the region Tuesday night. Precipitation may begin as a some ice but is likely to change to a period of rain Wednesday before colder temperatures arrive.

Powder, when can we get some ?
To make use of this upcoming chilly change we will need the powder and Thursday and Friday both appear powder free unfortunately. By Saturday a jet impulse responsible for providing some cold air reinforcement may bring a light accumulation of snow but a big weekend accumulation is unlikely. Though the pattern will turn colder the mean position of the trough axis will actually be off the east coast. This makes it tough on storms as they are not given the chance to strengthen along the coastline before they are swept out to see. Precipitation is thus confined to terrain-induced events and the random bomber, clipper or mauler that might sweep southeastward with limited moisture. March however is an unstable month. You can do a lot with a little. The strong winter winter jet stream is still present in March but surface temperatures are warmer thus reducing stability. Little and often very benign looking events can turn noteworthy and we will have to keep our eyes open for that beginning this weekend and persisting into next week.

Looking way out in the 10-14 day time frame, the ensembles show evidence of a split flow scenario with energy flowing through the southern rockies, advancing east across the plains and eventually into some of the colder temperatures along the east coast. This is our next chance of something sizable but it woudn't occur until sometime in between the 18th and 23rd of the month.

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