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Monday, November 28, 2011

Arctic Oscillation crippling early season hopes

The Arctic Oscillation is the king of teleconnection indices. The ability to achieve a pattern of sustainable cold in areas south of 50 North with a minimal risk of rain is best done through a negative AO. A negative AO indicates that the upper air pattern at the poles is blocked and consequently consists of higher surface pressures at higher latitudes and lower pressures in the middle latitudes. The AO for the last several weeks has been positive and is largely responsible for the prevailing warmth in Vermont and the rest of the northeast. That warmth has reached a climax in the waning days of November. Yes, there are some signs of colder weather and snow particularly later next week but the ability for any cold weather to be a permanent fixture on the weather map will be compromised severely by the AO and NAO which show a lack of upper air blocking and encourages zonal flow. The SCWB has tried to quantify the teleconnection indices with the creation of the highly unscientific "favorability index". The index is a cumulative measure of the PNA, NAO and AO indices but we flip the signs of the AO and NAO so that positive numbers indicate "favorable" and negative "unfavorable". Negative AO and NAO indices indicate the blocking described above while positive PNA indices indicate upper ridging in western North America. For example, a NAO index of -0.7, an AO index of -1.5 and a PNA index of -0.1 produces a favorability index of +2.1. In this case the 14 day favorability consists of a NAO index of +0.7, an AO of +2.0 and a PNA of -0.2. This equates to a -2.9 which is an inauspicious number in this early part of the season.

The upcoming 5-7 days will remain very mild with only a brief break. A very moist storm system in the Midwest will track up through the eastern Great Lakes which is not good and lacks any access to cold air which is also not good. The front associated with this storm has had its eastward progression halted because of the prevailing upper air pattern and honestly I am just glad this hasn't happened later in the year. Get this nonsense out of the way now because such a scenario which includes the combination of rain and high dewpoints can obliterate snowpack. Since we don't have much of one anyway there is not much to lose.

Colder weather Thursday will also give way to another surge of warmer temperatures by the end of the weekend. More rain showers will mark the end of the warm weather on Monday the 5th or Tuesday the 6th of December. Much colder air will then grip the region for a period of around 3-5 days. Embedded in this stretch will likely be a clipper system capable of producing light accumulations. The question will be whether this cold air can persist through the middle of the month without another big warm-up. Right now I think that answer is no since too many of the above mentioned teleconnection indices are working against us.

1 comment:

TQ said...

There's some new research about the predictive nature of Eurasia's October snowcover and the AO you might find interesting.