Saturday, February 12, 2005


The way these indices work is pretty easy to understand. The North Atlantic Oscillation is a measure of air pressure changes in the North Atlantic as a result of the east-west oscillation of the Icelandic Low and the Azores High (off the coast of Spain). When the NAO is negative, the Icelandic Low is farther west, this forces the northern jet stream out of it's normal home in Canada and into the Eastern United States. A strongly negative NAO means the northern jet has to really swing far south and then charge up the eastern seaboard to get back to it's traditional latitude. The reason for this is due to a blocking pressure system in the way somewhere in the North Atlantic, preventing the northern jet from making it's west to east trek. Thus storms head farther south, and bring cold air down from Canada with them. An even better setup is that the jet swings south, delivering fresh arctic highs into the Northeast in advance of a big storm. The other key is the PNA because it signals a blocking Pacific or Rockies high pressure ridge. This in turn directs the southern jet stream around the bottom, up across Baja California, into the Gulf and towards the northeast.

The end result of these two factors causes the jet streams to "PHASE" or merge together for a brief time. If this occurs far enough south, and in the right orientation, phasing develops giant snowstorms such as the February 2003, 1983 or March 1993 storms. While the two indices are now way out of their predicted ranges, it is only a matter of time before they correct that anomaly, and we begin the setup for another winter storm. With lots of cold air building in Canada as a result of the recent warm spell, you know the clock is ticking.

FOR HUNGRY POWDERHOUNDS... I can give you this tidbit. Upper air indications are that the ingredients described above will start coming together in the February 18-24 time frame for the next major event. However I will not roll out a forecast on this until I feel confident that a storm of this magnitude is definitely in the offing, and will not be a disappointment.

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