Thursday, December 9, 2010

Cold to dominate weather pattern pre-Christmas

It is becoming clear that the global circulation is not ready to cooperate with surface La Nina conditions.  Typically the expectation would be that a cold start to December would give way to a warm subtropical ridge of high pressure and a retreating jet stream. The end result would be a December 20-31 similar to 2007.  The first hint that something was going "wrong" was accurate model predictions of a new high latitude Ridge formation over Western Europe which would then retrograde (move west) into the -NAO zone and effectively reload the dominant cold mechanism this month.

Is there an explanation for why the NAO would reload negative when warm North Atlantic temperatures and cold equatorial Pacific would not typically favor such a move after mid-December? Even though colder than average is expected why is our region near record territory and Great Britain and France in a deep freeze which is smashing records?  The answer is that that the global atmospheric circulation as defined by the Global Wind Oscillation is indicating a progression away from La Nina to a death spiral in neutral territory.
GWO has death spiraled into no man's land.
Source: Earth Science Research Laboratory

 Until a jolt of sorts comes to get the circulation moving we can expect a persistence in the weather pattern.  Therefore cold will reload for a few days around Dec 15 and then renew it's assault on the Eastern US and Western Europe.  By that time the major storm tracks will have shifted further SE and will significantly raise the probability of a rare White Christmas.

Will it ever warm-up?  I am watching northern Australia for the onset of the rainy season. The below satellite image could be used as an argument that the NAO should be positive and our region gearing up for a nice warmer than average weekend.  My suspicion is that even though MJO Phase 5 is moderately  indicated by thunderstorm activity NW of the continent the detail of High Pressure nosing its way in from the east is clouding the whole signal.

Watching for onset of rainy season in N Australia
Source: Univ of Wisc SSEC
MJO 5 has good correlation to Positive NOA and widespread warmth.
Source: NOAA Climatic Prediction Center

If the weather was simple then there would not be so many of us and billions of dollars in computers trying to figure it out.  I am concerned that my promises of long warm segments this season could be frozen out if sufficient snowcover combines with precipitation in the deep south to setup a situation that favors further persistence of the cold.

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