An historic December is likely to spill over into the new year. While Keene has not been directly affected, you've likely heard all about the mammoth blizzard that pounded the Mid Atlantic region up into southern New England, from Washington D.C. through Boston. We've seen a number of cut off lows develop over the nation, the blizzard being one of them, assisted by a dominant convoluted flow.
The primary cause of this pattrn can be traced to a literally record breaking Arctic Oscillation. The Arctic Oscillation (AO) is a measure of blocking in the polar region. The AO is directly related to something called an annular mode which is characterized by pressure anomalies over the polar region and midlatitudes. A negative AO indicates blocking, or higher pressures near the north pole. In these instances, the polar vortex is displaced southward. The vortex disrupts the flow, making it more convoluted and favorable for storm development.
Getting back to the current situation, the AO crashed around the 10th of December, reaching a record low of -5.668SD on December 21st. Since then, the AO has remained below -4SD (off the CPC charts), and December 2009 has placed 7 days on the top 10 lowest December AO list.
The AO shows no signs of recovering through the first week of January.
What does this mean? Well, we will see no shortage of cold or storm potential. Currently, the period of January 1-3 is being highlighted by model guidance for a major to historic storm. The ECMWF in particular, has been advertising a storm moving slowly up the coast and being captured and stalled near Long Island. Verbatim: snowfall measured in feet, and blizzard conditions for days. This storm potential is about 7 days away, so there is plenty of room for changes. The GFS has lost the storm for now, however this is a common action by the model in this time range.
There is also uncertainty as to the timing of the storm. Some of the guidance develops the system earlier than the ECMWF. The differences are a result of making different shortwaves phase. This is in fact a very good sign for storm potential in my opinion: The idea that this storm may be produced in multiple ways increases the probability that it will happen.
|posted by Sam Lillo @ 12/26/2009 01:30:00 PM
