Thursday, December 16, 2010

Significant snow a real probability this weekend

Could we be on a path to a historic winter?  December is on a pace to possibly the one of the coldest ever here in the Raleigh area.  While I fully expected the usual below average 2-3 weeks of December that is normal in a La Nina season the intense cold and the prospects of lingering into the New Year is definitely a surprise.  

Now that today's snow, ice, and rain are behind us it is time to take advantage of a clearer look at the potential winter storm Saturday night and Sunday which is capable of dropping a serious snowfall.  Today, models have trended west and depict a stronger system.  An outcome like this is not yet a certainty but could indicate 3-5" of snow.
Short Range Ensemble Forecast (SREF) indicates a
a mean precipitation total near .5" in 12 hours.

A big question is: Why are models trending westward with more precipitation?  Global Atmospheric Angular Momentum has been in a freefall.  In our case weather systems will tend to have a more longitudinal (N-S) component than E-W,  As AAM goes increasingly negative over an area weather systems can move backwards, called retrograding.  Below you will see a pattern like last December with moderately positive AAM in the tropics and negatives to the north, not typical of La Nina at all.
Source: NOAA Earth Science Research Laboratory
The easy way to understand the diagram is to know that weather systems are progressing west to east in the tropics and subtropics.  Once they gain latitude to about 30N (our Latitude!) the systems turn north and even northwest and amplify.  Areas underneath the green and blue also areas tend to have persistent, repetitive weather patterns. 

Therefore I expect the storm to be close to the coast and have great concern that winter storm snowfalls in excess of 4" will blanket Central NC and VA this weekend.  There will be plenty of cold air available and a real chance for a rapidly intensifying system as it moves up our coast targeting the mid-Atlantic States.

Could something go wrong and cancel the snowstorm?   Yes, it is an unconventional setup and the system could do as many have done in the past .... blow out to sea or focus its snow on eastern NC and VA.  Another possibility is that the storm system could fail to progress far enough north to reach the negative AAM region and thus be weaker and progress quickly out to sea.  Finally, AAM is measured 2 days in the past.  Could the negative region no longer extend this far south?

Stay tuned...

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