Temporary pattern reversal possible next week

By: NYCvort , 9:20 PM GMT on December 27, 2011

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The models have been suggesting the possibility of a strong positive PNA ridge forming over western North America late this coming weekend into early next week.



If this occurs, we could see a storm ride up the eastern seaboard and drive in some much colder air.

While the ECMWF is going all in, the GFS is not so quick to jump on the bandwagon and remains much flatter and more progressive. I am a little suspect on the idea of such a strong PNA ridge forming. In favor of the PNA ridge would be the MJO orbiting into phase 7, as being forecasting by some of the models like this one:

Here are the corresponding 500mb anomaly composites for a phase 7 MJO in January (courtesy of Allan Huffman):

Note the anomalous trough in the eastern Pacific, western North America ridge, and trough in the east, roughly similar to that being forecasted by the ECMWF. As I mentioned in my winter forecast, the MJO is the biggest factor capable of temporarily reversing the PNA into a positive state during La Nina.

Unfavorable aspects:
Continued positive (though improving) AO—will cause any storm that should develop to be lacking in cold air to tap into overall:


Continued positive NAO—lack of overall blocking:


So the potential storm would be driven solely by the changes in the PNA and the associated phase. This is something that is much too early to call, but it is the first time period of the official winter season worth keeping an eye on. My best guess at this point would be that if a storm should form, the most likely areas for accumulating snowfall would be the interior north/west of I-95, based on the lack of blocking in the pattern. But right now we need to continue to monitor how the models trend on the PNA ridge and the phase.

-NYCvorttumblr page counter

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1. originalLT
10:27 PM GMT on December 27, 2011
Thanks NYCvort, for keeping on top of things.Of course over in Blizz's blog, he is on it too. Between the both of you, things should come into focus over the next few days.
Member Since: January 31, 2009 Posts: 0 Comments: 7873

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About NYCvort

I am a meteorologist from New York who has been studying and forecasting the local weather for years. I especially enjoy tracking winter storms.