Thursday, March 21, 2013

What is Driving this March Cold Spell?

At this point in the winter (now early Spring), we are all getting very annoyed with the cold. A few weeks ago when highs approached 70, the general populous assumed that this signified the end of the cold. The meteorological gods have had other ideas.

So what is driving this cold? The answer lies in the teleconnections derived from the pressure patterns in the higher latitudes. The North Atlantic Oscillation (NAO), Arctic Oscillation (AO) and the Pacific North American Index (PNA). I wrote extensively about each of these indices last year during our mild winter (2010-11) which you can read here. Even late in the season, these 3 teleconnections can still have a HUGE impact on the weather across the US.  As I write this, we are watching clusters of lake effect snow dropping several inches of "partly cloudy" across parts of the north coast

Back to the THREE indices...the more negative the NAO is, the stronger the northwest component which most times leads to a much stronger cold pattern. The AO is the NAO's close cousin. The more negative the AO, the better the propensity for colder periods and frequent clipper like systems. 

How strong is this NAO signature?  I looked back at Marches that had more than 2 straight days with NAOs below -1.5.  Why did I pick -1.5? Currently, the NAO is well below -1.5 and falling so -1.5 was a good starting point. Checking all of the data, I could only find 6 YEARS that match:  2011, 2001, 1980, 1977, 1962 & 1952.  I didn't use 2001 because those instances occurred in early march.  I plotted the NAO from March 10th through April 4th (to get some data overlap on both ends of the plot) to see if there was a pattern. Notice that in each year, the NAO bottomed out at the end of the month in EACH YEAR. Whether this is a statistical correlation or something more atmospheric, I'm not sure. More analysis would be needed to figure this out.

Where does this year rank in comparison to these years?  WAY BELOW THIS GRAPH. In fact, one model yesterday had the NAO dropping to -4.

Even if this model is a bit aggressive in its NAO forecast, we are still looking at temperatures staying WELL BELOW NORMAL for a while.

Is this cold spell some sort of record for this time of year? Surprisingly, not even close. Here is where this year's cold spell (starting after that warm Sunday a few weeks back) ranks ALL-TIME...How about 40th!

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