March 2012 vs. March 2013

Posted: March 11, 2013, 1:28 pm by tcollow

Remember last March. Temperatures soared to as much as 20 degrees above normal. Fast forward a year later and many places are struggling to get to near normal temperatures. For example, on March 8, 2012 temperatures topped out at a very warm 68 in Boston. However, on March 8, 2013 it was a chilly day with a high of only 42. It wasn’t just Boston that saw such a dramatic shift in temperatures from last March. Cities all along the northeast saw similar swings. For the entire month of March 2012, Boston picked up only 0.6” of snow all within the first ten days. This March we have already seen 13.1” most thanks to a recent nor’easter. Generally March is meant to be a transition month for the atmosphere, starting off chilly and ending mild with the average temperature increasing over a span of 10 degrees throughout the month. However, last March’s temperatures were unprecedented. A late March warm spell last year put the cities of Boston, Philadelphia, and Baltimore as well as surrounding areas above 80 degrees. This would be more typical of late May or early June. So why is there such a big difference this year?

The Major Players:

The North Atlantic Oscillation (NAO): The NAO always seems to be sneaking in to these discussions. If you don’t think it influences our weather yet, maybe you will after reading this…

It looks pretty clear the the NAO regime was highly positive early last year but transitioned into a negative phase later last year and remains there currently. A negative NAO keeps cooler air trapped in the northeast which is what we are seeing this year.

Arctic Oscillation (AO): The AO is essentially a measure of the degree at which arctic air enters the mid-latitude zones by means of pressure variations between the mid-latitudes and the arctic. A negative AO signifies an increase in arctic air flow and a positive AO keeps it warm. I’m sure you know where this is heading. Last year the AO was highly positive and this year it is negative. Like the NAO, this has a major impact on our temperatures.

500 mb Pattern: Last March the eastern United States was dominated by strong ridging which kept it mostly sunny and well above normal. Now we see a switch to more of a trough in the east for this month which is keeping it cooler and also giving us more precipitation.

What’s in store for the rest of the month?

Looks like any warm up be brief. As we head into the second half of March, the AO and the NAO remain negative which will result in temperatures at or even below normal.

Above is output from the European model for March 20th. Looks like a trough in the east remains in place. Perhaps winter isn’t over yet as a nor'easter is showing up on this run. Could we get another shot of snow before the season ends? I’d say it’s definitely plausible.

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