Wednesday, March 20, 2013

Taking Arctic Oscillations For The Block

In our game meme of posts today, going from monopoly to Hollywood Squares or Tic Tac Dough, you pick your X and O game show of choice, is a nice segue to work with.

March's cold weather has been driven by a rather intense blocking pattern in the high latitudes.  Both the Arctic (AO) and North Atlantic (NAO) Oscillation have shown a strong blocking tendency, with high pressure aloft over the high latitudes and the North Atlantic, which yields a trough in the East and a nice highway of cold from Western Canada down into the US.

Over the next five days, most of the country will deal with some level of below average temperatures, with the core of cold across the Northern Plains and Midwest.  Similar levels of chill are common throughout Europe, Russia, and Northeast China, all of which are also dealing with below average temperatures for this time of the year.

Because of the magnitude of blocking in the atmosphere over the Arctic and over Greenland, the result is that Greenland and Northeast Canada are significantly warmer than average.  This is the typical result in a blocky pattern like the one we are in -- temperatures over Northern Quebec and Greenland are unseasonably mild by their standards while we're colder than average.  Granted, it's March and at very high latitudes so temperatures up there aren't "warm" but modeling suggests highs perhaps 15 or more degrees above average over the next few days according to the GFS and Euro.

Going back through our climatic record on the AO, the level of blocking recorded by the AO this month may exceed the old record set back in 1962.  The three lowest AO levels on a monthly average for March were 1962, 1958, and 1984, all of which were below -2.3 on the AO scale.  The lower the number, the greater the magnitude of high pressure over the Arctic.  In those three years, the level of blockiness in March does not continue into April...we do see things gradually "normal" out or even reverse course.  There are exceptions in some years like 1996, which featured a persistent -AO into mid April.

All three years had six inch snowfalls during the course of the month -- two of the three had noteworthy storms (1958 and 1962).  This is not to say that the pattern we're in will yield a snowstorm but the pattern in March does favor more "lousy Smarch weather", which means chances of snow showers or even more than that.  That said, we've seen a much more wintry March this year than we have last.