Wednesday, November 23, 2011

Comparing NOV/EARLY DEC 2009 vs 2010 vs 2011

Comparisons are always tricky. Go ahead and pick the data and sure enough, you can find some correlation or causal relationship to the current weather situation.  The operative word here is "CAUSAL". Is the information or data giving you an accurate snapshot of what is driving the current weather? In other words, is there a direct cause and effect relationship?

The index we look at as a direct measure of the strength of the arctic air to the north is the ARCTIC OSCILLATION.  Pressure differences over the arctic and temperatures high in the atmosphere can make the arctic air more or less changeable for us living around the Great Lakes. If the AO index is negative, the arctic air becomes unstable and has a tendency to drive southward. If its positive, its stays locked up in the higher latitudes of the arctic.

Look at 2009 from the fall into early December.  Notice how the AO was strongly negative into late November and early December.  We had 20 of the 31 days in December with highs in the 20s and 30s!!!
Now look at 2010, same ultra-low negative AO index reading. We had 28 of 31 days in December with highs in the 20s and 30s!!!

Look at the early December snow from last year:

Why is this year so different? Check out the AO index recently.  Its been strongly POSITIVE with no big signals that it will drop sharply negative. While this isn't the ONLY measure of a potential cold air shifts into Northern Ohio, it is a strong one.  So if you are going out to any one of the many Christmas farms in Medina County to cut down your tree, the chances of their being a prolonged snowfall are much less than in the last 2 winters. 

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