You often hear the statistic that our region receives an average of 18"-20" of snow each winter season. This is indeed true. But does this average truly convey the actual behavior of a typical snow season? Far from it.
This past week, the NWS released a simple graph, showing annual snowfall recorded at D.C.'s Reagan National Airport, from 1992 through 2011. A graph for BWI looks very similar, with slightly higher amounts across the board. Here is the graph, which is called a time series chart:
Washington's annual snow has varied anywhere from nearly zilch (1997-1998, 2011-2012) to 50"-60" (1995-1996; 2009-2010). There is tremendous annual variability in our seasonal snow - so much, in fact, there there is very little predictive power in trying to glean one year's expected snowfall from the previous. A "Snowmageddon" season is very often preceded or followed by a "Snow Drought" year. Basically, it's all or nothing here. Meteorologists term this type of behavior "episodic". In fact, most of our years appear to hover around the 10" annual snowfall mark.
The 18"-20" snow average is really an artifact of averaging lots of very lean years, with a very few episodes of heavy seasonal snowfall. The average creates a false expectation that our "normal" snow should be in the 18"-20" range. In fact, 17 of the 22 years above fall closer to 10" than 20". That's a big difference. So the intermittent nature of our blockbuster snow seasons has inflated an average value, to the point of conveying little meaning.
Beware, you must, of those wily statistics!
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