Rainfall Deficit Continues to Grow

Washingtonians have been fortunate to have warm and dry conditions for much of the fall to this point.  However, gardeners and allergy sufferers have a heightened awareness of just how dry it has been.

The drier than average fall in the Nation’s Capital continues the trend that began in August 2005. Since then, 21 of the last 27 months (including this month) have been drier than average.  Consequently, the Nation’s Capital has a rainfall deficit of 11.72” for that period (through October 24).  Washingtonians have a rainfall deficit of 6.05” just since October 1, 2016.  

This year has featured some moderately improved conditions, with May, July and August all being significantly wetter than average in Washington, D.C.  In fact, July 2017 was DC’s wettest since 1969 with 9.15”.  Consequently, Washington, D.C. actually has a rainfall surplus since January 1, 2017 of 0.7”.  However, that total is down significantly from September 1, 2017, when DC residents had an annual rainfall surplus of 3.25”.  

This turn of events can be directly attributed to how dry this fall has been in the Mid-Atlantic Region.  The dry spell has coincided with warmer than average temperatures as well.  Through October 24, Washington, D.C. has had six days this month with high temperatures in the 80s, the highest such total since 2007 (11).  

NOAA recently issued a winter weather outlook which calls for a “55- to 65-percent chance of La Nina developing before winter sets in.”  The opposite of El Nino, La Nina typically favors warmer than average conditions in the Mid-Atlantic Region during the winter.  So, snow-lovers tend not to like La Nina winters in the DC Metro Area.  

This warm October, combined with the expected La Nina, may or may not be a good predictor of winter.  Of the 10 warmest Octobers on record, six of the following winters saw at least a weak La Nina.  Seven of the following 10 winters saw below average snowfall in the Nation’s Capital.  Since this October is well-positioned to finish among the warmest on record, it will be interesting to see if the 2017-2018 winter is also warmer with less snow than average.  

However, a significant snowstorm can form under the right conditions in an otherwise warmer and drier than average winter.  DC residents can think back to January 2016 when the fourth largest snowstorm on record occurred.  The 2015-2016 winter was a warmer than average winter and outside of that record storm, DC residents only saw 4.4” of snow (less than a third of the 15.4” seasonal average according to NOAA).