Drought vs. Rainfall Deficit

Maryland Drought Monitor Index (May 16, 2017)

May 2017 has officially become DC’s first wetter than average month since last May following yesterday’s rainfall that brought the monthly total to 4.18”.  Over the first three weeks of May, there have been 12 days with rain.  More rain in the forecast could help make this the first cooler than average month of 2017.

Prior to this month, Washington, D.C. has had only three wetter than average months since August 2015.  That helped create a rainfall deficit of 13.75” as of April 30, 2017.  Over the same period of time, the Nation’s Capital has also experienced a significant amount of warmer than average temperatures.  That includes three of DC’s warmest months on record (December 2015, February 2016 and April 2016).  The combination of above average temperatures and below average precipitation over a sustained period of time led to moderate drought conditions for much of the Mid-Atlantic Region.

As recently as the March 28, 2017 Drought Monitor Index, a portion of the DC Metro Area was under “severe drought” conditions.  However, near average April rainfall helped eliminate the severe drought conditions.  Since April 20, 5.04” of rain has occurred at National Airport, more rain than Washingtonians got from September 1 through November 30, 2016.  Although that hasn’t made a significant dent in the area’s rainfall deficit, it did help eliminate the drought conditions according to the most recent Drought Monitor Index.  

Now while there aren’t any current drought conditions in the Mid-Atlantic Region, there is still a rainfall deficit dating back to 2015.  It is possible for an area to have a long-term rainfall deficit and drought conditions.  However, if there are a wet few weeks that eliminate drought conditions, the deficit could remain depending on how large it is.  It’s important to point out that if an area has a sustained period of below average rainfall than the entire deficit cannot be effectively eliminated in a short period of time.

The Mid-Atlantic Region has had a short-term rainfall deficit on several occasions in recent decades.  For example, Washingtonians experienced rainfall deficits at different stages of the hot 1995, 1999 and 2011 summers.  However, those rainfall deficits were quickly made up when the remnants of decaying tropical systems produced a significant amount rainfall.