The Growing Rainfall Deficit in Washington, D.C.

1600 Pennsylvania Avenue, NW

A significant portion of the Mid-Atlantic Region is experiencing moderate to severe drought conditions according to the latest Drought Monitor Index.  That largely results from 15 of the last 18 months being drier than average, dating back to August 2015.  The last wetter than average month in the Nation’s Capital was May 2016.

Washington, D.C.’s rainfall deficit is more than 13”, dating back to August 1, 2015.  Acerbating the dry conditions is the fact that 15 of the last 18 months have also been warmer than average.  Washingtonians had their warmest December (2015) and warmest February (2017) on record during this time.

Washingtonians also endured their third hottest summer on record in 2016 and the first triple-digit heat since 2012.  Over the last 10 years, Washington, D.C. has had seven of the warmest months on record.  Having such frequent warmer than average weather in conjunction with below average rainfall only serves to heighten the drought conditions.  That’s what makes days like this past Monday-Tuesday so beneficial since more than an inch of liquid precipitation occurred at National Airport (1.23”).  In fact, the 0.89” on Tuesday alone made March 14 DC’s wettest day since December 6 (1.03”).

The Mid-Atlantic Region has experienced periods of hot and dry weather many times over the course of history.  What makes the current dry conditions unusual is the length of time they have been occurring.  The summers of 1995, 1999 and 2002 were all warmer and drier than average in the Nation’s Capital, with triple-digit heat in 1999 and 2002.  The combination of hot and dry weather led to water restrictions for parts of the DC Metro Region during those years.  No water restrictions are currently in effect locally.

The remnants of Hurricanes Dennis and Floyd made September 1999 one of the wettest on record in Washington, D.C. and helped bring an abrupt end to that year’s dry conditions.  The 2017 hurricane season is still several months off, so it would take a significant shift in the weather pattern to bring above average precipitation to the Mid-Atlantic Region.  To prevent flooding or runoff, it’s important that rainfall comes gradually over the course of several weeks.

What happened in California this winter is fascinating.  The state had an extreme multi-year drought until this winter.  Then a series of Pacific storms brought beneficial rain and mountain snowfall and an abrupt end to the drought in much of the state.  Although the rainfall deficit still exists in many parts of California, the widespread drought conditions are over.  If the Mid-Atlantic Region was to see a wetter than average spring, that would go a long way toward mitigating the drought conditions here. 

However, several consecutive wetter than average months would be needed to make up the rainfall deficit that’s developed over the last 18 months.  NOAA’s current outlook is only for near average precipitation over the next three months.