One Extreme to the Other

As the month of March comes to a close, it’s a good time to analyze an impressive weather trend in the Nation’s Capital.  Most area residents know 2018 finished as the wettest year on record in Washington, D.C.  Since weather records kept by NOAA date back to the 1870s, setting an all-time record is a significant achievement.  The wetter than average trend has continued during the first quarter of 2019.

Some may forget, however, that the DC Metro Area was experiencing a prolonged stretch of drier than average weather prior to last April.  The 31-month period from August 2015 through March 2018 included 24 drier than average months in Washington, D.C.  In that time, the Nation’s Capital accrued a rainfall deficit of 16.42”.  Washingtonians experienced its fourth driest January (2018) and February (2017), the third driest June (2017) and fifth driest December (2017).  Consequently, there were intermittent periods of “abnormally dry” to “moderate drought” conditions for parts of the DC Metro Area.

There has been a rather dramatic shift in the weather pattern for the Mid-Atlantic Region since April 1, 2018.  11 of the last 12 months have been wetter than average, including March 2019.  Washingtonians experienced the fifth wettest May (2018) and September (2018), fourth wettest July (2018) and wettest November on record (2018). 

The calendar year of 2018 also set a record for highest number of days (24) with at least one inch of rain.  The 4” of rain that occurred on July 21, 2018 was DC’s wettest day in nearly eight years, since September 30, 2010 (4.66”).  The 4” rainfall total was also more rain than occurred in all but six months during the drier than average period from August 2015 – March 2018.  Such a major shift from a long period with below average rainfall to being wetter than average over the last year can be attributed to an overall shift in the weather pattern.  That includes factors such as the position of the jet stream and ENSO status (El Nino vs. La Nina), as well as other oceanic factors such as the North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO).

The DC Metro Area just had its wettest March since 2015.  That’s contributed to making DC’s rainfall surplus an astounding 29.67” since April 1, 2018.  The Nation’s Capital averages 39.74” of rain in an entire year, so DC’s rainfall surplus over the last year is truly remarkable, especially considering the prolonged stretch of drier than average weather that preceded it.  There was a net-swing of 46.09” of rain between the 31-month period through March 2018 and the record-setting rain in the year that’s followed it.