Start of 2019 vs. Start of 2018

May 2019 has gotten off to a warm and wet start in the Nation’s Capital.  Scattered showers and thunderstorms later this afternoon will make this Friday the 12^th one with measurable rainfall in Washington, D.C. since January 1.  There has been 1.69” of rain in the Nation’s Capital since May 1 and that’s more than 40% of the May average of 3.99”

The weather pattern so far this month has also been a mild one for the DC Metro Area.  In fact, the coolest daily low temperature was 59° on May 1 and May 6.  High temperatures have been above average on eight of the first nine days of the month, but the balmy overnight low temperatures have been the primary reason DC’s average temperature so far this month is more than 6° above average.  The warmest temperature in the Nation’s Capital over the first week and a half of this May was 87° on May 2.  However, Washingtonians experienced three consecutive days in the 90s through May 4, 2018.  

In fact, May 2018 was DC’s third warmest May on record and also the sixth wettest.  That combination is a little unusual and was achieved through a unique combination of events.  There were four days last May that the Nation’s Capital had at least an inch of rain.  Meanwhile, there were four other days high temperatures reached the 90s.  

Four of DC’s 10 wettest May’s have occurred over the last 16 years.  However, in 2003, 2008 and 2009 May was also cooler than average.  More recently, May 2016 and May 2017 were wetter than average in the Nation’s Capital, but they were also cooler than average.  May 2015 happened to be DC’s warmest on record and featured only 1.92” of rain.  

What makes last May and this May so remarkable is how warm and wet the DC Metro Area has been since April 1, 2018.  DC has a rainfall surplus of 28.89” over the last 13 months.  However, the key difference between the start of last year and the start of this year is that DC had a rainfall deficit of 1.81” from January 1, 2018 through May 9, 2018.  By comparison, since January 1, 2019, DC has a rainfall surplus of 1.69”.  That means it’s been 3.5” wetter this year over the same period (January 1 through May 9) last year.

A wet start to a year isn’t a reliable indicator of whether or not the entire year will be wetter than average.  After all, two of the first three months of last year were much drier than average and 2018 ended up being DC’s wettest year on record.