May Gets Underway

Germantown, Maryland (Courtesy: Susan Granzow)

April 2020 finished as DC’s seventh wettest on record with 6.30” of rain, more than double the monthly average of 3.06”.  It also finished 1.5° cooler than average.  Those factors combined to make April 2020 only DC’s third cooler and wetter than average April since 2001.  Last month’s below average temperatures were partially a function of the 16 days with measurable rainfall.

Nine of the last 20 May’s have been cooler than average in the Nation’s Capital.  However, that hides how streaky May weather can be.  While seven of the 10 May’s between 2000 and 2009 were cooler than average in Washington, D.C., eight of the following 10 were warmer than average through 2019.  Average high/low temperatures in the Nation’s Capital range from 71°/52° on May 1 to 80°/61° on May 31.  DC’s hottest May temperature remains the 99° temperature on May 31, 1991.  By contrast, DC’s coolest May temperature on record is 33° from May 11, 1906.  

On average, the month of May is DC’s wettest month of the year with 3.99”.  Eleven of the last 20 May’s have been drier than average in Washington, D.C.  However, four of DC’s 10 wettest May’s have occurred just since 2003.

May weather can be very streaky as Washingtonians saw in 2018.  That May got off to a dry start with no measurable rainfall during the first 11 days of the month.  However, May 2018 finished as DC’s fourth wettest on record with a total of 8.73” of rain.  Nearly 70% of the monthly rainfall total occurred on just four days that each had over an inch of rain.  

Despite the abundance of rainfall, May 2018 finished as DC’s third warmest.  The big difference between last month and May 2018 is that 17 of the 30 days last month were cooler than average, while all but three days in May 2018 were warmer than average.  That helps illustrate that while rainy and cool weather often go hand in hand, they aren’t always positively correlated.  

With the exception of yesterday’s average high of 72° and today’s anticipated highs in the 70s, much of the upcoming week is going to be cooler than average.  NOAA’s Climate Prediction Center expects May to be both cooler and wetter than average in the DC Metro Area.  May 2017 was the last time Washingtonians experienced a cooler and wetter than average May.