May’s Merry Weather

Washingtonians may wonder what kind of weather they can expect in May.  It’s the third and final month of meteorological spring and there is a big jump in average temperatures over the course of the month.  Average high/low temperatures in the Nation’s Capital range from 71/52 degrees on May 1 to 80/61 on May 31.

Washington, D.C.’s hottest May temperature was 99 degrees on May 31, 1991.  Meanwhile, May’s coolest temperature remains the 33 degrees that occurred on May 11, 1906.  Official weather observations for the Nation’s Capital have been made at National Airport since it opened in 1941.  The coolest May temperature since then was 34 degrees on May 9, 1947.  May averages 3.99” of rain in Washington, D.C.

Following the warmest April on record in the Nation’s Capital, there are some interesting weather trends worth discussing.  The average temperature was 63.3 degrees last month – well above the average of 56.8 degrees.  Also, last month was the second “warmest month on average” this year in the Nation’s Capital (after February).  In the last six years, Washingtonians have now seen six of their twelve warmest months on record along with: July 2011, March 2012, May 2015, December 2015 and February 2017.

What helped make April so warm was two-fold.  Not only did 10 days last month feature high temperatures of at least 80 degrees – including the first 90-degree April day in the Nation’s Capital since 2013 – but overnight low temperatures were unusually warm.  Low temperatures in Washington, D.C. remained at or above 60 degrees on eight days last month.  That includes April 29-30 when low temperatures were 70 and 71 degrees (more characteristic of early July).

Moreover, Washingtonians tied the record for hottest April day this past Saturday, April 29, when the high/low temperature of 91/70 made for a daily average temperature of 80.5 degrees.  Saturday’s high of 91 degrees also tied the 1974 record high.  At one point Sunday morning, April 30, the dewpoint was a sultry 72 degrees which tied the highest value ever reached in April (on April 19, 2002).  The dewpoint is the temperature at which the air becomes saturated and when the temperature and dewpoint are the same, the relative humidity is 100%.  A dewpoint temperature of 70 degrees or higher is considered tropical.

NOAA’s outlook for May calls for a chillier than average first half of the month in the DC Metro Area.  They also expect a higher than average probability for above average precipitation this month for the Mid-Atlantic Region.  That would be excellent news considering the rainfall deficit of 13.75” in Washington, D.C. from August 1, 2015 through April 30, 3017.  Ironically, the last month to feature above average precipitation in the Nation’s Capital was May 2016.