DC’s May Weather Extremes

 

After a warm start to May, there has been a significant shift in DC’s weather pattern with several cooler than average days ahead.  That’s not uncommon as May is a transitional weather month in the Nation’s Capital.

Temperatures have been warmer than average on four of the first six days of the month, including highs in the 80s on three days.  By comparison, DC’s first 80° day last year didn’t occur until May 15.  The first half of May, in particular, can feature a wide range of weather.  For example, DC’s first heat wave of 2018 occurred from May 2 – May 4 when high temperatures reached the 90s.  A “heat wave” is defined as a minimum of three consecutive days with highs in the 90s. 

By comparison, last May not only didn’t have any 90° heat, but was also DC’s coolest May since 2005 with three days of highs in the 50s combined with two days of lows in the 30s.  While the Nation’s Capital has averaged roughly one May day with highs in the 50s dating back to the early 1990s, DC has averaged only one May day with lows in the 30s per decade in the last 30 years.

DC residents can expect an average of one to two May days in the 90s.  May 2015 was DC’s warmest May on record with a total of seven days of 90° heat, including two heat waves.  That was followed by May 2016, which had a combined 16 days with below-average highs in the 50s and 60s. 

Being DC’s wettest month of the year on average, May can either have a lot of rainy days or a few days with heavier amounts of rain.  For example, May 2018 finished with four days of 1” or more of rain.  Meanwhile, May 2019 had 13 days of measurable rainfall in the Nation’s Capital without any exceptionally wet (1”/+) days.  May 2021 has gotten off to a similar start with three days of light rain and several more days of occasional light rain in the forecast during the next week. 

May also tends to be an active severe weather month across much of the United States.   Some communities in the DC Metro Area can see very heavy rainfall depending on where thunderstorms occur.  Such was the case this past Monday, May 3, when severe weather, including a tornado, occurred in Frederick County, Maryland.  While that damaging weather event occurred north of the immediate DC Metro Area, only 0.06” of rain was officially measured at National Airport.  This underscores the need to pay close attention to the latest weather forecasts during the spring and summer months when showers and thunderstorms often threaten.