A Preview of Summer?

After a cooler and wetter than average start to the month, Washingtonians are in store for a significant warm up.  In fact, DC residents could have their first heat wave of 2017 by the end of the week.  A “heat wave” is defined by the National Weather Service as a minimum of three consecutive days with temperatures at or above 90°.  The last May “heat wave” in the Nation’s Capital was May 29-31, 2013.

Based on National Weather Service data, the hottest time of year in the Nation’s Capital is from July 7 – 22 when the average high/low temperatures are 89°/71° respectively.  By comparison, this month’s highest average temperature in Washington, D.C. is 80° on May 31.  May’s hottest temperature in the Nation’s Capital is 99°F that occurred on May 31, 1991.  May 1991 is an important month to remember when discussing unusual May heat.  That’s because May 1991 held the record for DC’s warmest May until 2015.

Although May 1991 and May 2015 both saw seven days of 90° heat in the Nation’s Capital, there was a subtle difference.  What helped May 2015 break the 1991 record for warmest May are the warm overnight temperatures that were common during the month.  In other words, daytime temperatures were not only very warm, but temperatures at night also remained very warm in the DC Metro Area.  A total of eight record high low temperatures were either tied or broken at National Airport in May 2015.  

May temperatures don’t necessarily equate to what the upcoming summer will be like.  For example, last May was 2.1° cooler than average, while the 2016 summer ranks as one of DC’s hottest on record.  The first 90° day of 2016 didn’t occur until June 11 (96°) while 2017 saw its first 90° day on April 29 (91°).  Now that doesn’t mean this summer will be as hot as last summer was.  It does, however, illustrate how much weather variability the month of May can feature in the Mid-Atlantic Region.

May 2016 was also the last wetter than average month in Washington, D.C.  Through May 14, 2017, this month was more than 2” wetter than average in addition to being 2.4° cooler than average in the Nation’s Capital.  However, today’s high temperatures in the low 80s will pale in comparison to the heat that’s on tap starting tomorrow.  That’s when Washingtonians could experience record-challenging temperatures.

Record Highs (Fahrenheit)

May 17

DCA: 92° (1974)
IAD: 89° (1986)
BWI: 93° (1896)

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May 18

DCA: 96° (1877)
IAD: 91° (1987)
BWI: 97° (1962)

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May 19

DCA: 96° (1997)
IAD: 92° (1997)
BWI: 98° (1962)* (monthly record occurred on 3 other dates)