DC’s Longest Heat Wave Since 2016 Finally Ends

July 23 will finish 15° - 20° cooler than July 22  (Source: WUSA9)

DC’s longest heat wave since August 2016 came to a conclusion yesterday after temperatures reached the 90s on a twelfth consecutive day.  The heat wave began on July 11 with a high temperature of 91° at National Airport.  DC’s hottest day since then was Sunday, July 21 with a high/low temperature of 99°/80°.  As hot as the last week and a half have been, no record highs were set in the Nation’s Capital.

No triple-digit heat occurred during this memorable heat wave.  The last time Washingtonians experienced 100° heat was August 15, 2016.  That’s not to say the last several days haven’t been dangerously hot as excessive heat warnings were in effect July 19 – 21.  Official weather measurements for Washington, D.C. are made at National Airport along the Potomac River.  There was a south wind blowing for much of the last few days, which helped keep National slightly cooler than it might have otherwise been.  DC’s hottest days occur with a west wind blowing, which is often slightly drier than a southerly wind.  

Most people would say the weather the last few days was as humid as any time in recent memory.  Dewpoint temperatures reached the mid-70s and that, combined with temperatures in the mid to upper 90s, produced dangerous heat indices of 105° - 115°.  It also provided plenty of fuel for heavy rain to fall in the thunderstorms that developed.

Although National didn’t reach 100°, Baltimore (BWI Airport) did on July 20 and 21.  Dulles Airport also reached 100° on July 21.  In both cases, it was the first triple-digit heat since July 25, 2016.  However, no record highs were set at either BWI or Dulles Airports.  It is worth pointing out that Dulles had hotter high temperatures than National on five days out of the 12-day heat wave.  That underscores the impact that National being on the Potomac River can have on air temperatures.  

The Nation’s Capital has had two previous 12-day heat waves over the last decade prior to the current one.  One occurred from August 10 – 21, 2016 and the other on July 14 – 25, 2010.  The key difference between those earlier heat waves and this one is that triple-digit heat occurred in DC during the two earlier heat waves.  Consequently, the current heat wave resembles the earlier ones in length only.  As is, the final four days of the most recent heat wave featured high temperatures between 95° and 99° for the longest such streak since July 19 – 22, 2017.