A Break from the Heat?

Keeping Cool on Labor Day, Rockville, MD

While Hermine has dominated weather headlines in recent days, people may not realize that temperatures have been relatively comfortable in Washington, D.C. since September 1.  That’s in stark contrast to last September when seven of the first nine days were 90 degrees or hotter.  In fact, every September since 2012 has featured several days at or above 90 degrees in the Nation’s Capital – significantly warmer than September’s highest daily average temperature of 84 degrees.

Although longtime Washingtonians have grown accustomed to occasional hot days in September, those are still the exception rather than the rule.  The Nation’s Capital will begin another heat wave today following highs in the 80s over the first five days of September.  A “heat wave” is defined as a stretch of three or more consecutive days with temperatures at or above 90 degrees.  Washington, D.C. has already had 51 days at or above 90 degrees in 2016, compared to an annual average of 36.  

Some record highs may be set at Dulles Airport this week, but record highs downtown should be safe.  It’s easier to break records at Dulles since the pool of data is much less than for downtown Washington, D.C. because records began in 1963 at Dulles vs. 1871 downtown.  The impact of the urban heat island effect is also less significant at Dulles Airport than at National Airport.  For example, only 2 record highs have either been tied or set in Washington, D.C. over the last decade during the month of September compared to six at Dulles Airport.  Also, in the last decade there have been a total of 11 record high low temperatures tied or set at Dulles compared to seven downtown.  That illustrates how increased urbanization has helped keep downtown areas warmer at night than they used to be.

Eight of the last 10 Septembers have been warmer than average in the Nation’s Capital – and include two of the top 10 warmest overall.  Since 1996, there have been 10 wetter than average and 10 drier than average Septembers.  That trend can largely be attributed to the years where the Mid-Atlantic Region saw the remnants of tropical systems such as Fran in 1996, Floyd in 1999 and Isabel in 2003.  Fortunately, there are no new tropical systems on the horizon.

Record Highs

September 7:

Washington, D.C. (National Airport – DCA): 104 (1881) --> monthly record
Dulles Airport, Sterling, VA (IAD): 95 (1985)
Baltimore, MD (BWI Airport): 101 (1881) --> monthly record

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September 8:

Washington, D.C. (National Airport – DCA) 100 (1939)
Dulles Airport, Sterling, VA (IAD): 93 (2015)
Baltimore, MD (BWI Airport): 100 (1939)

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September 9:

Washington, D.C. (National Airport – DCA): 98 (1872)
Dulles Airport, Sterling, VA (IAD): 95 (1985)
Baltimore, MD (BWI Airport): 94 (2015)

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September 10:

Washington, D.C. (National Airport – DCA): 98 (1983)
Dulles Airport, Sterling, VA (IAD): 99 (1983) --> monthly record (also occurred 9/2/1980, 9/11/1983)
Baltimore, MD (BWI Airport): 98 (1983)