Feeling Like Summer

U.S. Supreme Court, Washington, D.C.

Washington, D.C.’s first heat wave of August officially began today when high temperatures reached the low 90s for the third consecutive day.  The last heat wave in the Nation’s Capital was July 10 – 12.  However, high temperatures over the last few weeks have largely remained below average, with a rainy weather pattern that has dominated local weather headlines.  

Since Washington, D.C. had a record-setting rainy period with 11.32” between July 17 and August 3, high temperatures only reached 90°/+ twice during this period.  By comparison, DC residents sweat through 16 days of 90°/+ during the same period in 2016 and 11 such days last year.   The combination of high temperatures in the low to mid 90s combined with high relative humidity will make the heat index (or “feels-like” temperature) feel more like 100° for the next few days.

August is traditionally the second hottest month of the year in Washington, D.C.  Over the last 25 years, there has been a wide range in the number of 90° days during the month of August with as many as 20 in 2016 to as few as one such day in August 2004.  The Nation’s Capital has averaged approximately 10 – 11 days of 90° during the month of August since 1993, including an average of three days reaching 95°/+.  With 90° temperatures on tap for much of this week, DC residents will be close to the recent August average by the upcoming weekend.

A month can be warmer than average even if daily high temperatures remain at or below average for a majority of the time.  That’s because in recent years there have been more above average overnight low temperatures.  That can largely be attributed to the increased amount of urbanization around metropolitan areas, such as Washington, D.C., as well as Philadelphia, New York City and Chicago.  Denser urban areas contribute to an enhanced “urban heat island” effect.

Despite the recent rainy period, daily low temperatures largely remained above average.  For the 17-day period from July 17 – August 3, Washington, D.C. experienced record rainfall, and daily low temperatures remained warmer than average on 11 days.  By comparison, high temperatures were above average on only four of the 17 days.  NOAA expects above average temperatures to continue in the DC Metro Area through the middle of August.