Feeling Like Summer

Fall officially arrived this past Friday with the autumnal equinox.  However, the second half of September has felt more like August in the Nation’s Capital.  That’s following a cooler than average start to the month.  September is typically a transitional weather month in the Mid-Atlantic Region with summer-like temperatures possible, but cooler temperatures more and more frequent as the month wears on.

This month has featured two distinct weather patterns.  From September 1 – 13, all but one day saw cooler than average temperatures in Washington, D.C.  In fact, the high and low temperature of 65°/57° on September 2 was DC’s coolest weather during the first half of September since September 6, 2011 (67°/64°).  The coolest temperature so far this month occurred on September 10 (53°). 

September also got off to a wet start with 1.39” of rain through September 6.  Since then, only 0.04” of rain has fell at National Airport.  As active as the tropics have been the last few weeks, the DC Metro Area hasn’t seen significant precipitation from former tropical systems.  However, hurricane season runs through the end of November and Hurricane Maria is still east of the Bahamas and bears watching.  Given how wet July and August were in the Mid-Atlantic Region, Washington, D.C. has a rainfall surplus of more than 1.5” since January 1.

Nine of the last 10 days have been warmer than average in the Nation’s Capital.  Despite these above average temperatures, DC’s average temperature since September 1 (combining daily high and low temperatures) remains 0.4° below average.  That’s poised to change with above average temperatures on tap for much of the upcoming week.  The warmest temperature so far this month occurred yesterday when it was 89° at National Airport.  Since 1993, Washington, D.C. has averaged nearly four days of 90° heat and one day of 95°/+ temperatures during the month of September. 

Washingtonians sweat through seven days at 90°/+ in September 2016, while DC’s hottest September temperature last year was 98° on September 8.  There have been as many as 10 days of 90°/+ heat in the Nation’s Capital during the month of September twice in the last 25 years (1998, 2010).  The record high for this date in Washington, D.C. is 99° set in 2010.  By comparison, it hasn’t been 99° in the Nation’s Capital since August 15, 2016 (100°). 

Despite the recent above average temperatures, Washingtonians haven’t experienced any 90° heat since August 22 (92°).  That could change today as high temperatures climb to the upper 80s across the DC Metro Area, with 90° temperatures possible in spots.  According to NOAA, DC’s last September without 90° heat came in 2009.  That was also the last time Washingtonians experienced a cooler than average September.