Late-Season 90° Heat

Great Falls, MD is a great place to cool off !

The frequency of 90° temperatures in Washington, D.C. drops precipitously in September.  Average high temperatures fall from their highest level (89°) on July 22 to 84° by September 1.  As the transition from summer to fall gets underway in late-August and early-September, Washingtonians can expect to see less and less 90° heat.

While 90° becomes less common as August gives way to September in the Nation’s Capital, the last several Septembers have featured a fair amount of such heat.  Last August and September were the second warmest on record. Washingtonians experienced 23 days of 90°/+ in August 2016, while September had 7 days of 90°/+.  According to the National Weather Service, the last 90° day to occur last year was on September 23 (90°).

Having 90° well into September has been a fairly regular occurrence over the last decade.  For example, it was 99° on September 24, 2010 at National Airport.  That was not only a record high for the date, but remains Washington, D.C.’s hottest observed temperature on or after September 9.  Longtime residents may recall that record high temperatures occurred in the DC Metro Area on October 8 (91°) and October 9 (94°) in 2007.  The Nation’s Capital has had 90° as late in the year as October 11, but that’s the exception and not the norm.

The last August to be both cooler and wetter than average was in 2014.  That’s unusual, though, since 14 of the last 20 August’s have been warmer than average in Washington, D.C.  Meanwhile, the last September to be both cooler and wetter than average in the Nation’s Capital was in 2006.  In the Mid-Atlantic Region, impacts from decaying tropical systems can contribute to above average rainfall.  Although this summer has been wetter than average, that hasn’t been due to tropical systems. 

June and July were both warmer than average, but this month is poised to become only the fifth cooler than average August since 2001.  Yesterday’s high temperature of 92° at National Airport was the hottest day in the Nation’s Capital since July 22 (96°).  My colleagues and I on the WUSA9 weather team are expecting cooler than average temperatures over the final week of August.