DC’s unofficial start to fall

 

NOAA's September temperature outlook

Last month finished as DC’s first cooler than average August since 2017.  It also featured a below average number of 90° August days with nine in the nation’s capital compared to the 30-year average of 11 – 12.  Last month also finished as slightly wetter than average with 3.71” of rain in Washington, DC.  NOAA’s August rainfall average for the nation’s capital is 3.25”.

September features some of the best weather of the year in the nation’s capital as fall officially gets underway.  Average daily high/low temperatures drop from 85°/69° on September 1 to 75°/59° by the end of the month.  The amount of daylight also continues to decline throughout September.  Sunset will occur at 6:53 p.m. on September 30.  The combination of reduced daylight and a lower sun angle contributes to the cooler average temperatures as September wears on. 

Five of DC’s 10 warmest September’s on record have occurred since 2005.  DC’s hottest September temperature on record is 104° on September 7, 1881.  Triple-digit heat during September is a rarity in the nation’s capital and has occurred only four times since 1871.  The most recent was 101° on September 2, 1980.  By comparison, DC’s coolest September temperature on record is 36° from September 23, 1904.

Although Washington, D.C. averages 3.93” of September rainfall, the amount of rain can fluctuate wildly year-by-year.  September can be a very wet month if decaying tropical systems bring appreciable rainfall.  Such was the case with Floyd in 1999, Isabel in 2003 and Florence in 2018.  This month can also be very dry in the Nation’s Capital.  The driest September on record in 2005 (0.11”) was followed by the wettest October on record (9.41”).

The cooler autumn weather that occurs as September wears on is a relief to many Washingtonians following the typically hot and humid summer conditions.  However, the first half of September often sees a continuation of summer-like heat in the DC Metro Area.  That’s going to happen over the next few days with near-record heat through the middle of the upcoming week. 

This September has an above average chance for warmer than average temperatures in the nation’s capital combined with an equal chance for above or below average rainfall, according to NOAA’s Climate Prediction Center.