The Arrival of Fall

Whether its meteorological fall that gets underway on September 1 or astronomical fall that arrives on September 23, autumn officially arrives in September.  September is also the most active month of the Atlantic hurricane season because atmospheric and environmental conditions are the most favorable.  The Mid-Atlantic coastline had a glancing blow from Hurricane Dorian, which had a destructive trek through the Bahamas earlier in the week.

Other tropical systems over the last 20 years have had significant impacts in the Mid-Atlantic Region during September like Floyd in 1999 and Isabel in 2003.  However, Dorian wasn’t such a storm for the DC Metro Area.  According to the National Weather Service, the Nation’s Capital averages 3.72” of rain in September.  Ten of the last 20 September’s have been wetter than average.  Many DC Area residents should remember that September 2018 was DC’s fifth rainiest on record with 9.73”.

Average high/low temperatures in Washington, D.C. fall from 84°/67° on September 1 to 74°/57° on September 30.  September’s hottest temperature on record in the Nation’s Capital remains the blistering 104° high on September 7, 1881.  By comparison, DC’s coolest September temperature is 36° from September 23, 1904.  Early September can be quite hot, however, as Washingtonians experienced earlier this week with a record high temperature of 96° on September 4.  That was DC’s hottest September temperature in nearly three years.

DC residents have experienced at least one day of 90° heat every September since 2010.  Not surprisingly, Washingtonians endured three of the five warmest Septembers on record over the last decade (2010, 2016, 2018).  As the Nation’s Capital has gotten more developed and seen a commensurate increase in automobile traffic, overnight low temperatures are warmer than they used to be.  There have been 15 record high-low September temperatures in Washington, D.C. during the last two decades.  A “high-low” temperature means the daily low temperature on a particular day was unusually warm.  By comparison, the last time DC residents had a record low temperature in September was in 1988.

September 2019 has gotten off to a warmer and drier than average start in the Nation’s Capital.  NOAA’s outlook for the month is for near average temperatures with a small probability for a wetter than average month.  If September finishes as a wetter than average month in Washington, D.C., then it would be the first time since 2010 – 2012 that the Nation’s Capital has had consecutive wetter than average September’s.