September arrives and Meteorological Fall gets Underway

Meteorological fall is comprised of the three-month period of September, October and November.  September can be a very warm month with a lot of summer-like heat, but can also be a cool and wet month.  The Mid-Atlantic Region often has a high degree of weather variability during the month of September as the transition from summer-like heat to more autumn-like weather gets underway.

Average temperatures in the Nation’s Capital range from a high and low of 84°/67° on September 1 to 74°/57° on September 30.  The hottest September temperature on record in Washington, D.C. is 104° on September 7, 1881.  However, Washington, D.C. has been 100°/+ only once since National Airport opened in 1941 (101° on September 2, 1980).  September’s coolest temperature in the Nation’s Capital was 36° (September 23, 1904). 

Rainfall in September often varies widely from one year to the next.  September is the busiest month of the Atlantic hurricane season.  Consequently, Washingtonians have felt the impacts of several decaying tropical systems over the last twenty years.  Hurricanes Floyd (1999) and Isabel (2003) as well as the remnants of Tropical Storm Lee in 2011 all contributed to wetter than average Septembers.  Heavy September rainfall can also occur absent tropical systems such as on September 30, 2010 when a daily record of 4.67” occurred at National Airport.  During the last 20 years, there have been 11 drier than average Septembers in Washington, D.C.

If a month is wetter than average than it is sometimes cooler than average due to the increased cloud cover and precipitation.  Over the last two decades, Washingtonians have had 13 warmer than average Septembers, including 8 of the last 10.  The wettest September on record was in 1934 (17.45”), while the driest occurred in 2005 (0.11”).  It’s interesting to note that while September 2005 was the driest on record, Washingtonians experienced their wettest October the following month (9.41”).  That’s another illustration of the high degree of weather variability that occurs in the DC Metro Area.

Some Washingtonians may recall that September 2016 was the second warmest on record (behind only 1980).  However, this month has gotten off to a significantly cooler than average start.  Yesterday’s high and low temperatures of 65°/57° were significantly below the average high and low temperatures for September 2 (84°/67°).  It was also DC’s coolest day since May 13 (61°/50°).  NOAA’s outlook for September is for an increased potential for a cooler and wetter than average September in the Mid-Atlantic Region.  That would be the first such September in Washington, D.C. since 2009.