The Arrival of Fall

August 2020 finished as DC’s thirteenth wettest with 8.73” of rain that also made it the wettest August since 2011.  Last month also finished 1.5° warmer than average in the Nation’s Capital.  For those of you tired of the heat and humidity of summer, September ushers in the start of meteorological fall (that runs from September 1 – November 30).  Meanwhile, astronomical fall gets underway on the autumnal or fall equinox, which is Tuesday, September 22.  

While there is a discernable decline in temperatures during September, plenty of hot weather can still occur.  Daily average high/low temperatures in the Nation’s Capital range from 84°/67° on September 1 to 74°/57° on September 30.  DC’s hottest September temperature was 104° on September 7, 1881, while its coolest temperature was 36° on September 23, 1904.  It’s worth pointing out that weather records were kept downtown prior to when National Airport opened during World War II.  DC’s warmest September temperature since then was 101° on September 2, 1980.  DC’s coolest September temperature at National Airport was 39° that occurred twice on September 24-25, 1963.  

Every September since 2010 has been warmer than average in the Nation’s Capital, with four of DC’s 10 warmest Septembers during that time.  Although triple-digit heat is a rarity in the Nation’s Capital in September, with only four such occurrences on record and none since 1980, there have been many 90° days.  For example, DC residents endured a sweltering total of nine 90°/+ days last September.

Washingtonians average 3.72” of September rainfall, but six of the last 10 have been drier than average.  September rainfall can vary depending on whether or not the DC Metro Area sees the influence of tropical systems.  Longtime Washingtonians may remember September 1999, 2003 and 2011 were all quite rainy due to the impact of Hurricanes Floyd and Isabel, as well as the remnants of Tropical Storm Lee.  Meanwhile, last September finished as DC’s fourth driest September with only 0.25”.

While there has been a near 50/50 split between wetter and drier than average Septembers in the Nation’s Capital over the last decade, September has been consistently warmer than average during that time.  No record low temperatures have occurred in Washington, DC during the month of September since 1988.  On the other hand, five record highs have occurred since 2010.  NOAA’s Climate Prediction Center expects near average temperatures with an above average chance for a wetter than average September 2020 in the DC Metro Area.