A Soggy and Warm Start to Autumn

This month has become DC’s fifth rainiest September on record with 9.73” at National Airport.  Measurable rain has fallen on a record 16 days since September 1.  To the pleasure of many, the final weekend of September is dry with partly sunny skies.

This month is also DC’s sixth consecutive wetter than average month; the longest such streak in the Nation’s Capital since 2003.  That’s remarkable when you consider the unusually dry streak Washingtonians endured prior to April 1.  During the 32-month period from August 2015 through March 2018, Washington, D.C. had 25 drier than average months.  In that time, DC accrued a rainfall deficit of 16.42”.  Dulles Airport in nearby Sterling, Virginia built a similar rainfall deficit of 11.58” during the same 32-month period.

However, over the last six months there has been a dramatic change from a very dry to a very wet weather pattern.  September’s 9.73” of rain made this month the third of 2018 that DC has had at least 8” of rain for only the second time on record.  DC’s wettest day this month occurred on September 7 when 2.89” fell at National Airport.  

The 9.73” of September rainfall also pushed DC’s rainfall total since January 1 to 49.83”.  Since the Nation’s Capital averages only 39.74” of rain for the entire year, 2018 has already exceeded that.  If DC were to simply have average rainfall the rest of the year, then the Nation’s Capital would finish with 59.45” – making 2018 DC’s fourth wettest year on record.

September 2018 has also been warmer than average with a monthly temperature (combining daily high and low temperatures) of 75.7° as of September 29.  The September average is only 71°.  That leaves DC well positioned for this month to also rank among the warmest September’s on record.  Making that especially impressive is the record number of rainy days that Washingtonians have had in September.  

September got off to a very hot start with a five-day heat wave and highs in the low-to-mid 90s through September 7.  DC’s hottest September temperature of 95° occurred on September 4.  Temperatures largely remained warmer than average thereafter, with DC’s coolest temperature of 59° not occurring until September 27.  In a rare occurrence, that was actually the only time temperatures fell below 60° this month.

DC’s Five Wettest September’s  (Source: National Weather Service)

1.  17.34” (1934)

2.  12.36” (1975)

3.  10.81” (1876)

4.  10.27” (1999)

5.  9.73” (2018)

DC’s Ten Warmest September’s

1.  78.2° (1931)

2.  77.1° (1980)

3.  76.0° (2016, 1930)

5.  75.5° (2010)

6.  75.4° (1998)
7.  75.0° (2005, 1970)
9.  74.9° (2015)
10.  74.3° (1973)

DC’s Five Wettest Year’s

1.  61.33” (1889)

2.  60.83” (2003)

3.  60.09” (1878)

4.  58.17” (1886)

5.  57.54” (1948)