DC’s Wettest Years

NOAA's Precipitation Outlook for November 2018

November 5 was the 20^th day of 2018 with at least 1” of rain in the Nation’s Capital.  That’s just one off Washington, D.C.’s annual record of 21 – the highest number of 1” daily rainfalls in a given year occurred in 1878.  According to National Weather Service records, 1878 was also DC’s third wettest year on record.  By comparison, a total of 55.90” of rain has occurred at National Airport since January 1.  That already makes 2018 DC’s sixth wettest year on record as of November 6.

The period of April 2018 through September 2018 was the first period of at least six consecutive months that Washington, D.C. had above average rainfall since 2003.  DC’s rainfall surplus over this six-month period is 20.97”, compared to the average annual rainfall of 39.74”.  That means Washington, D.C.’s rainfall surplus over this rainy six-month period is roughly 53% of what DC averages over the course of an entire year.

Consequently, it should come as no surprise that 2018 will almost certainly finish among DC’s five wettest years on record.  With the exception of 2003, DC’s five other wettest years on record all occurred in 1948 or earlier.  Weather records in Washington, D.C. date back to 1871, while they date back to 1964 at nearby Dulles Airport.  Since January 1, Dulles Airport (IAD) has had 56.12” of rain compared to its annual average of 41.54”.

Although October 2018 was the first drier than average month in Washington, D.C. since March, November has gotten off to an exceptionally wet start.  From November 1 – November 5, a total of 2.22” of rain occurred at National Airport making it the third wettest first start of November on record.  DC’s rainfall total of 3.01” over the first six days of November is already more than the combined total of 2.76” from November 2016 and November 2017.  This rainy start to November means there is a good chance this month will become DC’s first wetter than average November since 2009.

January 2018 and March 2018 were also drier than average months in Washington, D.C.  Had merely average rainfall occurred during those months, than DC would have a year-to-date total of 59.33”.  Since NOAA’s outlook for November and its three month outlook both call for an increased chance of above average precipitation in the Mid-Atlantic Region, there is also a chance 2018 could become Washington, D.C.’s wettest year on record.  If D.C. were to see only average rainfall the rest of the year, then 2018 would finish with 61.55”.  

Washington, D.C.’s Ten Wettest Years (Source: NOAA)

1.  61.33” (1889)

2.  60.83” (2003)

3.  60.09” (1878)

4.  58.17” (1886)

5.  57.54” (1948)

6.  54.49” (1937)

7.  52.95” (1891)

8.  52.92” (1906)

9.  52.59” (1877)

10.  52.03” (1942)

Average: 39.74”

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Dulles Airport’s Five Wettest Year’s (Source: NOAA)

1.  65.69” (2003)
2.  59.05” (1972)
3.  58.09” (1996)
4.  55.43” (1979)
5.  52.64” (1975)

Average: 41.54”