A July for the Record Books

The first half of July 2018 was the driest on record in Washington, D.C. with no measurable rainfall.  The second half of the month has been the complete opposite and has propelled July 2018 to rank among DC’s Top Five wettest on record.

Since April, May and June were wetter than average in the DC Metro Area, it was quite noticeable how dry the first half of July was.  However, given the above average rainfall during the previous three-month period, no drought conditions developed in the Nation’s Capital.  Fortunately, the record rainfall that occurred on July 17 (2.79” at National Airport) was gone before the Major League Baseball All-Star Game got underway at Nationals Park.

The five-day period from July 21 – July 25 was one of the wettest on record for the DC Metro Area.  A total of 6.4” fell at National Airport, 7.7” at Dulles Airport in Sterling, Virginia and Baltimore, Maryland (BWI Airport) had 11.17” of rain.  By comparison, the National Weather Service says Washington, D.C. averages only 3.73” of rain for the entire month of July.  National Weather Service records in the Nation’s Capital date back to 1871.

The frequent, heavy rainfall had an impact on temperatures during the second half of the month.  High temperatures reached the 90° mark only twice in the Nation’s Capital since July 17.  On the other hand, a total of five days in the last two weeks had high temperatures of 85° or cooler, several degrees below average.  Nevertheless, July 2018’s average monthly temperature will finish almost a full degree warmer than average.  The last cooler than average July in Washington, D.C. was in 2014.

Many of the wettest months on record in Washington, D.C. saw the influence of decaying tropical systems, such as the remnants of Hurricane Agnes in June 1972.  However, none of this month’s heavy rain can be attributed to a current or former tropical system and that makes July 2018 that much more unique.  Absent the widespread heavy rain that commonly occurs with tropical systems, this month’s rainfall has been more localized.  Washington, D.C. had 9.73” of rain so far this month, good for fourth wettest July on record.  Dulles and BWI Airports have also had their wettest July’s with 11.21” and 16.73” of rain, respectively.

Washington, D.C.’s Wettest July's (Source: NOAA)

1.  11.06” (1945)

2.  10.63” (1886)

3.  9.95” (1905)
4.  9.73” (2018)

5.  9.59” (1922)

6.  9.44” (1969)

7.  9.41” (1917)
8.  9.15” (2017)

9.  8.40” (1891)

10.  8.37” (1878)

Weather Quiz

True or False.  No two consecutive July’s have been as wet in Washington, D.C. as July 2017 and July 2018 dating back to 1871 when official weather records began.

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Answer to July 15 Weather Quiz question.

With some irony I say the answer is true.  After no measurable rainfall occurred in the Nation’s Capital from July 1 – July 16, more than 9” of rain has occurred just since July 17.

DC’s Wettest Months

If July were to end today, then it would be the seventh wettest on record in Washington, D.C. with 9.19” of rain.  That’s less than 2” from the all-time record for wettest July of 11.06” in 1945.  Many people assume that DC’s wettest months are largely the result of heavy rainfall from decaying tropical systems, but that’s not always the case.

July 2017 (9.15”) and July 2018 didn’t see any rain from tropical systems.  However, both months featured slow-moving thunderstorms that produced heavy rainfall.  Earlier this month, a rare July Nor’easter also brought record rainfall.  More than a third of last July’s rainfall occurred on a single day (July 28, 2017: 3.31”) and more than half of this month’s rainfall occurred on just two days (July 17: 2.79”, July 21: 4.00”).  According to the National Weather Service, Washington, D.C. averages 3.73” of rain in July.

June 2006 was the wettest June on record in Washington, D.C. with a monthly total of 14.02”.  None of that rainfall was caused by a tropical system.  Almost 75% of that total (10.34”) occurred in just three days (June 25-27).  A stagnant weather pattern that resulted from a stalled jet stream over the Appalachian Mountain chain, combined with a steady feed of moisture from the Atlantic Ocean, led to DC’s record June 2006 rainfall.  

June 2015 was similar in that no tropical systems influenced the Nation’s Capital, but three days still saw record daily rainfall totals of more than 2”.  That contributed to making June 2015 DC’s second wettest June (11.94”).  May 2008 was also a wetter than average month with a total of 10.66” at National Airport.  That was largely the result of severe weather and an unusual May Nor’easter.

On the other hand, tropical systems contributed to making September 1999, August 2011 and September 2011 significantly wetter than average in the DC Metro Area.  Weakening Hurricane Floyd brought DC 3.67” of rain on September 16, 1999.  That was after the remnants of Hurricane Dennis produced heavy rainfall earlier in the month.  The Mid-Atlantic Region also saw record-setting rainfall in August-September 2011 due to Hurricane Irene’s landfall in North Carolina and the remnants of Tropical Storm Lee.

There is a chance Washington, D.C. could add to this month’s total of 9.19” of rain.  In any event, July 2018’s place in weather history is secure.