How Unusual Was Monday’s Rainfall ?

Driving on I-270 South towards Rockville on July 8

DC Area residents and tourists alike experienced a deluge of rare proportion this past Monday morning, July 8.  In less than an hour, 3.3” of rain accumulated at National Airport.  In all, a total of 3.44” of rain fell in the Nation’s Capital on July 8.  That obliterated the previous daily rainfall record of 2.16” from 1958.

Monday’s rainfall was roughly equivalent to getting a month’s worth of rain in just one day.  The month of July is DC’s wettest of the year with a monthly average of 3.73”, so Monday’s total of 3.44” is more than 90% of that total.  Moreover, 3.44” is greater than DC’s monthly average rainfall in seven other months.  In addition, Monday’s rainfall was the only instance of DC’s highest hourly precipitation report to occur in the morning, dating back to 1936.

Flooding was inevitable with such a high volume of rain in such a short period of time.   Monday’s rainfall was the latest wave of rain over an especially rainy past several weeks in the DC Metro Area.  It rained on 20 out of 30 days in June 2019, followed by rain on five of the first six days of July prior to Monday.  That left the ground saturated and unable to absorb any additional rainfall.

Consequently, the combination of saturated ground after a steady stream of rain since Memorial Day and Monday morning’s deluge produced widespread flash flooding across the DC Metro Area.  That resulted in impassable roadways, countless stranded cars and flooded buildings.  As quickly as the flash flooding occurred, the water dissipated leaving destruction in its wake.  A prime example is the sinkhole that developed on Belfast Road in Potomac, Maryland.

The majority of DC’s wettest days can be attributed to current or former tropical systems such as the combination of Hurricane Irene and Tropical Storm Lee in 2011.  However, the weather pattern in the Mid-Atlantic Region over the last 15+ months has been unique.  Washington, D.C. has had an astonishing a total of 80.93” of rain from April 1, 2018 – June 30, 2019.  That’s 30.36” above normal for the time period!  Making that rainfall surplus even more remarkable is that the Nation’s Capital averages only 39.74” of rain for an entire year. 

Monday’s rainfall guaranteed that July will finish wetter than average for a third consecutive year.  DC’s rainfall surplus will also continue to grow.

DC’s Seven Wettest July Day’s  (Source: NOAA)

1.  4.69” (July 9, 1970)
2.  4.35” (July 22, 1969)
3.  4.25” (July 30, 1878)
4.  4.12” (July 30, 1876)
5.  4.00” (July 21, 2018)
6.  3.68” (July 5, 1908)
7.  3.44” (July 8, 2019)