Heavy rain Friday night causes flooding across parts of DC region

The Washington D.C. metro area saw a dose of heavy rainfall overnight. Four inches of rain poured down at Reagan National Airport — 3 inches of which fell between 1 and 3 a.m. — and some areas saw even more. 

Doppler radar estimated as much as 6 to 8 inches of rain deluged parts of Montgomery County, though that may be an overestimate by about 15 to 20 percent. Still, suffice to say some communities saw close to half a foot of rain.

Considering that D.C.'s average July rainfall is 4.33 inches, that means some places saw roughly a month's worth of rainfall in just a few hours' time.

There were instances of urban and small stream flooding across the DMV, but rainfall was tapering down west to east around sunrise. Roads were reportedly flooded near the Interstates 295 and 695 interchange in the District. 

Witex Road in Montgomery County was reportedly closed due to a flooded bridge over Rock Run. 

Heavy rainfall is a staple of the summertime, but there is evidence to suggest heavy rainfall may be increasing in frequency and intensity. The average summertime rainstorm nowadays drops about 9 percent more water than was typical during the 1950 to 1980 window. Top 10% July rainfall events are now 20 percent more common, and the top 1 percent of rainfall events — which used to correspond to 1.63 inches of accumulation — are now closer to 1.71 inches on average.

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That all matches up to a 3.8 degree increase in air temperature since WWII. As warmer air can hold more moisture, it's no surprise that rainfall events are heavier.