The Washington DC metro area experienced a dose of heavy rain overnight. Four inches of rain poured into Reagan National Airport — 3 inches of which fell between 1 and 3 a.m. — and some areas saw even more.
Doppler radar estimated that 6 to 8 inches of rain flooded parts of Montgomery County, although this may be an overestimate of about 15 to 20 percent. Still, suffice it to say that some communities saw nearly half a foot of rain.
Considering DC’s average July rainfall is 4.33 inches, that means some locations saw around a month’s worth of precipitation in just a few hours.
There were instances of urban flooding and small streams across the DMV, but precipitation decreased from west to east around sunrise. Roads were reportedly flooded near the interchange of highways 295 and 695 in the district.
Witex Road in Montgomery County was reportedly closed due to a flooded bridge over Rock Run.
Heavy rain is a summer staple, but there is evidence to suggest that heavy rain may be increasing in frequency and intensity. Today, the average summer rainstorm drops about 9% more water than was typical during the 1950 to 1980 window. The most frequent 10% of July rain events are now 20% more frequent, and the top 1% of rainfall events – which previously corresponded to 1.63 inches of accumulation – are now closer to 1.71 inches on average.
Tree down along Belfast Road in Potomac
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All this corresponds to a 3.8 degree increase in air temperature since World War II. Since warmer air can hold more moisture, it’s no surprise that there’s more precipitation.