“I’ve been here 40 years,” said airport observer Nicholas Parrell. “I’ve never had that.”
“That certainly is a very heavy amount for a short period of time,” said Chris Strong, warning coordination meteorologist at the National Weather Service office in Sterling, Va., in an email.
In the afternoon, 0.95 inches of rain fell in just nine minutes and 1.22 inches fell in 15 minutes. Such rainfall intensities are expected only once every 10 to 50 years on average.
As torrential as the rainfall was, it did not quite equal the intensity of what happened in Ellicott City during the first of its two big floods in 2016 and 2017. “Ellicott City’s 2016 flood had 3.2 inches in 30 minutes measured at the government center rain gauge there,” Strong said.
The exceptional rainfall today resulted from an extremely moist atmosphere. An indicator of atmospheric moisture, known as total precipitable water, was at near-record levels.
A number of locations in the immediate metro area posted rainfall amounts of 1½ to 2½ inches in under an hour, including sensors in Alexandria, Arlington, the District and Cheverly, Md.
A daily rainfall record was not only set at Reagan National Airport, but also at Dulles and Baltimore-Washington Marshall airports where 1.05 and 3.34 inches fell, respectively.