Torrential rain fell overnight into early this morning, resulting in pockets of flash flooding in the Washington, D.C. area. Generally, 1-4 inches fell, with isolated amounts to six inches in a swath from around Warrenton to Centreville to Dulles Airport, which set a record for its wettest July day Thursday.
Rainfall rates, in some locations, exceeded one inch per hour.
Dulles Airport officially received 3.71 inches Thursday, a record for not only the date (crushing the old July 11 record of 1.12 inches from 1977) but also for the month, besting the 3.5 inches that fell July 27, 1994. Remarkably, 3.55 inches fell in just three hours. (Note: Dulles’ period of record is relatively short, dating back to just 1963.)
Including the rain which has fallen through 7 a.m. this morning (Friday)., 4.36 inches of rain has fallen in the last 24 hours at Dulles.
Reagan National Airport, by comparison, has received just 1.25 inches from this event and BWI Airport, a modest 0.3 inches.
Here are some additional rainfall totals, via the Cocorahs, citizen observer weather network, through 7 a.m.:
Fort Washington 2.6″
Oxon Hill 2.7″
Potomac 2.4″Takoma Park 1.56″
The cause of the heavy rain
A lumbering area of low pressure and pool of cool air at high altitudes over West Virginia clashed with the very warm, humid air over the D.C. area. A stalled front draped over the region served to focus an area of very slow-moving, heavy rain. In some locations, thunderstorms tracked over the same spots repeatedly, a process known as training.
The heaviest of the rain has shifted east of I-95, where we expect the greatest potential for heavy rain and the opportunity for flash flooding the remainder of today. However, there remains the chance that new showers and storms could fire up again along I-95 and to the west this afternoon, and flash flood watches remain in effect for the entire region. Because the ground is so wet in many locations, not much rain is required for more flooding.