Why Thursday’s torrential rain didn’t prompt a flash flood warning in Loudoun, Fairfax

Thursday’s midsummer deluge sent streams bursting out of their banks, inundating roadways and causing travel headaches in parts of Loudoun and Fairfax counties in Virginia — so why wasn’t a flash flood warning issued?

A slow-crawling nuisance of a storm cell brought over 2 inches of rain in less than an hour from Dulles International Airport east through Fairfax County.

With images of commuters stranded on top of their islanded cars fresh in memory, it’s worth a look at what sets the National Weather Service’s two non-coastal flood warnings apart.

Jason Elliott, a senior hydrologist at the weather service’s Baltimore-Washington office, said it depends on how much rainfall an area sees in a certain period of time and how sudden a rush of water it causes.

Flash floods rise quickly after intense rainfall in a short period of time, overwhelming storm drains and threatening life and property near streams and on roadways within minutes. The July 8 D.C. area flood and thousand-year rainfall events in Ellicott City, Maryland, are two recent examples of flash flooding.

Standard flood warnings, meanwhile, are slower to develop. They still pose a threat if not heeded, especially in areas that typically flood, but the floodwaters they describe generally aren’t as swift and rise more gradually.

On the whole, Thursday’s storms, though packing considerable downpours, managed to move quickly enough to avoid widespread flash flooding. The storm briefly slowed down over Fairfax County but sped up enough in time to prevent a more serious situation from unfolding, Elliott said.

“If we’d seen that heavy rain continue for a longer period of time, then it absolutely would have gotten to flash flood warning level,” Elliott told WTOP. “But as it was, it was more of an urban flooding situation with water that just couldn’t drain fast enough because it was raining so hard.”

But the area wasn’t sparred from damage: All lanes of U.S. Route 50 in Middleburg, Virginia, were closed after powerful winds downed a tree. Severed transmission lines sent sparks into the air near Oakton. Fairfax and Annandale both saw major roadways obstructed by high-standing water.

The weather service is working to reformat its flash flood warnings, tweaking its verbiage to emphasize local impacts and make them easier to understand, Elliott said. It hopes to debut the revamped warning by the end of the year.

WTOP’s Nick Iannelli contributed to this report.

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