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How a flash flood engulfed Leesburg on Thursday night and the risk for more deluges today in the D.C. area

August 7, 2020 at 1:28 p.m. EDT
Leesburg radar at 9:07 p.m. Thursday. (RadarScope)

* Flash flood watch until late tonight *

A complex of intense storm cells parked themselves over Leesburg on Thursday night, unloading up to 5 inches of rain in just a few hours, leading to dangerous flash flooding.

A very humid air mass over much of the Washington region combined with a slow-moving front means heavy downpours capable of triggering flash flooding could occur again this afternoon and tonight. The National Weather Service has issued a flash flood watch for the fourth time in five days.

Nine people in Loudoun County required rescue in high water due to Thursday’s night’s torrent. The flooding also closed numerous roads near and just north of Leesburg, where the heaviest rain occurred.

The heavy rains began a little before 8 p.m., and by 9:06 p.m. the area was under a flash flood warning. National Weather Service then upgraded the warning to a flash flood emergency, the most severe flood alert, at 10:13 p.m. “THIS IS AN EXTREMELY DANGEROUS AND LIFE THREATENING SITUATION!” the Weather Service warned. “Avoid all unnecessary travel!” The rains finally let up after 11 p.m., and the warning was discontinued.

Nine rescued amid flash-flooding as creek rises 9 ½ feet in Loudoun County

The downpours spurred a rapid rise in water along Limestone Branch in Leesburg, rising nearly 10 feet in about three hours, reaching major flooding stage and nearly its record level.

“It’s insane here in Leesburg,” wrote Erin Brown on Capital Weather Gang’s Facebook page Thursday night. “Our road is flooded. … This got dangerous QUICK.”

A weather observer 4.4 miles west of Leesburg measured 4.65 inches of rain.

Several of the high-water rescues occurred on and near the Route 7 Leesburg Bypass, according to Dave Dildine, a WTOP traffic reporter. He tweeted that Route 7 was closed between Routes 15 and 9.

The Leesburg Police tweeted a picture of a van that washed under a bridge. There were no reports of injuries.

Several roads remained closed due to flooding or downed trees in Loudoun County on Thursday morning.

How the flash flood in Leesburg happened

Thursday at 6 p.m., the National Weather Service group that handles flood forecasting issued a special discussion highlighting the possibility of flash flooding for much of the Washington region. As shown in its graphic below, forecast models were highlighting the potential for at least 4 inches of rainfall through the evening.

In that diagram, the green shade indicates a plume of deep, rich moisture streaming from southwest to northeast across the region. Additionally, note the stationary front draped across Virginia, which focused the moist, low-level winds into a convergent pattern over much of Virginia. The converging, moist air was thus forced to rise over northern Virginia.

Not shown were additional elements. Deep convective clusters, including the storm cluster over Leesburg, were being energized by a moderately unstable air mass, with that unstable air persisting well into the evening.

The wind pattern through deeper atmosphere was progressive, meaning individual cells were moving steadily through the region from the southwest, and feeding off modest amounts of wind shear (the increase in winds with altitude). A passing jet streak (pocket of strong winds in the upper atmosphere) also helped foster the uplift of moist air across the region.

The combination of shear and instability was ideal for the formation of persistent, multicellular clusters. As shown in the radar loop below, it appears that one of these clusters essentially became “parked” over the Leesburg area for several hours in the evening.

The larger cluster can become stationary, even though individual cells continuously erupt and move through the cluster. Typically, the storm’s “outflow boundary” (the blast of cool air created by cloud downdrafts) will serve as an anchor point, allowing for the continual regeneration of cells in one location.

From the loop, it also appears that there was some “backbuilding” as well, whereby new cells formed upwind — in this case, toward the southwest — which caused the entire complex to slowly propagate in that direction.

Today’s flooding potential

Today, the same pesky front that caused flash flooding problems in Leesburg continues to linger across the region, as shown in the forecast map for 8 p.m.

An approaching disturbance in the jet stream will trigger formation of a weak low-pressure region on this front. Moist, low-level air converging into this system will enhance the coverage of showers and thunderstorms. The surface wave and upper-level energy will slowly migrate through the region overnight.

We again anticipate widespread convective storms late this afternoon and evening. Because of very saturated soils in the wake of Tropical Storm Isaias, the Weather Service again has highlighted the region for a slight risk of flash flooding.

Weather Service forecasters believe there may be less-extensive coverage of heavily raining cells today and reduced longevity of storm clusters. This is because the jet stream disturbance aloft is weakening as it passes through.

Additionally, parameters that help optimize the organization of multi-cell clusters are weaker today than on Thursday; for instance, the wind shear (increase in winds with altitude) is weaker, and early day cloud cover will perhaps limit the degree that the atmosphere will destabilize.

Nevertheless, the combination of a moist, tropical atmosphere, proximity of frontal boundaries, and a weak low-pressure system mean that we cannot let down our guard with regard to flash flood possibilities.

Images from Leesburg area flood