Tuesday, October 4

Flood watch: Heavy storms probable in D.C. area this afternoon and evening


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Since August began, the Washington region has felt like a steam bath, culminating in Tuesday’s oppressive high of 97 degrees. But our 10-day run of sweaty weather is about to end.

Seldom does the air mass change from hot and muggy to dry and comfortable without thunderstorms. And numerous storms are expected this afternoon and early evening; some could be quite heavy.

The National Weather Service has issued a flood watch from 2 p.m. to 11 p.m. in anticipation of the downpours.

“Showers and numerous thunderstorms are expected this afternoon into this evening,” the Weather Service says. “Rainfall amounts will average around 1 to 1.5 inches across the area, but locally higher amounts of 2 to 4 inches are likely and much of that may fall in a one to two hour time frame.”

Areas most vulnerable to flooding include those near creeks, streams and zones where there is poor drainage. July and the start of August have been wetter than normal in most parts of the region, which increases the potential for flooding since soils are already wet.

In addition to the heavy rain, storms will also bring dangerous lightning and very strong localized wind gusts. The Weather Service has placed our area in a marginally elevated risk zone for severe storms because of gusts that could cause tree damage.

Why there aren’t National Weather Service warnings for lightning

Short-term models project numerous storms in our western areas between 3 p.m. and 5 p.m., close to the Beltway between 4 p.m. and 6 p.m. and into our eastern suburbs between 5 p.m. and 7 p.m. However, more isolated storms could develop early this afternoon (especially west of Washington) and linger past sunset (especially south and east of Washington).

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The forecast weather map for early evening (shown below) depicts the very slow-moving front (and in fact, additional fronts to approach from the Northwest later tomorrow). The slow-moving front is expected to focus heavy shower and thunderstorm development across the greater D.C. region starting as early as midafternoon today, and continuing into the evening.

The mountain terrain to our west and bay breeze circulations to our east will add to a focused uplift of a moist, unstable air mass. A weak high-altitude disturbance is also approaching and will boost the uplift more broadly.

The first widespread threat is for torrential rain that may lead to flash flooding. The atmosphere is exceptionally moist through a deep level, and storm cells will move slowly — given very weak flow aloft. Additionally, those weak winds are aligned parallel to the frontal boundary — creating an ideal setup for repeated passage or “training” of cells over the same regions.

A second, more marginal threat will be for a few storms to attain severe levels, in terms of isolated wind gusts reaching or exceeding 55-60 mph. These so-called “microbursts” can occur when the heavy, wet cores of storm cells collapse — creating a chilled outburst of violent wind at the ground.

Multi-fatality lightning strikes are rare, but most have this in common

As always, it’s worth reminding folks to be lightning aware. The sobering loss of life from severe storms last week near the White House is a reminder that all it takes is one strike. We’ve had an especially stormy summer and in our densely-populated region, everyone should be mindful not to neglect lightning as a distinct storm hazard.

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