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Jul 27, 2018

Intense storms may unleash flash flooding and damaging winds through this evening I The Washington Post I Weather Alert

Updates: Intense storms may unleash flash flooding and damaging winds through this evening

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* Severe thunderstorm watch until 10 p.m. | Flash flood watch 3 to 9 p.m. *

3:20 p.m. – Watching main line of storms developing out to west

The main line of thunderstorms of concern is now near Interstate 81 and should passing through that area over the next 90 minutes or so, before affecting the metro region between 5 and 10 p.m. (details provided below).
But we still have isolated storms around, one between Reston and Potomac, and large cluster in southern Charles County which has prompted a severe thunderstorm warning through 4 p.m. This particular storm has a lot of lightning and may also some pockets of damaging winds.

1:52 p.m. – Severe thunderstorm watch issued, runs until 10 p.m.

The National Weather Weather has issued a severe thunderstorm watch for all of the region. It warns storms may produce damaging wind gusts up to 65 mph and a few instances of large hail about to 1.5 inches across. A watch means conditions are favorable for severe storms, but not a guarantee. But if a warning is issued for your location, that means a storm is imminent or occurring and that you should seek shelter in a strong building.
In addition to the wind and hail threat, flash flooding is a significant concern, as explained below.

Original article from 1:04 p.m. (expired storm updates at the bottom of this post)
The air is thick and the ground waterlogged. As a vigorous front squeezes out the moisture from the sky and violently hurls it down onto our sodden terrain, wind and flooding hazards may become a real issue for the region this evening.
Strong to severe thunderstorms with torrential rain and highs winds are expected between about 5 and 10 p.m. This is a situation where some areas may feel a vicious blow, while others are mostly passed over. But given all of the recent rain, our region is vulnerable.
The saturated ground and potential deluge have prompted the National Weather Service to issue a flash flood watch for the entire area. The soils are so swamped, only an additional half to 1 inch of rainfall is needed to tip parts of the region back into a flooded state. The Weather Service says the hardest hit areas may see 2 to 3 inches in a short time.
Motorists during the evening rush hour and a little afterward will have to be vigilant for rapid ponding on the roads. During heavy rain and the immediate aftermath, avoid routes near creeks and streams that may hastily overflow, rising up to 5 to 10 feet in minutes. Such rapid rises occurred in places earlier this week, like when Four Mile Run in Alexandria rose 6.3 feet in 11 minutes, according to the National Weather Service.
If you encounter a flooded road, turn around — the water level is frequently difficult to judge and a stranded vehicle places the passengers and first responders in danger.
Additionally, the soaked ground has weakened tree root systems to the point that even nonsevere wind gusts, i.e. those in the 40 to 50 mph range, may cause more trees to fall than normal. This means moving to the lowest floor of a dwelling if you hear the wind suddenly come up, and be prepared for power outages.

Storm dashboard

Approximate arrival time for storms:
  • Interstate 81 area: 4 to 7 p.m.
  • West of Beltway: 5 to 9 p.m.
  • The District and inside Beltway: 6 to 10 p.m.
  • East of Beltway: 6 to 11 p.m.
(Isolated storms could develop earlier, especially east of Interstate 81.)
Storm duration: 45 minutes
Chance of measurable rainfall in any location: 60 percent
Storm motion: West to east
Likely storm effects: Heavy rain, lightning, strong winds, flash flooding
Possible storm effects: Damaging wind gusts, small hail
Very small chance of: An isolated tornado, large hail
Rainfall potential: Average 0.5 to 1.0 inches, highest west, but highly variable. Localized amounts of 2 to 3 inches possible.

HRRR simulation of total rainfall, but this is just illustrative and localized amounts will vary considerably.


This afternoon and evening, a cold front will slowly approach the D.C. region from the west, accompanied by a disturbance in the upper atmosphere that will increase wind speeds in the middle and upper levels of the atmosphere. Additionally, the jet stream aloft will be aligned in such a way as to enhance the uplift of air over the Mid-Atlantic.

(National Weather Service)
With plenty of sunshine and a humid lower atmosphere, the atmosphere is expected to destabilize to fairly significant levels by mid-to-late afternoon. The unstable atmosphere, combined with winds increasing aloft, and uplift along the front, will set the stage for some strong to severe thunderstorms.
The high resolution prediction models, in general, develop these storms first over the mountains and highlands to our west, during the mid afternoon. By 5 to 6 p.m., a broken line of showers and storms advances into the far western suburbs, then across the D.C. and Baltimore metro corridor around 7 or 8 p.m. The line is expected to be narrow and may not be continuous.

Simulated radar loop of storms between 4 p.m. and midnight. This is just a model so the actual timing and locations of storms are likely to differ some from this.
With abundant buoyant energy (unstable atmosphere), storm updrafts will be strong and wind shear will further increase the strength and organization of storm drafts.
Some of the embedded storm cells could produce damaging winds, intense lightning and small to medium-size hail. We expect these storms will organize into longer-lived clusters, and a few could even display supercell-like characteristics for a short time. This would mean the potential for larger hail (perhaps golf ball size), microbursts, and/or a brief tornado.

Expired storm updates

1:42 p.m. – Severe thunderstorm watch likely on the way

We’re already seeing a severe storm develop northwest of Fredericksburg and the National Weather Service indicates there’s an 80 percent chance a severe thunderstorm watch will be issued for the region. “Both severe wind gusts and marginally severe hail will be possible with the stronger storms,” the Weather Service said in a statement.

2:25 p.m. – Severe thunderstorm warning for area around Manassas

A severe thunderstorm in east central Fauquier County, about 10 miles east of Warrenton, is moving northeast in the general direction of Haymarket and Manassas. These storm could produce wind gusts up to 60 mph and hail through around 3:15 p.m., when the warning in effect expires.

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Jason Samenow Jason Samenow is The Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association. Follow
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