Severe thunderstorm threat in Washington, DC area Thursday - The Washington Post

Twice this week, on Monday and Wednesday, severe storms have charged through parts of the Washington region, unleashing damaging winds and even some hail. Today may mark the third occasion, as a strong cold front approaches while an energetic weather disturbance transits the region.

The National Weather Service has cautioned there is a chance of a “significant severe weather outbreak” if ingredients come together. In this scenario, thunderstorm could generate damaging winds, hail and even some tornadoes, in addition to heavy downpours and lightning.

The Washington region has been placed in a Level 2 out of 5 risk zone for severe storms by the Weather Service, while the Baltimore area has an “enhanced” Level 3 risk.

Storms may come through the area in two rounds: the first late this afternoon and early this evening; the second closer to midnight. The first round has the most potential to produce severe storms, especially if sunshine breaks out through midafternoon, energizing the atmosphere.

At a glance

Timing: The first round is most probable between 3 and 7 p.m., moving from west to east across the region; the second, after 9 or 10 p.m., as the cold front comes through.

All clear: After 2 a.m.

Storm coverage: Scattered, with greatest concentration north of the District

Storm duration: 30 minutes on average

Likely impact: Very heavy rain, frequent lightning, gusty winds

Possible impact: Damaging winds, small hail

Small chance of: Large hail, a few tornadoes, flash flooding

Rainfall potential: Highly variable, up to 1 to 2 inches in the heaviest storms but averaging 0.25 to 0.5 inches. Some areas may get grazed or missed, especially into our southern suburbs.

Discussion

The setup involves a surface low pressure system tracking across the Great Lakes (shown below), with a warm front pushing northward across Pennsylvania and New Jersey, and a cold front approaching from the Ohio Valley. In between these fronts, the D.C. region lies in the warm sector with warm, southerly flow and abundant humidity.

Higher into the atmosphere, the situation is dynamic with a pocket of energy moving through the jet stream (called a shortwave trough; see image below). Cooler air in the core of the trough aloft will help destabilize the atmosphere, and increase the winds in middle levels of the atmosphere. The resulting shear will help intensify storm cells and organize them into longer-lived complexes.

The combination of an unstable atmosphere, plenty of shear, energy aloft and an approaching front is a potentially volatile combination. These factors favor clusters or arc-shaped complexes of strong to severe thunderstorms, and possibly a few supercells or rotating thunderstorms.

However, there are a few aspects that may help temper storm intensity:

First, abundant mid- and upper-level cloud cover has been streaming into the area from overnight and early morning storms that swept through the Great Lakes. The thickness of the cloud cover tends to increase as you head from Northern Virginia to Baltimore. Depending on the persistence of these cloud layers, heating from the sun may be reduced in spots, lowering the maximum afternoon temperatures. This would prevent the atmosphere from fully destabilizing.

Second, the morning weather balloon launch revealed several “inversion layers” (thin layers where the air temperature is warmer aloft). These could rob rising cloud updrafts of some of their buoyancy, weakening thunderstorm cells. However, with the approaching shortwave trough and its cooler air aloft, these layers could be eroded enough to not hamper storm intensity.

Third, how dense and widespread the area affected by severe storms will be this afternoon and evening is questionable. The suite of high resolution (thunderstorm-simulating) forecast models suggests that cells may only be scattered in nature. Which means not everyone will experience severe weather. An example of one of these simulations is shown below.

Should severe storm cells develop, however, locations impacted will experience a torrential downpour and intense lightning. Hail may range from quarter to golf ball size, wind gusts may exceed 70 mph, and tornado activity is possible.

Everyone needs to be vigilant this afternoon and evening for severe weather impacts, and we will be monitoring the situation closely.

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Severe thunderstorm threat in Washington, DC area Thursday - The Washington Post
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