Atmosphere primed for possible severe weather this afternoon and evening

1:45 p.m. update: The Storm Prediction Center says it is likely (80 percent chance) to issue a severe thunderstorm or tornado watch for the region.

(Storm Prediction Center)

(Storm Prediction Center)

cwg_storms_02

From 1:20 p.m.: The combination of heat, moisture and low-level spin in the atmosphere may foster an environment conducive to strong to severe thunderstorms this afternoon and evening.

The most likely time frame for vigorous storms is between 3 and 8 p.m., with 5-7 p.m. perhaps most likely in the immediate metro area.

HRRR model radar simulations shows cluster of storms over the region around 6 p.m.  Note this is just a simulation which may or may not come close to matching reality. (WeatherBell.com)

HRRR model radar simulations shows cluster of storms over the region around 6 p.m. Note this is just a simulation which may or may not come close to matching reality. (WeatherBell.com)

The chance of storms varies from about 70 percent south and southwest of the District to 60 percent in the immediate metro region to around 50 percent north of I-70 in Maryland.

As always, recognize thunderstorm formation is somewhat chaotic and difficult to predict and adjustments to the forecast may be required.

Possible hazards from any severe storms that develop include lightning, damaging winds, hail, isolated tornadoes (probably short-lived and low-end), and heavy rain. The chance of severe storms is highest to the west and southwest of the metro area, but cannot be dismissed for the metro area itself. In northern and northeast Maryland, the air is more stable and the risk of severe weather is somewhat lower.

Area in yellow over the Mid-Atlantic declared under a slight risk of severe thunderstorms. (Storm Prediction Center)

Area in yellow over the Mid-Atlantic declared under a slight risk of severe thunderstorms. (Storm Prediction Center)

National Weather Service’s Storm Prediction Center has declared the region under a “slight risk” of severe storms.

Even though the threat of severe weather may wane somewhat after dark, additional showers and storms with heavy rain and lightning could develop overnight.

Technical discussion

Morning cloud cover north of the District has limited the development of instability. But around the District and southwestward, the sun has broken out of the cloud and is destabilizing the atmosphere.

The models are predicting very impressive instability area-wide this afternoon. At midday, Convective Available Potential Energy (CAPE) was already exceeding 2,000 just southwest of the District and will increase some in the next few hours.

Wind shear predicted to be moderate, in the 30-35 kt range, with strong veering (turning of wind with altitude in a clockwise fashion from the southeast to more westerly) in the low levels. The veering (directional change) is courtesy of a backdoor frontal zone which has set up shop across central Maryland.

Most models including the NAM and WRF-AFR develop robust convection this afternoon, first to the south of DC, and also to the west over the higher terrain. Convection from both regions advances into the D.C. metro late in the afternoon.

There is a decent prospect that part of our region will have to contend with severe weather today. We see the strongest threat to the immediate south and west of D.C., including NOVA, and also including D.C. proper. Central to northeastern MD may too close to the backdoor front, where cloud and more stable marine air have persisted.

Convection will likely take the form of organized multicells, and a few supercells may also develop. In the stronger/longer-lived cells, expect the possibility of severe wind gusts, small to medium hail, and a weak to possibly moderate tornado. The airflow aloft is faster today and the moisture has decreased a bit, so flash flooding is less of a concern. However, areas (northern PGCO) that received tremendous rain yesterday are saturated, so it won’t take much heavy rain today to potentially re-flood those zones.

CWG will remain vigilant about the situation through the day and update as necessary. It would not surprise us to see a severe thunderstorm or tornado watch issued by mid-afternoon.

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