Radar projection for 8 p.m. Tuesday evening.

Strong to locally severe thunderstorms are in the forecast Tuesday afternoon and evening, and the potential exists for this to be a significant severe weather event.

The atmosphere will have many of the ingredients necessary for supercell thunderstorms to form and track across the region. Such storms have persistent, rotating updrafts and can unleash tornadoes as well as produce large hail and damaging winds.

“Tomorrow afternoon and evening present a potentially volatile situation along the I-95 corridor,” says Jeff Halverson, Capital Weather Gang’s severe weather expert. “The Storm Prediction Center has upgraded its outlook to Enhanced (level 3/5 on the threat scale), which is significant for the Mid-Atlantic; very infrequently do we get elevated to that level.”

Threat area

The entire DMV region will be at risk tomorrow for damaging winds, large hail and a few tornadoes as severe storms form to our west and generally track to the east as the day goes on.

Timing

There could be a cluster of showers and thunderstorms moving east out of the Ohio Valley on Tuesday morning, bringing the chance for rain into the forecast early in the day, between rush hour and noon. This will not be the main severe threat, however.

Other thunderstorms are expected to fire rapidly around 2 to 4 p.m. over the Shenandoah Valley, shifting eastward thereafter and increasing in both areal coverage and intensity. In the D.C. area, we may be dealing with them from late afternoon straight through the evening commute and into the first part of the night.

If another line of storms develops trailing the initial batch, it would come through late in the day, keeping rain chances alive through midnight.


The setup

The severe weather setup will feature “ridge runners,” or shower and thunderstorm activity riding southeast down the eastern edge of a high pressure dome anchored to our west. That high pressure area is what’s been bringing a heat wave to the South, and increasing the humidity in the D.C. area as well.

“An approaching frontal system, a disturbance in the jet stream, and an unstable atmosphere and increasing winds aloft will raise the prospect for damaging wind gusts and even a tornado or two,” Halverson wrote. The damaging wind threat is the No. 1 concern.

Patrick Marsh, chief of the science support branch at SPC, agrees. “Any time you have strong flow with a moist unstable air mass, you run the risk for severe weather.” He explained that Tuesday could feature a few rotating thunderstorms.

Any supercells that do develop will ride along and just south of a warm front, and the position of this front will be crucial in determining where the highest risk will be for isolated tornadoes.

“I think it will set up just to our north,” said Cody Ledbetter, a meteorologist with the National Weather Service in Sterling, Va. It will “probably be near the Pennsylvania/Maryland border.” Some computer models, however, have the warm front struggling to make it even to the Beltway. As such, anyone from the Mason-Dixon Line and the Delmarva all the way to Fredricksburg should check back for later, more refined forecasts.


Weather models are highlighting the potential for rotating thunderstorms Tuesday afternoon in our area. Damaging winds are the main concern, with a secondary risk of isolated tornadoes. (Pivotal Weather)

After storms initially fire, they may remain discrete, individual rotating cells before merging into a line. Once that transition happens, it would decrease the tornado and large hail risks, but boost the odds of widespread, damaging straight-line winds. Storms should move quickly enough to limit flash flooding potential, but it’s something we’ll have to keep an eye on in case any storms repeatedly move over the same regions.

“Regardless, there’s a lot of moisture in the air, so the storms will be producing heavy downpours,” Ledbetter said.

Uncertainties

The amount of sunshine we get tomorrow will help determine how much severe weather materializes. If the morning stays cloudy and even rainy, the diminished solar heating could hold our severe risk at bay by limiting atmospheric instability.

If sunshine emerges, however, the atmosphere could become feisty.

The bottom line

The key takeaway is that tomorrow is a day to watch weather-wise. There have only been three other times when the Storm Prediction Center has placed the District in an “Enhanced” risk zone a full day in advance. It’s a sign to keep an eye to the sky, have a severe weather plan, and know what to do if a warning is issued for your location.