Radar courtesy MyRadar | © OpenStreetMap contributors

* Severe thunderstorm watch until 9 p.m. for immediate D.C. area and points south | Tornado watch for northern and northeastern suburbs *

6:15 p.m. — After producing several tornado warnings, storms exiting region

The scattered storms that transited the region late this afternoon and evening triggered four tornado warnings and 11 severe thunderstorm warnings. So far, reports of damage have been pretty isolated with just a few instances of downed trees and wires. In addition, a few reports of large hail, up to the size of golf balls, came in from Stafford County near a rotating storm that tore through Fredericksburg.

The National Weather Service received two reports of funnel clouds: one in eastern Howard County and another south of Fredericksburg. So far, no tornadoes have been confirmed, but Baltimore-Washington International Marshall Airport did clock a wind gust of 62 mph as a tornado-warned storm passed by.

It can take time until the extent of storm damage is known, however. And the National Weather Service would probably not conduct any surveys of damage to determine whether any tornadoes touched down until Friday.

This will be our last update in this article. Stay tuned for our PM Update for the forecast for the rest of tonight into tomorrow.

5:55 p.m. — Intense storms near Annapolis and Upper Marlboro pushing east-southeast; tornado warning discontinued around Fredericksburg.

The storm that was potentially tornadic has pushed southeast of Fredericksburg (see update below) and the tornado warning has been discontinued.

The two main storms of concern in the region are entering the Upper Marlboro and Annapolis areas and should push east-southeast over the next hour, gradually exiting our area. But heavy rain and strong winds are possible, especially through southern Anne Arundel and northern Calvert counties, where the Upper Marlboro storm has yet to pass.

5:25 p.m. — Tornado warning for Fredericksburg area until 6 p.m.

Radar indicates a rotating storm approaching Fredericksburg that could produce a tornado. It will be near Fredericksburg at 5:30 p.m. The storm may also produce large hail up to two inches in diameter. Seek shelter in the lowest level of a strong building away from windows if in this zone.

5:15 p.m. — Scattered storms, several severe, along Interstate 95 from Aquia Harbour in Virginia north to Baltimore

A broken line of severe thunderstorms, including rotating supercells, stretch from near Aquia Harbour (just north Fredericksburg) to north of Baltimore.

Here is a summary of the most intense storms:

1. The cell west of Aquia Harbour is pushing toward I-95, including the Quantico and Triangle area to as far north as Dale City and Woodbridge.

2. A large cell just east of Silver Spring into Beltsville and Greenbelt. It’s pushing east-southeast toward Bowie and Largo.

3. The cell in northern Anne Arundel County, warned for a tornado (see below) between Elkridge and Severn, is headed into Glen Burnie and pointed at Pasadena and Severna Park.

These three storms could all produce damaging winds in addition to very heavy rain and frequent lightning.

5 p.m. — Tornado warning from near Columbia to Glen Burnie until 5:30 p.m.

A severe storm capable of producing a tornado is near Columbia and is pushing east at 25 mph. The following locations are in the storm’s path: the south side of Elkridge, Severn and Glen Burnie, as it sweeps east. Seek shelter in the lowest level of a strong building away from windows if in this zone.

4:45 p.m. — Severe thunderstorm warning from Gaithersburg to Laurel until 5:30 p.m.

An intense storm has flared up around Gaithersburg that is sweeping southeast toward Aspen Hill. This storm will eventually end up near Beltsville, Greenbelt and Laurel around 5:30 p.m. In addition to heavy rain and lightning, it’s likely to produce some areas of damaging wind gusts and perhaps small hail. It also shows some rotation, so we cannot rule out a tornado spinning up within this cell.

4:35 p.m. — Tornado warning for northeast Howard County, including Columbia and Ellicott City until 5 p.m.

Radar indicates possibility of tornado in northeast Howard County that could be near Ellicott City at 4:50 p.m. Seek shelter in the lowest level of a strong building away from windows if in this zone.

4:25 p.m. — Severe storms focused north of Interstate 70 so far

The most intense storm activity so far is concentrated north of I-70 with several warnings between Frederick and Baltimore. In addition to the tornado warning in Carroll County, now focused east of Westminster, a severe thunderstorm covers the area around Sykes and Eldersburg, straddling the boundary between northern Howard and southern Carroll counties and extending east to near Ellicott City. This storm shows signs of rotating, so there is a bit of a tornado risk with it as it pushes east-southeast at 25 mph.

4:05 p.m. — Tornado warning until 4:30 p.m. in Carroll County, including Westminster

Radar indicates rotation and the possibility of a tornado approaching Westminster. If you’re in this area, seek shelter in the lowest level of a strong building, in an interior room away from windows.

3:45 p.m. — Showers and storms pushing east of Interstate 81 into Loudoun and Frederick counties, could near metro around 5 p.m.

A line of showers and storms stretches from Hagerstown to near Front Royal. For the most part, this activity is not severe. A few storms have also fired off ahead of this line in Frederick and Loudoun counties. Much of this activity is pushing east at 25 mph or so, which could bring it to near the Beltway a little after 5 p.m. The environment is favorable for some storms to intensify, especially in Maryland.

3:10 p.m. — Severe thunderstorm watch issued for immediate D.C. area and points south

Shortly after issuing the tornado watch for the District’s northern and northeastern suburbs, the National Weather Service issued a severe thunderstorm watch for the immediate area and points south, also in effect until 9 p.m.

“Thunderstorms are expected to develop soon along the mountains of western Virginia and spread eastward across the watch area,” the Weather Service wrote. “The strongest cells will be capable of damaging wind gusts and hail.”

While tornadoes are more probable north of the District, they cannot be ruled out in the immediate area.

A severe thunderstorm watch means conditions are favorable for severe storms, but depend on ingredients coming together. If a warning is issued, it means a severe storm is imminent or occurring and you should seek shelter.

2:55 p.m. — Tornado watch issued for Frederick, Carroll, Howard and Anne Arundel counties and points north until 9 p.m.

Storms are beginning to increase in coverage in eastern West Virginia and south central Pennsylvania and are pushing southeast. Anticipating that some of these storms may rotate, the National Weather Service has issued a tornado watch for much of central and eastern Pennsylvania, New Jersey, Delaware, and central and eastern Maryland through 9 p.m.

“A few supercells and bowing lines are expected, capable of damaging winds, hail, and perhaps a few tornadoes,” the Weather Service wrote.

A tornado watch means conditions are conducive to severe thunderstorms and the possible development of tornadoes, but not a guarantee. However, if a tornado warning is issued for your location, it means you should seek shelter immediately, preferably in the lowest floor of a strong building, away from windows.

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, a 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.