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After huge hail and flooding in Frederick, storms to let up across region overnight

* Flash flood watch until 1 a.m. Wednesday, except southern areas | Flood warning for in many north and west suburbs through 2:15 a.m. *

10:30 p.m. update: Wrapping up for tonight

As we no longer have severe storms and the remaining activity is expected to slowly weaken, this will be our last update of the night.

Ground-zero for tonight’s storms was most certainly Frederick, Md., which had both golf ball size hail and half a foot of rain in just over two hours – which led to severe flooding (see earlier updates). Parts of upper Montgomery County have also seen flooding according to the Maryland State Highway Administration.

Be careful if you’re out and about tonight as there are still areas of heavy downpours and lightning, which will only slowly diminish over the next few hours. We still can’t rule out some new areas of isolated flooding and flooding warnings are in effect for many of our northern and western suburbs through 2:15 a.m. due to rising streams.

For the outlook for the rest of tonight and Wednesday, see: PM Update: More rain likely Wednesday

The National Weather Service declared a flash flood emergency in Frederick, Md., on May 15. (Video: The Washington Post)

10:10 p.m. update: Severe thunderstorm watch canceled

As the intensity of storms has waned, the severe thunderstorm watch for the region has been canceled.

Despite the fact damaging winds and hail are no longer expected to be a significant threat, storms with heavy rain and lightning continue. The two most concentrated areas of activity are in western Montgomery County about to pass over Interstate 270 and just east of the Beltway and south of Rt. 50.

Light showers continue to skirt Frederick where severe flooding was ongoing in the last 90 minutes:

9:45 p.m. update: Storms starting to decay

We have widespread showers and storms around the region but no longer any warnings for severe weather. Storms that previously contained hail and strong winds have weakened some.

The other bit of good news is that the extreme rainfall over Frederick has pushed a bit south, even as occasional showers brush the area. That said, especially from Leesburg to Annapolis through the Beltway and then north to the Interstate 70 corridor, there are still heavy downpours and quite a bit of lightning. And it will take a few more hours for the weather to substantially calm; and, even then, some showers may linger well into the night.

9:30 p.m. update: Severe flooding continues in Frederick

The Flash Flood Emergency is Frederick is ongoing. The National Weather Service has confirmed up to six inches of rain has already fallen there and reports numerous water rescues, water inside buildings, and many roads flooded. NBC4’s Jackie Benson reports numerous cars have “stalled out in flash floodwaters”:

Graham Cullen, with the Frederick News Post, said six people were rescued from three trapped vehicles:

For the time being, the heaviest rain has slipped just south of the city but could expand back north. We will watch this.

9:20 p.m. update: Severe storm in District weakens, but big lightning producer

The severe storm warning for the Beltway and District has expired and not renewed as the storm has weakened slightly. Still, the storm is presently cutting through the District, Alexandria, and Mount Vernon with very heavy rain and a lot of lightning. See these pictures:

The Washington Nationals are in a rain delay.

9:05 p.m. update: Flash Flood Emergency in Frederick, Md.

The National Weather Service has declared a flash flood emergency for the city of Frederick, Md. through 11:45 p.m. This is the most severe alert for flash flooding. At least three inches of rain has fallen in a short amount of time and torrential downpours continue. Emergency management is reporting multiple water rescues in the area due to high water.


8:38 p.m. update: Severe storm to move inside Beltway, warning issued

An intense storm that has flared up between Burke and Vienna is moving east inside the Beltway, and prompting a severe thunderstorm warning through 9:15 p.m. for much of the Beltway and the District.

This storm could produce not only extremely heavy rain and frequent lightning, but also 60-mph wind gusts and quarter-size hail. Head inside.

8:25 p.m. update: Flash flood warning in Frederick while storms form close to D.C.

Doppler radar indicates at least 1 to 3 inches of rain has already fallen around Frederick and at least another inch could fall. The National Weather Service has thus issued a flash flood warning through 11 p.m. The water could quickly rise in creeks and flood roads. Do not attempt to drive through a flooded road, turn around.

Already, some flooding has been reported:

Meanwhile, ahead of the main line of storms, now right along Interstate 70 (from Frederick to Baltimore), the outflow or cold pool that the line is generating is triggering showers and storms from the District north. These are not severe, but are expecting to increase in coverage and produce heavier rain as time wears on.

8:00 p.m. update: Storms edging toward D.C.’s far northern suburbs. Warning for ‘golf ball size’ hail

Storms with a history of producing golf ball size hail around Frederick (see below) are slowly drifting south. A severe thunderstorm warning covers southern Frederick, northern Loudoun and northern Montgomery counties through 8:45 p.m.

In addition to the potential for large hail, wind gusts could reach 60 mph. The rainfall is extremely heavy, falling at over 2 inches per hour in spots and could lead to flooding.

7:45 p.m. update: Weather Service says up to 4 inches of rain could fall

As this vigorous squall line pushes south out of Northern Maryland and slows down, it has the potential to dispense a remarkable amount of rain, especially from the District and to the north and east. In a special statement, the National Weather Service said rainfall rates could reach 2 to 2.5 inches per hour with localized totals to 4 inches. This kind of rain would lead to serious flash flooding potential.

Already, some areas northeast of Frederick have received at least 2.5 inches of rain.

Meanwhile, we just received a report of golf ball-sized hail in Frederick.

7:25 p.m. update: Large hail, heavy rain pounding Frederick

The storm hitting Frederick is intense and is unleashing marble-sized to quarter-sized hail or larger.

One Twitter follower, Kevin Quinlan, reported almost an inch of rain in less than 30 minute and “crazy lightning.”

All across northern Maryland the storms are producing a ton of lightning, very heavy rain and, likely, some very strong wind gusts.

When do we expect these storms to reach the Beltway? The storms seem to be slowing down as they push south and the latest models suggest maybe closer to 9 or 9:30 p.m. rather than 8 or 8:30 p.m. We’ll keep you posted on their progress.

7:00 p.m. update: Storms rapidly expanding south into northern Maryland; Warning from Frederick to Baltimore

The southern part of the line of the storms which less than an hour ago was confined to Pennsylvania has rapidly expanded south and southwest and is now bearing down Frederick, Maryland and much of northern Maryland east to Baltimore.

The National Weather Service has issued an expansive severe thunderstorm warning from Frederick to east of Baltimore, including northern Howard County and northern Montgomery County through 8 p.m.

Storms along this line, pressing south at 20 mph, could pack pockets of 60 mph gusts and hail, in addition to torrential rain and lightning. Head inside.

6:13 p.m. update: Severe thunderstorm watch issued

Because of all of the atmospheric fuel available for storms (see the 4:10 p.m. update), the National Weather Service has issued a severe thunderstorm watch for the D.C. and Baltimore regions through 11 p.m.

“Thunderstorms are sagging southward across Pennsylvania and into northern Maryland, while other storms form to the west,” the watch says. “Conditions are favorable for locally gusty/damaging wind gusts and hail in the strongest cells.”

In the most severe storms, which should be isolated, wind gusts could reach as high as 70 mph and hail two inches across cannot be ruled out.

Remember that a severe thunderstorm watch means conditions are favorable for severe storms, but they may or may not actually form. On the other hand, if a severe thunderstorm warning is issued, it means severe weather is imminent or happening, and you should seek shelter in a strong building, away from windows.

We think, for the immediate metro area near the Beltway and the District, the window for the most intense storms would be between about 8 and 9 p.m. or so; somewhat earlier to the west and north, and later to the southeast.

Storms are already moving into northern Frederick County, where there is a severe thunderstorm warning through 6:45 p.m.. To the southwest, the area from around Front Royal area to western Fauquier County is also under a severe thunderstorm warning which expires at 7:00 p.m.

Per our earlier updates, once storms move in, they may stick around for a while and track over some of the same areas repeatedly while unloading torrential rain – hence the flash flood watch.

Our next update will be around 6:45 or 7:00 p.m.

5:40 p.m. update: Will storms to our southwest and north connect?

In short, probably.

A wide view of weather radar shows an intense squall line from south central Pennsylvania through eastern New England, and a second broken line of storms bubbling up to our southwest along Interstate 81 in Central Virginia. Model indicate they will try to connect between 6 and 8 p.m. in the northern and western parts of our region. The Virginia line will grow to the northeast while the Pennsylvania line expands to the southwest.

By 8 or 9 p.m., storms should be numerous in the immediate area.

4:10 p.m. update: Atmosphere is juiced for storms

Temperatures have soared into the upper 80s in Washington and it is very humid (dew points in the low 70s). This signifies a very moist atmosphere that storms moving into later will be to draw from. While our greatest concern is the potential for heavy rain and flooding, there is more than enough atmospheric instability not to rule out a damaging wind threat, especially for storms that arrive before sundown.

Short-term models show the line of storms that begin in south central Pennsylvania and continue into New England expanding southeast toward our region between 6 and 8 p.m.

Original post from 2:15 p.m.

For the fifth time in six days, the Washington region faces the likelihood of thunderstorms late Tuesday, mostly between about 6 p.m. and midnight. A few storms could be strong to severe, but damaging winds and hail should be less widespread compared with Monday.

Flash flooding is the biggest concern. Washington and many surrounding areas have received over two inches of rain in the past few days, and soil is becoming saturated.

On Tuesday evening, storm cells may pass repeatedly over some of the same locations. This could cause the levels of creeks to rise rapidly and standing water on roads. If you encounter a flooded road when driving, it is unsafe to attempt to drive across as it is difficult to judge the water depth. Turn around, don’t drown.

While we can’t pinpoint where the heaviest will fall, one to three inches of new rainfall is possible this evening within a county radius of the District and to the north. The heavy rain potential has prompted the National Weather Service to issue a flash flood watch from 3 p.m. Tuesday until 1 a.m. Wednesday for this area.

Rain and storms could make getting to and leaving the Washington Capitals playoff game a nuisance (allow extra time) and result in postponement of the National-Yankees showdown.

While a few pop-up storms could develop in the late afternoon, we expect the bulk of the rain to occur between 6 p.m. and midnight, arriving first in our northern and northwest areas.

Storm dashboard

  • Approximate arrival time for storms
    • 5 to 7 p.m.: Northern Maryland
    • 6 to 8 p.m.: Montgomery, Loudoun and Howard counties
    • 7 to 9 p.m.: Rest of the metro region, excluding far-southern suburbs
    • 8 to 10 p.m.: Far-southern suburbs, including Southern Maryland
  • Storm duration: Up to several hours
  • Chance of measurable rainfall in any location: 50 to 90 percent from south to north
  • Storm motion: Northwest to southeast
  • Likely storm effects: Downpours, lightning, areas of flooding
  • Possible storm effects: Strong winds, small- to medium-size hail
  • Very small chance of: Damaging winds, large hail
  • Rainfall potential:
    • Average of one inch along Interstate 66 and Route 50 north. Locally up to three inches or more.
    • Decreasing amounts to the south, 0.1 to 0.75 inches.


Our region’s atmosphere is recovering from Monday and early Tuesday morning’s thunderstorm activity. A large pool of cooler downdraft air (called an outflow) has been left behind areawide, which has temporarily stabilized the atmosphere. However, warm air arriving on southerly winds and strong sun under largely cloud-free skies will rapidly destabilize the atmosphere this afternoon. Computer models suggest we will reach low 90s in spots, with high humidity.

There is some evidence that Monday’s frontal boundary, which served as the focus for storms, has retreated into central Pennsylvania. Forecast models suggest that the front will shift back south through the afternoon, crossing through the District this evening, stalling south of the region Wednesday morning. With this front returning and plowing into an unstable air mass, we are again set for evening thunderstorms.

The amount of buoyant energy available to fuel thunderstorms areawide is expected to become significant later today, reaching a level at which a few strong to severe storms are possible. The front is likely to organize these storms into a broken line, oriented WSW-ENE and sagging southward across the Mason-Dixon Line during early evening. Additionally, a cluster or two of storms may crop up across the greater D.C. region (south of the line) in the very unstable air mass, triggered by air converging along Monday’s remnant outflow boundaries.

The National Weather Service Storm Prediction Center (SPC) suggests that the area of greatest severe threat (Enhanced Risk, Level 3 out of 5) will lie far to our northeast in the vicinity of a wave of low pressure. The environment northeast of us also contains stronger wind shear (winds increasing with altitude). We are in a more marginal risk zone for severe storms.

The threat of severe storms will increase from south to north, from Northern Virginia and the District toward Pennsylvania. Regionally, the widespread risk is probably greatest along the Mason-Dixon Line.

By the time the possible line of storms works south toward the District during the evening, the atmosphere will probably stabilize, reducing storm severity. Any scattered storms that erupt ahead of this line in the late afternoon and early evening hours may reach strong to potentially severe levels. Here, the degree of instability and wind shear will be ideal for organized, multicell storms.

SPC suggests a 5 to 15 percent risk of damaging wind from south to north in our region (much less than Monday’s 45 percent chance) and 5 percent chance of large hail.

As mentioned, the hazard of greatest concern is heavy rain and flooding as westerly winds blow parallel to the front oriented west to east over the area, causing what’s known as training — when storm cells track over the same areas repeatedly.