7 p.m. - Storms exit to the east, light to moderate showers linger

This evening’s storms, which moved through fast enough to avoid significant flooding despite the heavy downpours, have pushed off to the east and southeast of the metro area. A lingering band of light to moderate showers will continue across the region from west to east during the next couple of hours before we start to dry out.

This is our last planned update for this post. Keep reading for Ian Livingston’s forecast through tomorrow...

Through tonight: Once showers and storms end, it’s partly cloudy with temperatures settling to a range of mid-60s to low 70s. Winds turn to come from the northwest by dawn. Since winds are on the light side, some patchy fog can’t be ruled out late tonight.

Tomorrow (Friday): Skies are mostly sunny and the air dries out a bit over time as a breeze from the west and northwest continues. Humidity is down, to the moderately high level, as highs reach the upper 80s to low 90s.

6:15 p.m. - Some downpours moving through but widespread flooding has not materialized. End of storminess in sight.

The showers and storms moving through the region since this afternoon have moved quickly enough that we’ve avoided flash flooding in most of the area. Also, this second wave of storms moving through, at least north of downtown, isn’t as intense as feared because the energy it could draw from was used up some from the first wave.

That said, we are seeing heavy rain and frequent lightning south of downtown Washington through Fredericksburg and these vigorous storms will sweep through Southern Maryland over the next hour or so.

North and west of the Beltway, we just see light to moderate rain which should pass through the region over the next couple of hours. By 8 or 9 p.m., many of us should catch a break in the rain.

5:45 p.m. - Second wave of storms arriving with very heavy rain and gusty winds, especially southern areas

The leading edge of the second wave of storms is moving into the immediate area and is about to move inside the Beltway. It is currently stretching from Frederick to Dale City and then curling to the southwest into central Virginia. The heaviest activity is along the southern half of this area of storms from just north of Manassas to the south and southwest. It is pushing east at around 25 mph and will move inside the District by 6 p.m. and reach the east side of the Beltway around 6:15 p.m.

Expect the heaviest rain as well as some strong gusts winds (up to 40 to 50 mph) generally along and south of Interstate 66 and Route 50 as this pushes east.

“Locally heavy rainfall ... may cause ponding of water on roadways and cause small streams to go out of their banks,” the Weather Service cautions. “Be alert for the potential of standing water and hydroplaning.”

5:00 p.m. - First wave of storms exiting but second, larger wave approaching from west and southwest

The first round of storms swept through the immediate area quickly, and has already reached Anne Arundel County having exited the Beltway. Its fast forward speed (25 mph) prevented widespread flooding although we did see some isolated reports of high water. We also saw a couple reports of downed trees from high winds from the intense storm that passed through Takoma Park, Silver Spring and College Park.

Now the next wave is moving in from west and southwest, stretching from central Loudoun south through Fauquier County. This activity is moving east at 20 mph and should effect all of our areas just west of the Beltway including Leesburg, Poolesville, Manassas, Centreville, Reston and eventually Fairfax over the next 30 to 45 minutes. It should reach the Beltway between 5:30 and 5:45 p.m.

This incoming area of rain and storms, which is larger than the first, has a higher flood potential. So allow extra time if you plan to commute over the next hour or two and remember to turn around and find a different route if you encounter high water.

In addition to very heavy rain, some wind gusts of 40 to 50 mph are possible with these inbound storms.

4:00 p.m. - Storms from Silver Spring to Mt. Vernon, passing through District; severe thunderstorm warning from College Park to Odenton until 4:45 p.m.

Storms span from Silver Spring through the District to Mt. Vernon. They are especially heavy between downtown Washington and Silver Spring but are moving fast enough (to the east at 25 mph) that flooding has yet to be reported. This entire line of storms will exit the east side of the Beltway in the next 30 minutes. It should hit Bowie, Clinton and Waldorf between 4:15 and 4:30 p.m.

The Weather Service has issued a severe thunderstorm warning until 4:45 p.m. for the zone from Silver Spring to Odenton, including Landover and Bowie, where wind gusts could hit 60 mph. Radar indicates some very strong winds between Silver Spring and Greenbelt, right around College Park and pointed at Greenbelt.

3:35 p.m. - First wave of storms entering the Beltway with heavy rain and lightning

A line of storms stretches from Woodbridge through Annandale, McLean, Bethesda and up to Olney with locally heavy rain and lightning. So far, no flash flood warnings have been issued but expect visibility to drop as these storms pass and some ponding of water on roadways, especially in poor drainage areas. Some locally strong winds are also possible, with gusts over 30 mph.

The storms are moving east-northeast at around 10 to 15 mph and should cross Interstate 95 and pass through the District over the next hour or so.

This is just the first wave of storms and a second round is expected to move in from the west starting around 5 p.m.

3:00 p.m. - Weather Service warns of the potential for “significant” flash flooding as first wave of storms moves into area

An area of storms has developed between Manassas and Gaithersburg, with pockets of very heavy rain and frequent lightning. The most intense activity is southwest of Centreville moving northeast toward central Fairfax County. We’ll need to watch this storm for flash flood potential.

This area of storms is ahead of the main line of storms in eastern West Virginia expected to move through the region this evening.


(National Weather Service)

Meanwhile, the National Weather Service has issued special discussion for our region with the headline “flash flooding likely” and that storms “will likely produce rainfall rates of 2″/hr or more in an exceptionally moist environment." The discussion says models simulate “pockets of 4” of rainfall, if not locally higher" and that “flash flooding could be significant” if storms pass over the same areas repeatedly.

Original post

The thick humidity of summer has returned, and a low-pressure system passing to the north along with its associated weather features will probably help ignite a round of strong to severe thunderstorms this afternoon into evening.

The most likely timing for thunderstorms is about 3 p.m. in our western areas to around 7 or 8 p.m. well to the south and east. In the immediate area near the Beltway, the most likely window is roughly 4 to 6 p.m., with a focus around 5 p.m. — coinciding with the commute.

Given all the rain that has fallen recently, areas of flooding are a concern.

“Torrential rainfall may lead to totals exceeding two inches in a short period of time. Isolated locations may receive three or four inches,” the National Weather Service cautions.

However, we do not expect an event as intense and widespread as the major flash flood event Monday. That said, it would be wise to consider delaying your commute if it is raining heavily and remember never attempt to drive through flooded roads. Turn around, don’t drown.

In addition to the hazards posted by heavy rain and dangerous lightning, these storms may produce some pockets of damaging winds.

Storm dashboard


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Approximate storm arrival time:

  • 2 p.m. to 4 p.m. in western areas.
  • 4 p.m. to 6 p.m. in the immediate area, including the Capital Beltway.
  • 5 to 7 p.m. in areas east of Interstate 95, and as late as 8 p.m. or so in southern Md.

Isolated storms are possible earlier, starting in the early afternoon throughout the region.

All clear: 8 to 10 p.m. from west to east, showers possible after.

Storm duration: 30 to 60 minutes, although rain may linger longer.

Chance of measurable rainfall in any location: 75 percent.

Storm motion: West to east.

Likely storm effects: Heavy rain, lightning, isolated flash flooding, gusty winds.

Possible storm effects: Scattered flash flooding, damaging wind gusts, hail.

Rainfall potential: Considerably variability; average may be as little as 0.5 inches, but some areas could see one to three inches and others very little.

Discussion

Today the likelihood of widespread showers and thunderstorms is high, given an approaching front and strong upper-level disturbance in the jet stream flow. Additionally, the air mass is quite moist. With the recent extreme rains across the D.C. region, the threshold for flash flooding is low. The most likely time frame for these storms is roughly 3 p.m. west to 8 p.m. south or east.

A solid line of convection has formed west of the Appalachian spine and is moving steadily toward the east-southeast. This is our main focus. Models such as the high-resolution HRRR and NAM hold the line together while regenerating it east of the mountains. It then sweeps across the D.C. region during the afternoon and evening.


Radar simulation for 5 p.m. from the high resolution NAM model.

So what can we expect when this line of storms arrives?

Despite cloud cover in parts of the region (from midmorning showers), the atmosphere is forecast to be sufficiently unstable to support strong to perhaps severe storms. One caveat comes from the morning weather balloon launch at Dulles, which showed a fairly stable (or warm) temperature layer in mid-levels of the atmosphere. If this prevails through the afternoon, it could lessen the intensity of the storms.

It is likely that the most significant hazard this afternoon is the prospect of more torrential rain.


Rain may be heavy, but also possibly a bit hit or miss.

The entire D.C. area is under a flash flood watch, and the Weather Prediction Center (WPC) has also placed it in a moderate risk zone for excessive rainfall. This is due to a combination of factors, including the arrival of organized storms, atmospheric moisture near its maximum and other processes that will be promoting plentiful uplift of air.

This line of storms should pass quicker than what was seen with the flooding on Monday, thus we are not expecting pockets of four to six inches of rain. However, a quick one to two inches is all that may be necessary to cause flooding problems again. Those amounts are certainly a possibility and we can’t entirely rule out isolated amounts over three inches.

The Storm Prediction Center has also placed our region under a slight risk for severe thunderstorms, which is a Level 2 out of 5 on the likelihood scale. The combination of an unstable atmosphere and modest wind shear (increase in winds with altitude) is ripe for multicell thunderstorms, which tend to be more intense and sustained than ordinary “pop-up” cells.

A combination of instability, heavy rain and shear may produce a few wet microbursts with isolated wind damage.

We’ll update as the situation unfolds.