There are some heavier thunderstorms well to the east near the Bay and into Southern Maryland, and those should move to the east and/or weaken over the next couple of hours.
In the immediate area, just some passing showers are possible through sunset, mainly near and south of Interstate 66 and Route 50. Severe weather is not expected.
Here’s our updated forecast:
Tonight: In the wake of any evening showers, we’ll trend partly to mostly clear later at night. There’s no major change in air mass, which means warm and muggy, though perhaps a bit cooler than recent nights. Lows range from about the mid-60s to the low 70s.
Tomorrow: Rather than fully passing the region, the cold front that helped kick up today’s storms manages to meander locally. It should be sunnier than today, but considerable cloudiness may still develop with time. An isolated storm or two may pop up in the afternoon. Highs are mainly in the mid-and-upper 80s. Humidity is on the moderately high side, with dew points in the upper 60s, making heat indexes approach the mid-90s in spots. Winds are variable around 5 to 10 mph.
5:30 p.m. - Intense storms firing up, but mainly in the far southern parts of the region. Tamer showers more likely in the immediate D.C. area
Strong to severe storms are tracking through Orange and Spotsylvania counties in central Virginia as well as Calvert County in Southern Maryland. This activity is generally headed to the east and northeast. A severe thunderstorm in Spotsylvania County is headed northeast into Fredericksburg and southern Stafford County
Meanwhile, showers to the north, which may move into the immediate Washington region, are considerably weaker. Clouds over our area have robbed some of the energy which might have fueled heavier and more widespread storms. If these showers hold together, they’d pass through the Washington between about 6:30 and 8:30 p.m; there’s some chance they dissipate and the area stays mostly dry.
3:30 p.m. - Weather Service highlights heavy rain threat, but storm timing pushed back
The Weather Service released a special discussion on the potential for heavy rainfall into this evening, noting the possibility of rain rates of at least one inch per hour.
However, radar is pretty quiet in the Washington region and short-term modeling suggests the best chance of storms may be pushed back toward 6 to 9 p.m. whereas we had earlier anticipated 4 to 7 p.m.
1:40 p.m - Severe thunderstorm watch issued for region
The National Weather Service has issued a severe thunderstorm watch for the region, due to the possibility that some storms could produce some damaging wind gusts in addition to heavy rain and lightning. The watch covers a large region from Washington all the way northeast to Boston.
A severe thunderstorm watch has been issued for parts of CT, DE, DC, MD, MA, NJ, NY, PA, RI, VA until 9 PM EDT pic.twitter.com/fzUH5HeGOm
— NWS Severe Tstorm (@NWSSevereTstorm) July 31, 2019
A severe thunderstorm watch means conditions are favorable for severe storms to develop but are not a guarantee in any particular location. However, if a severe thunderstorm warning is issued for your location, it means a severe storm is imminent and you should seek shelter.
Original post from midday
The clash between humid air lodged over Washington and an approaching cold front will trigger numerous showers and storms across the region this afternoon and evening, between about 3 p.m. and 8 p.m.
Anticipating intense storms that unload very heavy rainfall in a short time, the National Weather Service has issued a flash flood watch until 8 p.m. for the District, Fairfax, Montgomery and Prince George’s counties, as well as counties to the northeast.
“[H]eavy rain may lead to localized total rainfall amounts of up to 3 inches,” the National Weather Service writes.
In addition to heavy rain, a few storms could also produce some pockets of damaging winds. Some of these storms are likely to coincide with the commute home, so allow extra time or consider delaying travel until they pass.
Storm dashboard
Approximate arrival time:
- 3 to 4:30 p.m. in western areas.
- 4 to 5:30 p.m. in immediate area, including the Capital Beltway.
- 4:30 to 6 p.m. in areas east of Interstate 95.
Isolated showers and storms possible before these times
All clear: after sunset.
Storm duration: 30 to 45 minutes or so.
Chance of measurable rainfall in any location: 60 percent.
Storm motion: west to east.
Likely storm effects: heavy rain, lightning, gusty winds.
Possible storm effects: damaging wind gusts, small hail.
Rainfall potential: highly variable. Locally up to 2 to 3 inches in heaviest storms.
Discussion
We have reasonable prospects of experiencing scattered strong to severe thunderstorms this afternoon and evening. Factors in favor include an unstable air mass, high humidity air, an approaching cold front and a weak upper level disturbance swinging through the region.
The approaching front and region of severe weather outlook (dark red shading) is shown on the forecast weather chart below.
The bull's eye of humidity is also depicted below, with 1.6 inches of precipitable water centered on the I-95 corridor. Precipitable water is a measure of the amount of moisture between the cloud height and the ground. Values over 1.5 inches typically indicate the potential for heavy rainfall.
The one negative is fairly extensive, low-level cloud along and ahead of the front, as shown in the image below. The pervasive nature of this cloud (although allowing some filtered sun) may prevent the air mass from reaching maximum instability.
High resolution forecast models such as the HRRR below suggest scattered showers and thunderstorms will develop along a broad swath of the Interstate 95 corridor, ahead of the front.
So with enough instability, high moisture content and a few lifting mechanisms (front, prefrontal trough, elevated terrain, weak upper disturbance), odds are better than 50-50 that most locations will experience showers and thunderstorms.
The severity of storms will depend on how fully the atmosphere can destabilize. Wind shear is another determinant of severity. The higher the value, the greater the tendency for organized, long lived and damaging cells. We like to see values in the 30-40 knot range for widespread severe. Today’s value will be in the 20-25 knot range.
The combination of shear and instability is enough to generate what we call “multicell” storms that can sustain themselves for longer time periods. If this cells aggregate and organize “upscale” to larger aggregates, where downdrafts can merge and intensify, then a few pockets of damaging wind (downbursts) may be on the table. The National Weather Service Storm Prediction Center has placed our region in a slight risk zone for severe storms.
Additionally, given high precipitable water and a slow-moving front, repeated waves of storm cells/passage of multiple cells over the same areas (echo training) could lead to isolated pockets of flash flooding.
The Capital Weather Gang will monitor for any issuance of a severe thunderstorm watch and warnings later this afternoon.
