4:30 p.m. – Intense storms to exit eastern areas next hour. All-Star Game rain chances low.
The heavy storms have pressed well southeast of Washington to the counties along the western side of the Chesapeake Bay. A severe thunderstorm warning is in effect for southern Calvert County until 5:15 p.m. where some pockets of damaging winds are possible.
This storm activity should exit by 5:30 p.m.
Looking ahead, while we cannot rule out an isolated shower or thundershower through around 10 p.m. this evening, we think the chance of rain meaningfully affecting the All-Star game is low. The atmosphere is pretty worked over from the previous storm activity. But we’ll continue to monitor activity and will post any updates, if needed, in our PM Update to be published around 5 p.m. on our blog front page.
4:10 p.m. – Heaviest storms east of Interstate 95, remarkable rain totals to the west
The most intense storms have exited the Beltway. To the east, the severe thunderstorm warning that had been in effect over south central Prince George’s County has been discontinued although radar continues to indicate very strong winds southeast of Upper Marlboro. Heavy rain stretches from near Annapolis south into northern St. Mary’s County.
The rainfall that swept through the city was extremely impressive. We logged nearly an inch at The Washington Post’s rain sensor downtown.
But the most extreme rainfall report, if it is correct, came from Reagan National Airport. The airport posted 2.63 inches of rain in 42 minutes. “I’ve been here 40 years,” said airport observer Nicholas Parrell. “I’ve never had that.”
3:45 p.m. – Severe thunderstorm warning for south central Prince George’s County
While the heaviest rain downtown starts to exit, a very intense storm is pushing through south central Prince George’s County, where a severe thunderstorm warning is in effect until 4:15 p.m. Radar shows potentially damaging winds around Camp Springs and Clinton, possibly exceeding 60 mph. Anyone in this area should move inside away from windows.
3:20 p.m. – It’s pouring. Flood warning in northern Anne Arundel County
Thunderstorms with heavy rain cover much of the area inside the Beltway and are lined up along Interstate 95 from Baltimore through Dale City. The rain is falling hard enough in northern Anne Arundel County to trigger a flash flooding warning through 6 p.m., although the rain should exit that area by 4 p.m.
At our weather station here at The Washington Post downtown, our temperature has dropped 12 degrees with the rain and we’ve received over 0.5 inches.
All of this rain should last 30 to 60 minutes in any one area from start to finish. Our eastern suburbs are up next for these storms, including Prince George’s and Charles counties.
3:00 p.m. – Storms approach Beltway, dry spell about to end
No measurable rain has fallen at Reagan National Airport in 19 days but, with heavy storms surrounding the Beltway, this streak is about to end.
Radar shows storms from Aquia Harbor to the south up through Manassas and then curling to the northeast through Reston and Silver Spring. All of this activity will push through the Beltway over the next hour. Locally heavy downpours and lightning are likely with these storms.
2:05 p.m. – Storms moving into western suburbs
Radar shows widespread showers and thunderstorms pushing into eastern Loudoun, western Prince William, western Fairfax, and Montgomery counties. They contain locally heavy downpours and lightning, but are not severe. We should expect them to move inside the Beltway by around 3 p.m., perhaps somewhat earlier on the northern side.
Our next update will be around 3 p.m. or earlier, if conditions warrant.
Washington hit 98 degrees Monday, the hottest day of the summer so far. It’s stifling again today, but thunderstorms are likely to erupt this afternoon and evening, when a strong cold front sweeping across the Eastern United States collides with this muggy air.
The storms may come in two waves. We are most confident in numerous showers and storms developing during the mid- to late afternoon.
A second wave of storms is also possible later this evening, between about 7 and 11 p.m. These storms, if they develop, could cause a rain delay at the MLB All-Star Game at Nationals Park. But this second round has just perhaps a 20 to 30 percent chance of forming.
The main hazards in these storms will be downpours and lightning, but a few could become severe and unleash some damaging wind gusts.
Approximate arrival time for storms:
- Interstate 81 area: Noon to 3 p.m.
- West of Beltway: 2 to 5 p.m.
- Beltway and east: 3 to 6 p.m.
Note: Small chance of second round between 7 and 11 p.m. from west to east, which would target the Beltway around 9 p.m.
Storm duration: 30 to 60 minutes
Chance of measurable rainfall in any location: 60 percent
Storm motion: West to east
Possible storm effects: Heavy rain, lightning
Possible storm effects: Damaging wind gusts, isolated flash flooding
Rainfall potential: Average 0.1 to 0.5 inches, but highly variable. Localized amounts of over one inch possible.
An unstable air mass continues to build on warm southerly flow ahead of an approaching cold front. The air mass also features a significant amount of water vapor, nearly two inches of precipitable water (the total integrated water vapor from surface to top of the troposphere) — a very high value, even for summer in the Mid-Atlantic.
Showers and thunderstorms are expected to erupt over the mountains by midday, aided by uplift along the cold front. A broken to fairly solid line of storms should congeal and sweep eastward into the metro region starting midafternoon.
The chance of widespread severe storms for Washington is on the low side. It is in a marginal risk zone for severe storms, whereas the risk increases to northeast from Baltimore through New England.
The reason for the limited severe risk around Washington is primarily weak wind shear. Wind shear is the increase in winds with altitude. Values from the morning weather balloon launch at Dulles International Airport were quite weak. Winds will freshen a bit through the afternoon at higher altitudes, which should increase the shear but not to the more significant levels for a damaging squall line or supercell thunderstorms.
There may be just enough wind shear to cause the storm cells to cluster and become longer lived, raising the prospect of some isolated to scattered stronger storms. The main severe threat will be strong to briefly damaging wind gusts, but we do not anticipate this on a widespread basis.
The other threat is locally heavy rain, due to the very high moisture content of the air. However, there are two limiting factors that preclude a wider flood threat: (1) the storms will be progressive, i.e. not stalling or training; and (2) the ground is fairly dry in many locations.
While Washington’s streak of 19 days without measurable rain is likely to end, models suggest amounts exceeding even half an inch will be pretty isolated.
We expect the main batch of storm to form in the mid- to late afternoon hours followed by a break in activity. There is the outside possibility a second line of storm develops in our western areas around 7 or 8 p.m. and then sweeps across the Beltway around sunset. This possible second line would be associated with the passage of the cold front itself.
Our confidence that this line will materialize is low because it is only simulated in one model, the high-resolution NAM. Other modeling suggests all precipitation will shut off across the region by 7 or 8 p.m.