5:50 p.m. update: A severe storm has tracked from near Frederick to near Baltimore late this afternoon into early evening, and a hefty shower went up and recently got warned north of the city for the potential of strong winds. Other than that it’s been rather quiet. At this point it seems safe to say widespread storms are unlikely. We could still see an isolated cell or two pop up and get big, though. Look for the forecast through tomorrow with PM Update in the next hour.
4:10 p.m. update: Storms that may target the region remain few and far between. So far, it seems that a warm layer aloft is doing a good job suppressing activity around here.
We’ll have to see if that remains the case, but more recent high resolution model data seems to also suggest lower storm coverage this evening than earlier anticipated. Given a strongly unstable air mass at the surface, we can’t yet say this threat has waned entirely, but it seems wise to lean somewhat toward a low coverage event at this point. Anything that does form can be intense, with all the hazards noted below.
From 1:30 p.m....
Powerful thunderstorms again threaten the area today. Although an isolated storm is possible prior, the expected timing for the most intense activity locally is focused on the early-to-mid evening, or roughly from 5 p.m. west through 8 p.m. east.
High heat and humidity have mixed with lots of sun today. This sets the stage for the potential that some storms will turn severe. The main risk locally on that angle is damaging winds. Any storm that forms will also carry a risk of heavy rain and dangerous lightning.
Approximate arrival time for storms:
- 5 to 6 p.m. western areas
- 6 to 7 p.m. immediate area, including the Capital Beltway
- 7 to 8 p.m. areas east of Interstate 95
All clear: 9 to 11 p.m. from west to east
Storm duration: 30 to 60 minutes for the most intense
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, hail, brief tornado
Very small chance of: Large hail
Rainfall potential: Variable. Locally up to an inch or so in persistent storms.
It feels as if summer is in full swing. Another hot and humid air mass is overhead, and there are late-day prospects for thunderstorms. An approaching cold front and a significantly unstable atmosphere coupled with strong winds aloft only up the ante for the storms expected to affect the area this evening.
Not everyone will experience severe weather, but the threat is high enough that the NWS Storm Prediction Center has included the D.C. area in a level three out of five, or “enhanced.” This means there is a 30 percent chance of damaging wind gusts and large hail within 25 miles of a point. There’s also a low but nonzero chance of a brief tornado.
Today there is strong sunshine in place to heat the lower layers of the atmosphere, and significant moisture is streaming in on southerly winds. Meanwhile, a frontal boundary lies north of the Mason-Dixon Line and a wave of atmospheric energy is approaching our region.
One element that could delay or even hamper formation of storms, is a stable warm layer about two miles up in the atmosphere. Stable (especially warm) layers aloft act like a “lid.” This can rob developing cloud updrafts of buoyancy. That stable layer stretched west to Ohio late morning but is giving way from west to east as the atmospheric wave approaches.
The forecast models eventually erode this layer by mid-late afternoon around here, which may be just in time to allow the pent-up buoyant energy to surge upward as deep thunderclouds. The NWS forecast office in Sterling, Va., will release a special weather balloon at 2 p.m. to assess whether this erosion process is taking place. Weather models do suggest it will.
Brisk winds aloft, aligned parallel with the frontal boundary, have generated significant wind shear, which is an increase in wind speeds with altitude. The shear is not nearly as strong as last Thursday, when a destructive squall line of storms came through. It is still enough for severe weather potential.
If the atmosphere destabilizes sufficiently, in combination with the shear, the favored mode of storm will be of the multicellular type, which tends to be longer-lived and more intense than ordinary storms.
These multicells are likely to develop over the mountains during the early to midafternoon, and congeal into a line of storms that will then cross the Washington area, perhaps as soon as 5 p.m. well west, but most likely in the 6-to-8 p.m. time frame for most spots. Some rain and thunder could linger for a while after.
Any bowing line segments that develop will contain the strongest wind gusts. Lightning could be particularly intense in spots, along with small hail and brief, torrential downpours. A tornado is not particularly likely but also not impossible. It would probably be brief if one occurs.
Given relative agreement among models on timing and intensity, our confidence is increased that the event will unfold as outlined here.
It’s probable the Storm Prediction Center will issue a severe thunderstorm watch for later today. We will keep you updated on all stormy weather developments as they happen.