Severe thunderstorm watch until 8 p.m.
Flash flood watch in effect until 9 p.m.

4:35 p.m. update: The heaviest rain has pushed northeast of the metro from roughly Severna Park to Baltimore. But to the south and west, we still have widespread showers working through the region.

When does it all end? Radar and short-term computer models suggest showers and storms will continue passing by through 7-10 p.m., ending first in our northwest areas and last in our southeast areas. There could even be some heavier downpours to pass by between 6 and 8 p.m., and perhaps a bit of thunder.

Additional showers and storms shouldn’t be severe with the possible exception of areas in Southern Maryland where the air is still fairly unstable.

We could see another 0.25-0.5 inches of rain on top of what has fallen. So far rainfall has totaled about 0.5-1.0 inches in the metro, with higher amounts to the northwest.

Unless more significant storms come through, this will be our last update.

Below are some reports of spotty wind damage received from throughout the area (also note, Dominion Power reported about 26,000 customers without power around 4 p.m.):

See our PM Forecast Update for the forecast for the remainder of this evening and Tuesday.

4:00 p.m. update: The worst of the storm has passed downtown. We had a 40 mph wind gust on the roof at The Washington Post (1301 K St. NW) and the temperature dropped ten degrees in ten minutes. The severe thunderstorm warning in effect for the immediate metro area has expired but very heavy rain and gusty winds are pushing toward the eastern side of the Beltway to the northeast toward Laurel and Bowie. A severe thunderstorm warning has just been issued through 4:45 p.m. for Washington’s eastern suburbs:

Due to the rain and storms, expect a slow commute home.

3:50 p.m. update: Much of the close-in D.C. metro area remains under a severe thunderstorm warning through 4 p.m. The main impacts with this storm continue to be very heavy rain, and some pockets of strong, gusty winds. We’ve seen at least a couple reports of downed trees. Below are some telling visuals of the storms coming through:

For earlier storm updates, scroll down to the bottom of this post.

Original post

Severe thunderstorms and flash flooding are possible in the D.C. region today. An approaching cold front and high humidity will combine to generate numerous showers and thunderstorms, some of which will likely reach severe levels. Damaging gusts and torrential downpours are the main threats in today’s storms.

Timing: 2 to 8 p.m.
Coverage: Widespread
Impacts: Scattered damaging wind gusts, frequent lightning, small hail, torrential rains with possible flash flooding.

Here is a brief breakdown of the types of severe weather we expect this afternoon:

  • Locally very heavy rains — This is due to a combination of factors, chiefly the very moist air mass and the movement of storm cells, which are expected to stream from southwest to northeast along the front, setting up a the potential for “training” or repeated passage of cells over the same locations. The ground is not saturated, but there is the potential for a quick one to two inches, and isolated three-to-four-inch amounts are not out of the question.
  • Intense lightning — As always, thunderstorms mean lightning strikes, and those engaged in outdoor activities (i.e. school kids attending summer camps should be especially vigilant this afternoon).
  • Damaging gusts — This is less of a widespread threat than the heavy rain, but any one location has a 30 percent chance of experiencing the effects of damaging wind, including downed limbs, trees and utility lines. The mechanism here is localized, wet microbursts.

The Storm Prediction Center put the D.C. region in an “enhanced” risk of severe weather, which is the third-highest on the five-point scale, which means we have a pretty decent chance of seeing widespread severe weather. Wrapped up in the risk is a 30 percent likelihood of experiencing damaging gusts, 5 percent chance of large (damaging) hail and a 2 percent tornado risk.

The entire Interstate 95 corridor from Richmond to New York City is at enhanced risk.

Additionally, the Weather Service has placed a large segment of our region under a flash flood watch until 9 p.m. A severe thunderstorm watch is also likely this afternoon.

Our region is in a zone of strong winds from the south ahead of a cold front, ushering in abundant low-level humidity and warm air. Combined with strong solar heating through the morning, the atmosphere is becoming moderately unstable. Additionally, the atmosphere is very humid through a deep layer extending upward to about four miles. The so-called “precipitable water” — an integrated value of how much water can theoretically rain out in any one location — is very high. This is shown in the graphic below, with a nearly two-inch bull’s eye over the greater D.C.-Baltimore region.

(National Weather Service)

The approaching cold front will provide the means to lift this humid air mass, releasing the instability. Winds will increase aloft through the afternoon, as a trough (disturbance) in the jet stream crosses the East Coast. The freshening winds will cause wind shear (the difference in wind speed between the ground and regions aloft) to increase, to about 30 to 40 knots. Wind shear of this strength helps storm cells become stronger and longer-lived, and enables them to organize into clusters called multicells.

The high resolution forecast models such as the HRRR (shown below) predict that a broken line of intense thunderstorm cells will develop along the cold front and sweep across the I-95 region, potentially as early as 3 to 4 p.m. A convective line has been ongoing across West Virginia and western Maryland this morning. The line may wane a bit as it crosses the spine of the Appalachians, but then is expected to re-intensify on the eastern side.

(National Weather Service)

Earlier storm updates, now expired

3:38 p.m. update: A severe thunderstorm warning remains in effect for the much of the immediate D.C. metro area. Storms have become a bit more intense on approach to the Beltway. There was a report of a 58 mph gust in Chantilly, and doppler radar shows 40-50 gusts on the leading edge of the storms now inside the Beltway moving into Arlington, northwest Washington, and Bethesda.

3:20 p.m. update: We’re hearing of power outages in Fairfax County, likely due to strong wind gusts. It looks like the wind in these storms has finally picked up just in time to hit the immediate metro.

If you’re out driving — or about to leave — take it slow and pull off the road when things get too rainy/windy. Once this line comes through, we’ll be storm-free (although showers will linger).

3:11 p.m. update: This is an example of that “torrential rain” we’re talking about.

3:04 update: Storms are approaching the immediate metro area, and some of them could be producing strong wind gusts. So far we haven’t seen much in the way of damage reports, but it’s possible that winds will pick up as they track over the Beltway between 3:30 and 4 p.m.

Heavy rain still seems to be the biggest impact at this point. The National Weather Service will continue to issue severe thunderstorm warnings as the line advances.

Severe t-storm warnings outlined in yellow. (Radarscope)

2:30 p.m. update: The Storm Prediction Center posted a discussion noting storms could strengthen as they approach the I-95 corridor. Along the front edge, winds could gust at 40-50 mph. We’re also concerned about microbursts, which would also cause damaging winds.

Let us know what you experience as these storms move east.

Part of the SPC note we mentioned:

2 p.m. update: The line of thunderstorms continues to move in from the west, though it’s not making speedy progress. Frederick has been dealing with severe-storm conditions for the past 30 minutes or so, and the slow progress also means rainfall rates will be high.

There are warnings coming in left and right now. We assume most of this line will have a severe thunderstorm warning on it as it comes through the Beltway. The alert will serve as a good heads-up that the line is close.

Based on radar, we suspect storms will arrive in the Beltway in the 4 p.m. hour with the potential to disrupt the evening commute. Light rain could linger in the immediate metro for a couple of hours before departing by 7 or 8 p.m., which is what the HRRR is suggesting.

12:50 p.m. update: A severe thunderstorm watch has been issued for the entire D.C. area, effective until 8 p.m. Storms are already starting to enter the area starting in the northwest suburbs. They will continue moving toward the Beltway over the next couple of hours.