* Wind Advisory for most of region until 4 a.m. *

Updates

11:40 p.m. — Tornado watch cancelled for metro area as gusty showers give way to much cooler air

As the storms exit to our east, here’s our forecast for the rest of the night and tomorrow...

Through tonight: Once the front passes by around 1 or 2 a.m., the remaining showers should come to an end as winds continue to kick up from the northwest. A wind advisory is in effect until 4 a.m. as winds blow around 20-30 mph, with gusts in the 40 to 50 mph range, before calming somewhat toward dawn. As skies clear, temperatures fall. Lows are near 40 in the cool spots to near 50 in the city.

Tomorrow (Friday): It’s a rude awakening with a wind-chilled start. On the bright side, it will be rather bright with plenty of sun. Temperatures are down a good 20 degrees or so from today and winds make it feel even colder. Highs are mainly in the mid-and-upper 50s. Winds are out of the northwest around 15 to 20 mph, decreasing late.

11 p.m. — Strong to severe storms moving east of the Beltway as gusty showers trail behind

The most intense storms have now pushed east of the Beltway and I-95, with a Severe Thunderstorm Warning still in effect mainly east of the Beltway to the Bay (including Bowie and Annapolis) until 11:30 p.m. Trailing behind the band of strong storms, areas of gusty showers will continue to move through the region from west to east through 1 or 2 a.m. A Wind Advisory has been issued for the entire area (except for Fauquier, Prince William, Charles counties and points south) until 4 a.m. for the threat of wind gusts up to 50 mph that could blow down tree limbs and cause some power outages.

10:15 p.m. — Severe thunderstorm warning for the District and immediate surroundings

A second line of storms behind the first is now reaching the Beltway and has prompted a Severe Thunderstorm Warning until 10:45 p.m. for a significant portion of the immediate metro area mainly south of I-270. This will bring one last burst of torrential rain during the next 45 minutes to 1 hour, and the risk of damaging winds that could bring down trees.

9:40 p.m. — Torrential rain leading to threat of flash flooding in some areas

Another Flash Flood Warning now for central Montgomery County including Germantown, Montgomery Village, and Gaithersburg until 12:30 a.m. Do not attempt to drive or walk through high water.

9:20 p.m. — Strong to severe storms tracking east across the DMV

Strong to severe storms are nearing the west side of the Beltway and will head east across D.C. and I-95 during the next couple of hours. Severe Thunderstorm Warnings for the risk of damaging 60 mph wind gusts are in effect until 9:45 p.m. across the immediate northern and western suburbs, coinciding with the strongest part of the line. A Flood Warning has also been issued in upper Montgomery and lower Frederick counties. All three warnings are pictured below.

8:25 p.m. — Strong to severe storms getting closer to the Beltway as they advance east

Thus far the heavy storms have focused on our far north and west suburbs, with very heavy rain across much of Frederick and Loudoun counties including the cities of Frederick and Leesburg. Can’t rule out some flash flooding as the rain continues there. The leading edge of the storms is now edging east into upper Montgomery County and approaching Reston and Manassas, and should arrive inside the Beltway around 8:45 to 9:15. Damaging winds are possible as well as an isolated tornado, along with heavy rain. Winds are gusting 30-40 mph as the storms approach, and stronger than that after the storms pass.

7:30 p.m. — Severe storms pushing through northwest parts of the area, others approaching from southwest

A severe thunderstorm warning has been issued for a good chunk of the northwest part of the region, including parts of Montgomery, Frederick, Loudoun, and Howard counties. It runs until 8:15 p.m. The main hazard is the potential for damaging wind gusts around 60 mph.

This intense line of storms is moving into the Washington region from the west and more is headed this way from the southwest. Storms will bring torrential rain, strong winds, and the potential for isolated tornadoes. Trick-or-treaters in D.C. should seek shelter by 8 p.m. Storms may approach with little or no lightning or thunder.

7 p.m. — Storms moving into western and northern suburbs

As the storms start to push into our western and northern suburbs, we have changed the “get inside by” times to 1 hour earlier as seen below. You may have a bit more time than the map indicates below, but it’s difficult to pinpoint the timing of this line, and we want to err on the side of caution with so many folks out and about for Halloween.


(Capital Weather Gang) (CWG/CWG)

6:30 p.m. — Storms entering northern Virginia

Storms are somewhat disorganized at this time, in part because the initial line in West Virginia has weakened some as storms out ahead of it gather strength. One area of storminess is lined up along the Blue Ridge to the south of the area, moving this way, as the original line passes the Interstate 81 corridor. Although these storms are not producing much or any lightning, they will continue to pose a damaging wind and isolated tornado threat. Intensity should also try to increase somewhat as they push east into more unstable air. Western suburbs are getting in on the action over the next hour. While the reformation east may push up the start time somewhat, the line still seems on target to reach the Beltway around 9 p.m., but perhaps between 8 p.m. and 9 p.m. if the front runners take over.

5:40 p.m. — Storms closing in on Interstate 81

Weather radar shows a bowing line of storms in west central Maryland and northeast West Virginia, close to the Virginia line, racing east-northeast at 55 mph. An intense thunderstorm is about to hit the area around Berkeley Springs. This area of storms will hit the Interstate 81 corridor between about 6 and 6:30 p.m. Arrival time near the Beltway still seems like it will be close to 9 p.m.

5:00 p.m. — Weather Service warns to expect ‘a rapid increase in severe weather potential’ this evening

In a special discussion, the National Weather Service Storm Prediction Center writes that it expects showers and storms in eastern West Virginia to consolidate into a squall line and intensify as they push east of the mountains toward the Interstate 81 corridor:

A rapid increase in severe weather potential with developing thunderstorm activity is expected east of the Blue Ridge Mountains, including at least western portions of the Greater Washington D.C. metro area, by 6-8 PM EDT. This will include the risk for fairly widespread damaging wind gusts, and perhaps a couple of tornadoes.

In its more technical discussion, it notes “a rapid development and intensification of thunderstorm activity is expected, which may include supercells and an evolving squall line.”


3:45 p.m. — Dangerous squall line has formed and is headed toward the DMV.

A squall line with a history of damaging wind gusts has organized across West Virginia, and is moving our way. Analysis of the atmospheric conditions ahead of the line is revealing exceptionally strong wind shear values (winds of different speed/direction with height). This raises the concern that a few damaging wind gusts in the 60-70+ mph range may be possible in our area, particularly in line segments that take on a bow shape. A few weak tornadoes are also possible, and we cannot rule out a tornado or two that reaches strong category.

Detailed briefing

An intense line of storms is expected to barge through the Washington region Halloween night. The squall line is likely to arrive suddenly, with a burst of rain and widespread strong to damaging winds, possibly up to 75 mph in some isolated locations. A few tornadoes, possibly including a few intense tornadoes, could occur, according to the Storm Prediction Center. A tornado watch is up for the entire D.C. metro area through midnight as storms progress from west to east throughout the evening.

“All the ingredients remain in place for a potentially dangerous severe weather event tonight," wrote the National Weather Service office serving the Washington region.

Models are in reasonably good agreement that these storms will reach the Washington region after dark, between 7 and 11 p.m. from west to east. For Halloween trick-or-treaters, the safest plan of action is to be home between 7 and 8 p.m., especially if you live west of Interstate 95. For anyone staying out well after dark, have a safe place to seek shelter quickly.


Strong to locally severe storms will approach from the west this evening. (CWG)

Here is our best estimate on storm arrival times, by area:

  • Interstate 81: 6 to 7:30 p.m.
  • Frederick, Loudoun and Fauquier counties: 7 to 8:30 p.m.
  • Howard, Montgomery, Fairfax, Prince William and Stafford counties: 8 to 9:30 p.m.
  • Interstate 95, the District, the Beltway, Prince George’s and Charles counties: 8:30 to 10:00 p.m.
  • Anne Arundel and Calvert counties: 9:30 to 11 p.m.

Estimate of approximate timing of storms from National Weather Service. We believe this is a reasonable prediction. (National Weather Service)

Through the evening, it will be very important to monitor radar to see where the storms are, or to have a way to receive storm warnings, which are likely to be issued. We will be providing updates right here in this article as the storms approach and move through, and will post updates to our Twitter feed (with radar) for those out and about.

We also recommend securing or bringing inside any loose outdoor items, including Halloween decorations, that could turn into projectiles in strong winds.

The storms will sweep through quickly, lasting no more than an hour in most spots (light sprinkles may linger behind the storms for another hour or so), but they could drop a quick inch or two of rain and even trigger a few pockets of flooding near streams and poor-drainage areas. Conditions are likely to be hazardous for motorists and pedestrians as these storms roll through.

After the storms push by, high winds caused by a rapid rise in air pressure and movement of cold air behind the front could bring strong to isolated damaging wind gusts as well, perhaps lasting several hours overnight.

The National Weather Service Storm Prediction Center has placed the region in an “enhanced risk" zone for severe weather, Level 3 out of 5, which is somewhat unusual this late in the year.


(National Weather Service)

Storm dashboard


Storm duration: About an hour.

Storm arrival times: See above.

All clear: By around 11:30 p.m. west of Interstate 95. By around 1:30 a.m. east of Interstate 95.

Chance of measurable rainfall in any location: 95 percent.

Storm motion: West to east.

Likely storm effects: Heavy rain, lightning, gusty winds.

Possible storm effects: Damaging wind gusts over 50 mph.

Small chance of: A few tornadoes, flash flooding.

Rainfall potential: Generally half an inch to an inch, but locally up to two inches.

Storm discussion

The stage is set tonight for a classic autumn severe weather setup across much of the Mid-Atlantic. Storms will feed off the clashing air masses along a powerful fall cold front sweeping across the country. Temperatures fell 13 degrees in 20 minutes this morning in Tullahoma, Tenn., as the front plowed through.


Weather map at 2 p.m. on Thursday. (National Weather Service)

The approaching storm system features very impressive wind dynamics, which will probably sculpt storms into a semi-continuous squall line along the front.

Ahead of this line, strong southerly winds will gust in excess of 30 mph, and could even lead to a few falling tree branches during the late afternoon and evening. It’s these winds that will pump in a steamy, moisture-rich air mass, with temperatures in the mid-to-upper 70s. A pocket of broken cloud cover over West Virginia may also bring a few glints of sunshine to parts of the region, destabilizing the atmosphere further and boosting the storm risk.

The winds will also enhance something known as shear, or a change of wind speed/direction with height. A cranking low-level jet will bring a river of strong southerly winds a few thousand feet up. Above that at the mid-levels, winds veer more southwesterly and eventually even westerly. That turning motion will foster a few rotating elements within this evening’s thunderstorms, as well.

Some embedded areas of storm rotation are most likely within any “kinks in the line.” Sometimes, squall lines like this feature downdrafts that surge sections of them outward, the bowing segments locally enhancing the wind field enough to stir up a few areas of spin. That will present the risk of a few tornadoes. Usually tornadoes in these kinds of squall lines are short-lived but can still cause damage.


Simulated radar at 9 p.m. in Washington region from high-resolution NAM model.

Otherwise, widespread 50 mph to 60 mph winds are possible as the squall blasts through.

Because the storms don’t have the classic summertime heat and humidity to work with, they won’t be very tall. In fact, they probably won’t produce much lightning. Don’t wait to “see” the storm before heading indoors. Moreover, the fast-moving squall line’s arrival will be abrupt. So it’s imperative to have a plan to quickly seek refuge indoors as the line approaches.

Temperatures will drop sharply after storms pass, up to 10 degrees to 15 degrees within an hour.