Gusty storms exit region, paving way for quieter, less humid Wednesday

Behind the storms overnight, somewhat cooler and drier air begins to filter in and clouds vanish with time. Temperatures settle in the upper 60s and lower 70s for lows. Winds out of the northwest around 10 mph this evening become light and variable overnight.

Mostly sunny skies greet Wednesday. That’s the basic story from start to finish. Humidity is down somewhat, though there’s an outside shot of a quick late-day shower as highs mainly reach the mid-80s. Winds are from the west around 5 mph.

7:45 p.m. — Storms pushing south and east of the District

A curved line of storm bends from near Laurel in Maryland to the southwest through western Prince George’s County across the Potomac through Mount Vernon and Dale City. Severe thunderstorm warnings cover this zone and areas to the south and southeast where these storms are headed through between 8 and 8:15 p.m.

Heavy downpours and strong, gusty winds are expected as the line continues pressing southeast at a 40 mph clip. A few isolated pockets of damaging winds that could bring down trees cannot be ruled out.

North and northwest of the District, the severe weather threat has ended.

7:10 p.m. — Severe thunderstorm warnings for much of southern half of region, including District

Storms just north of Interstate 66 have expanded eastward, some into northern Fairfax County, and are racing southeast. The National Weather Service has issued a severe thunderstorm warning for much of Fairfax County until 7:45 p.m. Heavy downpours and brief strong gusts of wind are likely as these pass from northwest to southeast at 40 mph.

A severe thunderstorm warning was also issued for our southwest suburbs, covering most areas south of I66 from southern Prince William County through central Fauquier County until 8 p.m. This warning includes Manassas and Warrenton.

Finally, a third warning covers areas from downtown and to the south and east until 8 p.m. Storms are pretty spotty and not intense in this zone, but may expand a bit.

6:30 p.m. — Widely scattered storms west of Washington; a couple are intense

An organized area of strong to severe storms originating from West Virginia and western Maryland is pushing southeast along the Interstate 81 corridor from around Winchester to Front Royal. These storms, which may contain pockets of damaging winds, will cross Interstate 66 in the next 30 minutes into northern Prince William County and may expand into the southern part of western Fairfax County. These areas are now under a severe thunderstorm warning until 7:15 p.m.

The storms are moving fast to the southeast (at 40 mph) but, as they moved through West Virginia, brought down quite a few trees.

To the north and northeast, a few widely scattered storms were located around Frederick and Gaithersburg and were pushing southeast toward southern Montgomery County and the Beltway. They are not currently severe but contain heavy downpours and lightning and could strengthen.

Generally, the worst of this thunderstorm event may pass west and south of the immediate Washington area, but we will monitor.

We’ll post another update in an hour or sooner if any severe weather develops close to the immediate area.

3:45 p.m. — Severe thunderstorm watch issued until 11 p.m.

The National Weather Service has issued a severe thunderstorm watch for much of the Washington and Baltimore region until 11 p.m. The watch covers a large area from southwest Virginia through northern New Jersey.

“Clusters of severe thunderstorms will move southeast through this evening with a risk for damaging wind gusts and isolated instances of large hail,” the Weather Service writes.

A severe thunderstorm watch means conditions are favorable for severe storms, but not guaranteed, and that you should stay weather-aware. If a severe thunderstorm warning is issued for your location, it means a severe storm is imminent and you should head inside.

Radar and short-term modeling suggest storms will sweep through the region between 6 and 10 p.m. from northwest to southeast. They may be hit or miss rather than widespread. We will update this story as storms develop.

3:30 p.m. — Severe thunderstorm watch likely to be issued

Storms are developing in western Pennsylvania and the Ohio Valley and headed in this direction. The National Weather Service indicates that as these storms move into the region, some may contain damaging wind gusts. In a special bulletin, it stated that there is an 80 percent chance it will issue a severe thunderstorm watch.

Original article from Tuesday morning

As a cold front approaches the Washington region from the northwest and interacts with the hot, muggy air overhead, strong to severe thunderstorms may erupt Tuesday evening.

Storms could unleash torrential rain, frequent lightning and, in some areas, damaging wind gusts. But it is not clear how widespread these storms will be. The most likely timing for storms is between about 5 and 10 p.m., although isolated showers and storms could pop up earlier.

The Storm Prediction Center (SPC) has placed our region in an enhanced risk (Level 3 out of 5 on its threat scale) zone for severe storms, with damaging winds the main hazard of concern. This translates into a 30 percent chance of damaging winds within 25 miles of any location.

The region may deal with another round of storms on Wednesday, when it is under a marginal to slight risk of severe weather.

Discussion

This afternoon and evening, a combination of strong winds aloft, an unstable air mass and an arriving cold front will be sufficient to trigger scattered to widespread showers and thunderstorms. Some of these storms could be strong to severe, with the greatest threat being damaging wind.

The image below, the forecast surface map for 8 this evening, shows some of these elements. A cold front is sagging our way from the north. Out ahead lies a hot and humid air mass. The area shaded in red suggests the location of potentially severe storms.

Storms are most likely to form into intense clusters and bowing line segments. These segments would be most concerning, since they may contain damaging winds (downbursts) at their apex.

The morning runs of high-resolution computer models all suggest a stormy evening, but the degree of storm coverage varies considerably from model to model. The model shown below, the high-resolution NAM, suggests widespread coverage, with storms erupting along the Mason-Dixon Line, just ahead of the cold front. It projects storms passing through the immediate area between about 7 and 10 p.m., from northwest to southeast.

The simulation from the WRF-ARW model shows a line of storms arriving from the northwest about 5 p.m. The seeds of that line can be traced to a cluster of storms in the vicinity of Lake Michigan, which erupted in the early morning.

A long-track line of storm developing to our northwest is not universally simulated by the models. It is one possibility of how the scenario could evolve. The HRRR model, not shown, shows more widely scattered and rather disorganized storms in the region this evening.

One factor arguing against more widespread or coherent storm lines is high pressure, situated just to the south of our region. This pressure cell is directing surface winds from the west-northwest at the surface. That causes airflow to descend the mountains, leading to drier and subsiding air on the lee (east) side, and tends to suppress development of convective storms.

That westerly wind is also not conducive to directing the inflow of unstable air into storms advancing from the west-northwest. A low-level wind with a more southerly component would help better fuel advancing storms.

Another possible limiting factor for later Tuesday is the arrival of an early batch of showers, thunderstorms and cloud cover between midday and midafternoon, especially in our northern areas. This could limit solar heating for a few hours and reduce some of the destabilization.

Nevertheless, it’s quite possible that the National Weather Service will issue a severe thunderstorm watch for our region. We will be closely monitoring the situation later this afternoon and evening.