Threat of severe weather diminishes with just scattered showers and storms through this evening

Hit or miss showers and storms remain possible into the overnight hours and there may be a more organized line of activity that comes through with the cold front closer to midnight. However, this activity probably won’t be severe.

This update ends our coverage of these storms. See our PM forecast update to be posted on our section front after 4:45 p.m. for the forecast for the rest of tonight through the weekend.

3:40 p.m. — Storms in northwest part of region weaken. Watching activity to the southwest.

The storms affecting Frederick County, Md. have weakened and are no longer severe as these pass through downtown Frederick. All warnings have been dropped. However, they are still likely to produce heavy rain and gusty winds.

We are monitoring some widely scattered showers and storms to the southwest of Washington, between Charlottesville and Culpeper, which could affect our western areas later this afternoon or early this evening.

3:10 p.m. — Severe thunderstorm warnings for northern Loudoun and Frederick (Md.) counties through 3:45 p.m.

An intense line of storm stretches from the border of Virginia and the West Virginia Panhandle through Frederick County in Maryland. These storms will pass through the city of Frederick over the next half hour and may contain damaging winds and small hail in addition to very heavy rain and lightning.

These storms should be approaching the northwest Montgomery and Carroll counties shortly before 4 p.m.

2:40 p.m. — Thunderstorms forming and intensifying well north and west of Washington

Radar shows thunderstorms, some of which could produce one-inch hail and winds to 60 mph, forming along and just southwest of the Mason-Dixon Line. These storms are moving from southwest to northeast, and more storms are expected to form further southwest as the afternoon goes on.

2:25 p.m. — Severe thunderstorm watch issued for Washington’s northern and northwest suburbs

The watch, in effect until 9 p.m., includes Carroll, Frederick, Howard, Loudoun and Montgomery counties. While areas to the south and east are not included in the watch area, a few strong storms could still occur in this zone.

A watch means conditions are conducive for some storms to become severe and you should remain alert. It means severe weather is possible but not assured. However, if a severe thunderstorm warning is issued for your location, it means a severe storm is imminent and you should seek shelter.

Original article

Scattered strong to severe thunderstorms are likely Friday afternoon and evening across the Washington area as a cold front advances into a warm and humid air mass. These storms will bring the potential for strong to locally damaging winds, a few instances of hail, very heavy rainfall and perhaps some sporadic urban flooding.

The storms will be scattered in nature, and not everyone will be impacted — but where storms do strike, conditions will be favorable for severe weather.

Storm risk at a glance

• Storm timeline: Thunderstorms will break out to the northwest shortly after lunchtime, potentially affecting the Interstate 81 corridor as early as 2 or 3 p.m. Thunderstorms, some severe, will arrive in the D.C. area between 4 p.m. and 7 p.m., with the potential for additional storm cells lingering well into the night.
• Storm hazards: Strong to damaging winds are the main concern, with the risk of some large hail as well, especially across western areas. Isolated instances of brief urban or small-stream flooding are possible if and where storms train over the same areas repeatedly, which is favored in the northern Shenandoah Valley.
• Areas affected: Showers and thunderstorms will be scattered across the region, so not everyone will be affected. The greatest risk of severe weather is from the D.C. to Baltimore corridor and points west, where the National Weather Service Storm Prediction Center has highlighted a level 2 out of 5 “slight risk” for severe weather. A level 1 “marginal risk” is assigned farther east to the Delmarva Peninsula.

Situation overview

A cold front was located across the Midwest and Tennessee Valley around sunrise Friday. This will serve as the focus for thunderstorm development. Cold air at the upper levels of the atmosphere will precede the front, which will help a few strong thunderstorms crop up in the heat of the day even before the cold front arrives.

Storms during the afternoon will also be aided by a pocket of mid-level spin, or “vorticity,” lifting to the northeast toward the Mid-Atlantic.

Over the Washington region, humid southerly air flow and temperatures in the mid- to upper 80s will yield ample fuel for storms to work with. Meanwhile, a change of wind speed and direction with height, a phenomenon known as “wind shear,” will encourage a couple of the thunderstorms to rotate initially, posing a threat of hail and damaging winds.

Wind shear promotes storm cells that are longer lived, more intense and more organized.

Any storms that exhibit weak rotation will be capable of producing hail to the size of quarters, although the tornado risk is very low. More pressing is the potential for thunderstorms to tap into those strong, high-altitude winds, transferring some of that momentum down to the surface in the form of damaging wind gusts.

Individual storms will move northeast, meaning they may be visible to our west before moving close enough to urban areas.

Storms may merge into clusters or line segments during the evening hours, which pose a greater risk of gusty winds.

There are also indications that a few storms may bubble up and stick around through midnight until the front finally clears. That front will usher in a more crisp, refreshing air mass Saturday.

Technical discussion

While we are on the southeastern fringe of the Slight Risk zone, we are concerned that the atmosphere will destabilize strongly by mid- to late afternoon, given the exceptionally high moisture (dew points in the 70s area-wide), abundant sunshine and cooling middle layers of the atmosphere.

Furthermore, with the arrival of the weather disturbance aloft, winds are picking up from the southwest, meaning the shear — or increase in those with winds with altitude — is on the increase.

The forecast models suggest that storms will begin popping over the mountains to our west during the late afternoon, then glide from southwest to northeast across the metro region. The best timing suggests a window from 5 to 9 p.m.

The coverage of these storms is not extensive, so they may be hit-or-miss, and brief. They will most likely be organized into small clusters or bowing line segments. The biggest severe threat includes small- to medium-size hail (perhaps up to golf ball-size), and locally severe gusts exceeding 60 mph.

Additionally, given the rich moisture content of the air, torrential downpours are a concern, considering our recent rainy spring. Lightning could be particularly frequent in some of these cells.

The one mitigating factor for severe storms is the late-day timing suggested by the models. If storms do not encroach on our region until early evening, the decrease in solar heating will cause available instability to start to wane. This could take the “edge” off these storms, such that they mostly remain sub-severe.