Severe weather threat eyes Mid-Atlantic on Wednesday evening – The Washington Post, The Washington Post

February (at 1:) (PM) A potential round of strong to severe thunderstorms, along with heavy downpours, is possible for some in the Mid-Atlantic on Wednesday afte rnoon and evening. The area at greatest risk stretches from central Virginia north toward the Mason-Dixon Line and east across parts of the Delmarva Peninsula. Gusty to locally damaging winds, small hail and a low-end risk of an isolated tornado are possible if storms materialize – which in and of itself isn’t a certainty. Here, we break down the setup, timing and impacts of this challenging forecast. Timing: Late afternoon into the evening hours for most

The NAM model simulates storms approaching from the west late Wednesday. (WeatherBell)

Save for some morning drizzle, most of Wednesday will be dry with overcast skies. By late afternoon, a few sprinkles or showers will work from south to north across the Washington region. These will precede the broken band of heavy downpours and possible thunderstorms that will develop to our west around nightfall.

The storms should intensify as they cross over the Blue Ridge, passing across the Interstate 480 corridor sometime between 8 pm and Around this time, the downpours or storms may become capable of producing severe weather, mainly in the form of damaging wind gusts.

any strong storms that do form should arrive in the DC metro area sometime between 18 PM and midnight. The timing for areas north and east of the District would be later, with the Delmarva Peninsula looking at a threat focused around or just after midnight . (Potential impacts)

The HRRR model hints at a corridor of tornado potential on Wednesday night. (WeatherBell) Any storms that form will probably be low-topped, meaning they won’t produce much lightning. But thanks to cold temperatures and strong winds aloft, small hail and gusty winds are possible. The winds, perhaps in the 50 – to 60 – mph range, will arrive abruptly. There may be embedded pockets of 60 – to – mph winds or stronger, especially in the Appalachian foothills east of the mountains.
There is also a low-end risk of an isolated tornado. However, the dynamics with this event are not as impressive as they were on Feb. 7, when five tornadoes touched down in the DC area , since this time around the low-level jet stream begins to really intensify only after it’s moved out of our area. The strong winds from this feature are needed to increase the wind shear and encourage storm cells to spin.

The The Storm Prediction Center opted to include our area in a “marginal risk” for severe weather in their Tuesday midday update.

(Uncertainties)

The HRRR model, left, projects that the amount of available energy for storms to work with (blue) will be far greater than what the NAM model, right, simulates. (WeatherBell) There is considerable uncertainty as to how far north the warm front, which will precede the potential round of strong to severe storms, will progress. The most favorable environment for any storms to blossom will be along and south of the warm front, so where that feature sets up will ultimately determine the zones most at risk for any storms. “If the warm sector tracks a bit further north, this could put the Washington DC region in play for a few strong to possibly severe storms, ”said Jeff Halverson, Capital Weather Gang’s severe weather expert, in an email.

How close the mild air gets to the DC area will ultimately determine the city’s risk for severe weather. This is a schematic weather map produced for Wednesday night by the National Weather Service. (NWS / NOAA)

Moreover, instability – the equivalent of “fuel” to generate storms – is often a limiting ingredient in cold-season severe weather episodes. Temperatures in the s simply don’t compare with the toasty, oppressive and highly unstable air masses we see on hot summer afternoons. Because of this, storms could be “elevated,” and rooted in a layer of warm air above the ground, rather than at the slightly cooler surface. That would reduce the risk of damaging winds making it down to the surface, though it would not eliminate this threat entirely. However, if that warming with height can mix out ahead of time and allow ground temperatures to warm into the mid-to upper s, storms could become surface-based, with an increased wind and a subtle tornado risk.

“We have to be vigilant for Wednesday, since at times there is a tendency to underestimate the potential for severe cells in these very dynamic, wintertime storm systems, particularly when unstable air appears to be in limited supply, ”Halverson said. “There is significant wind energy just a few thousand feet above the ground that may get mixed down to the surface in rainy downdrafts.”

(The GFS) Model projects a strong low-level jet will race north on Wednesday night. (WeatherBell)

Wednesday’s setup could be boom or bust. While several of the ingredients needed for strong or severe storms are anticipated in prolific quantities, others are barely present at all. The Storms are likely to be triggered by a large dip in the jet stream that will be approaching from our west. Cold air nestled within this southward jaunt of the jet will help encourage surface air to rise, while the strong and changeable winds with height will encourage storms to rotate.

A conveyor belt of strong southerly or southwesterly winds will scream northward about a mile above the ground, while surface winds may be more southerly or southeasterly. That’s in response to a weak surface low-pressure area that could form in the lee of the Appalachians. If surface winds become southeasterly, that could enhance the amount of “shear,” or the change of wind speed and / or direction with height, available.

A shallow, broken squall line of storms could develop beneath the approaching low. Gusty winds will surge east with the line, while small transient circulations can sometimes establish themselves along bends in the line.

“The line could take on an undulating, bowed shape in spots – where small zones of rotation develop, feeding off the shear,” Halverson said.

An example of a similar setup occurred the morning of Feb. 7, when five tornadoes swirled just west of the District on a day when no thunderstorms were in the forecast. Forecast parameters on Wednesday are similar, although the wind energy present this time around is not as strong as it was during the previous event.

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