An early season round of severe weather is possible in parts of the Mid-Atlantic region on Wednesday evening and night, with the chance of damaging winds being the primary threat. At greatest risk are portions of Virginia, western Maryland and Pennsylvania, with the immediate Washington area being a bit of a wild card as to whether the severe weather makes it this far east.
Washington-area storm snapshot: Most likely timing: 10 p.m. to 1 a.m. | Storm duration: under 45 minutes | Storm movement: southwest to northeast | Likely impacts: brief downpours, gusty winds (30 to 49 mph) | Possible impacts: damaging winds (50+ mph) | Very small chance of: brief tornado | Rainfall forecast: 0.25 to 0.33 inches, with locally heavier amounts
Timing is starting this evening in the west, extending into the night
The trigger for these storms is a rapidly intensifying storm moving from West Virginia northeastward into Canada. This storm is dragging a cold front across the Mid-Atlantic while dragging mild air northward along with strong winds above the surface.
This setup is likely to result in some severe weather, but questions remain about the extent and intensity of these storms.
- Storms will develop over the Appalachians, near the West Virginia-Virginia border into south-central Pennsylvania around 7 to 8 p.m. They will quickly organize into a broken line that will progress east.
- The line of storms, probably not producing much, if any, lightning and thunder, will arrive near Interstate 81 by about 9 or 10 p.m.
- Washington and Baltimore will be affected by heavy downpours and gusty winds around midnight. Lancaster, Pa., and Philadelphia, along with the Delmarva Peninsula, could see heavy rains and gusty winds.
Ahead of the main squall line, a few rain showers are possible, with heavier downpours or isolated storms developing in southern Virginia. If such storms do develop in advance of the main line, they could pose a very low risk of an isolated tornado, particularly in the Roanoke to Richmond corridor.
Thereafter, the main line — several arcing segments of storms and heavy downpours — will form and move east. Along the entirety of the line, an abrupt wind shift, heavy downpours and brief wind gusts near or above 40 mph are likely. A quick quarter to a third of an inch of rain is possible, although heavier amounts are not out of the question.
A few pockets of more intense 50-to-60 mph wind gusts or even an isolated tornado can’t be ruled out, especially in northern/western Virginia (around the Interstate 81 corridor) and the central panhandle of Maryland (near Hagerstown).
The National Weather Service Storm Prediction Center has placed much of the region in its marginal risk zone for severe weather, which is level 1 out 5:
This type of weather setup is notoriously difficult to predict. That’s because while some ingredients for severe weather are present, there’s a lack of others.
Sometimes, this results in a boom, with destructive storms blossoming and at times catching forecasters by surprise. That happened on Feb. 7, when five tornadoes swirled through areas to the north and west of Washington, on a morning that was supposed to feature little more than a passing shower.
But more often than not, these setups underwhelm, with the shallow “low-topped” rain showers failing to take on severe characteristics.
The reason? A lack of instability, or fuel for severe storms. Thunderstorms feed off comparatively warm, moist air.
That’s the case leading into Wednesday evening’s forecast. Temperatures are only anticipated to be in the mid-50s in most areas, indicating little, if any, instability present. That means there won’t be much support for the air to rise. It’s also part of the reason any storms that do form won’t tower to the height of typical warm-season storms; they probably won’t even grow tall enough to produce thunder or lightning.
“Any chance of some strong or even severe storm cells developing this evening is highly dependent on whether the D.C. region sneaks into a brief surge of milder air for a few hours,” said Jeff Halverson, CWG’s severe weather expert, via email.
“The highest resolution model runs that we use suggest a bit of instability may make a close brush with our region … but best chances would be to the south and east, i.e. Fredericksburg, Richmond.”
On the other hand, plentiful wind energy aloft — in the form of a powerful jet stream strengthening overhead late this evening — argues in support of severe weather. In addition to trucking in milder air from the south, the strong winds, which feature a change in speed and/or direction with height, could foster some rotation within the storms. That change in wind speed and/or direction with height is known as wind shear.
“However, we are reticent to let our guard down. This is because the exceptionally strong dynamics involved (deepening low pressure, strengthening jet stream) may to some extent compensate for the lack of buoyant air,” Halverson said.
That will bolster any acute damaging wind risk or very low-end tornado risk to the west, especially over the higher elevations of north-central Virginia and west-central Maryland. It may also enable strong winds to occur further east, into metro Washington.
It’s worth noting that the winds above the ground at a few thousand feet altitude will be considerably weaker than the Feb. 7 event. As such, the tornado risk is dramatically lower.
A look at current conditions can help us to key in on areas that may see an increased risk over other locales later Wednesday evening.
Temperatures along the eastern slopes of the Appalachians are warming in response to clearing. There’s the potential for those temperatures to reach a couple degrees higher than model forecasts.
However, a stubborn onshore flow has helped push a bank of low clouds and fog across much of the Mid-Atlantic to the east. This will limit solar heating, cutting back on severe weather chances.
Warmer air to the west could be transported north and east later on Wednesday afternoon, but it’s unlikely the overcast skies will significantly erode. This indicates the greatest risk of severe weather remains to the west.
There could be a zone to watch in northwest Maryland or the Potomac Highlands into extreme south-central Pennsylvania, where a steep temperature gradient — or a change of temperature with distance — could help focus storm activity.
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