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Monday, December 11, 2023

East Coast faces unusually dangerous thunderstorm threat Monday

A serious and uncommonly widespread outbreak of severe thunderstorms is expected in the eastern United States, affecting more than 50 million people from Georgia to New York. The storms could unleash destructive winds, hail and a few tornadoes, mainly in the late afternoon and evening hours.

The National Weather Service Storm Prediction Center has taken the extremely unusual step of declaring a Level 4 out of 5 risk of severe storms in parts of the Mid-Atlantic region, connoting the potential for a very high-impact event.

The zone of greatest risk includes Washington, Baltimore, Roanoke and much of the Appalachians toward the Cumberland Plateau of Tennessee. The National Weather Service is warning that “a severe weather outbreak is possible … with widespread damaging winds, locally destructive, and isolated tornadoes.”

It’s the first time since June 12-13, 2013, that Washington has been included in that tier of risk.

A level 3 out of 5 “enhanced” risk, which still represents a formidable threat, covers the zone from Atlanta to just west of New York City. That is where damaging winds and a few tornadoes are still possible — but the coverage and intensity may not quite reach the level of that in the Mid-Atlantic.

The storms are most probable along the Interstate 81 corridor in the mid-to-late afternoon, around the Interstate 95 corridor early in the evening and nearer the Atlantic coast by sunset.

It’s highly probable that tornado or severe thunderstorm watches will be issued early Monday afternoon ahead of the storms. More targeted warnings will be issued when storms are imminent or occurring.

The parent storm system already brought at least two tornadoes to central Illinois on Sunday, including a potentially significant one that initially did not include a tornado warning. A couple tornadoes also formed in Iowa on Saturday.

  • Damaging to destructive straight-line winds will be the primary hazard.
  • Storms will produce violent downbursts, particularly in the Mid-Atlantic. The National Weather Service noted that a few gusts of 80 mph are possible in the strongest storms. It’s impossible to predict in advance what particular areas will be hardest hit.
  • A swift, potent jet stream will work overhead. Storms will tap into that momentum and mix it to the surface in the form of damaging/destructive gusts.
  • A layer of dry, dense air at the mid-levels may mix into thunderstorm downdrafts. That would cause pockets of rain-cooled air to experience “evaporative cooling,” drying out and becoming heavier — and then accelerating toward the ground with even stronger winds.
  • Tornadoes are possible with any of Monday’s storms. The risk will be greatest in the Mid-Atlantic from the central Appalachians through Northern Virginia into central Pennsylvania.
  • Initial storm cells in northwest Virginia, northern West Virginia, the Panhandle of Maryland, northern Maryland, southern Pennsylvania and perhaps western New Jersey may be rotating supercells. Those would present the risk of hail, straight-line winds and a few tornadoes.
  • Storms will eventually merge into a fierce line known as a “QLCS” — or a quasi-linear convective system. That will feature widespread straight-line winds and embedded kinks of rotation, any of which could produce erratic, quick-forming and quick-hitting tornadoes. Those tornadoes would come with little warning.
  • Hail would be relegated to initial discrete (lone, isolated) cells, the majority of which will form within 50 miles of the Pennsylvania-Maryland border.
  • If a rotating supercell forms, hail to half-dollar size would be possible. Otherwise, the strongest storms would contain penny- to nickel-sized hail.
  • Storms will probably be progressive in nature, meaning they’ll probably move west to east quickly enough to lessen the flood risk. However, torrential downpours could cause brief flooding where there’s poor drainage, especially in urban areas.
  • A somewhat elevated flood threat could occur east of Interstate 95 into the Delmarva Peninsula on Monday evening. As the intense squall line passes I-95, it will encounter a jet of moisture from the south. Toward sunset, that could intensify rainfall and foster the redevelopment of storms. If training occurs, or the repeated movement of storms over the same area, then a few locations could see a quick 2 to 3 inches of rain, causing flooding.

Storms will form west of Interstate 81 around 1 to 3 p.m. They’ll precede the cold front, forming instead along a broad strip of low pressure ahead of the front known as a “lee trough.”

They’ll increase in coverage and intensity during the mid- to late afternoon, taking advantage of daytime heating. They should approach Interstate 95 in the Mid-Atlantic during the height of the evening commute in the 5 to 7 p.m. window. Major travel disruptions are likely.

  • Charge devices/batteries ahead of possible strong winds.
  • Secure or move inside loose outdoor items.
  • If possible, avoid parking vehicles near large trees.
  • If there is a tornado warning or warning for destructive winds, shelter in an interior room, away from windows, at the lowest level of a strong building.
  • Plan to be off the roads by the time severe weather commences.
  • Have a way to be notified if warnings are issued for your location.

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