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Severe storms, isolated tornadoes possible Monday across Northeast

A couple of dangerous thunderstorms could form along the Interstate 95 corridor

The Storm Prediction Center's outlook for severe weather on Monday. (WeatherBell)
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A powerful cold front will bring the risk of strong to severe thunderstorms across much of the Northeast and Mid-Atlantic Monday. In addition to large hail and damaging straight-line winds, a couple of tornadoes are possible along the Interstate 95 corridor. A few rotating thunderstorms, known as supercells, may even form.

D.C.-area forecast: Severe storm threat today, amazing Tuesday, late week heat blast

The National Weather Service Storm Prediction Center has drawn a level 3 out of 5 “enhanced risk” of severe weather to convey this threat, with an orange zone on its forecast maps that includes a number of big-name cities. Philadelphia, Baltimore, Washington, Binghamton and Albany, N.Y., and Scranton, Pa., all are in the enhanced category, with a level 2 out of 5 “slight risk” closer to the coast in places like New York City and Virginia Beach. Boston has a level 1 out of 5 marginal risk, since storms will probably weaken en route to the chilly Atlantic.

About 60 million people face a substantially elevated threat of severe storms.

A severe thunderstorm watch was issued for central until 2 p.m. for Pennsylvania, where storms were already cooking Monday morning. Additional severe thunderstorm or tornado watches will probably be issued around lunchtime. More targeted warnings will be issued for imminent storm threats indicated by radar.

A slightly cooler, more refreshing air mass will set up behind the front starting Tuesday before significant heat builds in to round out the workweek.

The setup

A zone of low pressure spanned from the Hudson Bay to central Quebec on Monday morning, with a cold front trailing to the south. A secondary area of low pressure had developed along the front near Cleveland.

The low was located within a dip in the jet stream known as a trough; as the jet stream swiftly moves overhead, a change in wind speed and/or height, known as wind shear, will be felt in columns of atmosphere ahead of the front. That will exert a turning or twisting force in any pockets of air that grow tall enough to ascend through multiple layers.

In advance of the front, warm, humid air is streaming north from the Gulf of Mexico and spreading over the Eastern Seaboard. That will contrast with approaching frigid air aloft. As a result, scattered thunderstorms, some of which will rotate because of the wind shear present, are expected to bubble up after lunchtime.

Areas impacted

Areas along Interstates 95 and 84 from the Virginia Tidewater north into central New York state have the greatest risk of severe weather Monday. That’s where the air masses will clash most dramatically without any influence from the cooler waters of the Atlantic.

Storms will begin to weaken as they pass the Tri-State area into western and southern New England, but there’s a low-end risk for severe storms in the Connecticut River Valley, Vermont and most of New Hampshire. The slight risk extends into North Carolina too, with a shot of severe storms as far south as Raleigh.

Hazards

Damaging winds: Gusty to damaging straight-line winds between 40 and 60 mph are the main concern with thunderstorms as they draw down wind energy from the jet stream. The bull’s eye of maximum likelihood for strong winds is in eastern Pennsylvania and western New Jersey, where confidence is greatest in a continuous, unbroken squall line blasting east. As thunderstorms enter the Garden State, strong winds may precede any lightning or thunder.

Hail: Thanks to cold air aloft, hail the size of quarters to half dollars is possible with the strongest thunderstorms, though it will be isolated to widely scattered at most. Hail is usually less prevalent in linear storms, but if any supercells form ahead of the main line, large hail would be more probable.

Isolated tornadoes: The majority of thunderstorms along the cold front will be QLCS, or quasi-linear convective system, in nature. That means they’ll take the form of clusters or line segments with embedded kinks of rotation. It’s possible that one or two circulations tighten up enough to warrant the issuance of a tornado warning.

In advance of the main squall line, a couple of rotating supercells are expected. High-resolution model simulations have been a bit more aggressive in simulating a few. Any of them could pose a greater risk of prompting tornado warnings. That will be especially true near and south of the Mason-Dixon Line in Maryland, D.C. and perhaps Northern Virginia.

Timing

Showers and a few thunderstorms were already moving through eastern Ohio and western Pennsylvania to start the day. These downpours will become juiced-up by the warming atmosphere and increase in coverage and intensity throughout the late morning. They’ll eventually grow into the main squall line of storms, which will probably become severe by 1 p.m. at the latest. That squall should reach the New Jersey border around the time of the afternoon rush hour.

Discrete cells, a couple of which could be supercells, will blossom around 1 or 2 p.m. over Virginia and Maryland and shift north and east.

The severe threat should diminish markedly after about 6 p.m. in the nation’s capital, and 7 or 8 p.m. as storms clear the coastline through New York City.

The decaying squall line will still be marching through New England, but in a weakened state.

There could be also a second line of storms in the evening across the eastern Mid-Atlantic.

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