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Severe storms, tornadoes possible in Southeast, Mid-Atlantic today

The zone from Atlanta to Richmond is most at risk from severe storms, while flooding could affect areas just to the north

Surrounded by tornado damage, Paul Campbell, founder of the Academy of Seminole, sits outside of the school in Seminole, Okla., on May 5. (Sarah Phipps/Oklahoman/AP)
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A major storm system, which has brought severe thunderstorms and flooding to many parts of the Central United States, is rolling eastward. The Southeast and Mid-Atlantic are next in line.

On Friday, the National Weather Service declared a level 3 out of 5 enhanced risk for severe thunderstorms from southern Georgia to southeast Virginia, including Atlanta, Raleigh, N.C., and Richmond.

“Damaging winds, hail and a few tornadoes will be possible, with the primary threat expected between late morning and early evening,” the Weather Service’s Storm Prediction Center wrote.

A lesser, though still elevated risk of severe storms, expands as far south as central Florida and as far north as southwest Pennsylvania. Overall, about 68 million people live in a zone with at least a 1 out of 5 risk of intense storms.

The Weather Service issued tornado watches for much of southern and central Virginia, North Carolina, Georgia and western South Carolina, in effect into Friday evening.

The storm threat comes after a swarm of tornadoes terrorized Oklahoma and north Texas on Wednesday, before more severe storms blitzed the Missouri Valley on Thursday. The Storm Prediction Center received more than 150 reports of severe weather between the two days, including 10 twisters.

The storm has also unloaded extreme rainfall with up to 10 inches near Tulsa. A broad 4 to 6 inches came down in the past 48 hours from east central Oklahoma into the Ozarks. Resultant flooding triggered dozens of high water rescues.

Additional heavy rain from the storm could trigger flooding in the northern Mid-Atlantic late Friday into early Saturday.

Severe weather risk on Friday

Areas affected

The level 3 out of 5 enhanced risk zone from Georgia to Virginia, by itself, affects 24 million people. Surrounding it is a lesser, but still noteworthy, level 2 out of 5 risk, encompassing New Orleans; Birmingham, Mobile and Huntsville in Alabama; and Tallahassee.

Southern parts of the Mississippi Delta, coastal Mississippi and Alabama and the Florida Panhandle will see their threat of severe thunderstorms wane during the afternoon as the system shifts to the east.

Washington, D.C., was set to straddle the line of how far north the system’s “warm sector” may reach, meaning Washington may not tap into the warm, humid and unstable air mass to the south. Subsequently, only a level 1 out of 5 “marginal risk” of severe weather is up for the nation’s capital. Nevertheless, it’s under a flood watch for potentially heavy rain.

Flood watch issued for D.C. area ahead of soggy, cool weekend

The setup

On Friday morning, the center of a low pressure zone was draped from near Memphis to southwest Kentucky, nudged up against the Ohio River. Low pressure in the Northern Hemisphere spins counterclockwise, so southerly flow ahead of the low was reinforcing a warm, humid air mass to the east. That will provide ample CAPE (convective available potential energy), which is essentially thunderstorm fuel.

At the same time, a change of wind speed and/or direction with height, known as “wind shear,” will impart a twisting force on any thunderstorms that grow through multiple layers of the atmosphere. That will cause individual thunderstorms to rotate and potentially unleash tornadoes and large hail. The effect will be most dramatic for any lone, discrete “supercells” or rotating thunderstorms that are isolated from their neighbors, and less pronounced with squall lines or clusters of thunderstorms that form along the cold front.

There is some uncertainty associated with Friday’s severe weather episode.

If morning rain showers/cloud debris linger for much of the day in the system’s warm sector, the air near the ground will struggle to reach its “convective temperature” before the main line of storms comes through. That’s the temperature needed to get a bubble of surface air to rise into a storm. So the number and intensity of any supercells that may form in advance of the main line is unclear.

Hazards

Jet stream momentum aloft may be mixed to the surface by strong thunderstorms in the form of damaging straight-line wind gusts to 60 mph. Any supercells that form could contain winds to 70 mph.

Likewise, dime- to quarter-sized hail may accompany the worst clusters or lines of storms, though a rogue supercell or two could drop hail to golf-ball size.

Lastly, any supercell would yield a tornado threat, particularly between south central Virginia and northern North Carolina. The Storm Prediction Center indicates there’s a 10 percent chance of a tornado within 25 miles of any location in this area, which is an unusually high probability.

Heavy rainfall

There is the risk of heavy rainfall and flash flooding along the stalled warm front, with “training” downpours, or cells that repeatedly move over the same areas, expected intermittently through the weekend. The nation’s capital could find itself facing 2 or more inches of rain, with totals likely to surpass 3 inches in parts of Pennsylvania and New Jersey. The Big Apple could face heavy rainfall too. Flood watches are up from southwestern Pennsylvania to the Chesapeake Bay.

Looking ahead

Saturday could feature some very isolated severe weather in north central Nebraska or southwestern South Dakota, but the risk is quite low.

Otherwise, Sunday will be characterized by extreme instability but also an extreme cap — meaning there is plentiful fuel to brew very severe thunderstorms, yet mild air a mile above the ground will prevent any pockets of surface air from rising, maintaining only blue skies.

On Monday, strong southwesterly winds aloft could support some severe weather over the Upper Midwest or extreme northern Mississippi Valley, but the details are hazy. Thereafter, a general uptick in severe weather over the Plains is probably in the cards, but no further details can be sifted out at this time.

Update on storms that hit Wednesday

The National Weather Service announced Thursday that the tornado that slammed Lockett, Tex., on Wednesday, damaging homes and battering a storm chase tour van, was a mile-wide EF3 on the 0 to 5 scale for twister intensity.

A number of other tornadoes affected the region, including a pair of tornadoes in Earlsboro, Okla., about 50 miles east of Oklahoma City. One of the pair traced a loop-de-loop east of the town center. A tornado also touched down outside of Earlsboro on Monday, meaning the town was hit by three tornadoes in three days.

A tornado in Oklahoma traced a near perfect loop-de-loop

Jason Samenow contributed to this report.

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