Severe storms sweeping across Deep South

• Southwest and south central Louisiana, until 7 p.m. Central time.
• Northeast Louisiana, eastern Arkansas, northern and western Mississippi and western Tennessee, including Memphis, until 8 p.m. Central time. Here, “a couple intense tornadoes are likely,” the Weather Service said, warning that “storms are expected to result in widespread wind damage.”
• Southeast Louisiana, including New Orleans, until 9 p.m. Central time. New Orleans was just struck by an EF3 twister last week, the city’s strongest on record.
• Southeast Missouri, southern Illinois, southwest Indiana and western Kentucky, until 9 p.m. Central time.

Around 4:00 p.m. Central time, a line of vigorous thunderstorms stretched from southeast Missouri to the central Louisiana Gulf Coast. The most intense storms covered the zone from just north of Memphis to around Jackson, Miss., where several tornado warnings were in effect.

Over 100,000 customers were without power in Louisiana, Mississippi and Tennessee, mainly due to high winds. The Weather Service had received 36 reports of damaging winds and 2 reports of tornadoes.

The storms, charging eastward at 30 to 50 mph, were pointed at western parts of Kentucky, Tennessee and Mississippi, along with southeast Louisiana for the the late afternoon and early evening hours. After sunset, storms may still be severe as they approach Alabama but should gradually weaken.

Additional severe thunderstorms are possible along the East Coast on Thursday, and there are signs that April is favored to feature considerable above-average severe thunderstorm and tornado activity across the Lower 48.

“Review your severe weather safety procedures for the possibility of dangerous weather today,” the Storm Prediction Center urged in a public severe weather outlook released early Wednesday.

Even outside the destructive squall line that’s expected to form, ambient winds may gust over 50 mph ahead of any storms.

“Damaging winds will blow down trees and power lines,” wrote the National Weather Service in Mobile, Ala. “Widespread power outages are expected.”

The entirety of Mississippi, Alabama, Louisiana, Tennessee and Kentucky are encompassed within wind advisories and high-wind warnings — alerts that also extend across a broader swath of the South and the Ohio Valley.

Amid the dry, windy air ahead of the storm system and in its wake, there is an elevated risk of fast-moving fires Wednesday in portions of the Tennessee Valley as well as Texas, where several such blazes erupted Tuesday.

Daylight dawned on severe thunderstorms over the Ozarks of southwestern Missouri, western Arkansas and extreme southeastern Oklahoma, as well as in northeastern Texas east of Interstate 35. Those storms fired up late Tuesday along a dryline, or the leading edge of dry air from the Desert Southwest. That dryline trailed southward from a low pressure system over the Corn Belt that was being energized by an approaching upper-air disturbance.

That same deepening surface low-pressure zone will strengthen south-southeasterly winds ahead of the front, dragging north a mild and moisture-rich air mass from the Gulf of Mexico and draping it across the South. Cloud cover left over from Tuesday’s storms, however, will inhibit daytime heating, meaning the air mass won’t be “juiced up” as much as it otherwise could be.

Despite the comparatively modest fuel for severe thunderstorms, shear, or a change in wind speed/direction with height, is extreme. That’s thanks to a roaring low-level jet stream, or a river of swiftly moving air a mile or so above the ground, screaming north into the surface low. That means any clouds that grow sufficiently tall will have a tendency to rotate.

The Level 4 out of 5 red zone encompasses the entirety of Mississippi, western Alabama, eastern Louisiana and southwestern Tennessee.

Memphis; Tupelo, Starkville, Hattiesburg, Meridian and Jackson, Miss.; Tuscaloosa, Mobile and Montgomery, Ala.; and Monroe and Alexandria, La., are in the greatest-risk category. A lesser, but still formidable, Level 3 out of 5 enhanced risk covers Nashville, Little Rock, New Orleans, Birmingham and Huntsville, Ala.

It’s unclear how far east the line will remain intact before fragmenting and weakening late Wednesday or early Thursday.

Storms that developed Wednesday morning had take the shape of a QLCS, or quasi-linear convective system, as they worked into Louisiana midday. That’s essentially a squall line with embedded kinks of rotation, each of which could become tornadic.

Given the setup, a few things are worth noting:

• Thunderstorms will move very quickly across the Deep South. That may cut back on how much advance notice of dangerous weather can be provided.
• With strong jet stream winds aloft, it will be easy for thunderstorms to mix momentum down to the surface. That means widespread 60 to 80 mph winds within thunderstorms.
• Even apart from any thunderstorms, winds might gust up to 50 mph ahead of the squall line and then over 35 mph in the wake of the storms as winds switch around out of the northwest.
• There is sufficient wind shear to support scattered quick-forming, brief and erratic tornadoes along the QLCS. Because of the swift forward motion of the line, it wouldn’t take much for a couple to cause EF2+ damage. There could also be a few embedded rotating thunderstorms or supercells in southern portions of the line over southern Mississippi, the Delta in Louisiana or southern Alabama.

Thunderstorms will be racing east at speeds topping 50 mph. Current projections suggest:

• Thunderstorms will cross the Mississippi River around 3 or 4 p.m. Central time, affecting cities including Memphis or Greenville, Natchez and Vicksburg, Miss.
• The squall line will be plowing through central Mississippi during the evening commute around 6 p.m.
• New Orleans could see storms around 6 to 7 p.m.
• Thunderstorms will be on the Mississippi-Alabama border around 7 or 8 p.m.
• Storms reach Tuscaloosa by 8 or 9 p.m. and Birmingham about an hour later.
• Thunderstorms hit Nashville around 8 or 9 p.m.
• Mobile, Ala., could see storms around 9 p.m., with storms continuing into the Florida Panhandle thereafter.

Thunderstorms will be moving so quickly that they won’t “realize” they have outrun the conditions fueling them until long after they have progressed east. That means they are unlikely to fade much until they approach the Georgia border close to midnight.

On Thursday, vigorous storms are possible from New York state to Florida. The zone of greatest risk for severe weather, classified as a Level 2 out of 5, covers the Mid-Atlantic from Richmond to Scranton, Pa., including the Washington-Baltimore region and northern Florida to southern South Carolina.

“Damaging wind gusts and hail are the primary threats, but low-level vertical shear is strong enough to support a tornado or two,” the Storm Prediction Center wrote.