Severe storms predicted in the South with a risk of strong tornadoes

Damaging winds and tornadoes are expected across the South on Monday as a band of severe thunderstorms forms along a cold front. A few of the tornadoes could be intense and/or long-lived.

“Tornadoes (some strong), damaging thunderstorm winds, and isolated large hail are expected from this afternoon across east Texas and parts of Louisiana, through tonight across the Delta region into portions of Mississippi and Alabama,” the National Weather Service wrote Monday.

The Weather Service has declared a level 3 out of 5 “enhanced” risk of severe weather in northern Louisiana and western Missisippi — the area most threatened.

An unusually intense jet stream disturbance originating from the Southwest will swing over warm, humid air present across the Mid-South and Mississippi Delta region. Any thunderstorms will be highly sheared, or encouraged to spin, yielding a classic recipe for tornadoes.

The tornado threat will continue into the overnight hours, which forecasters say underscores the importance of having multiple ways to be notified of severe weather warnings . Nighttime tornadoes are most perilous because visual cues are absent and many people are asleep.

More severe thunderstorms, as well as the possibility of a couple tornadoes, are expected across Alabama and the Southeast on Tuesday .

The parent storm system, which began as an upper-air disturbance over the Southwest, already produced one tornado — oddly enough, in Gila County, Arizona. It touched down in the mountain town of Star Valley, about 75 miles northeast of Phoenix, or 80 miles south-southeast of Flagstaff. The twister, which damaged 10 buildings, was rated a EF-1 on the 0 to 5 Enhanced Fujita scale, only the fourth tornado this strong to occur in Arizona during November since records began in 1950.

While the spring months are most commonly known as peak time for tornadoes, the autumn is known as a second season as warm and cold air clash ahead of winter. So far this year, this second season has been quiet, but Monday’s storms could change that.

Areas at risk

level 3 out of 5 enhanced risk of severe weather covers areas near and south of Interstate 20 in Louisiana and western and central Mississippi. This is the zone at greatest risk for strong tornadoes. Included are Jackson, Hattiesburg and Vicksburg in Mississippi, and Monroe, Shreveport and Bossier City in Louisiana. Extreme eastern Texas, primarily east of Lufkin, is in the zone too, including around the Angelina and Sabine National Forests.

level 2 out of 5 risk spans a broader area surrounding the level 3 zone, and begins just east of Interstate 35 between Dallas and Waco. Then it blankets eastern Texas, the remainder of Louisiana, the southern half of Arkansas, much of Mississippi and extreme southwest Alabama. Texarkana, Ark.; Mesquite, Tex.; New Orleans; Baton Rouge; Starkville and Meridian, Miss.; and Mobile, Ala. are among the cities in this area.

Houston, Dallas, Little Rock and Memphis are in a level 1 out of 5 marginal risk area for severe weather.

What to expect

A squall line with damaging winds and embedded kinks of spin, which could produce quick-hitting tornadoes, will likely take shape by midafternoon. It will progress east across the risk area. Winds could top 60 mph as storms tap into jet stream energy aloft and mix momentum to the surface.

Ahead of the broken line, a cluster of rotating supercell thunderstorms may materialize. These will be the most primed to produce significant and/or long-track tornadoes. Changing winds with height will foster deep rotation within storms; that so-called “shear,” or driver of spin, will increase into the evening as the jet stream approaches overhead.

Storms will continue well past dark, with at least some risk of tornadoes lingering into the early hours of Tuesday morning as the cold front triggering the storms pushes toward the Alabama border.

What’s driving the severe weather risk

A pocket of high altitude cold air, low pressure and spin, known as a is nestled within a prominent dip in the jet stream over the western U.S. This upper-air disturbance, which will incite the storminess, is ejecting east out of New Mexico.

That disturbance will help intensify a developing area of low pressure over Oklahoma. That low will draw warm, humid air northward across a sliver of Texas, Arkansas, Louisiana and Mississippi. This is known as the “warm sector” ahead of the cold front. That moist, unstable air will fuel the atmosphere and allow thunderstorms to form.

The storms won’t be particularly tall, but they will enter a strongly sheared environment. Winds at the surface will be from the south and southeast, with they blow from the southwest and west at higher altitudes. Any storm that grows through multiple layers of atmosphere will rotate. As jet stream energy arrives into the evening, that twisting force will grow.

That’s why it’s likely that supercells will evolve, rather than erupt. In other words, there probably won’t be a sudden explosion of afternoon thunderstorms that immediately rotate which is sometimes the case in the Plains. Instead, existing thunderstorms will be subject to increasing shear until, eventually, some of them grow into supercells with persistent rotation. It’s those storms that will pose the risk of strong tornadoes — a threat that’s predicated on them remaining isolated or discrete and not being forced to compete with neighboring storms.