The National Weather Service Storm Prediction Center has issued a level 4 out of 5 risk for severe weather, highlighting the threat of “a potential outbreak of severe storms including several long track strong tornadoes.”
They noted the potential for a rare upgrade to a level 5 out of 5 “high risk” of severe weather, citing “the very favorable environment forecast across [the] region” and the shot at “multiple strong to potentially violent, long-track tornadoes.”
Cities such as Starkville and Jackson, Miss., and Tuscaloosa, Birmingham and Huntsville, Ala. are among those most at risk for dangerous and destructive storms Thursday, as are locales north of Interstate 20 and along the U.S. Highway 82 and Interstate 22 corridors in Mississippi and Alabama.
The Storm Prediction Center also upgraded north Alabama and parts of Middle Tennessee, including Nashville and Memphis, to a level 4 out of 5 categorical risk; it now appears that a strip of warm air will surge farther northward, fueling the threat of rotating thunderstorms even into Music City during Thursday evening. A strong tornado can’t be ruled out.
Eastern Mississippi and much of Alabama were subjected to a rare “high risk” tornado threat last Wednesday, rated 5 out of 5, during which 49 twisters touched down. Despite the widespread tornadoes, there was no loss of life — perhaps because of a well-communicated forecast and because the twisters largely spared major population centers; their damage was rated no stronger than EF2 on the 0 to 5 Enhanced Fujita scale.
How Thursday’s storm threat may evolve
Computer models are still in slight disagreement about the exact shape and strength of the upper-level weather system that will trigger the anticipated severe weather outbreak, but for places such as northwest Alabama, “significant severe weather appears likely regardless of the outcome, particularly near and northwest of I-59,” wrote the Weather Service in Birmingham.
It’s uncertain just how far north the favorable atmospheric setup will extend into Tennessee, but some flooding is possible regardless of tornado and wind potential; two to three inches of rain may fall by Friday morning.
The impetus for severe weather could be found Wednesday morning in the Southwest, United States, where a counterclockwise-swirling ebb of cold air and spin was bottled up within a dip in the jet stream. Those chilly temperatures at high altitudes, along with increasing spin, will foster rising motion and generate scattered to widespread thunderstorms Thursday as the storm system sweeps eastward. Because winds will be blowing at different speeds and directions at different levels of the atmosphere, that will increase the likelihood of thunderstorm rotation.
Thursday may begin with a band of downpours and a few hail-producing thunderstorms lifting north across Mississippi, Alabama and Tennessee along the storm system’s warm front. Rain will probably last two or three hours in most spots and should clear places such as Jackson and Birmingham by 2 or 3 a.m. with a few renegade showers left behind.
Thereafter, the Deep South will emerge into the volatile “warm sector” of the approaching storm system — the slice of warm, steamy air ahead of the cold front within which rotating thunderstorms or supercells will blossom.
In the model simulation shown below, the warm sector is prominent in areas colored yellow and orange, indicating high levels of convective available potential energy Thursday afternoon, a fancy way of describing the fuel available for storms.
The low level jet stream, a river of swiftly moving humid air sweeping north from the Gulf of Mexico, will determine how far north the warm sector gets, and subsequently where the northern extent of the severe thunderstorm threat will exist. That may remain unclear until midday or early afternoon Thursday. Some models show the warm sector penetrating as far north as southern Tennessee, while others only bring it to around Huntsville, the Weather Service in Huntsville wrote in its forecast discussion Wednesday.
Current models indicate a cluster of thunderstorms, some rotating, will take shape across Mississippi shortly after lunchtime Thursday, shifting east and expanding into Alabama during the afternoon into the evening.
The amount of energy to fuel storms, as well as rotation in the atmosphere, is more than sufficient to produce tornadoes, as well as higher-end tornadoes. Whether that threat is realized boils down to storm mode — whether clusters, squall lines or individual supercells form. Supercells or rotating thunderstorms are able to tap into the atmosphere’s full fury in a way that other storm modes struggle to do. It’s often challenging to pinpoint storm mode in advance, lending uncertainty to the forecast.
“The worst case scenario includes the potential for a ‘violent’ (EF-4) tornado in the event of the most favorable storm evolution,” cautioned the Weather Service in Birmingham.
The Storm Prediction Center noted uncertainty in storm mode as well.
“Concern does exist regarding the effect of... potential for destructive interference from too many storms developing across the warm sector at the same time,” they wrote. In other words, competition between a larger number of storms would result in weaker storms.
There’s a chance that, assuming supercells form ahead of the cold front punching eastward, multiple rounds of dangerous storms could occur. Scattered supercells ahead of the cold front will have strong tornado, wind and hail potential, with an evolving line of storms along the cold front later arriving with wind and tornado risk.
The threat should wind down in Mississippi by 6 or 7 p.m., around the time it may be at its worst in northern and western Alabama and Middle Tennessee. Storms will begin to lose some of their punch east of Interstate 65, and should wane considerably by the time they reach the Chattanooga to Gadsden, Ala. corridor.
Storms will move into northeast Alabama, the Appalachian foothills of Georgia and the Cumberland Plateau of Tennessee by 10 or 11 p.m., with only an isolated severe risk lingering overnight. The weather should be comparatively tranquil, save for an outside chance of isolated severe storms in the Carolinas, on Friday.