Cities such as Jackson, Miss., Memphis and Jackson, Tenn., and Birmingham, Ala., look to be at greatest risk for potential tornadoes this time.
The National Weather Service Storm Prediction Center has outlined much of the Deep South, especially from south-central Mississippi north through southwest Tennessee and east into western Alabama, as having a Level 3 out of 5 enhanced risk for severe weather Thursday. There’s a likelihood that, if trends continue, that risk category may need to be upgraded further. As it stands, the prediction includes the risk for “significant” severe weather.
How the storm threat may evolve
Storms ongoing across the Mississippi Valley on Tuesday morning will clear to the east overnight into Wednesday morning, allowing an influx of warm, moist air from the Gulf of Mexico to overspread the region in their wake. Simultaneously, a storm system ejecting from the Great Basin of Nevada and diving over the Four Corners region will work as a triggering mechanism to induce severe storms by Thursday.
That upper-level system is wrapped up within a dip in the jet stream, which contains cold air at the upper levels as well as an eddy of counterclockwise swirl. That will simultaneously destabilize the atmosphere, or cause pockets of air to rise, and bring about a change of wind speed and/or direction with height known as wind shear. It’s a textbook recipe for rotating thunderstorms.
Storms will flare up Thursday morning in the northern half of Mississippi, northwest Alabama and southern Tennessee as a warm front lifts north. Fog and overcast skies will be present, too. If any sunshine breaks out south of the warm front, it will act to heat the air and destabilize the atmosphere even more, bolstering the risk of strong to severe storms.
“All of this would support damaging winds, larger hail and tornadoes, some of which could be strong,” wrote the National Weather Service in Jackson, Miss.
There are indications that storms could be early bloomers, breaking out by as early as 1 or 2 p.m. Central time around the Interstate 20 corridor of Mississippi. Storms ahead of the cold front should quickly expand east, perhaps entering western Alabama by early evening.
It’s impossible to offer more specifics yet, since there remains a wide array of options on the table for this potential “boom or bust” event. The American model simulates a less potent upper-level disturbance that, while possibly brewing severe weather, would be tame in comparison to what the European model projects. The European is calling for a highly energized upper-air system that would result in more widespread thunderstorms, significant tornadoes and a serious impact in the South.
“Contrary [to the GFS], the ECMWF has trended very aggressively toward a major severe weather event and tornado outbreak across [Mississippi] and northern/western parts of Alabama as multiple long-track supercells are shown evolving within an open warm sector,” wrote the Weather Service in Birmingham.
An additional source of worry stems from the potential for nocturnal tornadoes in northern Alabama and central and eastern Tennessee. The low-level jet stream will ramp up in intensity after dark Thursday night in response to quickly strengthening low pressure, providing the needed wind dynamics to spawn scattered tornadoes within a squall line or evolving messy cluster of storms in the Tennessee Valley.
That region is known for its vulnerability to tornadoes, due to housing types, particularly at night when they are difficult to spot.
The risk for severe weather will wane Friday as the most favorable upper-level dynamics outrun surface heating, though a few strong to locally severe storms cannot be ruled out in the southern Ohio Valley and parts of the Southeast.
Another chance of severe weather may materialize early next week.
Parts of the South are still picking up the pieces from a tornado outbreak last week that dropped more than four dozen twisters heavily concentrated in western Alabama and surrounding areas. None were ranked as strong (EF3 or greater), and there was no loss of life, most likely the product of advanced warning and communication by forecasters as well as luck; most tornadoes missed more heavily populated areas.
More rounds of dangerous storms can be expected as we head into April and May — peak tornado season.