The American HRRR model simulates a line of supercell thunderstorms moving east around nightfall. (WeatherBell)

This story has been updated.

Severe thunderstorms and strong tornadoes are possible from the Ozarks to the Tennessee Valley on Thursday along a cold front draped across the region. Multiple rounds of storms are likely, the strongest of which may continue after dark and present an overnight risk of dangerous weather. It’s a setup somewhat similar to nine days ago, when violent tornadoes swept through Middle Tennessee, killing 24 people.

Middle Tennessee is, once again, at risk.

A tornado watch was issued around lunchtime for portions of Illinois, Indiana, Kentucky, Missouri, and Tennessee. Nashville was included on the far southeastern portion of the watch. The Storm Prediction Center advised of a “high” likelihood for tornadoes, specifically noting of “a couple intense tornadoes [were] possible.”

The watch is valid until 5 p.m., but in eastern areas, the risk will likely persist later.


A tornado watch is in effect for portions of the region until 5 p.m. local time Thursday evening. (NOAA/SPC)

Later on this evening, areas farther south into Arkansas may also face dangerous storms.

Atmospheric ingredients will allow for the development of supercell, or rotating, thunderstorms. Those will prove the most perilous for a tornado and hail threat, with damaging winds likely in other storms.

The National Weather Service Storm Prediction Center cautions that significant tornadoes may occur.

Storms had already begun to form over Missouri around midday, lumbering along the Ohio River as a second line develops to the south and west. Severe weather was also entering northwest Tennessee, prompting severe thunderstorm warnings near Memphis.

Additional storms, the most dangerous, will line up to the southwest on the cold front and race east during the afternoon into the nighttime hours into the Tennessee Valley.

“Confidence has continued to build that severe thunderstorms will impact Middle TN this afternoon into tonight,” writes the Weather Service office in Nashville. “We have big concerns about the threat of nighttime tornadoes.”

Having a severe weather plan and multiple ways to be notified of warnings is imperative this afternoon and evening for those in the affected areas.

Areas at risk


Severe thunderstorm risk as outlined by the National Weather Service. (NWS)

The Storm Prediction Center has declared an enhanced risk (Level 3 out of 5) for severe thunderstorms in central and northeast Arkansas, northwestern Tennessee and eastern Missouri north into much of western Kentucky, parts of southern Illinois and even a sliver of extreme southern Indiana. Cities such as Evansville, Ind., Paducah and Bowling Green, Ky., and Clarksville, Tenn., are encompassed by this zone.

Within the enhanced risk zone, the Storm Prediction Center has outlined a hatched area where there is at least a 10 percent chance of significant tornadoes, rated EF2 or higher on the 0 to 5 scale for intensity, within 25 miles of a point.

Nashville straddles the line between the enhanced risk and slight risk zones (level 2 out of 5), but exactly where a line is drawn can be somewhat arbitrary.

A slight risk for severe weather will even trail southwest into Texas, while Dallas lies in a marginal risk zone (Level 1 out of 5). Severe thunderstorms are possible there as well in a narrow strip ahead of the cold front, but with less areal coverage than anticipated farther north and east.

Timing and Impacts

Storms were already erupting over eastern Missouri at lunchtime, organized in a messy cluster with heavy rain to the north. Along the southern edge of that impulse, a few severe thunderstorms will continue to develop through mid-afternoon. Those initial storms will pose a tornado, damaging wind, and hail risk as they trek through southern Illinois, southern Indiana and western Kentucky.

That same cluster could also feature heavy rain, posing the risk of flooding. A flash flood watch is in effect for much of northern Kentucky.

The dynamics of the atmosphere could support an isolated strong tornado with any storms that form.


President Trump tours damage from last Tuesday's tornado in Cookeville, Tenn. (Tom Brenner/Reuters)

Thereafter, a broken line of potential supercell thunderstorms will ignite along the cold front draped southwest to northeast, initiating in southeastern Missouri, southern Illinois and northeastern Arkansas by 5 or 6 p.m. Those storms will move into western Tennessee and Kentucky by sunset, accompanied by large hail, possible tornado activity, and potential damaging winds. Once again, a strong tornado can’t be ruled out.

There are some indications the second line of storms may be the stronger of the two.

Storms will move into Middle Tennessee as night falls. It’s unclear the extent to which storms will maintain their strength, but it’s likely that at least some severe threat, perhaps significant, will linger through at least midnight.

Setup


The American HRRR model simulates the buildup of “instability,” driven by warmth and moisture, to fuel storm growth this afternoon and evening. (WeatherBell)

Quickly intensifying low pressure over the northern Plains on Thursday will translate north and meet another low-pressure system north of the Great Lakes early Friday. Ahead of them, breezy southerly winds carried north a warm front that will allow much of the Tennessee Valley to climb into the 70s Thursday.

That juiced-up atmosphere will provide fuel for rapid storm growth as the low to the north swings a cold front through late Thursday afternoon and evening. At the same time, strong jet stream winds dipping south and screaming back north again will enhance the amount of wind shear available. Wind shear describes a change of wind speed or direction with height. That’s the perfect recipe for rotating storms.

As the sun sets and daylight heating wanes, there are questions about whether storms will survive as they continue barreling east. Storms may enter a “high shear/low CAPE” environment, meaning that the exceptional amount of wind shear present could compensate for the loss of daytime heating and reduction in storm-sparking “instability.” Setups like that are notoriously difficult to predict, but are common in places like Tennessee.

Action items

For those in the affected areas, having a severe weather plan is crucial. Knowing how to implement it is also key, as is communicating the plan with family and loved ones who it affects. Ask yourself what you’ll do if you have one minute, two minutes, five minutes, or 10 minutes of warning that a tornado or other serious threat is imminent. Know how you’ll use that time to get to shelter, and make sure your shelter of choice is available.

Moreover, have a way to be notified — even at night. Remain aware of the situation and keep abreast of an evolving forecast. Charge cellphones and weather radios; don’t rely on sirens to alert you of impending threats. And if a warning is issued, don’t waste time seeking environmental cues or confirmation. Use any possible lead time to move to a place of safety.

“Media partners, please remind people to have multiple ways to get important warning information,” the Weather Service in Nashville writes. “NOAA Weather radios are best, but another option is needed, such as WEA [Wireless Emergency Alerts] alerts or reliable apps for smartphones. Phones need to have volume up!”