Widespread severe weather will continue this afternoon and into the overnight hours ahead of a highly potent upper-level low pressure system and surface cold front. This upper-level low already instigated severe weather across the Plains Wednesday and Thursday. The low has since intensified, and is capable of producing widespread severe thunderstorms. Powerful wind gusts, colossal hail, and tornadoes are all possible with Friday’s storms.
As of this afternoon, strong thunderstorms were already firing over Indiana behind a larger ongoing complex of storms in Ohio. Southeasterly surface winds are delivering air from the Gulf of Mexico to the surface, whereas northwesterly winds aloft are delivering dry air from the Rocky Mountains. This vertical structure of winds has resulted in strong wind shear and the buildup of extreme instability, a perfect environment for thunderstorms to grow in.
Given the high degree of instability, any nudge upward will be sufficient to generate thunderstorms. Abundant sources of lift from existing outflow from earlier storms, the cold front, and the upper low’s vigorous jet streak will ensure ripe conditions for the for widespread development of severe weather.
Tornadoes and supercell thunderstorms that generally produce them prosper when an unstable atmosphere with strong wind shear is vertically stretched. Those conditions will occur Friday afternoon over southern Indiana, southwestern Ohio, western Kentucky, and northwestern Tennessee. This places big cities like Indianapolis, Cincinnati, Louisville, and Nashville in the most direct line of fire for powerful straight-line winds, hail more than 2″ in diameter, and numerous destructive tornadoes. Tornadoes and large hail are also possible in areas surrounding this corridor, including cities like Fort Wayne, Memphis, and Huntsville, but instances of destructive weather will be less widespread than in the aforementioned corridor.
Discrete thunderstorms will continue to initiate Friday afternoon from Arkansas to Kentucky. Many of these storms will develop rotating updrafts and become supercell thunderstorms, capable of producing violent severe weather. A few of these thunderstorms could be capable of devastating small towns this afternoon and evening.
Throughout the evening, discrete storms will merge to form large clusters and bowing segments that will travel east-southeast into West Virginia, eastern Tennessee, northern Alabama and northwestern Georgia. These bowing segments, known as bow-echoes, have strong straight-line winds at the center of the segment. Downed trees, power lines and other debris will accumulate in the overnight hours as these storms pass.
Residents of communities in the widespread threat zone should be sure to listen for tornado sirens and to pay attention to automated warnings from the National Weather Service. There is generally only fifteen minutes or less of time between a warning and impacts from a tornado. Immediately seek shelter after noticing rotation in the clouds or after being alerted of severe weather. The safest place to be during a tornado is a storm shelter or a basement. If lacking either, seek shelter in a windowless room on the lowest-level, preferably in a bathroom or under sturdy furniture.
After the storms subside early Saturday morning, the upper-level low pressure system that generated them will approach a wall of high pressure that will impede it from traveling further east. It could bring a few strong thunderstorms to parts of the Southeast this weekend, but the most consequential reaction to the stalling of the upper-level low will be the inundation of tropical moisture towards the East Coast through next week.