...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS IN THE MID/DEEP SOUTH TO TN VALLEY AND CUMBERLAND PLATEAU... ...SUMMARY... Multiple rounds of severe thunderstorms are expected today through this evening, centered on the Mid/Deep South, Tennessee/Lower Ohio Valleys, and Cumberland Plateau. Tornadoes, some of which could be strong, along with scattered to widespread damaging winds, and large to isolated significant severe hail will be possible. ...Synopsis... Multiple shortwave impulses and attendant mid-level jetlets will rotate through the southern to eastern portion of the broad mid/upper trough over the central states. A belt of 60-75 kt 500-mb southwesterlies should be centered on the Ark-La-Tex to Lower OH Valley by late afternoon. This should weaken thereafter, as the next shortwave impulse strengthens flow across the central Great Plains by evening. Primary surface cyclone should initially drift east from the IA/MO border into IL through the day, before accelerating eastward across IN/OH tonight. ...Mid/Deep South to the TN/Lower OH Valleys and Cumberland Plateau... An extensive swath of convection, currently from the Mid-MS Valley to the Red River, will likely be ongoing across parts of KY to the Ark-La-Miss at 12Z. The northern portion of this convective swath is consistently progged to persist into the diurnal heating cycle, aided by low-level warm theta-e advection along with mid-level height falls, and downstream destabilization. The latter is a primary source of uncertainty given the outpacing of convection thus far relative to 00Z CAM guidance. It does appear that there should be at least modest surface-based buoyancy being maintained downstream to warrant potential for a severe-producing QLCS into the afternoon. All severe threats are possible, but damaging winds may be the predominant coverage hazard. Tornado potential should peak where the QLCS approaches the surface warm front, before an abrupt drop-off in surface-based instability to its northeast. Greater boundary-layer destabilization is expected to the west-southwest of the early-day activity with relatively rapid airmass recovery amid low 70s surface dew points. Mid-level lapse rates will initially be steep over the Mid-MS Valley, but be more muted with southern extent into the Lower MS Valley. Still, moderate to large buoyancy with MLCAPE of 2000-3000 J/kg and minimal MLCIN should be common by mid-late afternoon. During that time frame, scattered supercells will likely develop along and just ahead of the eastward-moving cold front from the Lower OH Valley to the Ark-La-Miss. The modest mid-level lapse rates will be a limiting factor, but isolated significant severe hail (up to around tennis ball-size) will be possible in the initial supercell corridor. A broad swath of 30-40 kt low-level southwesterlies should be sufficient for at least a few strong tornadoes as supercells mature, most likely from western/southern KY across TN into northern portions of MS/AL. Guidance is insistent on upscale growth by early evening though, which would yield small-scale bows and potentially multiple damaging wind swaths. Embedded supercells should still persist well into the evening as storms spread east-southeast in the Southeast and southern Appalachians. But the aforementioned relative decrease in mid-level winds and muted lapse rates lower confidence on significant severe wind and greater tornado probability highlights. ...IL/IN... Within the left-exit region of the aforementioned mid-level jetlet, a dry slot and attendant corridor of steep lapse rates will likely evolve across portions of MO to IL/IN from midday into the afternoon. To the north of this, a cluster of sustained storm development is initially anticipated towards mid-afternoon, centered on central IL ahead of the surface low. Additional storms should develop farther south in time towards the Lower OH Valley. A mix of supercells and multicell clustering is anticipated, with isolated to scattered coverage of severe and all hazards possible. ..Grams/Weinman.. 05/20/2025
