A major severe weather outbreak is expected tonight in the lower Mississippi Valley, where several tornado watches have already been issued. A powerful middle and upper air trough over the Southwestern United States is plowing into a warm, moist and moderately unstable atmosphere over the mid-south. This environment is favorable for violent tornadoes, especially in Arkansas, western Tennessee, northern Mississippi, extreme southeast Missouri, western Kentucky and far southern Illinois. Widespread straight-line wind damage could reach parts of Alabama toward daybreak.
The numerical model guidance is in fairly good agreement about the evolution of the large-scale pattern on Wednesday. Even some details about the mesoscale environment are agreed upon, giving us moderate to high confidence in the forecast. The upper-level trough is expected to lift to the northeast, which will focus the strongest forcing for ascent over the Ohio Valley and Central Appalachians. That tells us that whatever severe threat we receive on Wednesday will not be as bad as the debacle that's expected to our west tonight. The short range ensemble forecast (SREF) models still forecast some vertical motion this far south on Wednesday afternoon. This will support updraft development since the environment will be modestly unstable, as dewpoints reach the mid to upper 60s. Vertical wind shear will be strong over the entire area and that supports storm organization. The instability; however, is expected to weaken dramatically, especially east of U.S. Highway 319. The various computer models show the best juxtaposition of wind shear, instability and large-scale vertical motion to be over the western one-third of our viewing area (basically along and northwest of a line from Moultrie to Tallahassee to Apalachicola). As the surface cold front moves eastward, we expect either several line segments or perhaps a broken line of strong to severe storms to move into the area during the afternoon and early evening. Our locally-run workstation WRF model and the Super Microcast model agree on this exact scenario unfolding, with the strongest storm cells located west and north of Tallahassee. The storms are expected to be weaker as they move toward Perry, Live Oak, Madison and Valdosta areas if the latest model simulations are accurate. The primary severe threats will be straight-line wind damage, but a brief/weak tornado cannot be ruled out.
The severe threat should end shortly after dark Wednesday evening. We will upload our video weather blog this evening with all of the pretty graphics and our latest thoughts on the situation.