William’s Severe Weather Zone for Wednesday:
At the surface, 11Z analysis showed a double-structured front, with the southern boundary from southwestern VA across middle TN, northwestern MS, northern LA, east TX, and south-central TX. The MS/LA/TX portion of this boundary denotes the northern rim of the moistest, most completely modified Gulf boundary layer, and should move northward to merge gradually today into the northern frontal zone. The northern front was quasistationary, analyzed from northern/western KY through central AR and northeast TX, to a weak frontal-wave low between DFW-GLE, then west-northwestward past CDS. The low should move east-northeastward along the merging frontal corridors, across central AR and western TN through this evening, reaching northeastern KY by 12Z tomorrow. Behind the low, the combined boundary should move southeastward as a cold front tonight, reaching middle TN, northern MS, northern LA, and east TX by the end of the period. …Lower Mississippi Valley, Arklatex to Southern Appalachians… Isolated to scattered thunderstorms are expected in multiple episodes through the period — primarily in a corridor just ahead of the front corresponding to the alignment of the “enhanced” and surrounding “slight” probabilities, but also, in a belt of subtle yet sufficient warm-sector convergence farther south over parts of MS/AL. A few tornadic supercells are possible, along with sporadic damaging gusts and isolated severe hail. The greatest combined concentration and intensity of severe potential still appears to be in the corridor closer to the front, extending from southern AR to where the two regimes will overlap in parts of the Tennessee Valley region. A conditional significant- tornado potential appears roughly collocated with the “enhanced” area, with forecast hodographs on the fringes of that parameter space; one may need to be added in a future outlook if sustained supercell potential appears in more-confident large-hodograph setting. Parts of the “slight” outlook have been extended somewhat southward of I-20 for the warm-sector convergence regime, and on the east end into central NC, for a narrow plume of late-period low-level destabilization supporting at least marginal severe potential there. Ample low-level moisture is evident already over much of LA, western/southern MS and southwestern AL, with surface dew points commonly from the upper 60s to around 70 F. Continued warm/moist advection will extend favorable theta-e northward into parts of eastern AR and southern TN. The airmass will become much more stable in the northern fringes of the outlook area because of frontal and precip effects. Deep shear will be favorable, with 50-60-kt effective-shear magnitudes being common in the corridor from AR to eastern TN, decreasing to 40-50 kt across southern MS/AL. Forecast hodographs over most of the AR-Tennessee Valley corridor are long but not particularly large for most of the period, though they may enlarge for at least a few hours over the northeastern MS/northern AL/southern mid TN area late this afternoon into evening, with passage of a 45-55-kt LLJ overlying mostly southerly surface flow. Forecast soundings suggest effective SRH may reach 250-350 J/kg in the latter environment. ..Edwards/Gleason.. 12/29/2021 $$
