Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 21 2023 12:36:19 ACUS01 KWNS 211236 SWODY1 SPC AC 211234 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0634 AM CST Tue Nov 21 2023 Valid 211300Z - 221200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN ALABAMA...SOUTHWESTERN GEORGIA AND PARTS OF THE FLORIDA PANHANDLE... ....SUMMARY... A few tornadoes and isolated damaging gusts are possible today across southern Alabama, southwestern Georgia and parts of the Florida Panhandle. Isolated damaging gusts are possible, along with a marginal tornado threat, from there northeastward to the coastal Carolinas. ....Synopsis... In mid/upper levels, a split-flow pattern will develop this period, as a strong shortwave trough now over northwestern MX digs southeastward and evolves into a cut-off cyclone, centered over Sinaloa at 12Z tomorrow. Meanwhile, moisture-channel imagery indicates a vorticity max and formerly closed circulation over northern IL, on the northern part of a trough extending southwestward to southwest TX. Though now in phase with the MX perturbation, the IL one will break northeastward across the Lower Great Lakes, ahead of an amplifying shortwave trough now over MN. The MN feature will dig southward to the Ozarks today, then pivot eastward across the Mid-South and lower Mississippi Valley tonight as a closed (or very nearly closed) 500-mb low. Ahead of all that, and the associated mean trough, a broad belt of southwest flow aloft will shift slowly eastward with height falls over parts of the Southeast and southern Atlantic Coast. The 11Z surface analysis showed a primary synoptic low -- associated with the IL mid/upper perturbation. The associated cold front was analyzed near LAF. A cold front was moving southeastward over eastern/southern MS, southeastern LA, and the northwestern Gulf. A warm/marine front was drawn from a secondary/triple-point low near CBM southeastward across the PNS area and northeastern Gulf, becoming quasistationary near the southwest coastline of Florida. The warm front is forecast to shift diffusely northeastward across eastern AL, southern/eastern GA and parts of the Carolinas through the period, while being overtaken from north-south by a belt of precip/thunderstorms. The cold front is forecast to move eastward/southeastward by 00Z to the Appalachians of western VA/NC/SC and northern GA, southwestward over southeastern AL, the west-central FL Panhandle and central Gulf. Overnight cyclogenesis should occur along this boundary, moving northeastward up the Atlantic Seaboard, with the surface low over NJ by 12Z. The cold front should extend southwestward across eastern NC -- where early-stage/secondary frontal-wave low development is possible ahead of the Mid-South shortwave perturbation. The front then should extend from there across northern FL to the south-central Gulf. ....Northeastern Gulf Coastal Plain... An ongoing, rather nebulously organized swath of precip and isolated embedded thunderstorms is apparent from near the mouth of the Mississippi River northeastward across parts of southern/eastern AL, the FL Panhandle and western GA. Within or behind this activity, diurnal destabilization and low-level convergence should support reorganization of the convective belt today, and especially from midday through afternoon. This may come about as convection forming nearer to the front moves eastward and merges with what is left of the ongoing belt. Either way, a better-focused corridor of thunderstorms is expected to take shape over parts of AL and the FL Panhandle and move eastward, while the warm front diffusely shifts inland, and moisture transport and theta-e advection continue from the Gulf. Forecast soundings today suggest a plume of 500-1200 J/kg MLCAPE within about 100 nm of the coast, weakening/narrowing northward. Before prefrontal flow veers to southwesterly, southerlies are expected through much of the boundary layer, veering with height above about 1 km, and contributing to large hodographs. Around 150-300 J/kg effective SRH may develop, about 75-150 J/kg of which is in the lowest 500 m. The associated parameter space should support some tornado potential with any sustained supercells or the strongest line-embedded mesovortices. ....Remainder of GA-Carolinas... The main band of convection should extend from the Gulf Coastal Plain northward into the southern Appalachians by mid/late afternoon, the move eastward over more of GA/SC this evening into tonight. Already modest buoyancy should diminishing northward and with time for the most part, given less-unstable low-level trajectories apparent in progs out of the northern FL Peninsula. Large-scale forcing should weaken as the Great Lakes-area trough aloft shifts away from the region, with mid/upper flow largely parallel to the swath of low-level lift supporting the convection. Yet favorable low-level and deep shear will persist. As such, a more conditional and isolated severe potential is apparent northeast of the 15%/"slight risk" area. Though presently remaining in the range of unconditional marginal probabilities, a secondary/conditional ramp-up in severe threat may develop late tonight across eastern/coastal NC. Sufficient southerly flow component in the lower boundary layer should be maintained to yield a persistent fetch off warm Atlantic waters -- and especially if the frontal-wave development is a bit stronger than currently forecast. This should yield a corridor of mid-60s F surface dewpoints over eastern NC the last several hours of the period, and perhaps mid/upper 60s along the immediate coast. That should offset weak, nearly moist-adiabatic deep-layer lapse rates enough to yield 300-800 J/kg MLCAPE. Shear will favor embedded supercells and/or mesovortices with the main line, as well as storm-scale rotation for some cells ahead of it moving off the ocean. Forecast soundings generally depict about 30-40 kt effective-shear magnitudes, and long lowest-km hodographs beneath a 50+ kt LLJ. At this time, weak instability and uncertainty on coverage of favorable storm mode precludes higher probabilities. ...Edwards/Leitman.. 11/21/2023 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .