Subj : DAY1SVR: Day 1 Convective To : All From : Mike Powell Date : Sat Apr 29 2023 08:33:00 ACUS01 KWNS 291252 SWODY1 SPC AC 291250 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Sat Apr 29 2023 Valid 291300Z - 301200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF FLORIDA AND SOUTHERN GEORGIA... ....SUMMARY... Two rounds of severe thunderstorms, with damaging winds, large hail, and a few tornadoes, are possible across parts of Florida and southern Georgia -- today, then again late tonight. ....Synopsis... Mid/upper-level mean troughing will remain over the CONUS through the period, with broadly cyclonic flow from the Rockies to the East Coast by 12Z tomorrow. This will result from deep cyclogenesis over the Upper Great Lakes region, as a strong shortwave trough now over the upper Mississippi Valley region evolves a closed low over WI by 00Z. By 12Z, the low should decelerate over Lake Michigan, then move only slowly eastward across Lower MI through day-2. In response to those developments, an intense shortwave trough -- apparent in moisture-channel imagery over eastern OK and north to southwest TX -- will pivot eastward across the Deep South/Gulf Coast States, and deamplify gradually. However, it still should be a well-defined and strong perturbation as it passes the lower Mississippi Valley/Delta region around 00Z, reaching GA, the FL Panhandle and the northeastern Gulf at 12Z. At the surface, 11Z analysis showed a low over southeastern AR, with cold front across central LA and the TX offshore waters, to northeastern MX. The cold front was preceded for about 100-150 nm by convective outflow over much of the northwestern Gulf. A quasistationary front was drawn near a line from BTR-MSY-PAM-VLD- MYR, and should move slowly northward/inland as a diffuse warm front today. Farther north, a low near Saginaw Bay will wrap northwestward under the developing mid/upper cyclone, while the southern low moves diffusely eastward across the Mid-South, Tennessee Valley and southern Appalachians, to the Carolinas through the period. By 00Z, the cold front should extend from a low over AL southward over the central Gulf. By 12Z, the front should reach from the low in the CHS/MYR area off the GA Coast to the western/ central FL Peninsula. ....Southeastern CONUS... Multiple primary convective episodes are possible across the outlook area today and tonight, offering the threat for severe hail, damaging to severe gusts, and a few tornadoes. These episodes will overlap spatially to a great extent, and so are numbered chronologically below. Each is more conditional and uncertain than the one before. 1. Eastern FL/GA today: Of the episodes, certainty is greatest in the development of strong-severe thunderstorms today along the East Coast sea breeze from southeastern GA to east-central/southeastern FL. This activity will occur on either side of a northward-returning, subtle frontal zone described below, with enough low-level moisture/heating to support MLCAPE reaching the 1500-2500 J/kg range. WIth strong ,id/upper winds overhead , sufficient effective and cloud-layer shear will be in place to support organized multicells and at least transient supercells. This may be followed by the eastward extension/progression of an MCS from the Gulf, whose early-stage development already is apparent. 2. Eastern Gulf into FL this afternoon/evening: The second scenario still is somewhat conditional, but confidence is increasing based on current convective/boundary trends. A separate, weaker baroclinic zone from the western Gulf cold front was drawn across the north-central/east-central Gulf and over south-central FL -- south of which resides the most optimal maritime/tropical air with surface dewpoints in the mid 70s F. To its north, conditions were still moist, with upper 60s to low 70s dewpoints over the northeastern Gulf and central/northern FL. This boundary is being reinforced by associated thunderstorms evident in satellite and distant-radar loops, roughly along a line from 27N84.5W west- northwestward, linking with the northeastern "warm-advection wing" of a broken line of convection over the western Gulf. Though the southern part of the line produced severe wind over deep south TX several hours ago, the northern portion of the line -- south of LA -- appears to be taking over as the most intense area of convection, based on lightning and IR cloud trends. Behind the latter activity, an elongated vorticity lobe -- perhaps containing two MCVs -- is apparent well off the TX Coast, moving eastward over the northwestern Gulf. As the midlevel lobes progress eastward, prefrontal convection should continue to organize/expand and may develop into a substantial thunderstorm complex over the northeastern Gulf today. Its apex may move along the northward-shifting boundary toward western FL, reaching the coast mid/late afternoon before crossing the peninsula and perhaps merging with whatever is left of the sea-breeze activity that hasn't yet moved off the Atlantic Coast. Though increasingly dense cloud cover spreading off the MCS will temper afternoon heating over land, sufficient instability and moisture should be in place to sustain this activity inland, along with its threat of damaging wind and perhaps a tornado or two. A relatively maximized corridor of wind potential may become apparent across central and/or northern FL, near the north-ward-shifting maritime/tropical boundary, where it is intercepted by maximized lift on the leading thrust of the MCS. If confidence increases in this scenario, a smaller-scale wind-probability increase may be needed in a succeeding outlook. 3. MS/AL this afternoon to GA/FL/SC overnight: Finally, a broken arc of thunderstorms may develop this afternoon near the cold front and south of the surface low, from southern AL across the north-central/central Gulf, ultimately expanding/building southward into the Gulf. The afternoon phase will overlap cold midlevel temperatures with residual boundary-layer moisture 50s F surface dewpoints) and pockets of afternoon heating, with weak low-level flow but strong mid/upper winds and marginally favorable deep shear. A few organized multicells and perhaps marginal supercells are possible, offering isolated large hail and/or damaging gusts. The southern part of this area of convective forcing should build into a more-favorable air mass over the Gulf. A 45-60-kt LLJ is progged to develop across the northeastern Gulf and into northern FL/southern GA tonight, ahead of the shortwave trough and cold front, as flow aloft also strengthens considerably. Associated, rapid low-level theta-e advection will destabilize the air mass to its east, supporting continuation of some parts of the thunderstorm arc into GA, central/northern FL and perhaps SC by the end of the period. Hodographs swollen by the LLJ yield effective SRH values of 200-400 J/kg in forecast soundings, amidst 50-65-kt effective-shear magnitudes. With a substantial component of flow aloft across the zone of convective forcing, tornadic supercells are possible in such a setting, as well as bowing segments and LEWP/mesovortex features. However, the degree of destabilization is quite uncertain behind what likely will be a robust cold pool produced by the earlier Gulf MCS. Pending more confidence in specific location of corridor of greatest shear/CAPE parameter-space overlap, a broad 5% tornado risk is kept for this cycle. Still, a corridor of greater threat (perhaps including significant tornado potential) conditionally may evolve somewhere from central FL to southern SC after 06Z. ...Edwards/Mosier.. 04/29/2023 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .