SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0744 AM CDT Sun Apr 30 2023 Valid 301300Z - 011200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF SOUTH FLORIDA...AND PARTS OF THE CAROLINAS/TIDEWATER REGION... ...SUMMARY... Damaging thunderstorm winds, a tornado or two and isolated large hail are possible through midday over parts of south Florida, and through the afternoon over parts of the Carolinas/Tidewater region. ...Synopsis... A mid/upper-level mean trough will remain over east-central North America, anchored by a large, synoptic-scale cyclone now centered over Lake Michigan. The associated 500-mb low will drift eastward across lower MI through the period, while broader-scale cyclonic flow covers the CONUS from the Rockies to the Atlantic Coast. That cyclonic flow also extends southward across much of the Gulf of Mexico and FL, thanks to a strong, basal shortwave trough now apparent in moisture-channel imagery over western GA, the central/eastern FL Panhandle and the northeastern Gulf. This perturbation will deamplify today, but still remain well-defined, as it eject northeastward to the Delmarva Peninsula and adjoining Atlantic by 00Z. The perturbation then will accelerated northward across New England and dissipate by around 12Z tomorrow. At the surface, 11Z analysis showed a cold front from a low over central SC southward just offshore from the GA Coast, then southwestward over west-central FL and the central Gulf. A double-structured warm-frontal zone was drawn, east and northeast of the low, with the southern branch representing the northern rim of best-modified Atlantic marine air, and the northern branch extending near the Fall Line to eastern VA, then across the Delmarva Peninsula. The northern warm boundary should drift northward, while the marine front moves inland and becomes somewhat more diffuse, as the surface low ejects northeastward. By 00Z, the low should move/redevelop up the Fall Line to the eastern PA/NJ vicinity, with cold front across the Delmarva/Chesapeake Bay region and offshore, to the Straits of Florida. ...FL... A corridor of strong-severe thunderstorms will continue to sweep eastward across the remainder of the peninsula and Keys through midday, ahead of the cold front, with a tornado or two still possible, along with damaging gusts and isolated hail. See remaining portions of SPC tornado watch 184 and related mesoscale discussions for near-term guidance. The air mass ahead of this activity has recovered behind yesterday's MCS, amidst a rapid mass response to the shortwave trough that includes an intense/50-65-kt LLJ. Surface dewpoints commonly in the upper 60s to mid 70s will support prefrontal MLCAPE of 1500-2500 J/kg across south FL and the Keys, where the old MCS outflow never reached, or quickly retreated. The LLJ decreases southward, but still will support enlarged low-level hodographs throughout the remaining outlook area before the convection passes and flow veers. ...Carolinas/Tidewater region... Supercells offshore from the southern NC Coast this morning should remain so, though a roughly triangular wedge of the high-theta-e air mass supporting them may spread inland ahead of the next area of substantial convective potential. As that occurs, multiple rounds of widely scattered to scattered thunderstorms should move northeastward across the outlook area through the afternoon, offering sporadic damaging gusts, isolated large hail and the threat for a few tornadoes. Over the next few hours, warm/moist advection will weaken MLCINH substantially, even without much diurnal heating, offsetting modest midlevel lapse rates enough to support 500-1000 J/kg MLCAPE. Enlarged hodographs enabling 150-300 J/kg effective SRH, and around 50-kt effective-shear magnitudes, will support supercell potential. Behind this activity, cooling aloft and related steepening of low/middle-level lapse rates, in the proximal DCVA/ascent zone preceding the mid/upper trough, will help to maintain favorable buoyancy for more convection prior to passage of the low and cold front. Somewhat weaker low-level shear, but stronger winds aloft, are expected with 50-60-kt effective-shear vectors and long, somewhat straight hodographs. The severe threat should terminate with cold frontal passage from southeast to northeast. ..Edwards/Mosier.. 04/30/2023
