SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0638 AM CST Tue Dec 15 2020 Valid 151300Z - 161200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Thunderstorms are possible this evening and tonight over the central Gulf Coast region. ...Synopsis... In mid/upper levels, a split-flow pattern will prevail across the CONUS, with the northern stream belt over southern Canada and portions of New England through the period. The southern stream will buckle around a strong trough -- now evident in moisture-channel imagery from the NE Panhandle southward across eastern portions of CO/NM. This trough will shift eastward across the southern Plains and gradually assume more-positive tilt with time. By 12Z tomorrow, the 500-mb trough should extend near an axis from UIN-LIT-GGG-SAT. At the surface, 11Z analysis identified a low over the Llano Estacado northwest of MAF and southwest of LBB. This low should remain in continental/Polar (CP) air as it moves southeastward across central TX into this evening. A weak low was drawn over the Gulf southeast of CRP and northeast of BRO, with warm frontogenesis underway to its east, and north of a synoptic front that extends from south FL across the central/southwestern Gulf. As the mid/ upper trough approaches, the northwest Gulf low should deepen slightly and move to roughly south of LCH by around 00Z, still distinct from the TX inland low. The Gulf low will become the dominant one overnight, while moving east-northeast generally toward the coastal AL/FL line. A warm front should extend southeastward from that low, while the other frontal zone weakens from west-east, allowing both sea-air fluxes and theta-e advection to modify the Gulf low's marine warm sector gradually. A cold front will extend southwestward from the low, and move eastward across the northwestern/north-central Gulf. ...Central Gulf Coast -- LA to western FL Panhandle... A belt of convection, with isolated to scattered embedded thunderstorms, should develop by this evening near the low and southwestward along the cold front. Isolated to widely scattered thunderstorms also are possible over the modifying Gulf warm sector, which will be weakly capped. Convection should extend inland from the low along a low-level trough, atop a relatively stable near- surface layer with modest elevated buoyancy. This regime should shift eastward overnight, in step with the longitudinal progression of the low. The strongest DCVA/cooling aloft and associated steeper midlevel lapse rates should lag the regime, closer to the mid/upper trough. However, buoyancy may extend into icing layers supporting lightning production inland. Marginally surface-based inflow parcels may reach the immediate coastal areas of southeastern LA to the Mississippi River mouth, after 06Z and in a narrow plume with MLCAPE less than 500 J/kg. This potential is very conditional, especially given: 1. The great amount of open-water airmass modification still needed -- especially with ongoing, cold offshore cP flow still observed 18-24 hours out, and 2. Potential for partly modified warm-sector trajectories to return through precip before reaching inland, even if the warm front does reach the coast. While an isolated strong storm cannot be ruled out on that stretch of coast, the potential for severe appears too low, small in scale and conditional for an outlook area. Strong-severe convection -- including supercells -- are possible farther south, over open Gulf waters outside the outlook domain, and may impact aviation, shipping and rig interests there. ..Edwards.. 12/15/2020
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