SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1028 AM CST Thu Dec 31 2020 Valid 311630Z - 011200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS UPPER TX COAST ACROSS LA TO SOUTHWEST MS... ...SUMMARY... Severe thunderstorms with tornadoes and damaging winds as the primary hazards will initially be possible across the Upper Texas coast and southwest Louisiana through this afternoon, before expanding across the rest of Louisiana into Mississippi and Alabama this evening and persisting through tonight. ...Upper TX coast to MS/AL... A mid-level closed low over western Coahuila will amplify as it advances north into OK by early Friday, in response to an upstream shortwave trough digging south-southeast from southern CA to Sonora. At the surface, a 1004 mb cyclone should track north from the Middle/Upper TX coast to northeast TX. By 06Z, this cyclone will become increasingly occluded, yielding a T-shaped surface pattern with the apex of the warm sector across the Ark-La-Miss as a cold front sweeps east across the western Gulf and a west/east-oriented warm front advances north across central portions of MS/AL. The warm sector characterized by boundary-layer dewpoints in the mid-upper 60s will spread inland to the east of the cyclone and contribute to MLCAPE of 500-1000 J/kg across extreme southeast TX/southern LA through this afternoon, and into tonight across central/southern portions of MS and AL. Thunderstorm development has been focused near and offshore of the Upper TX coast this morning, with multiple supercells noted offshore. Convection will spread northeast through tonight along and just ahead of the surging cold front. An additional swath of convection will likely develop separately downstream across the central Gulf coast by early evening and persist north over eastern MS and AL through tonight within a robust low-level warm advection regime. Greater low-level shear and enlarged hodograph curvature will initially be confined to the TX/LA portion of the outlook region this afternoon before spatially expanding this evening as flow fields strengthen downstream of the amplifying low. The combination of linear forcing for ascent along the cold front and substantial cross-boundary flow of deep-layer shear vectors will support broken bands of supercells and line segments, capable of producing tornadoes and damaging winds. However, the magnitude of the tornado threat should be tempered some by rather modest low-level lapse rates inland, and less-than-ideal phasing with the ejecting mid-level trough and surface cyclone glancing the northwest side of the warm sector. 12Z HREF and recent HRRR guidance remain subdued with the intensity and coverage of rotation potential within simulated cells, lowering confidence for identifying a corridor of 10 percent or greater probability of significant tornadoes. ..Grams/Cook.. 12/31/2020
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