SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0638 AM CST Thu Dec 31 2020 Valid 311300Z - 011200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS TODAY INTO EARLY TONIGHT FROM THE UPPER TX COAST TO SOUTHWESTERN MS... ...SUMMARY... A few tornadoes, damaging winds, and isolated large hail will be possible today along the upper Texas coast, and through tonight across parts of Louisiana, Mississippi, and Alabama. ...Upper TX coast today to MS/AL overnight... A midlevel closed low south of the Big Bend will eject northeastward toward the Ozarks by early Friday, in response to an upstream shortwave trough digging south-southeastward from northern CA toward northwestern Mexico. At the surface, a 1004 mb low as of 12z will develop inland across the upper TX coastal plain today along a quasi-stationary front, and the low will continue north-northeastward toward the Ark-La-Tex tonight. The warm sector (with boundary-layer dewpoints in the mid-upper 60s) will spread inland to the east of the cyclone and contribute MLCAPE of 500-1000 J/kg today across extreme southeast TX/southwest LA, and into tonight across southern MS and southwestern AL. Thunderstorm development this morning will be focused along the front near the developing surface cyclone along the upper TX coastal plain. Convection will spread northeastward through tonight as the cold front surges eastward across LA/MS to the south and southeast of the cyclone. Effective bulk shear of 45-60 kt will be favorable for organized severe storms on the nose of a 70-90 kt midlevel jet, and low-level shear/hodograph curvature (effective SRH of 200-300 m2/s2) will be maximized in the northern portions of the warm sector in conjunction with a southerly 40-50 kt low-level jet. The combination of linear forcing for ascent along the cold front and substantial cross-boundary flow and deep-layer shear vectors will support a somewhat broken band of supercells and line segments. Embedded supercells will also be possible in a loose band along a secondary moisture plume from the southeastern Gulf of Mexico toward southwestern AL by tonight. Surface-based buoyancy and the strong vertical shear will support the potential for a few tornadic supercells, damaging gusts, and isolated marginally severe hail. However, the magnitude of the tornado threat could be tempered some by rather modest low-level lapse rates inland, and less-than-ideal phasing with the ejecting midlevel trough and surface cyclone glancing the northwest side of the surface warm sector. ..Thompson/Gleason.. 12/31/2020
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