SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook AMEND 1 NWS Storm Prediction Center Norman OK 0243 PM CST Thu Dec 31 2020 Valid 312000Z - 011200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE SABINE RIVER VALLEY AREAS EASTWARD/NORTHEASTWARD ACROSS THE CENTRAL GULF COAST STATES... AMENDED TO ADD MRGL RISK AREA TO A PORTION OF THE EASTERN CAROLINAS ...SUMMARY... Thunderstorms -- a few of which will likely be severe, with tornadoes and damaging winds as the primary hazards -- will continue from the Sabine River Valley vicinity across southwest Louisiana through this afternoon, and then will expand across the rest of Louisiana into Mississippi and Alabama this evening and tonight. ...Discussion... Thunderstorms continue across portions of east Texas at this time, and will continue spreading into western Louisiana over the next few hours. While severe risk has remained muted thus far, the environment downstream from the ongoing storms has become steadily more supportive of severe/rotating storms -- both thermodynamically and kinematically -- over the past few hours, as shown by changes in the LCH RAOBs from 12Z to 18Z. Thus, the ongoing outlook reasoning and areas appear to remain representative of the evolving situation, with the only changes which appear necessary at this time being to trim western fringes of the areas to account for convective advance. ...Northeastern South Carolina/southeastern North Carolina... A small cluster of thunderstorms has developed in marginally unstable air near the northeastern South Carolina coast. Ample shear observed across this area suggests limited/local risk for gusty winds or a brief tornado late this afternoon. Please refer to SWOMCD #1896 for additional short-term details. ..Goss.. 12/31/2020 .PREV DISCUSSION... /ISSUED 1028 AM CST Thu Dec 31 2020/ ...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.
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