SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0652 AM CST Thu Jan 07 2021 Valid 071300Z - 081200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Isolated thunderstorms are possible over parts of the Southeast. The severe-weather potential inland appears very minimal. ...Synopsis... In mid/upper levels, a fairly high-amplitude, complex and progressive pattern will continue over the CONUS, with these as the principal convectively pertinent features: 1. A cyclone with 500-mb low now located over northern LA, and forecast to move eastward to the southern Appalachians by the end of the period. 2. A strong, somewhat positively tilted shortwave trough -- now evident in moisture-channel imagery near 140W, west of the Pacific Coast. This perturbation, which has been producing considerable lightning over the north-central Pacific, should assume negative tilt before reaching the coast around 12Z tomorrow morning, and will be preceded by the risk for isolated thunderstorms in its plume of DCVA/cooling aloft. The surface manifestation of the cyclone at 11Z was centered just north of MLU, nearly vertically stacked with the mid/upper circulation center. An occluded front extended southeastward to between the mouth of the Mississippi River and GPT, with warm front southeastward over the east-central Gulf and cold front southwestward across the west-central Gulf. The surface low will move eastward in cool air and weaken through the period, while a "handoff" redevelopment process occurs with a second low forming off the SC/GA coastline by around 06Z. That will become the primary low and move northeastward just off the SC/NC coastline through the rest of the period. The cold front will move eastward across the rest of the northern/eastern Gulf, preceded by a convective band. The cold front should reach southeastern FL around 12Z. ...FL... The prefrontal band of convection, with isolated to scattered embedded thunderstorms, should continue to move eastward across the Gulf. Analyses, modified RAOBs, forecast soundings and planar progs all indicate that airmass recovery across coastal and inland areas of the FL Panhandle will be insufficient north of the warm front to support enough theta-e for surface-based effective inflow parcels. That, in turn, minimizes the potential for severe, given the unsupportive thermodynamic profile for hail. As such, any organized severe wind or tornado threat should remain offshore, though strong gusts may penetrate the stable boundary layer in the immediate coastal Cape San Blas/AAF areas as the northern end of the MCS passes overhead in a few hours. Farther east, the band of convection should approach western FL late this afternoon, likely reaching the coast in the PIE/SRQ area this evening, then further south with time tonight. While considerable airmass modification is possible in the boundary layer over the Gulf, stable layers and poor lapse rates aloft, combined with the persistence of trajectories from a lower-theta-e air mass over the peninsula, should keep all but very weak, perhaps mostly elevated buoyancy from extending inland. While favorable low-level hodographs and 0-6-km shear are forecast as the line approaches, a preponderance of guidance reasonably indicates that it should weaken rapidly soon before reaching the coast (losing organized severe threat) while penetrating the unfavorable thermodynamic profiles. ..Edwards/Leitman.. 01/07/2021
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