SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0745 AM CDT Wed Oct 28 2020 Valid 281300Z - 291200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF EXTREME SOUTHEASTERN LA...SOUTHEASTERN MISSISSIPPI...SOUTHERN ALABAMA...AND THE WESTERN FLORIDA PANHANDLE... ...SUMMARY... Hurricane Zeta may spawn a few tornadoes later today through tonight over parts of the central Gulf Coast States. ...Synopsis... In mid/upper levels, a strong cyclone is forecast to eject east-northeastward from its current central position over southeastern NM, with the 500-mb low over the CDS area by 00Z, and reaching southeastern OK by 12Z. At the surface, 11Z analysis showed a weak low near DUJ with arctic cold front southwestward across eastern portions of KY/TN, coming quasistationary over northeastern or central MS. The boundary then curved across southwestern LA and the waters offshore from BRO. As the mid/upper cyclone proceeds across the southern Plains, and shortly behind the passage of Zeta across the central/north-central Gulf, the boundary will move eastward across the northwestern Gulf as a cold front. Portions well inland over MS/TN may move slightly northwestward before Zeta passes those latitudes. ...Southeastern LA to western GA and FL Panhandle... The NHC forecast depicts Hurricane Zeta's center moving ashore southeastern LA late this afternoon. That will be preceded from midday through afternoon by increasingly favorable low-level winds near the coast, enlarging hodographs and SRH values. The favorable kinematic field will spread northeastward across the outlook area this evening and tonight as Zeta rapidly translates across southern MS and AL to northwest GA. The circulation will become embedded in a regime of increasing ambient deep shear, ahead of the ejecting southern Plains cold-core cyclone. The main concerns for the tornado threat instead are related to instability and convective coverage/mode. Convection not part of Zeta's circulation envelope has been noted during the past several hours, moving north-northwestward off the Gulf to between GPT-MOB and over portions of southeastern MS. A few cells therein exhibited horizontal cyclonic shear despite 1) the lack of substantial apparent surface boundaries, and 2) the modest to weak SRH apparent in 06Z RAOBs and VAD wind profiles for observed cell motions. Ambient hodographs have increased slightly since, based on the 12Z LIX RAOB, and as they continue to grow today, any such convection remaining in the area later this morning, or forming over the Gulf in a weak-CINH setting as broader low-level convergence increases ahead of Zeta, may pose a marginal tornado threat before Zeta's directly related outer bands begin to affect the coastal areas this afternoon. A favorably high-theta-e air mass, with low/mid-70s F surface dew points, has advected inland across parts of the central Gulf Coast and lowest Delta regions, gradually diffusing an old frontal zone that had been in place for a few days, and effectively displacing it inland in the direction of the arctic front described above. That process will remain incomplete, however, and both surface theta-e and SBCAPE will continue to diminish northward through AL/GA from the optimal CAPE/shear parameter space forecast to affect the 5%/"Slight" area. With its expected acceleration and fast forward speed, Zeta should outrun the greatest buoyancy inland tonight. As that occurs, the tornado threat will ramp down gradually, but still be nonzero well inland, across the "Marginal" area, where a tornado cannot be ruled out in what still will be a large-hodograph environment. ..Edwards/Mosier.. 10/28/2020
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