SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0748 AM CDT Tue Oct 27 2020 Valid 271300Z - 281200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Severe thunderstorms are not expected across the contiguous United States through tonight. ...Synopsis... The main upper-air feature influencing this period's forecast is a well defined synoptic-scale cyclone, currently centered over southern AZ per moisture-channel imagery. The associated 500-mb low is forecast to pivot roughly eastward and reach southwestern NM by 00Z, then move to near ROW by 12Z tomorrow. An extensive fetch of cyclonic flow aloft will accompany this feature from the southeastern Great Basin and Colorado Plateau across northern MX, the southern Rockies, and the southern Great Plains. At the surface, 11Z analysis showed a wavy, quasistationary frontal zone across northern FL, extreme southeastern LA, and the TX shelf waters of the Gulf. A slow-moving but strong cold front was drawn from the southern Appalachians across western MS, western LA, the middle TX coastal waters, and deep south TX, merging with part of the southern baroclinic zone over the northwestern Gulf. The western Gulf part of the combined boundary will move little for most of the period, while the eastern part remains stationary to northward drifting. ...Southern Plains... Large-scale forcing for ascent (elevated low-level warm advection already in place at first, then DCVA aloft shifting in from the west), juxtaposed with a favorable corridor of moisture transport, will spread eastward over the southern Plains through the period. These will contribute to multiple episodes of thunderstorms rooted atop the elevated frontal surface, beginning with the ongoing widely scattered thunderstorms embedded in a large area of convective precip offering winter mixed precip across parts of OK and northwest TX. Another major precip episode in the form of a coarsely north- south convective plume is likely late overnight across northwest, central and north-central TX into OK, also containing sporadic thunder. By that time, isolated thunderstorms may also begin across east TX and the Arklatex region, in a moistening regime of marginal buoyancy between the southern Plains plume and the Gulf activity discussed below. ...Gulf Coast States, Southeast... Tropical Storm Zeta (as of this writing) is forecast by NHC to reinvigorate to hurricane strength over the Gulf north of the Yucatan Peninsula today, its path curving toward a day-2 landfall across the southeastern LA area. Associated wind fields favorable for potentially tornadic supercells are expected to remain offshore through this period, but will affect portions of the Gulf Coast east of its track on Wednesday; see the SPC day-2 convective outlook for more details. In the meantime, the boundary-layer response to the approaching hurricane -- and indirectly, the ejecting Southwestern cold-core cyclone -- will foster early stages of an inland spread of increasingly well-modified return-flow air, both at and above the surface. This will support at least isolated general thunder potential over a broad swath of the Gulf Coast States and Southeast, in addition to antecedent potential across the FL Peninsula. Weaknesses of lapse rates, buoyancy and shear will preclude organized severe. See NHC advisories for latest track/intensity guidance and tropical watches/warnings related to Zeta. ..Edwards/Mosier.. 10/27/2020
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