SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0646 AM CST Sat Nov 28 2020 Valid 281300Z - 291200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Severe thunderstorms appear unlikely across the contiguous United States today and tonight. ...Synopsis... The dominant upper-air feature for this forecast will be a closed (but not quite cutoff) cyclone -- evident in moisture-channel imagery over NM. A separate, northern-stream trough -- initially over the Canadian Rockies -- is progged to move east-southeast and amplify, reaching the Dakotas and eastern WY by the end of the period. As that occurs, the southern-stream 500-mb low will move in an erratically eastward path, reaching the Red River region of southern OK or north-central TX by 12Z tomorrow. At the surface, 11Z analysis depicted a weakening cold front from Lake Ontario across WV, middle TN, and central LA, losing definition as it penetrates deeper into a poorly modified, continental/polar air mass left behind the slower-moving frontal zone analyzed near the FL/GA line, to near the mouth of the Mississippi River, just offshore from the western LA and upper TX Coasts, and across deep south TX. A weak frontal-wave low may develop by 00Z near the lower TX Coast, then ripple northeastward along the boundary, near or barely offshore from the middle/upper TX Coast overnight. The remainder of the southern front should linger quasistationary near the Gulf Coast from southern LA to northwestern FL, eastward across the northern peninsula. ...Southern States... Ahead of the NM cyclone, precursory DCVA is spreading across the southern High Plains. The related plume of ascent/cooling aloft is yielding increasing convective coverage and forming a baroclinic- leaf configuration on IR imagery. As the UVV field translates eastward over progressively greater (though still modest) moisture in an elevated, low/middle level convective inflow region, associated general thunderstorm potential will spread over most of TX to the east, as well as parts of the Red River Valley, southern OK, and the Arklatex region. Isolated subsevere hail cannot be ruled out, though forecast buoyancy and inflow-layer moisture content each appear too scant to support an unconditional/organized severe threat. Over south TX, convection should be mainly on the cool side of the front, with frontal forcing aiding weak large-scale ascent and overcoming minimal CINH to support convection. Isolated thunderstorms cannot be ruled out along and south of the front in a conditionally favorable CAPE/shear parameter space that ordinarily would cause some severe concern. However, weak lift will temper convective organization. Farther downshear, the broad fetch of elevated, low-level warm advection and moisture transport, along and north of the front, will support isolated to widely scattered thunderstorms near the central/eastern Gulf Coast and across southern GA/northern FL. Sufficient near-surface destabilization to support overland severe potential should delay until at least early day-2, when the frontal-wave low is forecast to move inland over LA and MS, and deepen. See the SPC day-2 outlook for more details on the severe threat after 29/12Z. ..Edwards/Broyles.. 11/28/2020
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