SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0656 AM CST Sat Dec 19 2020 Valid 191300Z - 201200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY ACROSS PARTS OF THE MIDDLE/UPPER TEXAS COAST... ...SUMMARY... Isolated strong to severe thunderstorms may occur today across parts of the middle/upper Texas Coast. ...Synopsis... A generally progressive upper-air pattern will prevail through the period, with broadly cyclonic flow spreading eastward from the central to eastern CONUS. The most important feature aloft will be a positively tilted trough -- initially evident in moisture-channel imagery from northern WI across eastern NE to far west TX and adjoining northern MX. This perturbation will maintain positive tilt while moving eastward and gradually weakening. By 00Z the trough should extend from Lower MI across the Ozarks to south- central TX. By 12Z tomorrow, it should reach southeastern ON, eastern KY and AL. Surface analysis showed weak lows over TX between MWL-SPS and between DRT-LRD, connected by a cold front. The current south TX low, or a new one, should move northeastward just of the mid/upper TX Coast this afternoon, turning eastward along or just south of the LA Coast tonight. Mesoscale convective effects may modulate the low track southward and/or somewhat faster. The trailing cold front will move across the TX Coast and northwestern Gulf, while extending southwest of the low. A weaker inland version of the front, and a collocated surface trough, will extend northeastward from the low. ...Mid/upper TX coastal plain... Scattered thunderstorms are expected to develop, increase in coverage and spread across the outlook area from mid/late morning into the afternoon, offering the potential for isolated strong- severe gusts and marginally severe hail. A tornado cannot be ruled out as well. The bulk of convection should occur in the form of a primary band developing in a zone of relatively maximized low-level lift near the front. With weak CINH in place, isolated thunderstorms also are possible farther east near the immediate coast and over the Gulf. This evolution should occur as the zone of lift encounters a boundary layer steadily destabilizing from a combination of: 1. Warm/moist advection of a still-modifying marine layer from the Gulf, and 2. Muted diurnal heating, amidst considerable cloud cover. Although the CAPE-shear parameter space will be optimized offshore, deep shear will increase over the outlook area as the mid/upper trough approaches, with 40-50-kt effective-shear magnitudes becoming common. Well-curved but marginally sized hodographs are expected, supporting 0-1-km SRH in the 100-200 J/kg range. Modest low/middle level lapse rates will keep peak preconvective MLCAPE around 500-800 J/kg inland near the coast, decreasing substantially northward. A few high-res mesoscale progs indicate that weak surface-based buoyancy (i.e., MLCAPE 200-500 J/kg) briefly may extend as far northward as parts of the HOU metro, immediately before the main area of convection moves through. As such, the marginal line has been adjusted somewhat northward in this area. Late this afternoon into evening, the convective band should shift eastward over the Gulf, Golden Triangle region of southeast TX, and portions of southwestern LA. This activity should outpace the eastward extent of inland surface-based destabilization east of the HOU area. By then, the most intense convection (including supercell potential) will be relegated to the Gulf -- where the most well- modified and unstable inflow air will reside -- posing some threat to the various offshore interests. ..Edwards/Smith.. 12/19/2020
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