SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0652 AM CST Fri Jan 01 2021 Valid 011300Z - 021200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN ALABAMA AND PARTS OF THE FLORIDA PANHANDLE TO PARTS OF SOUTH CAROLINA... ...SUMMARY... Isolated strong-severe gusts and perhaps a tornado are possible from eastern Alabama and parts of the Florida Panhandle to parts of South Carolina. ...Synopsis... In mid/upper levels, a binary cyclone system is evident in moisture- channel imagery, within a large area of cyclonic flow covering most of the Rockies and central CONUS. The leading cyclone is centered over southeastern OK, and is expected to eject northeastward and weaken through the period. By 00Z, the cyclone should be located over IL, while beginning to phase with a northern-stream shortwave trough now moving southeastward across northern SK. The low will devolve into an open-wave trough thereafter, accelerating east- northeastward to western NY and Lake Ontario by 12Z tomorrow. Meanwhile, the upstream 500-mb low -- now over Sonora -- will pivot across northern MX, becoming a strong, open-wave trough over the southern Plains and northeastern MX by 12Z. At the surface, 11Z analysis showed the main low (related to the eastern mid/upper cyclone) over eastern OK between MLC-FSM, with an elongated area of low pressure winding eastward along a wavy occluded front over central AR. The triple point was drawn near the northeastern corner of MS, with warm front across northern/eastern AL, central GA, southern SC and offshore NC. The cold front extended across western AL to the central Gulf. The original/ occluded low should move northeastward across the Ozarks to IL, close to the mid/upper cyclone center, while continuing to fill. Meanwhile, a low should form today over the northeastern AR/western KY/MO Bootheel/southern IL region and move northeastward toward the lower Great lakes overnight, itself becoming the anchor of the primary surface cyclone, while also increasingly stacked with respect to the circulation aloft. The trailing cold front will move slowly eastward as the upper low weakens and pulls away. This boundary should be located over eastern TN, southwestern AL and the central Gulf by 00Z. By 12Z the front should reach the western Carolinas, GA and the central/eastern FL Panhandle. Cold-air damming will retard warm-frontal advance over the southern Appalachians/western Carolinas region, but the front should move northeastward up the coastal Tidewater area through the period. ...Southeast... Isolated to widely scattered thunderstorms will remain possible within a loosely organized band of convection along/ahead of the front over portions of AL, GA and the FL Panhandle, in a nominally favorable CAPE/shear parameter space. The northeastern extent of this activity may reach parts of SC this evening before weakening. A tornado cannot be ruled out in any sustained supercell or wrapping notch on an embedded quasi-linear segment, and isolated strong- severe gusts also are possible. Warm and moist advection will continue in the boundary layer ahead of the primary convective swath, offsetting the modest lapse rates aloft enough to yield a narrow corridor of around 500 J/kg of preconvective MLCAPE today. Hodographs will be less elongated and more looped with southeastward extent, but also smaller with time and eastward extent over GA/SC. Effective shear will diminish southward and southeastward into higher theta-e as well, as the mid/upper cyclone fills and ejects northeastward away from the region as a deamplifying wave. The resulting dampening mass response will decrease low-level convergence with time, leading to concerns about coverage and persistence of the strongest convection. Given those factors, weaknesses of both low/middle-level lapse rates and lift at various scales may preclude sufficiently persistent deep convection to organize more than a marginal/ conditional severe threat. ..Edwards/Leitman.. 01/01/2021
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