SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0650 AM CST Thu Nov 12 2020 Valid 121300Z - 131200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL/EASTERN PARTS OF THE CAROLINAS... ...SUMMARY... A few strong to severe storms are possible across central/eastern parts of the Carolinas. ...Synopsis... The mid/upper pattern initially will be characterized by broadly cyclonic flow cross much of the CONUS, becoming more zonal through the period, except near substantial shortwave perturbations. One such perturbation is located over the central Rockies. This feature is forecast to move eastward across the central Plains today and this evening, phasing with an initially separate trough now over the Dakotas. The combined trough should reach eastern Lake Superior, western Lower MI and eastern IN by 12Z tomorrow. A more-subtle perturbation -- evident in moisture-channel imagery over portions of GA and the western Carolinas -- is expected to move slowly eastward today, then offshore from the Carolinas around 00-03Z. As heights fall over the central/southern Appalachians in advance of that mid/upper trough, T.S. Eta is forecast by NHC to continue weakening across northern FL today, then accelerate northeastward over the adjoining Atlantic this evening. Some re-strengthening is possible as a post-tropical/extratropical cyclone offshore NC into day-2. Late in this period, the cyclone may link with an outflow boundary or outflow-reinforced iteration of the synoptic surface baroclinic zone shifting offshore from NC. See latest NHC advisories for forecast track/intensity guidance on Eta. At the surface, 11Z analysis showed a synoptic cold front from southern New England across north-central/west-central NC, southwestern GA, and the north-central Gulf. This front is progged to proceed eastward to a 00Z position offshore from New England and the Mid-Atlantic, extending from there over southeastern NC, eastern SC, and Apalachee Bay. ...Eastern Carolinas... Numerous showers and scattered thunderstorms are possible across the outlook area today, generally moving northeastward as the regime shifts eastward over the area. The most intense/sustained cells may produce damaging winds approaching severe limits, and a tornado cannot be ruled out. Most of the closed circulation envelope of Eta is forecast to remain offshore, with some of the northern periphery brushing the coast. Its effects on severe potential over this region are largely indirect -- mainly in terms of: 1. Gradient flow helping to maintain a pre-existing, warm-sector fetch of rich low-level moisture/theta-e over the outlook area, 2. Related strengthening of a low-level convergence zone now evident to its northeast, from southern SC south-southeastward over the Atlantic, which will move north-northeastward and intersect a separate, pre-existing convective regime today over the SC part of the outlook area, and 3. Subtly increasing low-level shear over coastal NC/northeastern SC in particular prior to its passage abeam of the coastline, and In a very moist/low-LCL setting, hodographs and low-level shear vectors are forecast to enlarge to levels marginally supporting storm-scale rotation over the coastal plain. Time series of forecast soundings indicate effective SRH in the 100-200 J/kg range, locally/briefly higher, before low-level flow veers from northwest- southeast this afternoon and evening as the mid/upper trough passes. Cloud breaks may foster patches of localized heating sufficient to support 1000-1500 J/kg peak MLCAPE, despite modest, nearly moist- adiabatic low/middle-level lapse rates. Abundant cloud cover related to both Eta and separate convection over the Piedmont may keep the most-favorable diabatic heating patchy and short-lived in nature. The lack of greater low-level lapse rates, and potential for messy/outflow-driven convective modes, renders severe potential isolated and conditional with this regime. After 00Z, supercells may be enhanced where the outer northeastern/downshear rim of Eta's circulation interacts with any outflow/baroclinic boundary, which is currently shown to be offshore in most higher-res guidance. ..Edwards/Kerr.. 11/12/2020
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