SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0649 AM CST Thu Dec 24 2020 Valid 241300Z - 251200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE EASTERN CAROLINAS AND SOUTHEASTERN VIRGINIA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE FLORIDA PANHANDLE...AND SURROUNDING THE "ENHANCED" AREA... ...SUMMARY... The main threat for severe thunderstorms will be across portions of the Carolinas and Mid-Atlantic region this afternoon and evening, with tornadoes and damaging gusts possible. ...Synopsis... In mid/upper levels, broadly cyclonic flow will shift eastward from the central to eastern CONUS through the period, in step with a high-amplitude synoptic trough. As one associated shortwave eject northeastward across the upper Great Lakes, an upstream perturbation -- now evident in moisture-channel imagery over the MB/ON border -- will dig southeastward. Another perturbation initially over central TX will pivot eastward to the southern Appalachians. As this occurs, a closed 500-mb cyclone should develop over the lower OH Valley region during the 00-06Z time frame, its center moving eastward past SDF before 12Z tomorrow. At the surface, the primary, occluded synoptic low over the eastern Lake Superior region will fill and eject northeastward across northern ON. The associated cold front -- initially drawn across parts of OH, eastern KY/TN, AL, to near the Mississippi River mouth, will move eastward across the southern/central Appalachians through this evening. As the mid/upper cyclone closes to the west, a weak, closed area of low pressure over the eastern AL/northwestern GA region should move up the front to the eastern WV/southwestern PA corridor by 00Z, becoming the primary low over NY tonight and reaching southern QC by 12Z. By then, the trailing cold front will reach eastern PA, then offshore from the remainder of the CONUS Atlantic Coast, having crossed south FL around 06Z. ...Eastern/central Carolinas and vicinity... Widely scattered to scattered thunderstorms are expected to cross this region today into this evening in two primary regimes: 1. Ahead of the cold front, moving northward to north-northeastward from a favorably moistening/destabilizing marine boundary-layer region over the Atlantic. This convection will cross lesser but still sufficient/surface-based buoyancy across the eastern Carolinas, as the warm sector modifies and becomes more unstable, amidst strengthening deep shear. Warm-sector dew points in the 60s will offset modest midlevel lapse rates enough to contribute to MLCAPE generally in the 500-1000 J/kg range from eastern NC across the southern sounds/Outer Banks of NC, locally higher near the southern NC coastline. Discrete supercells are possible, offering tornadoes and isolated damaging wind. Then... 2. A near-frontal band of thunderstorms will sweep eastward to east-northeastward over the region, presenting a more-dominant wind threat, but with some line-embedded/QLCS tornado threat as well. Height falls and tightening height/thermal gradients aloft are expected to spread over the region near and just ahead of the front throughout the day, as the synoptic trough deepens to the west. A 65-75 kt LLJ will develop and shift across this region, leading to uncommonly similar, 40-50-kt magnitudes of shear vectors in the 0-1, 0-3, and effective layers. Despite the nearly unidirectional, southerly low/middle-level flow, even very slight hodograph curvature contributes to 200-350 J/kg effective SRH in forecast soundings. The air mass will become still more stable with northward extent past a diffuse warm front that will move northward over the Delmarva/Mid-Atlantic this evening and tonight. However, at least a marginal wind and tornado threat appears to be justified, given the strength of the ambient deep-layer flow and at least weak instability near the surface. ...FL/GA... See the remainder of tornado watch 517, mesoscale-discussion updates for that, and separate mesoscale discussion 1372 for more details on the severe threat with ongoing convection in the near-frontal squall line and warm sector over the FL Panhandle region. The main belt of convection will sweep eastward across the remainder of the outlook area through this evening, presenting a threat for sporadic damaging gusts and a tornado or two. Moisture and buoyancy will continue to be greatest over open Gulf waters near the Loop Current, where marine thermal/moisture fluxes optimize boundary- layer theta-e. Still, at least marginally favorable thermodynamic conditions will exist in a northward-narrowing corridor ahead of the main convective line, as partly modified parcels advect from the Gulf. This air mass, characterized by modest lapse rates but also minimal MLCINH, also may support maturation of cells developing in the warm sector before they cross into more-stable air. ..Edwards/Smith.. 12/24/2020
There’s more click here.