SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1149 AM CST Wed Dec 23 2020 Valid 241200Z - 251200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS FAR NORTHEASTERN SOUTH CAROLINA...EASTERN NORTH CAROLINA...AND SOUTHEASTERN VIRGINIA... ...SUMMARY... Severe thunderstorms are expected across the Carolinas and Mid Atlantic region Thursday afternoon and evening. A few severe thunderstorms are also possible across portions of southeast Alabama, southwest Georgia, and Florida Thursday. ...Synopsis... A strong upper trough with an expansive area of surrounding/cyclonic flow will shift steadily eastward into the eastern half of the U.S. Thursday, with a closed low gradually evolving across the Midwest/Ohio Valley. Very strong flow aloft will accompany this system, including mid-level south-southwesterly flow in excess of 100 kt which will spread across the Appalachians into the East Coast States during the period and an 80 to 90 kt southerly low-level jet from the Carolinas into New England. At the surface, a deep low initially near Lake Superior is progged to fill gradually as it moves northeast across Ontario, to be replaced by a deepening frontal low which is forecast to develop over the Carolinas during the afternoon and shift quickly northward to the Lower Great Lakes region by the end of the period. A strong/trailing cold front will sweep eastward across the Appalachians during the afternoon, and then off the middle and southern Atlantic Coast overnight. ...The Carolinas to the Mid Atlantic region... As a strong cold front crosses the mountains during the afternoon, pre-frontal low-level warm/moist advection will occur east of the Appalachian crest. However, weak lapse rates and very limited potential for any heating will hinder CAPE development across the warm sector. Still, sufficient destabilization for thunderstorm development is expected, and thus expect an increase in convective coverage and intensity across the region through the afternoon and into the evening. CAM runs continue to indicate that pre-frontal storms will affect the eastern North Carolina and southeastern Virginia vicinity, in response to strong low-level warm advection. Particularly during the late afternoon, as the upper system advances, evolution of a few of these storms into supercells appears likely. Given intense/veering flow with height spreading across the area, shear will be quite favorable for both mid-level and low-level rotation. As such, potential for tornadoes with this pre-frontal convection appears sufficient to upgrade to ENH risk/10% tornado probability. Later, a well-defined band of frontal convection will spread across the eastern seaboard. Given the very strong flow through the lower troposphere, locally damaging winds are expected along the convective line, along with the possibility for brief QLCS-type tornadoes. Risk will likely continue through the evening, with the convective band expected to eventually clear the coast near or shortly after midnight. ...Southeastern AL/southern Ga/FL vicinity... Strong thunderstorms will likely be ongoing at the start of the period, along an advancing cold front. Modest instability/weak lapse rates ahead of the boundary will likely limit storm intensity, as storms progress eastward. Still, strong/unidirectional southwesterly flow aloft is expected across the region, on the southeastern periphery of the strong upper system. This suggests potential for local updraft enhancement -- and associated risk for locally damaging wind gusts, and possibly a brief tornado. Storms will spread across central and eastern Georgia and into the Florida Peninsula through late afternoon/early evening, with any severe risk likely to diminish after dark. ..Goss.. 12/23/2020
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