Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0156 PM CST Sun Jan 22 2017 Valid 222000Z - 231200Z ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON AND EVENING ACROSS SOUTHERN GEORGIA...AND NORTHERN FLORIDA INTO THE WEST CENTRAL FLORIDA PENINSULA... ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS SURROUNDING THE HIGH RISK AREA...INCLUDING MUCH OF THE REMAINDER OF CENTRAL FLORIDA...THE REMAINDER OF CENTRAL AND SOUTHEAST GEORGIA AND MUCH OF THE SOUTH CAROLINA PIEDMONT AND COASTAL PLAIN... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SURROUNDING THE MODERATE RISK AREA...INCLUDING PORTIONS OF SOUTHERN NORTH CAROLINA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK AREA...INCLUDING PARTS OF NORTHEAST ALABAMA...NORTHERN GEORGIA...UPSTATE SOUTH CAROLINA AND MUCH OF THE REMAINDER OF SOUTHEASTERN NORTH CAROLINA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK AREA...THROUGH THE NORTH CAROLINA PIEDMONT AND PORTIONS OF SOUTHEAST VIRGINIA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON AND EVENING ACROSS PARTS OF THE CENTRAL CALIFORNIA COAST...INCLUDING THE SAN FRANCISCO BAY REGION... ...SUMMARY... An outbreak of severe thunderstorms, including the risk for strong tornadoes, and very strong damaging wind gusts, appears likely late this afternoon through tonight across southern Georgia into the Carolinas, and much of Florida. ...20Z Outlook Update... ...Southeast... In response to large-scale forcing for ascent, vigorous thunderstorm development is now underway within a narrow pre-cold frontal corridor, from portions of eastern Alabama southward into the northeastern Gulf of Mexico. Severe categorical and probabilistic lines remain generally unchanged outside of Florida, except to begin to account for the eastward progress of the front/evolving convective band. There remains some lingering uncertainty concerning the extent of boundary layer destabilization across parts of northern Georgia through the Carolinas, along/north of a northward retreating outflow boundary/warm front. Otherwise, the primary lingering uncertainty concerns the extent to which convective cells remain discrete within the pre-frontal convective band. The continued upscale growth of convection into a solidifying squall line seems most probable in association with an area of stronger mid/upper forcing for ascent likely to overspread much of northeast Florida and southeastern Georgia into South Carolina through this evening. If/when this occurs, severe wind potential is likely to increase considerably in the presence very strong low-level and deep layer wind fields. Latest model guidance continues to suggest an intense southerly 850 mb jet streak (50-70+ kt) developing/shifting northeastward from the northeastern Gulf of Mexico through this region. And heavy precipitation loading coupled with downward momentum transfer seems likely to support considerable damaging wind potential, including at least localized intense surface gusts in excess of 65 kt. Otherwise, strong to extreme low-level shear, with very large, clockwise-curved low-level hodographs will support the potential for long-lived/long track supercells, in the presence of weak to modest warm sector boundary layer instability. This activity may impact the Tampa/St. Petersburg metropolitan area by early evening, and much of the remainder of the central and southern Florida Peninsula through tonight. Severe probabilities have been increased in general across this region to account for this. ...Central California, including the San Francisco Bay area... No changes have been made, with models suggesting a 110-130 kt westerly 500 mb jet streak nosing inland across the coast late this afternoon and evening. Destabilization mostly within the post-cold frontal environment may become sufficient to support inland advancing low-topped convection/convective bands accompanied by a risk for potentially damaging surface gusts. ..Kerr.. 01/22/2017 .PREV DISCUSSION... /ISSUED 1028 AM CST Sun Jan 22 2017/ ...AN OUTBREAK OF SEVERE STORMS AND TORNADOES IS EXPECTED THIS AFTERNOON AND EVENING OVER PARTS OF NORTHERN FLORIDA, AND CENTRAL/SOUTHERN GEORGIA... ...Southern AL/North FL/Southern GA... The late morning surface analysis shows a rapidly deepening low near MOB, with a convectively reinforced warm front extending eastward from the low along the FL/GA border. This low is forecast to deepen over 12mb in the next 12 hours as it tracks northeastward. This rapid cyclogenesis will be accompanied by very strong low and mid level wind accelerations across parts of GA/FL. The result will be a zone of impressive shear profiles in the warm-sector of the low, along with ample low level moisture and rather steep lapse rates. Forecast soundings in the HIGH risk area are characterized by effective helicity values of 500-700 m2/s2 overlapping MLCAPE of 1000-1500 J/kg. This rare parameter space will support the risk of long-track strong tornadoes across the HIGH risk area. Also, very strong low and mid level winds and steep lapse rates suggest a significant risk of bowing structures capable of widespread damaging winds and large hail. The line of storms will eventually sag southward across the entire FL Peninsula overnight with a continued severe risk. ...Northeast AL/Northern GA... As the rapidly deepening surface low lifts northeastward, a plume of rather steep mid level lapse rates and strong winds aloft will wrap around the low. This may result in an arc of strong to severe storms affecting parts of northeast AL and northern GA later this afternoon and evening. Ample low level vorticity along this arc may be sufficient for a few tornadoes, along with large hail and damaging wind gusts. ...Carolinas... The convective evolution is uncertain over parts of SC/NC later today due to the widespread upstream thunderstorm coverage expected, and the persistent thunderstorm complex now over southeast GA. Given the strength of the wind fields and cyclogenesis, there remains a significant threat of severe storms spreading northeastward across much of SC and into southeast NC after dark, with damaging winds and a few tornadoes being the main threat. Its unclear how far north and west this threat will extend, but have lessened severe probabilities over parts of western/central NC where cool air and clouds are likely to persist. ...Central CA Coast... Multiple bands of showers and thunderstorms are expected to spread inland and affect the central CA coast this afternoon. Cool temperatures aloft and steep mid level lapse rates, combined with the strong west-southwesterly winds aloft, will support a risk of fast-moving bowing structures capable of locally damaging winds and hail.Read more
from SPC Forecast Products http://ift.tt/u32qoI
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