Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0654 AM CST Sat Jan 21 2017 Valid 211300Z - 221200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS TODAY AND AGAIN TONIGHT FOR SOUTHERN ALABAMA AND THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON/EVENING FOR THE ARKLAMISS REGION... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EXTREME NORTHEAST TEXAS AND SOUTHEAST OKLAHOMA EASTWARD TO PARTS OF THE SOUTHEAST ATLANTIC COAST... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK AREA... ...SUMMARY... Multiple rounds of severe thunderstorms are expected today through early Sunday from Louisiana/Arkansas eastward to north Florida, Georgia and South Carolina. The severe weather threat will cover the full spectrum from isolated strong tornadoes to very large hail and damaging winds. ...Synopsis... A strong mid-upper shortwave trough over the southern Rockies/northern Mexico this morning will amplify over the southern Plains this evening, and reach the lower MS Valley by Sunday morning. An associated surface cyclone will develop generally eastward across the Red River Valley to AR, as a trailing cold front begins to surge eastward from TX to LA late in the period. This complex synoptic system will support multiple episodes of potentially significant severe weather through tonight in two main corridors - one across southern AL/GA and another across the Arklamiss. ...Southern AL/FL Panhandle into GA today and again overnight... Two severe weather episodes are expected across this area, with significant tornadoes possible tonight. At this time, a well-developed MCS and associated mesolow continue to move east-northeastward across west-central AL. Enhanced moisture flux and low-level shear preceding the MCS/mesolow have contributed to an environment favorable for embedded supercells within the larger line/cluster. This convective regime will spread into GA during the day, with sufficient warming/moistening in advance of the MCS to maintain a risk for damaging winds and a few tornadoes, prior to weakening this evening near the GA coast and into SC. The rain-cooled boundary trailing southwestward from this convection today will begin to retreat northward tonight into southern AL and the FL Panhandle, in response to the approach of the synoptic midlevel trough and deepening surface cyclone from the southern Plains. The low-level jet is again expected to strengthen tonight and contribute to an increase in warm/moist advection, beneath the eastern extent of a steeper midlevel lapse rate plume. Supercells will develop in this warm advection regime, with low-level shear/hodographs becoming favorable for strong tornadoes overnight from southern AL and the FL Panhandle into southwest GA. Depending on the evolution of the mesoscale details through tonight, a moderate risk for the tornado threat may need to be considered for overnight. ...Arklamiss region this afternoon into early tonight... The outflow boundary in the wake of the ongoing MS/AL convection has stalled near the southwest LA coast this morning. This boundary will retreat northward during the day in response to cyclogenesis across the southern Plains with the approach of the strong midlevel trough from the southern Rockies. Steep midlevel lapse rates and relatively cool midlevel temperatures, above the returning boundary layer dewpoints in the 60s, will support moderate buoyancy this afternoon (MLCAPE of 1500-2000 J/kg). Thunderstorm development is likely by mid afternoon along a surface trough/developing cold front from extreme southeast OK into northeast TX. Convection will subsequently expand across southwest AR into northern and central LA, and then spread into MS this evening. Deep-layer shear strength and orientation relative to the surface trough will favor semi-discrete supercells, with sufficient low-level shear/moisture for the risk of tornadoes. Cool midlevel temperatures and steep lapse rates will likewise support the potential for very large hail, prior to the storms growing upscale into a broader cluster overnight across northern/central MS. Supercell coverage could be sufficient to warrant some consideration for an upgrade to moderate risk for hail in later updates. ..Thompson.. 01/21/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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