Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Sun Apr 23 2017 Valid 231300Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF CENTRAL/WESTERN GEORGIA AND EXTREME SOUTHEASTERN ALABAMA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF CENTRAL/EASTERN SOUTH CAROLINA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM THE WESTERN FLORIDA PANHANDLE TO NORTHERN FLORIDA AND EXTREME SOUTHERN NORTH CAROLINA... ...SUMMARY... Areas of strong-severe thunderstorms are expected over the Southeast today and overnight. The main threats appear to be damaging gusts over parts of western/central Georgia today, followed by potential for a few tornadoes and isolated wind damage overnight in parts of South Carolina. ...Synopsis... A generally progressive upper-air pattern will feature broadly cyclonic flow over the western U.S., and a slow-moving cyclone/trough over the Southeast. The latter feature -- now becoming a closed circulation and centered over western TN -- should deepen and move southeastward to western GA by 12Z. Meanwhile, a series of mostly low-amplitude shortwave troughs will be embedded within the western U.S. cyclonic flow, amidst a coupled upper-jet structure. Associated large-scale lift and related destabilization/ cooling aloft, atop weak low/middle-level moisture, will contribute to general thunderstorm potential over a broad swath of the West from the northern Rockies and northern Great Basin to the northern High Plains. At the surface, a cold front was analyzed at 11Z over the Atlantic, east of NC, to parts of eastern SC, becoming quasistationary from there westward to northern GA, then arching northwestward into a frontal-wave low near CHA. A cold front extended from that low southwestward across AL to near the mouth of the Mississippi River. The low should migrate generally southeastward across northern GA to near AGS by 00Z, with the cold front over southern GA and the central FL Panhandle, and a warm to stationary front across central/eastern SC. As the mid/upper-level low approaches, the surface low should move slowly out of eastern GA into western/ central SC by 12Z, with very slow northward movement of the warm front possible over eastern SC and perhaps into extreme southern NC. By the end of the period, the cold front should reach the GA coast and northern FL Peninsula. ...Southeast... Multiple rounds of scattered thunderstorms are possible through the period across the outlook area, with the two primary and best-organized convective groupings likely being: 1. From midday through at least late afternoon, along and ahead of the cold front, starting in southeastern and perhaps extreme east-central AL and shifting/spreading east-northeastward across portions of western/central GA. Though low-level flow will not be particularly strong - limiting hodograph size and low-level shear -- strengthening upper-level winds will help to ventilate convection and contribute to organized multicellular structures. At least transient supercell characteristics might occur in a few cells, given enough flow aloft to yield pockets of 35-40 kt effective-shear magnitudes. Available moisture and pockets of surface heating amidst cloud breaks should help to offset modest lapse rates aloft, yielding peak MLCAPE in the 500-1000 J/kg range in a narrow/ prefrontal corridor. A well-mixed boundary layer will support the potential for occasional damaging gusts to reach the surface. A pocket of relatively low-theta-e low-level air -- currently manifest by a dry axis in surface dewpoint obs from the spine of the FL Peninsula into central GA -- will only slowly erode and shift/deform eastward somewhat through the period as low-level trajectories veer ahead of the cold front. Diurnally initiated convection over western/central GA mostly should weaken as it encounters the remains of this theta-e min, though a marginal/ lower-coverage severe threat may persist across the gap between the two relative probability maxima represented by the "slight risk" areas. 2. Overnight as moist and unstable flow increases off the Atlantic into parts of central SC and the SC coastal plain, along and south of the warm front. This should occur amidst strengthening low-level and deep-tropospheric flow related to the approaching mid/upper cyclone, leading to the juxtaposition of a favorably moist and minimally capped boundary layer with increasing low-level and deep shear. Relatively backed surface flow and strengthening 850-mb southerlies are expected along the frontal zone, and somewhat southward from it into the warm sector, yielding increasing hodograph sizes in the 00-06Z time frame. A roughly north-south band of thunderstorms should evolve by the last few hours of the period, preceded by some discrete or semi- discrete cells, then perhaps containing some supercells as well, early in its development cycle. Forecast soundings suggest surface dew points in the mid 60s to near 70 F contributing to MLCAPE in the 500-1000 J/kg range, despite the time of night, amidst effective SRH/shear values in the 150-300 J/kg and 35-40 kt ranges, respectively. This should favor the potential for a few supercells and perhaps QLCS mesovortices. As such, the overnight tornado potential is ratcheted up one notch into 5%/categorical-slight level. ..Edwards/Peters.. 04/23/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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