SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0650 AM CST Thu Feb 18 2021 Valid 181300Z - 191200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE FLORIDA PANHANDLE INTO SOUTHERN SOUTH CAROLINA... ...SUMMARY... Isolated severe thunderstorms, with a threat of damaging gusts and/or tornadoes, are possible today from parts of the Florida Panhandle into southern South Carolina. ...Synopsis... A mid/upper-level longwave trough will begin to deamplify over the central CONUS and shift eastward, as a basal shortwave perturbation -- now over extreme northern MX and the TX Big Bend area -- pivots northeastward and reorients its vorticity structure. By 00Z, the 500-mb trough should be stretched, positively tilted, from the Mid-South to south-central TX and northeastern MX. By 12Z, this feature should extend from eastern portions of KY/TN across MS to the TX shelf waters of the Gulf. At the surface, weak lows were analyzed inland in cool air near the southeastern corner of AL, and over the Gulf southwest of PFN. A synoptic warm front -- separating a cold-air damming regime to the north from a partly modified (but still mostly stable at the surface) warm sector, was drawn from the northern low eastward then east-northeastward across southern GA to near SAV. This boundary should drift northward through the day. A marine warm front, south of which more-completely modified Gulf and Atlantic air was evident -- was drawn across northern FL then west-northwestward to an intersection with a cold front near PFN. The cold front extended from there southwestward through the southern low-pressure area to the central Gulf. The marine/warm front is already showing signs of becoming diffuse over northern FL amidst a patchwork field of moisture/theta-e, and should blend northward into the northern boundary through midday. The cold front will move slowly eastward over the outlook area, as the northern low slowly crosses southern GA. ...FL Panhandle to SC... A loosely organized, broken band of thunderstorms is evident mostly ahead of the surface cold front, from clearly elevated convection over central GA southwestward through the low-level frontal zone to clearly-surface-based thunderstorms over the Gulf, and a broad transitional zone in between. Low-level shear remains favorable near the southern/marine front, where backed flow enhances SRH, and boundary layer theta-e supports surface-based effective-inflow parcels. For near-term information, refer to watch 10 and related mesoscale discussions. Through the remainder of the morning into the afternoon, gradual inland destabilization (related to both diabatic heating and warm advection) should occur with northeastward extent across the eastern FL Panhandle, northwestern FL and GA. This will lead to a gradual increase in severe potential across those areas into early afternoon, as: 1. The main belt of convection and cold front proceed slowly eastward (with embedded cells moving northeastward), and 2. The marine warm front becomes shallower and diffuses northeastward toward the synoptic boundary. The latter will lead to increasing boundary-layer lapse rates/ buoyancy and surface-based effective-inflow parcels, with dew points increasing mostly into the mid/upper 60s F across the outlook area prior to cold frontal passage. Resulting near-surface theta-e should offset modest midlevel lapse rates (including a "CAPE robber" evident in the 12Z TLH RAOB) enough to yield peak MLCAPE in the 300-800 J/kg range, with near 1000 J/kg locally/briefly possible. Forecast soundings over southern GA show 200-300 J/kg effective SRH and 40-45-kt effective-shear magnitudes, before flow veers near the cold front. ..Edwards/Smith.. 02/18/2021
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