SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0656 AM CST Mon Feb 15 2021 Valid 151300Z - 161200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF THE FLORIDA PANHANDLE...NORTHERN FLORIDA...EXTREME SOUTHEASTERN ALABAMA...SOUTHERN GEORGIA...AND IMMEDIATE COASTAL EASTERN SOUTH CAROLINA AND SOUTHERN NORTH CAROLINA... ...SUMMARY... Severe thunderstorms are possible from the Florida Panhandle into southern Georgia through the evening, and over the coastal Carolinas overnight. Damaging gusts and a couple tornadoes will be the main concerns. ...Synopsis... A mid/upper-level longwave trough will persist over the central CONUS through the period, regulating a large belt of cyclonic flow now covering the Rockies and Plains States. The most important/ embedded perturbation influencing sensible weather will continue to be a synoptic trough -- now evident in moisture-channel imagery from south-central KS across western OK, central TX and Coahuila. This feature will eject northeastward across the mid Mississippi and lower Ohio Valleys through the period, while deamplifying. By 12Z tomorrow, the southern rim of the associated 500-mb vorticity field should be crossing central portions of MS/AL. At the surface, the associated frontal-wave low was analyzed over the Gulf approximately east of CRP and south of LCH, with cold front southwestward over east-central MX. A warm front was drawn from the low eastward across the northern Gulf near buoy 42365 then eastward over central FL, where it was quasistationary. The low is forecast to move across the Mississippi River mouth around 15Z then inland near PNS about 18Z, when a trough will extend northward to another weak low in the cool air over northern AL. The lows should combine and reach extreme eastern TN near TYS by 00Z, with occluded/cold front southward to near TLH then southwestward over the east-central Gulf. The warm front should move northward over the central/eastern FL Panhandle, extreme southeastern AL and southern Ga before being overtaken by the cold front. The northeastern limb of the warm front also should get very close to the SC/NC coast overnight. ...Coastal northeast Gulf... A band of scattered thunderstorms is forecast to evolve through the remainder of the morning and midday over the Gulf, near the surface low and trailing cold front, and extending poleward of the low on an elevated version of the frontal zone toward the MS/AL coastlines. This activity should move northeastward across the Gulf Coast from there eastward through the day. As it does so, encountering progressively more surface-based parcels near and south of the warm front, the associated severe threat will spread obliquely inland across the FL Panhandle and southeastern AL this afternoon, before reaching southern GA late afternoon and evening. Potential for isolated, marginally severe hail will extend somewhat more inland than the damaging-gust and tornado threats, with suitably moist elevated inflow parcels located under colder air aloft and favorable deep shear. Along and south of the warm front, the buoyancy/shear parameter space will be favorable for supercells and bowing segments within the main convective band, and isolated supercells cannot be ruled out to its east. Mid-60s to low-70s F surface dew points are expected in that regime, supporting 1000-1500 J/kg MLCAPE near the coast, decreasing northward to less than 500 J/kg just north of the warm front. Expect strengthening mid/upper winds supporting 50-65-kt southwesterly effective-shear vectors, oriented just slightly rightward of the main corridor of convective forcing. The southern fringe of a 45-60-kt LLJ will contribute to mildly curved but long hodographs with 200-300 J/kg effective SRH -- greatest along the warm front. ...Coastal Carolinas... Elevated thunderstorms should spread northeastward over interior areas tonight, as a northward extension of the Gulf regime. A separate, prefrontal low-level convergence zone is forecast to support increasing thunderstorms tonight, initially from roughly the GGE-ILM corridor southward across Atlantic waters, the northern portion of which should affect coastal areas from there past HSE through the end of the period. A damaging-gust threat may develop very near the coastline, where boundary-layer instability may be at least marginally favorable, with tornado potential from any mature supercell that can cross the coast. Farther northwestward, isolated strong-severe gusts may penetrate the near-surface stable layer; that potential will lessen with inland extent. The northwestern margin of the most substantial severe potential, relative to the coastline, may be just a few miles either way. Planar guidance across this region, from a preponderance of operational models and ensemble (HREF, SREF, GEFS) members, now indicates MLCAPE developing somewhat inland, with 250-500 J/kg values possible, and 1000 J/kg just offshore. Buoyancy will increase with proximity to the Gulf Stream, where vertical heat/moisture fluxes will provide the most sustained airmass modification. Scrutiny of forecast soundings from some of them still show shallow near-surface stable layers at the bottom of the lifted layer yielding surface- based effective-inflow parcels. Vertical shear will be favorable, with 45-55-kt effective-shear magnitudes and effective SRH around 200-300 J/kg. ..Edwards/Smith.. 02/15/2021
There’s more click here.