SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0200 PM CST Mon Feb 15 2021 Valid 152000Z - 161200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS INTO THIS EVENING FROM THE FL PANHANDLE INTO SOUTHWEST GA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVERNIGHT ACROSS PORTIONS OF THE COASTAL CAROLINAS... ...SUMMARY... Scattered damaging winds and a few tornadoes are likely across the Florida Panhandle into south Georgia through early evening. A strong tornado or two is possible. Severe thunderstorms are also possible across north Florida and overnight into the coastal Carolinas. ...FL Panhandle into southern GA... The primary change with this outlook is to trim severe probabilities and the general thunderstorm line in the wake of the convective band running from south AL into the FL Panhandle and the northern Gulf of Mexico. Otherwise, the previous forecast rationale remains valid. With boundary-layer dewpoints increasing into the mid-upper 60s dewpoints across the FL Panhandle and a very favorable wind profile in place, supercells capable of producing tornadoes, strong wind gusts, and isolated hail remain possible into the late afternoon/early evening, with the threat eventually spreading into southwest GA by early evening. See MCD 90 and the previous outlook discussion below for more information. ...North Florida... Scattered thunderstorms have developed across north FL this afternoon, in the vicinity of a surface boundary. A couple of supercells may eventually evolve in this area, posting a threat of all severe hazards. See MCD 92 for more information. ...Coastal Carolinas... A separate area of strong/locally severe thunderstorm development is still expected across portions of the coastal Carolinas late tonight. See the previous discussion below for more information. ..Dean.. 02/15/2021 .PREV DISCUSSION... /ISSUED 1030 AM CST Mon Feb 15 2021/ ...FL Panhandle to south GA... Upgraded to cat 3/ENH risk for the potential of a strong tornado or two along with scattered damaging winds likely, a few of which may be significant. Scattered thunderstorms are underway from the surface cyclone that is near the far southeast tip of LA into the north-central Gulf along a sharp cold front. Additional convection may also develop within the next couple hours ahead of this activity within pre-frontal bands of confluence evident in visible satellite imagery. As the surface cyclone deepens towards the AL/GA/FL border area, the warm front should advance inland across the FL Panhandle reaching southern GA by early evening. Mid to upper 60s F surface dew points are expected in that regime, supporting a pronounced gradient of MLCAPE from north to south, approaching 1500 J/kg along the coast. Expect strengthening mid/upper winds supporting 50-65-kt southwesterly effective-shear vectors, oriented just slightly rightward of the main corridor of convective forcing with an elongated mid/upper-level portion of the hodograph. The southern fringe of a 45-60-kt LLJ will contribute to low-level hodograph curvature with 200-400 m2/s2 effective SRH, greatest along the warm front. Pervasive stratus north of the warm front renders uncertainty over the degree of tornado coverage as near-surface lapse rates may remain less than ideal. However, the preceding favorable factors may compensate and a conditional risk for a strong tornado or two is apparent this afternoon. The tornado threat should subside some this evening as the primary surface cyclone occludes across the western portion of the Appalachians with the shortwave trough progressing rapidly northeast away from the region. ...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 and affect coastal areas 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, with potential diminishing with inland extent. Along the coast, buoyancy will increase with proximity to the Gulf Stream, where vertical heat/moisture fluxes will provide the most sustained air mass modification. Forecast soundings still indicate shallow near-surface stable layers at the bottom of the lifted layer yielding surface- based effective-inflow parcels. However, vertical shear will be favorable, with 45-55-kt effective-shear magnitudes and effective SRH around 200-300 J/kg.
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