SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0213 PM CDT Thu Oct 29 2020 Valid 292000Z - 301200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE CAROLINAS INTO SOUTHERN VIRGINIA... ...SUMMARY... Isolated severe convective gusts and perhaps a tornado may occur this afternoon into tonight from the Carolinas into southeast Virginia. ...Carolinas into southern Virginia... Little change to the outlook at 20Z as storms are expected to form later this afternoon over northwest SC where low-level lapse rates are steep. Storms should increase in coverage coincident with increasing large-scale ascent through the evening into western NC and southern VA. A southward-moving cold front will limit the northern extent of the threat area into southern VA, with a moist and marginally unstable air mass over the Carolinas supporting isolated severe storms. Poor lapse rates aloft will mitigate the overall severe risk, but re-adjusting wind profiles in the wake of Zeta may support mixed-mode storms with locally damaging gusts or a brief tornado. ..Jewell.. 10/29/2020 .PREV DISCUSSION... /ISSUED 1130 AM CDT Thu Oct 29 2020/ ...Carolinas into southeast VA through tonight... The remnants of tropical cyclone Zeta will move quickly east-northeastward over VA and reach the Atlantic coast by about mid afternoon. Strong wind profiles/shear will accompany this system across VA until it moves offshore, but buoyancy will be quite limited in the poor lapse rate environment with the weakening warm core cyclone. Thus, only isolated damaging convective gusts and/or a brief tornado may occur prior to 21z. Thereafter, the synoptic midlevel trough over AR will progress eastward to the southern Appalachians by tonight, and to the Mid-Atlantic coast by the end of the period. An associated, weak surface cyclone will move eastward along the VA/NC border, along a baroclinic zone in the wake of Zeta. A separate round of convection is expected to form this evening across the Piedmont in the zone of ascent near the surface cyclone and along the cold front trailing southward. The convection will subsequently spread east-northeastward across the Carolinas and southeast VA overnight. Lapse rates aloft will remain rather poor, but lingering boundary-layer dewpoints in the low 70s will support at least weak buoyancy rooted at the surface (SBCAPE around 500 J/kg) overnight. Long, relatively straight hodographs with effective bulk shear in excess of 70 kt and ground-relative winds of 50 kt within a few thousand feet of the surface will support a threat for isolated damaging gusts with the overnight convection. To the south, bands of convection persist from the FL Big Bend into southeast GA along a confluence zone trailing south-southwestward from Zeta. Gusty outflow winds may occur with the heavier convective cores during the next few hours given 40 kt low-level flow in area VWPs. However, poor lapse rates will limit updraft intensity, and relatively moist profiles suggest that strong downdrafts are also unlikely. Given the marginal thermodynamic profiles and some weakening of low-level flow expected this afternoon, the threat for severe/damaging winds appears to be quite small.
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