William’s Severe Weather Zone for Friday:
At the surface, 11Z analysis showed the low — related to the mid/upper cyclone — over west-central KY, with occluded front arching across northeastern KY, southern WV, southwestern VA and northwestern/north-central NC to an ill-defined triple point over east-central/southeastern NC. The cold front extended from there across Atlantic waters to south FL, with warm front over the southern Outer Banks. The occluded/warm front will move northward and northeastward over the Mid-Atlantic region today. …Eastern NC/Mid-Atlantic… A marginal convective-wind/tornado threat will develop today in an arc of convection (with isolated to widely scattered embedded thunderstorms) in and near the occluded/warm frontal zone. This activity will move northward to northeastward over the region. With time this evening, the area of strongest convection-supporting lift will outrun any favorable surface-based instability, with weakening convection expected. A narrow slot of superposition of relatively maximized boundary- layer moisture (and minimized MLCINH) with backed near-surface winds is apparent on the surface chart and in mesoscale diagnostics from the southern Outer Banks region west-northwestward to north-central NC. Buoyancy does — and should continue to — diminish gradually with westward/inland extent, as this regime shifts north- northeastward across the outlook area, largely in keeping with available theta-e in the prospective inflow layer. Abundant clouds/precip will preclude development of large low-level lapse rates, but convergence in and near the baroclinic zone should combine with continuing weak CINH and minor surface heating to boost convective coverage/intensity later this morning. MLCAPE of 200-800 J/kg is possible, though a pronounced stable layer between 650-700 mb (evident in the 12Z MHX RAOB) may limit development/coverage on the east end for at least a few hours. As the deep-layer cyclone moves toward the Appalachians, a long, cyclonically curved mid/upper-level jet streak will shift eastward across the NC/Mid-Atlantic region, bringing parts of the outlook area beneath its left-exit region for some additional large-scale ascent. Favorable deep shear is forecast, with 0-6-km vectors around 50-75 kt, though weak and somewhat shallow CAPE-bearing layers will keep effective-shear magnitudes smaller than that (30-50 kt). Downdrafts may transfer enough momentum from aloft to yield a localized, isolated threat for damaging to severe gusts. In the sliver where surface-based buoyancy and optimally backed surface winds may develop, effective SRH of 250-350 J/kg is possible, followed by quickly veering surface flow that will yield much less favorable hodographs. Any sustained/discrete activity that persists in that small, conditional, yet favorable environment may become supercellular, with a tornado possible given the favorable shear and low LCL. ..Edwards/Mosier.. 10/29/2021 $$
