SPC AC 291232

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0732 AM CDT Fri Oct 29 2021

   Valid 291300Z - 301200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   COASTAL NORTH CAROLINA INTO THE MID-ATLANTIC...

   ...SUMMARY...
   Isolated strong to severe storms may occur today over parts of
   coastal North Carolina into the Mid-Atlantic. Marginally severe wind
   gusts will be the primary threat.

   ...Synopsis...
   The main feature for convective potential this period will remain a
   large, deep-tropospheric cyclone covering much of the eastern CONUS,
   and initially centered over western KY in mid/upper-level moisture-
   channel imagery.  Associated cyclonic flow covers areas from the
   coastal Carolinas to the central/southern Plains, and the upper
   Great lakes to the Gulf.  The cyclone aloft is forecast to fill
   slowly through the period, as its core region (with multiple,
   intermittent 500-mb lows possible) pivots east-northeastward to the
   central/southern Appalachians. 

   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

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