SPC AC 250557

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1257 AM CDT Mon Oct 25 2021

   Valid 251200Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE
   CENTRAL APPALACHIANS INTO VIRGINIA...NORTH CAROLINA...AND SOUTHERN
   MARYLAND...

   ...SUMMARY...
   Thunderstorms associated with mainly isolated wind damage are
   possible over parts of the southern and central Appalachians into
   the Carolinas and Middle Atlantic region today.

   ...Synopsis...
   A progressive mid-level trough will traverse central Appalachia
   today, accompanied by a surface low which will impinge on the
   Mid-Atlantic Seaboard and merge with another surface low traveling
   up the Atlantic coastline. The initial surface low is expected to
   weaken with time. However, a 50-70 kt mid-level jet, along with 20+
   kts of 850 mb south-southwesterly flow, is expected to overspread a
   modestly mixed boundary layer across eastern portions of the Ohio
   Valley towards the Mid Atlantic and Carolinas, supporting scattered
   strong to severe thunderstorm development. 

   ...Eastern OH Valley into the Mid Atlantic/Carolinas...
   Severe potential is expected to develop within a broad confluence
   zone ahead of the cold front across the central Appalachians into
   Virginia/North Carolina during the afternoon, progressing towards
   the Mid Atlantic later in the evening. Modest veering of the
   surface-850 mb winds flow beneath the aforementioned 50-70 kts of
   mid-level flow will promote 40-50 kts of effective bulk shear. As
   scattered thunderstorms mature, line segments and transient
   supercell structures may develop given the presence of 500-1000 J/kg
   MLCAPE. Damaging gusts are expected to be the main threat,
   especially across Virginia and northern North Carolina, though a
   brief tornado could occur with the strongest, longest lasting
   updrafts.

   A second area of scattered strong to potentially severe storms may
   develop across portions of central/eastern OH into WV with a second
   band of low-topped convection immediately ahead of the surface
   low/cold front and 500 mb vorticity maxima. These storms will be
   strongly forced within a region of 30+ kt effective bulk shear, with
   organization into small line segments/supercells likely despite
   meager buoyancy (i.e. less than 500 J/kg MLCAPE). Nonetheless,
   damaging gusts and perhaps a brief tornado or two are possible with
   the stronger storms in closest proximity to the 500 mb vorticity
   maxima, and co-located with locally stronger buoyancy. The very
   modest buoyancy contained within a narrow spatial corridor suggests
   that the severe threat will be too localized and brief to support an
   upgrade to Slight Risk this outlook. However, an upgrade may be
   needed in future outlooks if low-level moisture recovery and
   boundary-layer destabilization appear more favorable than currently
   anticipated.

   ..Squitieri/Mosier.. 10/25/2021

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