SPC AC 251620

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1120 AM CDT Sun Sep 25 2022

   Valid 251630Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THROUGH THIS
   EVENING ACROSS MUCH OF THE MID-ATLANTIC...

   ...SUMMARY...
   Scattered severe storms producing damaging winds and severe hail are
   expected across portions of the Appalachians and Mid-Atlantic States
   this afternoon through evening.

   ...Mid-Atlantic and vicinity through this evening...
   A midlevel shortwave trough over the upper OH Valley will eject
   eastward toward the Mid-Atlantic and southern New England, in
   response to an upstream shortwave trough digging south-southeastward
   from MB toward the upper MS Valley.  Some deepening of a surface
   cyclone is expected in the vicinity of the lower Great Lakes in
   association with the ejecting midlevel trough, and a weak cold front
   will progress eastward across the Appalachians by this afternoon to
   the south of the deepening cyclone.  The cyclone warm sector will
   not be particularly warm or moist, but a narrow zone of cloud breaks
   from southern PA into WV should allow surface temperatures to warm
   into the 70s with dewpoints in the mid 50s to near 60.  The warming
   surface temperatures and notable midlevel cooling/steepening of
   low-midlevel lapse rates (observed at ILN from 00 to 12z) will boost
   MLCAPE into the 500-1000 J/kg range in a narrow zone immediately
   ahead of the weak cold front this afternoon.

   The specific location and evolution of the main severe threat is a
   bit uncertain given the presence of clouds and early convection,
   with the potential for the stronger storms to evolve from the
   leading edge of the ongoing convection in PA/WV, and farther west
   within the band of ascent.  The weak buoyancy and steepening
   low-level lapse rates will combine with strengthening midlevel winds
   to provide an environment favorable for damaging winds with
   fast-moving, low-topped clusters/bands of convection spreading
   eastward toward the Mid-Atlantic coast through late
   afternoon/evening.  Isolated large hail may also occur with any
   stronger storms/supercells given the long hodographs and cool
   midlevel temperatures, and low-level shear will be strong enough to
   support a low-end threat for a tornado or two.  

   ...Southern Appalachians this afternoon...
   The southern fringe of the OH Valley midlevel trough will move over
   the southern Appalachians this afternoon, along with the remnant
   monsoonal moisture plume.  Clouds/ongoing convection will tend to
   disrupt surface heating across northern AL/GA and southeast TN, and
   the southern fringe of the somewhat stronger forcing for ascent with
   the OH Valley trough is expected to pass over eastern KY/WV/VA. 
   Thus, with modest buoyancy and the aforementioned concerns, storm
   coverage into the southern Appalachians is uncertain.  Where storms
   form with afternoon heating in cloud breaks, there will be the
   potential for isolated damaging gusts and marginally severe hail.

   ...WI/southwest Lower MI this afternoon into early tonight...
   The shortwave trough now approaching northern MN will reach WI later
   this afternoon/evening, and then continue southeastward to Lower MI
   tonight.  Cooling midlevel temperatures, residual low-level
   moisture, and surface heating in cloud breaks will support the
   potential for low-topped thunderstorm development by mid-late
   afternoon across WI.  Lapse rates will be sufficiently steep and
   midlevel flow sufficiently strong for a low-end threat for wind
   damage.  Despite arrival after the diurnal cycle, the threat for
   isolated wind damage could persist across Lake MI into southwest
   Lower MI, where the relatively warm lake will help maintain weak
   buoyancy and low-level lapse rates after sunset.

   ..Thompson/Weinman.. 09/25/2022

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