SPC AC 200708

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0208 AM CDT Mon Sep 20 2021

   Valid 221200Z - 231200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS WEDNESDAY
   AFTERNOON AND EVENING ACROSS MUCH OF CENTRAL AND SOUTHEASTERN
   OHIO...PARTS OF WESTERN PENNSYLVANIA...EASTERN KENTUCKY...MUCH OF
   WEST VIRGINIA...SOUTHWESTERN VIRGINIA...AND NORTHEASTERN
   TENNESSEE...

   ...SUMMARY...
   Strong thunderstorms may impact a corridor from parts of the
   Cumberland into Allegheny Plateau vicinity Wednesday afternoon and
   evening, and perhaps pose a risk for a couple of tornadoes and
   damaging wind gusts.

   ...Discussion...
   Models suggest that the westerlies may begin to undergo
   amplification across the northern mid-latitude eastern Pacific into
   western North America during this period, with large-scale mid-level
   ridging building inland across the British Columbia coast and
   downstream troughing digging to the lee of the Canadian Rockies. 
   However, a blocked regime over the mid-latitude Atlantic will
   continue to impact flow across much of the United States.  It
   appears that elongated large-scale mid-level troughing over the
   middle and lower Mississippi Valley at the outset of the period will
   only slowly shift eastward Wednesday through Wednesday night.  It
   may broaden considerably, with models generally indicating that one
   significant short wave perturbation will contribute to a deepening
   embedded lower/mid tropospheric cyclone.  Spread among the various
   models remains large, however, with this taking place anywhere from
   the lower Ohio Valley into the lower Great Lakes region.

   It appears that this may be accompanied by modest surface
   cyclogenesis along a slowing cold front.  Of primary concern is the
   potential for this to be accompanied by enlarging, clockwise-curved
   low-level hodographs in a warm sector with surface dew points in the
   mid 60s to near 70F.  Although lower/mid tropospheric lapse rates
   may not become particularly steep, the environment might still
   become conducive to organized convection, including low-topped
   supercells, with the potential to produce a couple of tornadoes and
   damaging wind gusts.  This seems most likely to become focused in
   relatively close proximity to the low, during a period of more
   substantive deepening late Wednesday afternoon and evening, probably
   somewhere west of the Allegheny Front.

   ..Kerr.. 09/20/2021

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