SPC AC 271241

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0641 AM CST Mon Dec 27 2021

   Valid 271300Z - 281200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Thunderstorms are possible today over parts of the mid/upper Ohio
   Valley region and California Coast, and late tonight from the
   south-central Plains to the Ozarks.

   ...Synopsis...
   In mid/upper levels, a mean trough and associated cyclonic flow will
   persist across most of the western CONUS, traversed by several
   substantial shortwaves.  One of those perturbations is evident in
   moisture-channel imagery near coastal northwestern CA, and is
   forecast to cross the Great Basin to parts of UT/AZ by 00Z.  The
   trough then should pivot east-northeastward across the south-central
   Rockies, reaching the lower Missouri Valley by 12Z tomorrow.  The
   associated mass response will include a plume of low-level warm/
   moist advection and isentropic lift to LFC, supporting increasing
   thunder potential over the northeastern OK/southeastern KS and
   Ozarks vicinity from around 06Z onward. A weaker/downstream
   perturbation -- initially over portions of Lower MI and IN -- will
   move over OH/PA and the lower Great Lakes the remainder of this
   morning.

   At the surface, 11Z analysis showed a low over eastern ND, with
   occluded front southeastward to a triple-point low over north-
   central IL, and a warm front from there across central IN,
   southwestern OH, eastern KY, and western NC.  A cold front was drawn
   from central IL to southwestern MO, south-central OK, to between
   MAF-LBB, and over southeastern NM.  By 00Z the triple point should
   reach east-central/northeastern OH, with warm front southeastward
   over northern VA and cold front over western KY, central AR and
   west-central TX.  By 12Z, the occlusion triple point should move
   offshore from the Mid-Atlantic.  The cold front will extend over
   central/southwestern VA to middle TN, becoming a quasistationary
   front and weakening from there across northeastern/west-central AR,
   as a surface cyclone develops/deepens over the central Plains.

   ...Mid/upper Ohio Valley...
   Isolated to widely scattered thunderstorms are expected into early
   afternoon, within the western part of a swath of low-level warm
   advection, moisture transport and related precip -- near and north
   of the warm front.  The bulk of this activity will occur prior to
   the passage of the Great Lakes shortwave trough, above a stable
   near-surface layer.  Behind this convection and the trough aloft,
   and in between warm- and cold-frontal passages, the northern fringe
   of the warm sector will yield weak surface-based buoyancy (MLCAPE of
   300-700 J/kg based on modified model soundings).  Despite veered/
   southwesterly to west-southwesterly surface winds following the warm
   front, sufficient midlevel flow will persist to yield 50-60 kt
   effective-shear magnitudes.

   Given those conditions, some concern exists for a conditional
   severe-gust potential over southern OH, northeastern KY and western
   WV, for any daytime convection that can form in the warm sector or
   on the cold front. A marginal/5% wind area was considered.  However,
   current indications are that most, if not all, of that favorable
   parameter space will lag behind the substantial convection and
   shortwave trough during daylight hours.  Progged low-level
   convergence and mid/upper support each remain too weak to justify an
   unconditional risk area while warm-sector parcels still are
   surface-based.

   ..Edwards/Gleason.. 12/27/2021

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