SPC AC 201236

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0636 AM CST Sat Nov 20 2021

   Valid 201300Z - 211200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Severe thunderstorms are not forecast across the contiguous United
   States.

   ...Synopsis/Discussion...
   In mid/upper levels, a nearly zonal pattern over most of the CONUS
   will become less so by the end of the period (12Z tomorrow), as
   these four initial shortwave perturbations behind to phase into a
   larger-scale trough from Hudson Bay to CA:
   1.  A trough now over north-central portions of MB/SK that will
   develop into a closed cyclone over southwestern Hudson Bay;
   2.  A perturbation -- evident in moisture-channel imagery over
   northern BC and far northwestern AB -- forecast to amplify
   considerably while digging southeastward to the MB/ON border area
   and eastern ND;
   3.  A weak but long, positively tilted trough currently located frm
   the Yellowstone area across NV and the central CA coastline.  This
   feature should shift eastward to the central Great Plains and
   central Rockies by 12Z.
   4.  An initially very weak vorticity lobe/perturbation over southern
   OR, which will strengthen as it shifts southeastward and evolves
   into a 12Z trough from southern UT to coastal southern CA.

   At the surface, 11Z analysis showed a cold front related to the
   leading northern-stream mid/upper trough, from northern MN across
   parts of the northern Plains.  This boundary is expected to move
   southeastward and southward through the central Plains today,
   reaching WI, western MO, northwestern OK, and the TX Panhandle by
   00Z.  By 12Z tomorrow, this front should reach Lake Michigan,
   western IL, central OK, and southern NM.  An eastern CONUS
   anticyclone will move offshore and across the western Atlantic
   through the period, but with sharply defined ridging extending
   southwestward to the central/western Gulf Coast, and a substantially
   restricted return of richer subtropical moisture until day-2. 
   Elevated showers are expected to develop near/ahead of the low-level
   cold front late overnight from IL to parts of AR and the Mid-South,
   and a rogue lightning flash cannot be ruled out.  However, with only
   small MUCAPE (around 300 J/kg or less) barely reaching past the -20
   deg C environmental isotherm, thunderstorm activity (if any) is
   expected to be too isolated and brief to warrant an unconditional
   10% area prior to 12Z tomorrow.

   A slow-moving, diffuse frontal zone was evident over south FL and
   the eastern Gulf.  Isolated thunderstorms will be possible again
   today over portions of east-central/southern FL, in a moist (PW
   1.75-2 inches), weakly capped air mass, but with coverage and
   intensity limited by modest low-level forcing and weak midlevel
   lapse rates.

   ..Edwards/Jewell.. 11/20/2021

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