SPC AC 211243

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0643 AM CST Sun Nov 21 2021

   Valid 211300Z - 221200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Thunderstorms (a few strong) may affect parts of Arkansas, Louisiana
   and Mississippi today into this evening.  Organized severe weather
   is not expected at this time.

   ...Synopsis...
   A progressive, amplifying mid/upper-tropospheric pattern is expected
   through the period.  A set of shortwave troughs -- now apparent in
   moisture-channel imagery from central Canada to southern CA -- will
   phase enough to yield a large area of height falls from the
   southwestern/south-central CONUS to the Great Lakes.  By 00Z, a
   coherent mid/upper trough should extend from the upper Great Lakes
   to the Ozarks, southern High Plains, and southern CA.  By 12Z, the
   trough should extend from eastern ON to the southern Appalachians
   and south TX, while ridging breaks through over NM and the CA
   segment splits off and drifts northward.
    
   At the surface, 11Z analysis showed a low over central MO with cold
   front southwestward across northeastern OK, northwest TX and
   southern NM.  A second/reinforcing and faster-moving cold front was
   drawn from northeastern MN southwestward over eastern SD and
   northwestern NE.  By 00Z, the primary, combined cold front should
   extend across central/western KY, southeastern AR, northern LA, and
   south-central TX. By 12Z, the front should reach NJ, western NC,
   southern AL, and the northern/western Gulf. 

   ...Central AR to ArkLaMiss and vicinity...
   Isolated to scattered thunderstorms are expected to form near the
   front over AR and northern LA this afternoon, expanding in coverage
   into early evening as they move east-southeastward to southeastward
   across the lower Mississippi Valley.  A few cells may produce small
   hail or strong gusts; however, buoyancy and effective deep shear
   each appear too meager to support an unconditional severe-threat
   area.  Additional thunderstorms are possible from the lowest Ohio
   Valley and Mid-South to Tennessee Valley and deep south TX -- many
   of which could be post-frontal.

   Forecast soundings and planar progs reasonably suggest that
   warm-sector theta-e advection and cloud-muted diurnal heating will
   support a narrow sliver of weak surface-based buoyancy and
   marginally unstable boundary-layer lapse rates, just ahead of the
   front and as far north as central AR.  This should correspond
   roughly to the inland reach of 60s F surface dew points.  Despite
   the immature/incomplete nature of the return flow, poor mid/upper-
   level lapse rates, and lack of more-sustained/direct diabatic
   heating, MLCAPE around 200-500 J/kg is possible.  Lack of deeper
   CAPE will limit effective bulk shear, though vector values around
   30-40 kt can support a blend of multicells and isolated marginal
   supercells.  The largest low-level hodographs will be farther north,
   amidst/atop a stable surface layer, and forecast soundings suggest
   only up to about 100-150 J/kg effective SRH where inflow parcels are
   purely surface-based.  850-mb (LLJ-proxy) flow also is forecast to
   weaken with time through the day, while the bulk of large-scale lift
   aloft remains behind the surface front.  Given these factors, severe
   potential appears too low, conditional and unorganized for an
   outlook area at this time.

   ..Edwards/Jewell.. 11/21/2021

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