SPC AC 111258

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0758 AM CDT Sun Apr 11 2021

   Valid 111300Z - 121200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS TODAY OVER
   CENTRAL FLORIDA...

   ...SUMMARY...
   The main severe-thunderstorm threat today will be over portions of
   the Florida Peninsula, with damaging wind and occasional hail
   possible.  A tornado cannot be ruled out as well.

   ...Synopsis...
   The mid/upper level pattern will continue to feature split flow
   across central North America, but with some shift in the location
   and amplitude of the splitting.  That mainly will be related to two
   features:
   1.  A cyclone now centered near the IL/IN state line, and forecast
   to gradually shrink/fill as its circulation center crosses IN and
   northern OH through the period.
   2.  An initially open-wave, neutral-tilt trough over SK, eastern MT
   and northern WY, which will evolve into a closed cyclone as it
   crosses the northern Great Plains today.  By 12Z tomorrow, the
   500-mb low should be near the SD/ND/MN border confluence, along a
   trough oriented west-northwest to east-southeast.

   In the southern peripheral cyclonic flow of the leading upper low,
   two small but important shortwave troughs are apparent:  the first a
   convectively enhanced vorticity lobe over the northeastern Gulf
   south of the FL Panhandle, and the second upstream across southeast
   TX and LA.  These perturbations will proceed eastward across
   central/northern FL and the central northeast Gulf Coast regions
   through 00Z, with the trailing one crossing northern/central FL
   tonight.

   At the surface, 11Z analysis showed a primary occluded low over
   northern IL, with another near a triple point over northern Lower
   MI.  A cold front extended from there across eastern OH, to the TRI 
   and MOB areas, and across the northwestern Gulf.  The surface low
   (or lows, in a broad cyclone with more than one center possible)
   should meander around the states bordering Lake Michigan through the
   period.  Another cold front associated with the northern Plains
   upper trough should move southeastward over the northern/central
   Plains and southern High Plains.  The cold front related to the
   Great Lakes cyclone will shift southeastward over the Carolinas, GA
   and northern FL, where convective processes will be largely
   prefrontal.

   ...FL...
   Scattered to numerous thunderstorms are expected to evolve from the
   ongoing, initially loosely organized activity over the northeastern
   Gulf.  The resulting convection should sweep across and down the
   peninsula today in an increasingly organized, largely forward-
   propagational MCS.  The main threat will be damaging gusts, some
   severe, and a tornado or two cannot be ruled out.  (See SPC severe
   thunderstorm watch 91 and related mesoscale discussions for
   near-term situational coverage.)  The main threat area, with
   greatest concentration of severe-wind potential, currently appears
   to be portions of central and perhaps south-central FL that
   experience longer-duration diurnal heating and sustained moisture
   advection/transport ahead of the complex.

   Veering flow with time is expected in much of the pre-MCS boundary
   layer, and should have these influences:
   1.  Making deep-layer winds more unidirectional, with the mean-wind
   and deep-shear vectors assuming substantial component along the
   convective axis, thereby supporting denser, quasi-linear
   configuration,
   2.  Reducing low-level shear and hodograph size somewhat (but not
   enough to forbid QLCS tornado potential entirely) and
   3.  Advecting higher-theta-e Gulf air across the central/southern
   peninsula, supporting potential MLCAPE of 1500-2000 J/kg.

   The complex should reach South FL this evening at the latest, and
   may affect portions of the Keys as well.  Mesoscale trends as the
   MCS and its cold-pool/pressure perturbation evolves likely will
   compel further extension or adjustment of the probabilities today.

   ...Mid-Atlantic region...
   Widely scattered thunderstorms may develop this afternoon, mainly 
   along and east of a surface prefrontal/lee trough, moving eastward
   to northeastward over the outlook area.  Isolated damaging gusts
   and/or marginally severe hail are possible.

   For generating buoyancy, diurnal destabilization of the boundary
   layer behind the morning clouds/precip will help to offset some
   relatively weak midlevel lapse rates, along with surface dew points
   commonly in the 50s to low 60s north, low/mid 60s south.  Depending
   on the duration of heating, some mixing-related reduction in surface
   moisture is possible, tempering CAPE somewhat, but at the same time,
   yielding more evaporation-favoring subcloud layers for localized
   acceleration of downdrafts.  Peak MLCAPE in the 500-1000 J/kg range
   is possible -- locally higher in the southern parts.  The main
   factor precluding a better-organized threat will be lack of
   substantial vertical shear, with only 25-35-kt effective-shear
   magnitudes expected (also locally/briefly greater).

   ..Edwards/Broyles.. 04/11/2021

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