SPC AC 210728

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0228 AM CDT Thu May 21 2020

   Valid 231200Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SATURDAY AFTERNOON
   AND EVENING ACROSS MUCH OF CENTRAL AND SOUTHERN SOUTH DAKOTA AND
   NORTHERN AND CENTRAL NEBRASKA...

   ...SUMMARY...
   An organized cluster of thunderstorms may develop to the east and
   southeast of the Black Hills Saturday afternoon, before spreading
   into adjacent portions of the Plains and middle Missouri Valley,
   accompanied by a risk for large hail and strong wind gusts through
   Saturday evening.

   ...Synopsis...
   Models suggest a continued weakening of the westerlies across the
   U.S. Saturday through Saturday night.  Mid-level ridging is forecast
   to build northeast of the southern mid-latitude eastern Pacific,
   toward the U.S. Pacific coast.  As this occurs, initially amplified
   downstream cyclonic flow over the Intermountain West and Rockies may
   begin to split.  One embedded perturbation, including a vigorous
   remnant cyclonic vorticity center, appears likely to accelerate
   northeast of the northern Great Basin, into the higher northern
   plains by late Saturday night.  Another may evolve and dig across
   the Four Corners states.

   Downstream, ridging may continue to build across much of the lower
   Mississippi Valley and Southeast, while a remnant mid-level closed
   low digs off the Mid Atlantic coast.  Weakening troughing may also
   progress around the crest of the southern branch ridging, across
   parts of the Upper Midwest and Great Lakes region.

   In lower levels, boundary-layer moisture return into the vicinity of
   surface troughing across much of the plains appears likely to
   contribute to moderate to large CAPE, with daytime heating beneath
   steep mid-level lapse rates associated with warm elevated
   mixed-layer air.  In the presence of modest vertical shear, aided by
   veering of wind fields with height, this environment is expected to
   become conducive to scattered strong to severe thunderstorm
   development Saturday afternoon and evening.

   ...Plains...
   Models continue to suggest that the strongest and most organized
   convective development will become focused east and southeast of the
   Black Hills, beneath a strongly difluent upper flow regime, and
   perhaps aided by forcing associated with lower/mid tropospheric warm
   advection.  This is expected to be near the nose of the plume of
   warmer elevated mixed-layer air, where mid-level cooling will weaken
   inhibition.  Surface dew points increasing into mid 60s+ F, are
   forecast to contribute to mixed-layer CAPE in excess of 2000 J/kg,
   which should support vigorous updrafts capable of producing large
   hail.

   Deep-layer vertical shear probably will become at least marginally
   sufficient for supercells initially.  Low-level hodographs may
   become large enough to support a risk for tornadoes.  However,
   activity may tend to grow upscale fairly quickly, with strong wind
   gusts becoming the prominent severe hazard.  The evolution of an
   increasingly organized convective system is possible, which may
   persist through much of the evening, while propagating eastward and
   southeastward.

   ..Kerr.. 05/21/2020

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