SPC AC 211151

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0651 AM CDT Mon Aug 21 2017

   Valid 211300Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THROUGH EARLY
   TONIGHT FROM SOUTHEASTERN SD TO IA AND NORTHERN IL...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK AREA...FROM THE CENTRAL PLAINS TO PARTS OF THE OH VALLEY
   AND GREAT LAKES...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS
   AFTERNOON/EVENING FOR NORTHERN VA TO EASTERN PA...

   ...SUMMARY...
   Isolated large hail and damaging winds will be possible with a
   couple of rounds of thunderstorms through tonight from parts of the
   central Plains to the upper Mississippi Valley.  A low-end
   severe-storm threat will also extend eastward into northern Ohio,
   and from northern Virginia to eastern Pennsylvania.

   ...Central Plains to the upper MS Valley through tonight...
   A midlevel trough over the Canadian prairie provinces and northern
   High Plains will progress east-southeastward to the upper MS Valley
   by early Tuesday, as an associated surface cold front translates
   southeastward across the upper MS Valley and into the central
   Plains.  The distribution of steeper midlevel lapse rates and richer
   low-level moisture will be affected today by an ongoing MCS over IA,
   and a subtropical moisture plume spreading northeastward from the
   southern High Plains.  Though the IA MCS should weaken slowly
   through the morning, its associated outflow has already spread into
   southeastern NE and far northern MO.  Meanwhile, a midlevel
   vorticity center over the southern High Plains will continue to move
   northeastward, while associated mid/high-level moisture north-east
   of the vorticity center will overspread western/central KS and
   southern NE today.

   The increasing clouds will tend to slow surface heating by afternoon
   along and southwest of the residual outflow boundary, which brings
   into question the prospects for surface-based storm development. 
   Likewise, the widespread convective overturning in IA and
   reinforcement of the outflow will likely hinder destabilization and
   reduce the severe-weather threat farther north-northeast in MN/WI. 
   The more probable scenario appears to be for convection to increase
   along the synoptic cold front later this afternoon/evening across NE
   and northwestern IA, as forcing for ascent (as evidenced by the
   ongoing elevated storms across northern SD) overspreads the
   boundary.  A separate band of convection may form on the east edge
   of the subtropical moisture plume across KS where surface heating
   and buoyancy will be greater.  Multicell clusters and line segments
   will be the more probable storm modes, given that the stronger
   deep-layer vertical shear will reside generally north of the
   stronger buoyancy, and the cold front will be the primary initiation
   source.  Large hail and damaging gusts will be the main severe
   threats this afternoon into early tonight, when the severe-weather
   threat will begin to diminish.

   ...Northern VA to eastern PA this afternoon/evening...
   A subtle midlevel trough over northern OH this morning will reach
   eastern PA later this afternoon.  At the surface, a weak lee trough
   from northern VA into eastern PA will help to focus semi-discrete
   thunderstorm development this afternoon as the midlevel trough
   approaches.  Midlevel lapse rates will not be steep, but boundary
   layer dewpoints increasing into the upper 60s to lower 70s and
   daytime heating will drive MLCAPE values into the 1500-2000 J/kg
   range.  Effective bulk shear will also increase to 30-35 kt, which
   is representative of a low-end supercell environment where storms
   could produce marginally severe hail and isolated strong/damaging
   outflow gusts.

   ..Thompson/Smith.. 08/21/2017

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