SPC AC 211615

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1115 AM CDT Sat Oct 21 2017

   Valid 211630Z - 221200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   INTO TONIGHT FROM SOUTHEASTERN KS AND SOUTHWESTERN MO ACROSS
   NORTHWESTERN AR AND OK TO PARTS OF THE RED RIVER VALLEY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTH TX TO
   IA AND SOUTHEASTERN MN...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM CENTRAL TX
   TO PARTS OF THE MIDDLE AND UPPER MS VALLEY...

   ...SUMMARY...
   Scattered severe thunderstorms are expected especially across
   portions of the south-central Plains late this afternoon into
   tonight.  Very large hail, damaging winds, and a few tornadoes will
   all be possible.

   ...Synopsis...
   A pronounced midlevel trough over the Rockies will progress eastward
   to the Plains by tonight.  A surface cyclone in southern MB will
   move to the north-northeast, as a trailing cold front moves eastward
   into the upper MS Valley, and southeastward across the
   central/southern Plains and the Ozarks.  Low-level moisture
   continues to spread northward in advance of the cold front and
   beneath a plume of steep midlevel lapse rates, which will combine
   with increasing deep-layer vertical shear and lift along the front
   to support a threat for severe storms this afternoon into tonight
   from IA to north TX.

   ...Eastern KS to IA this afternoon through tonight...
   Initial severe thunderstorm development is expected by mid afternoon
   along the cold front in KS (possibly evolving from early midlevel
   convection), and storms will subsequently expand northeastward into
   IA through this evening.  Remnant steep midlevel lapse rates and
   moderate buoyancy will support an initial large hail threat with the
   frontal convection, and deep-layer vertical shear will be sufficient
   for supercells.  However, largely front-parallel, deep-layer shear
   vectors and flow aloft will tend to support rapid upscale growth
   into a squall line in the zone of linear ascent along the front by
   this evening.  As such, damaging winds will become the main threat,
   though limited surface heating and moistening profiles aloft with
   time may tend to limit the downdraft/damaging-wind threat with
   northeastward extent.  An isolated tornado or two cannot be ruled
   out with the more discrete initial storms, or with embedded
   circulations within the line.

   ...OK to north TX late this afternoon through tonight...
   More discrete supercell development will be possible across western
   OK and northwest TX in the 21-23z time frame along the dryline, near
   and south of a weak triple-point low at the intersection of the cold
   front and dryline.  MLCAPE of 2500-3000 J/kg, midlevel lapse rates
   greater than 8 C/km, and effective bulk shear in excess of 40 kt
   will favor supercells with the initial semi-discrete storm
   development.  A couple of tornadoes and very large hail will be
   possible with the initial supercells, though low-level shear in the
   warm sector will not be particularly strong to the west of I-35. 
   Upscale growth into an extensive squall line (likely merging with
   the convection farther north into KS) is expected this evening, with
   the severe-storm threat transitioning primarily to damaging winds by
   late evening.  A couple of tornadoes will remain possible this
   evening into tonight with embedded mesovortices from southeastern
   KS/southwestern MO into eastern OK, especially with more
   southwest-northeast oriented bowing segments in the line.

   ..Thompson/Leitman.. 10/21/2017

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