SPC AC 300519

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1219 AM CDT Thu Aug 30 2018

   Valid 311200Z - 011200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR SOUTHERN
   MINNESOTA...MUCH OF IOWA...AND INTO EASTERN PARTS OF NEBRASKA AND
   SOUTH DAKOTA...

   ...SUMMARY...
   Isolated to scattered severe thunderstorms are possible from
   portions of the central Plains northeastward into the Upper Midwest
   mainly Friday afternoon into Friday night.

   ...Central Plains into the Upper Midwest...
   A low-amplitude mid-level trough will move across the north-central
   U.S. on Friday before moving into western Ontario overnight.  A belt
   of moderate west-southwesterly mid-level flow will encompass the
   area from the north-central High Plains eastward into IA/WI.  The
   strength of flow will weaken considerably south of a west-east line
   paralleling the NE/KS and IA/MO borders as a mid-level anticyclone
   is centered from the southern Plains east through the Mid-Atlantic
   states.  A cold front/surface trough will sag into parts of the
   Upper Midwest and extend into the central High Plains where a weak
   area of low pressure is forecast.  

   It seems increasingly likely one or more thunderstorm clusters may
   be ongoing Friday morning over the southern half of MN and perhaps 
   northwest MO and adjacent states in response to a warm advection
   regime.  A marginal risk for a few strong thunderstorms (marginally
   severe hail) or a strong gust could occur before this activity
   probably dissipates during the mid/late morning as a the LLJ wanes. 
   However, models maintain a weakened but identifiable LLJ with a
   terminus over IA.  Additional storm development is possible on the
   leading edge of outflow with a severe risk possibly maintained as
   the boundary layer destabilizes and thunderstorms become
   surface-based.  

   Model guidance appears to suggest mid-late afternoon storm
   development probably occurring near the MN/northeast SD border
   region as a cap erodes near the surface boundary.  Forecast
   soundings show MLCAPE 1500-2500 J/kg with 40 kt effective shear
   which would promote rapid storm organization.  Large hail (perhaps
   significant) is possible especially early during the convective life
   cycle due in part to 8 degree C/km 700-500 mb lapse rates.  An
   evolution into a linear band will probably occur during the evening
   with damaging gusts becoming the dominant hazard.  

   Farther south, a more conditional severe risk is apparent with
   varying model solutions.  Nonetheless, a very unstable airmass with
   sufficient mid-level shear for supercells warrants maintaining
   15-percent severe probabilities over eastern NE.  It seems the
   chance for thunderstorms increases near the MO River and into
   western IA during the early evening perhaps on the residual outflow
   intersection with steep low-level lapse rates from the west.  A
   hail/wind hazard threat would accompany any sustained thunderstorm
   activity that manages to develop during the early evening and
   perhaps grow upscale during the night.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:   2%     - Marginal
   Wind:     15%     - Slight
   Hail:     15%     - Slight

   ..Smith.. 08/30/2018

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