SPC AC 191630

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1130 AM CDT Wed Apr 19 2017

   Valid 191630Z - 201200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS
   AFTERNOON/EVENING FOR SOUTHEAST NEBRASKA AND PARTS OF
   WESTERN/CENTRAL IOWA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK AREA...FROM SOUTHWEST WISCONSIN TO NORTHEAST KANSAS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK AREA...FROM NORTHERN OKLAHOMA TO LOWER MICHIGAN AND
   NORTHERN OHIO...

   ...SUMMARY...
   Severe thunderstorms capable of producing large hail, damaging
   winds, and a few tornadoes will be possible from the central Plains
   to the upper Mississippi Valley mainly from mid-afternoon into
   tonight.

   ...Northeast KS to IA and southwest WI through tonight...
   A low-amplitude midlevel trough over the central High Plains will
   progress eastward to the mid MO Valley by this evening, and continue
   to the upper MS Valley overnight.  An associated surface cyclone
   near the KS/NE border will develop east-northeastward along a warm
   front to southeast NE this afternoon and southwest WI by early
   Thursday.  Boundary layer dewpoints in the lower 60s are spreading
   northward south of the warm front, and will combine with daytime
   heating and steep midlevel lapse rates to support MLCAPE near 1500
   J/kg.

   Lingering convective inhibition suggests that thunderstorm
   development this afternoon/evening will largely be confined to the
   synoptic cold front, and in the zone of isentropic ascent north of
   the warm front and immediately in advance of the surface cyclone. 
   Moderate buoyancy and strong deep-layer shear (effective bulk shear
   near 50 kt) will support a risk for supercells, and storms should
   remain semi-discrete given substantial cross-boundary flow/shear
   vectors.  The strongest storms will be capable of producing isolated
   very large hail and damaging winds.  Low-level shear will be
   strongest near the surface warm front, where the tornado risk will
   be greatest for a few hours this evening when storms are most
   probable to remain discrete, and before convective inhibition
   strengthens with diurnal cooling of the boundary layer.  The
   northern extent of any tornado risk will be delineated by strong
   static stability immediately north of the warm front.

   Convection will likely develop southwestward along the cold front
   this evening across KS, and could persist into the overnight hours
   as the frontal convection sags southward.  Otherwise, it is not
   clear that the afternoon/evening supercells will persist overnight
   and very far east across IA, since the stronger forcing for ascent
   will tend to spread more toward WI in the warm advection zone, where
   isolated large hail will be possible.

   ...Southern Lower MI, northern IN and northern OH this afternoon...
   A possible weak MCV over northern IL will move eastward through the
   afternoon.  Modest destabilization of the boundary layer to the east
   may support a few small clusters of storms that will subsequently
   move eastward through the afternoon over southern Lower MI, northern
   IN, and northern OH.  Isolated damaging winds and marginally severe
   hail will be possible.

   ..Thompson/Cook.. 04/19/2017

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