SPC AC 251938

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0238 PM CDT Tue Sep 25 2018

   Valid 252000Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE
   MID-MISSISSIPPI VALLEY EASTWARD TO LOWER MICHIGAN AND NORTHWESTERN
   OHIO...

   ...SUMMARY...
   Scattered damaging gusts and isolated large hail will be possible
   this afternoon and evening from eastern Iowa to northern Illinois
   and southern Wisconsin, and at least isolated damaging gusts will be
   possible into the overnight hours across Lower Michigan.  A couple
   of tornadoes will be possible this afternoon from northeastern
   Indiana into northwestern Ohio.

   ...20Z Outlook Update...
   An ongoing MCS across northwestern Illinois will continue to pose a
   risk for damaging wind gusts and perhaps an isolated tornado or two
   through this evening.  These storms will be sustained by a
   moderately unstable downstream airmass along with roughly 40 knots
   of deep shear.  Ref WW 388 and associated Watch Status Messages for
   more detail.

   Downstream of this MCS, airmass recovery was ongoing across Indiana
   and Ohio in the wake of morning convection, although this recovery
   was tied primarily to advection as only a few breaks in the cloud
   deck are present via satellite imagery.  Models continue to suggest
   that convection evolving in upstream areas (i.e. Illinois and
   vicinity) will exhibit mixed modes (linear and cellular) while
   migrating toward the region, with low 70s F dewpoints and
   strengthening low-level flow supporting near-surface-based
   convection along with a modest increase in tornado potential.  The
   risk for damaging winds will also continue.  Thus, 5% tornado/15%
   wind probabilities remain in place for this outlook, with the bulk
   of the Indiana/Ohio/lower Michigan severe potential occurring
   near/after 00Z.

   ..Cook.. 09/25/2018

   .PREV DISCUSSION... /ISSUED 1125 AM CDT Tue Sep 25 2018/

   ...Synopsis...
   A weak surface cyclone in south central IA as of late morning will
   move northeastward toward southwestern QC and gradually deepen by
   the end of the period, in advance of a shortwave trough progressing
   from the northern Plains to the Great Lakes.  An associated surface
   cold front (now from central WI/IA to central KS) will move
   southeastward to north TX and the lower Great Lakes by 26/12z.  A
   broad swath of 68-72 F boundary-layer dewpoints has spread
   northeastward in advance of the cold front, though midlevel lapse
   rates greater than 7 C/km are confined to a small area of the
   central Plains, with lesser lapse rates to the east.  Ongoing
   clusters of storms from central IN southward are related to a
   remnant tropical moisture plume with diffuse embedded speed maxima. 
   Larger cloud breaks and stronger surface heating is expected west
   through northwest of the ongoing IN storms today.

   ...Eastern IN to northwestern OH this afternoon/evening...
   The ongoing storms across IN will spread northeastward today as weak
   embedded MCVs interact with the moist environment across the OH
   Valley.  Storm mode will likely remain somewhat messy clusters, and
   surface heating will be slowed by persistent clouds.  Still, the
   stronger low-level shear this afternoon should be focused from
   eastern IN across western OH with weak surface-based buoyancy, and a
   couple of tornadoes will be possible.

   ...Eastern IA/northern IL/southern WI this afternoon/evening...
   The threat for isolated strong/severe storms should evolve from the
   ongoing convection approaching central IA.  These storms have been
   elevated through the morning, but will gradually phase with the
   surface low/cold front and warm sector from eastern IA to northern
   IL/southern WI this afternoon.  Surface heating in advance of the
   cold front, combined with the frontal circulation and forcing for
   ascent preceding the midlevel trough moving eastward from NE/SD,
   should support some increase in storm intensity this afternoon. 
   Though low-level shear is not expected to be particularly strong
   (the stronger low-level southwesterly flow is confined to the OH
   Valley moisture plume), MLCAPE near 1000 J/kg and effective bulk
   shear near 50 kt will favor an organized band of storms (with bowing
   segments and embedded supercells) capable of producing damaging
   winds and isolated large hail.

   ...Northern IN to lower MI this evening through tonight...
   In the wake of today's ongoing convection across IN/OH, additional
   storm development will be possible this evening into tonight.  The
   remnants of the afternoon/evening storms in WI/IL could reach
   western Lower MI by early tonight, with additional storm development
   possible from IN into southeastern Lower MI.  A modest increase in
   low-level flow/shear is possible this evening across Lower MI,
   though forecast hodographs do not look particularly favorable for
   tornadic supercells.

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