SPC AC 250556

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1256 AM CDT Tue Sep 25 2018

   Valid 251200Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE MIDDLE
   MISSISSIPPI VALLEY INTO THE OHIO VALLEY AND SOUTHERN GREAT LAKES...

   ...SUMMARY...
   Severe thunderstorms are possible this afternoon into tonight across
   parts of the middle Mississippi and Ohio Valleys into the southern
   Great Lakes region.

   ...Synopsis...
   A mid-level trough moving into the Black Hills this morning will
   amplify during the period as a 90kt 500mb speed max develops by
   early morning Wednesday over Lake Michigan.  The positive to neutral
   tilt of the mid-level trough suggests cyclogenesis over the western
   and central Great Lakes will remain modest compared to autumnal
   climatology.  A cold front will extend southwestward from the
   developing low over WI southeast KS and into the TX Panhandle. This
   boundary will advance east and southeast reaching the lower Great
   Lakes and extending southwest through central TX by the end of this
   period.  A weak disturbance over the central Appalachians will move
   into the Northeast during the day.  A warm frontal zone will develop
   northward through the OH Valley and into the Great Lakes during the
   day.

   ...Middle Mississippi Valley into the Ohio Valley and Great Lakes...

   A warm/moist conveyer is forecast over the OH Valley as southerly
   low-level flow is maintained during the day across IN/OH into
   southern Lower MI.  A moisture-rich airmass will gradually
   destabilize during the day but strong heating will likely be limited
   by extensive cloud cover and intermittent showers at least through
   the early afternoon.  Models show upper 60s to near 70 degrees F
   dewpoints across this broader region to the east of the cold front
   over the MS Valley.  MLCAPE is forecast to range from 750-2000 J/kg
   with a relatively deep moisture profile.  As the mid/upper-level
   disturbance approaches from the west, deep layer vertical shear will
   support organized storms.  The primary uncertainty this forecast is
   the magnitude of destabilization centered on the area of northern IN
   and the hodograph structure.  The latest NAM and some of the
   convection-allowing models show southerly surface winds and some
   enlargement to the low-level portion of the hodograph during the
   22-03Z period, whereas the GFS and ECMWF exhibit smaller hodographs.
   This would have a consequential effect on the potential for
   low-level mesocyclone development from discrete storms forecast by
   some of the convection-allowing models.  A conditional risk for a
   few tornadoes appears centered over northern IN and surrounding
   portions of states.  Later during the evening, thunderstorms are
   forecast to move through the southern Great Lakes states coincident
   with the front.  The risk for damaging winds may continue as storms
   move east during the overnight as a southwesterly LLJ strengthens
   over the OH Valley/lower Great Lakes.

   Farther west, thunderstorms will likely develop near the front
   during the afternoon with upscale development eventually expected
   across northern IL during the evening.  Damaging winds are the
   primary hazard with this activity.  A couple of storms may develop
   into western MO and southeast KS during the early evening.  An
   isolated hail/wind threat may accompany the stronger storms before
   this activity diminishes during the evening hours.

   ..Smith/Squitieri.. 09/25/2018

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