SPC AC 240450

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1150 PM CDT Sun Sep 23 2018

   Valid 251200Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TUESDAY AFTERNOON
   INTO TUESDAY NIGHT ACROSS PARTS OF THE MIDDLE MISSISSIPPI AND LOWER
   OHIO VALLEYS INTO THE GREAT LAKES REGION...

   ...SUMMARY...
   Severe thunderstorms are possible Tuesday afternoon into Tuesday
   night across parts of the middle Mississippi and lower Ohio Valleys
   into the Great Lakes region.

   ...Synopsis...
   While subtropical ridging remains fairly prominent across parts of
   the western Atlantic through much of the Southeast, models indicate
   that large-scale troughing within the mid-latitude westerlies will
   continue to generally shift eastward across interior North America
   Tuesday/Tuesday night.  Guidance is suggestive that there will be a
   number of smaller-scale short wave perturbations progressing through
   the cyclonic regime, but generally low predictability concerning
   these features is evident into and through this period.

   Fairly strong surface cyclogenesis associated with one impulse may
   proceed well north of the Canadian/U.S border, from southern
   Hudson/James Bays through northern Quebec.  Guidance suggests that
   secondary wave development is possible along the trailing frontal
   zone to the south, generally from portions of the upper Midwest
   through and northeast of the Great Lakes region by late Tuesday
   night.  The evolution of this feature will considerably impact the
   convective potential for this period, and remains at least somewhat
   unclear due to apparent lingering differences among the various
   model output.

   ...Middle Mississippi/lower Ohio Valleys into Great Lakes...
   The northward return of upper 60s/lower 70s surface dew points
   through the region, ahead of the southeastward advancing front, does
   appear fairly certain.  However, this may be accompanied by
   considerable convection and rain, aided by general large-scale
   forcing for ascent, and the extent to which insolation may be able
   to contribute to boundary layer destabilization during the day
   Tuesday remains unclear.  

   Despite modest to weak mid-level lapse rates, guidance is suggestive
   that breaks in cloud cover could allow for CAPE to increase in
   excess of 1000 J/kg by late afternoon.  While stronger mid/upper
   flow may lag near and to the west of the cold front, 30-50 kt
   southwesterly, cyclonic flow around 500 mb probably will still
   contribute to sufficient deep layer shear for organized convective
   development, including supercells, across the potential warm sector.


   The focus for the initiation of storms remains unclear.  It is
   possible that the leading/northward edge of the richer boundary
   layer moisture return may remain ahead of the cold front, oriented
   in west/southwest through east/northeast fashion across Illinois,
   Indiana and Ohio.  But the cold front could still become the most
   prominent focus as it advances southeast of the Upper Midwest and
   adjacent portions of the Mississippi Valley late Tuesday afternoon
   and evening.

   Most model output has been suggestive that generally weak low-level
   wind fields and hodographs may result in severe wind and perhaps
   hail as the primary hazards.  However, earlier/more pronounced
   surface wave development than generally forecast could be
   accompanied by enlarging clockwise curved low-level hodographs
   supportive of increasing tornado potential, particularly across
   southern portions of the Great Lakes region into portions of the
   Ohio Valley.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:   5%     - Slight
   Wind:     15%     - Slight
   Hail:     15%     - Slight

   ..Kerr.. 09/24/2018

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