SPC AC 190537

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1237 AM CDT Thu Jul 19 2018

   Valid 191200Z - 201200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE MID MS
   VALLEY...

   ...SUMMARY...
   Severe thunderstorms will be possible later today across parts of
   the Mississippi Valley and central Plains. A few strong storms may
   also be noted near the Canadian border of the northern Plains and
   over the southwestern US.

   ...Discussion...

   Early-morning water-vapor imagery depicts a well-defined mid-level
   circulation over SD. This feature is forecast to shift into
   southwest MN as associated 500mb speed max digs into southwest IA by
   mid afternoon. Large-scale high-level diffluence downstream across
   the Mid-MS Valley will encourage deep convection along the eastern
   edge of Plains steep lapse-rate plume.

   While the large-scale pattern favors a focused corridor of potential
   severe, especially across portions of IA into northeast MO, pre-dawn
   convection across this region may prove disruptive regarding
   timing/placement of potentially more significant thunderstorm
   development later in the day. Will maintain higher severe probs for
   the more synoptically favorable portion of the upper trough but
   meso-scale features will undoubtedly influence severe development.

   Latest NAM guidance suggests strong heating will develop ahead of a
   surface front from KS into southwest IA. If this boundary is the
   dominant low-level feature, and forecast buoyancy is as expected,
   then thunderstorms should develop immediately downstream of the
   surface low position, potentially arcing southwest into portions of
   KS. There is some concern for supercell development across
   IA/northern MO and if discrete storms develop then large hail and
   damaging winds can be expected. While a tornado can not be ruled
   out, low-level shear is not forecast to be particularly strong. This
   activity will propagate southeast during the evening as the
   mid-level jet translates toward southern MO.

   ...Northern Plains...

   Strong surface heating is forecast across eastern MT into ND ahead
   of a surface front that will sag south of the international border
   during the afternoon. While instability is not forecast to be that
   significant, steep lapse rates and adequate deep-layer flow should
   support a few marginally severe thunderstorms capable of generating
   gusty winds and some hail. This activity will be strongly diurnal in
   nature.

   ...Southwest...

   Seasonally high-PW air mass will be maintained across northwest
   Mexico into the lower CO River Valley through the day1 period as the
   center of a mid-level high begins to drift toward the southern
   Rockies. While mid-level temperatures are forecast to be somewhat
   warm across AZ, strong surface heating and adequate buoyancy may
   prove sufficient for gusty winds with the strongest convection.

   ..Darrow/Squitieri.. 07/19/2018

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