Storm Prediction Center Aug 18, 2021 0730 UTC Day 3 Severe Thunderstorm Outlook

   SPC AC 180731

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0231 AM CDT Wed Aug 18 2021

   Valid 201200Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS EASTERN PORTIONS
   OF THE CENTRAL AND NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY
   AREA...

   ...SUMMARY...
   Scattered strong/severe storms are forecast to spread from eastern
   portions of the central and northern Plains, into the upper
   Midwest/upper Mississippi Valley region Friday, accompanied by risk
   for large hail, damaging winds, and a couple of tornadoes.

   ...Synopsis...
   As an upper-level short-wave trough digs south-southeastward across
   the western Canada coast and eventually into the Pacific Northwest,
   a second short-wave trough will exit the Rockies and shift quickly
   northeastward toward the upper Midwest as an increasingly negatively
   tilted system.

   At the surface, a deepening low will shift similarly northeastward,
   while a trailing cold front moves across the Plains states through
   the period.  

   ...Parts of the central and northern Plains across Iowa and
   Minnesota...
   As a vigorous mid-level trough ejects northeastward across the
   central/northern Plains, a deepening surface low is expected to
   shift northeastward out of western Kansas into the eastern South
   Dakota vicinity through the afternoon.  As this occurs, the airmass
   ahead of the low and associated frontal system will destabilize
   through the day, as a moist boundary layer coupled with daytime
   heating results in a zone of moderate warm-sector CAPE across the
   Mid Missouri Valley region.  The eventual result will be scattered
   afternoon storm development, which may extend as far southward as
   western portions of Oklahoma and western North Texas.

   As storms spread northeastward across the Mid Missouri Valley region
   and vicinity during the afternoon, aided by veering/substantially
   increasing flow with height, shear favorable for updraft
   organization/rotation will likely permit severe/supercell storms to
   evolve.  An all-hazards severe risk should therefore accompany the
   stronger cells -- including large hail, locally damaging winds, and
   a couple of tornadoes.

   As the low deepens further through the evening, and reaches
   northwestern Minnesota, severe potential will likely spread into
   Minnesota and Iowa, where an all-hazards severe risk will likely
   continue.  Potential for at least localized severe-weather episodes
   will likely linger into the overnight hours.

   ..Goss.. 08/18/2021

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