SPC AC 170736

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0236 AM CDT Mon Aug 17 2020

   Valid 191200Z - 201200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHERN INTERMOUNTAIN REGION...AND SOUTHWARD ACROSS THE HIGH
   PLAINS...

   ...SUMMARY...
   Isolated strong/locally severe storms will be possible from portions
   of Idaho and southwestern Montana eastward into parts of South
   Dakota and Nebraska, and southward across the central and southern
   High Plains.

   ...Synopsis...
   Deamplification of the upper flow field is expected across the U.S.
   Wednesday, as suppression of the western upper ridge continues. 
   Within the anticyclonic belt of enhanced mid-level flow around the
   ridge, several embedded vorticity maxima will help support a large
   area of western U.S. to High Plains convective potential.  Farther
   east, scattered -- though primarily sub-severe -- thunderstorms are
   expected generally east of the Mississippi River and south of the
   Ohio River, and northeastward into New England.

   At the surface, high pressure will prevail over much of the East,
   though a weak baroclinic zone will linger from the Carolinas across
   the Gulf Coast states to Texas.  Meanwhile, lee troughing will
   persist over the High Plains.

   ...Northern Intermountain region to the central and southern High
   Plains...
   Isolated afternoon thunderstorms are expected to evolve across a
   large portion of the western U.S., as modest CAPE and differential
   heating over higher terrain combine to support convective
   development.  Development will likely be augmented as a series of
   cyclonic disturbances move through the fast anticyclonic flow field
   aloft.  At this time, narrowing down these areas of greater storm
   potential remains difficult, hence a broad MRGL/5% risk area --
   primarily for afternoon and evening wind gust potential with
   stronger storms.

   An eastward extension of the risk area -- into parts of South Dakota
   and Nebraska -- is being included, where increased warm advection
   may evolve after dark in conjunction with a developing/southerly
   low-level jet, as hints that a more prominent upper disturbance may
   shift across the northern High Plains during the evening, supporting
   potential for an eastward convective expansion.

   ..Goss.. 08/17/2020

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