SPC AC 181721

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1221 PM CDT Tue Aug 18 2020

   Valid 191200Z - 201200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   NORTHERN INTERMOUNTAIN REGION...SOUTHWARD ACROSS THE HIGH
   PLAINS...AND WESTWARD INTO THE DESERT SOUTHWEST...

   ...SUMMARY...
   Isolated strong/locally severe wind gusts will be possible within a
   large, arc-shaped area from portions of Idaho and southwestern
   Montana eastward into parts of South Dakota and Nebraska, southward
   across the central and southern High Plains, and westward to
   southeastern Arizona.

   ...Synopsis...
   Upper ridging in place across much of the western CONUS early
   Wednesday is expected to dampen throughout the day as a series of
   low-amplitude shortwave troughs traverse its northern periphery. A
   few convectively augmented shortwave troughs will also likely move
   through the eastern periphery of this ridge across the Plains.
   Farther east, a shortwave trough will move from Quebec
   eastward/northeastward through the Northeast and Canadian Maritime
   Provinces, leading to some deamplification of the upper troughing
   across northeast Canada and the Great Lakes/Northeast. Broad upper
   trough will remain in place from the OH Valley southward across the
   Southeast and into Gulf Mexico, supported by a weak vorticity
   maximum moving slowly across the Mid-South/Lower MS Valley.  

   At the surface, modest lee troughing will exist across the High
   Plains while high pressure centered over the Great Lakes/Mid MS
   Valley extends over much of the eastern CONUS. A weak front will
   exists along the southern and eastern periphery of this surface
   ridging, from the Carolinas through the Gulf Coast states.

   ...Northern Intermountain West across the High Plains, and into the
   Desert Southwest...
   A largely homogeneous thermodynamic environment will arc from the
   northern Rockies/northern Intermountain West across the High Plains
   and back into the Desert Southwest. In these areas, a deeply mixed
   boundary layer will contribute to high-based thunderstorms and the
   potential for damaging downbursts with any stronger, more persistent
   updrafts. As a result, the higher probability risk areas will likely
   be colocated with areas where the highest thunderstorm coverage
   exists. However, confidently discerning these higher coverage areas
   is difficult given the convectively induced nature of any shortwave
   troughs, precluding any increased probabilities with this outlook.
   Most likely area of a potential increase in probabilities in a later
   outlook is from central ID into southwest MT.

   ...Southeast States into the Mid-Atlantic...
   Scattered to numerous thunderstorms are anticipated in the vicinity
   of a weak front extending from the lower MS Valley. Overall storm
   intensity will likely be limited by the poor mid-level lapse rates
   and weak low-level flow across the region. Even so, slightly
   stronger mid-level flow across the Mid-Atlantic could contribute to
   a few stronger multicell clusters capable of producing locally
   strong wind gusts.

   ..Mosier.. 08/18/2020

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