SPC AC 210730

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0230 AM CDT Tue Jul 21 2020

   Valid 231200Z - 241200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHEAST/MID-ATLANTIC...NORTHERN/CENTRAL PLAINS...AND NORTHERN
   ROCKIES...

   ...SUMMARY...
   Isolated strong to severe thunderstorms appear possible Thursday
   across parts of the Northeast/Mid-Atlantic, northern/central Plains,
   and northern Rockies.

   ...Northeast/Mid-Atlantic...
   Another in a series of shortwave troughs should move across the
   Northeast and Mid-Atlantic regions on Thursday. Mid-level
   west-southwesterly flow will be somewhat enhanced through the day,
   which may support at least some storm organization. Convection may
   increase in both coverage and intensity by Thursday afternoon
   along/ahead of a cold front. A mainly linear storm mode should
   occur, with strong to locally damaging winds probably the main
   threat before convection moves offshore.

   ...Northern/Central Plains...
   A shortwave trough is forecast to advance northeastward across the
   northern/central Plains on Thursday. Moderate to possibly strong
   instability should develop ahead of this feature as low-level
   moisture streams northward through the day. Most guidance suggests
   convective initiation will occur no later than early Thursday
   evening, and subsequently spread northeastward across the warm
   sector. At this point, the main limiting factor appears to be
   marginal deep-layer shear, as the shortwave trough is not expected
   to be overly strong. Will include a broad 5% area for now to
   encompass the most likely area that could see isolated to scattered
   storm coverage in a favorable thermodynamic environment, with some
   potential for greater severe probabilities if forecast shear
   strengthens any further.

   ...Northern Rockies...
   At least weak destabilization should occur Thursday afternoon across
   parts of the northern Rockies and vicinity as a large-scale upper
   trough advances slowly eastward across the western CONUS. A
   shortwave trough embedded within this larger-scale upper trough
   should encourage convection to develop over the higher terrain of
   central ID into southwestern MT through the day. Steep mid-level
   lapse rates may compensate for modest low-level moisture, and
   deep-layer shear should be sufficient for isolated strong to perhaps
   severe storms.

   ..Gleason.. 07/21/2020

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