SPC AC 220751

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0151 AM CST Wed Dec 22 2021

   Valid 241200Z - 251200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Isolated thunder is possible near the Pacific Coast, as well as over
   portions of the southern Great Basin and Colorado Plateau.

   ...Synopsis/Discussion...
   A series of shortwave perturbations will contribute to maintenance
   of mean mid/upper-level troughing near the West Coast through
   day-3/Christmas Eve.  In the northern stream, a strong shortwave
   trough is evident in moisture-channel imagery east of the Kamchatka
   Peninsula.  The leading part of this perturbation is forecast, with
   reasonable consistency among various deterministic progs and most
   ensemble members, to reach the WA coastline early this period then
   weaken, while its western lobe digs southeastward across the Pacific
   offshore from OR and northern CA.  Associated DCVA/cooling aloft
   should pivot ashore over OR/CA during the latter half of the period.

   Progressively colder air aloft will spread across the inland
   Northwest, Great Basin and northern Intermountain regions, but
   predominantly atop cool surface conditions.  Very isolated flashes
   cannot be ruled out across a vast area of the CONUS between the
   Rockies and Pacific coastal thunder potential.  A relatively focused
   (but still isolated) thunder potential may spread eastward across
   portions of the southern Great Basin and Colorado Plateau during the
   latter half of the period, as a Pacific southern-stream shortwave
   trough discussed in the day-2 outlook ejects east-northeastward out
   of the mean trough and across this region.  Accompanying DCVA/
   cooling aloft and low/middle-level moist advection above the surface
   may support sporadic flashes within a broad plume of precip/showers.
   This perturbation should weaken considerably as it moves past the
   south-central Rockies.

   Broad lee cyclogenesis/frontogenesis is expected over parts of the
   central/northern Rockies and adjoining High Plains beneath strong
   west-southwesterlies aloft, and as heights fall to the west. 
   However, a well-developed EML should cover most of the southern/
   central Plains into lower/mid Mississippi Valley.  Any substantial
   moisture return from the Gulf should be too displaced from the areas
   with strongest deep-layer forcing -- and from a weakly convergent
   cold front extending from the upper Great Lakes across the central
   Plains -- to support 10%+ thunder potential east of the Rockies.

   ..Edwards.. 12/22/2021

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   NOTE: THE NEXT DAY 3 OUTLOOK IS SCHEDULED BY 0830Z