SPC AC 170755

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0155 AM CST Thu Dec 17 2020

   Valid 191200Z - 201200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   The risk for severe thunderstorms appears negligible across the
   U.S., Saturday through Saturday night.

   ...Discussion...
   Much of North America will remain under the influence of branching
   westerlies emanating from the mid-latitude and subtropical Pacific,
   but broad mid/upper ridging developing within this regime may spread
   inland of the Pacific coast during this period.  Downstream, as one
   short wave trough digs across the southern Rockies and another
   southeast of the Canadian Prairies, it appears that a short wave
   trough, initially over the southern Great Plains at 12Z Saturday,
   will pivot east-northeastward through the lower Ohio and Tennessee
   Valleys by late Saturday night.  

   A significant surface cyclone, initially developing to the lee of
   the Canadian Rockies, is forecast to accompany the northern
   perturbation into northwestern Ontario by the end of the period. 
   Models indicate that this will trail an initially more significant
   cyclone over Hudson Bay, with a developing cold front within weak
   surface troughing trailing southward across the upper Great Lakes
   into the southeastern Great Plains.  However, they suggest little in
   the way of substantive wave development along this boundary as it
   continues to shift east of the Mississippi Valley.

   Modest moisture return emanating from the southwestern Gulf of
   Mexico may reach northwestern Gulf coastal areas by 12Z Saturday. 
   While this may spread inland across upper Texas coastal areas,
   mostly above a residual surface based stable layer, it appears that
   it will only contribute to very weak CAPE as far north as the lower
   Mississippi Valley.

   ...Southeastern Great Plains into Gulf States...
   Weak destabilization associated with pre-cold frontal elevated
   moisture return may support increasing convection capable of
   producing lightning in a corridor across northeastern Texas through 
   central Arkansas early Saturday.  This likely will be aided by
   forcing for ascent ahead of the approaching short wave trough.

   As the short wave impulse turns east-northeast of the southern Great
   Plains, models suggest that one tightening low-level thermal
   gradient ahead of the cold front (roughly in the 925-850 mb layer),
   near its intersection, may provide a focus for continuing weak
   thunderstorm activity.  Due to spread among the various model
   output, how far east remains a little unclear, but low probabilities
   for thunderstorms may extend as far east as north central Georgia by
   late Saturday night.

   Models suggest that another corridor of favorable ascent may support
   thunderstorm development southeastward into better low-level
   moisture and instability across southeast Texas and southern
   Louisiana.  However, any intensification into more vigorous
   thunderstorms capable of posing an appreciable severe threat
   probably will be confined to northwestern into north central Gulf of
   Mexico.

   ..Kerr.. 12/17/2020

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