SPC AC 131815

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1215 PM CST Mon Dec 13 2021

   Valid 151200Z - 161200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF IOWA AND VICINITY...

   ...SUMMARY...
   A band of strong storms is expected to evolve over the mid Missouri
   Valley region during the afternoon, shifting quickly toward the
   upper Mississippi Valley into the evening.  A couple of damaging
   wind gusts will be possible.

   ...Synopsis...
   A vigorous upper low -- initially crossing the Four Corners region
   -- is forecast to eject rapidly northeastward out of the Rockies and
   into the central Plains by evening.  The system will then continue
   northeastward to Upper Great Lakes by the end of the period.  This
   ejection will occur as a second trough shifts southeastward toward
   -- and eventually into -- the western states through the second half
   of the period.  Meanwhile, ridging will prevail over the East.

   At the surface, low pressure deepening over the Colorado/Wyoming
   area at the start of the period will shift into/across Nebraska
   through the afternoon, while a trailing cold front shifts across the
   central Plains/southern High Plains region through sunset.  By the
   end of the period, the low should reside in the vicinity of Lake
   Superior, with the trailing front extending from Lake Michigan
   southwestward to central Texas.  

   ...Mid Missouri Valley/upper Mississippi Valley region...
   Notwithstanding questions regarding the degree of pre-frontal
   moistening/destabilization which may occur during the day Wednesday,
   rapid low- to mid-level atmospheric modification will occur ahead of
   the deepening low/frontal system.  It appears at this time that a
   narrow frontal band of relatively low-topped convection will
   develop, most likely over eastern Nebraska, which would then move
   quickly northeastward across Iowa and possibly southern Minnesota,
   depending upon how far northward any near-surface-based
   destabilization can occur.  

   Despite the questionable/minimally supportive thermodynamic
   environment, strong flow aloft accompanying this system will support
   very fast storm motion.  Given southwesterly low-level (850mb) flow
   in the 70 to 80 mph range anticipated, damaging wind gusts would be
   a possibility with any stronger storm or line segment.  Given this
   potential, a MRGL/5% risk area will be introduced, centered over the
   Iowa area.  Adjustments to area and/or probability level will likely
   be required in later outlooks, as the scenario -- particularly the
   rapid evolution of the thermodynamic environment ahead of the front
   -- becomes a bit more certain.

   ..Goss.. 12/13/2021

   CLICK TO GET WUUS03 PTSDY3 PRODUCT

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