SPC AC 151625

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1025 AM CST Wed Dec 15 2021

   Valid 151630Z - 161200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   INTO EARLY TONIGHT FROM EXTREME EASTERN NE ACROSS IA INTO
   SOUTHEASTERN MN AND EXTREME SOUTHWESTERN WI...

   ...SUMMARY...
   Widespread severe wind gusts of 60-75 mph along with at least a few
   tornadoes are likely from mid afternoon through early tonight across
   the Mid-Missouri Valley to the Upper Mississippi Valley. Embedded
   gusts of 75-100 mph and a strong tornado or two are also possible,
   particularly from extreme eastern Nebraska across western to
   northern Iowa and southeastern Minnesota.

   ...A rapidly evolving outbreak of severe storms with widespread
   damaging winds and a few tornadoes is expected to begin by mid
   afternoon across eastern NE and continue northeastward into the
   upper MS Valley by early tonight...

   ...Mid MO and upper MS Valleys this afternoon into early tonight...
   An intense midlevel shortwave trough and associated deepening
   surface cyclone will eject rapidly northeastward today from the
   central High Plains to the upper MS Valley by tonight.  A developing
   dryline will surge east-northeastward across KS/NE into western
   IA/northwestern MO to the south of the cyclone, prior to being
   overtaken by a cold front tonight.  An unseasonably moist air mass
   is spreading northward from the western Gulf of Mexico, with
   boundary-layer dewpoints near or above 60 F already into central IA.
   The moistening is occurring beneath a plume of steep midlevel lapse
   rates near 8 C/km per regional 12z soundings.  Destabilization will
   continue northward through the day across eastern NE/IA into
   southern MN in advance of the deepening cyclone, with MLCAPE
   expected to reach or exceed 1000 J/kg.

   Though the elevated mixed layer plume will act to cap the moistening
   boundary layer, strong forcing for ascent with the surging dryline
   and within the left-exit region of the intense mid-upper jet streak
   will quickly remove convective inhibition in a narrow zone along the
   dryline by mid afternoon (20-21z), when convective initiation is
   likely across east central/southeast NE and northeast KS.  Extremely
   strong wind profiles will accompany the ejecting midlevel trough,
   with speeds approaching 125 kt at 500 mb and 70-80 kt not far above
   the ground.  Effective bulk shear will also be quite strong at 70-80
   kt.  The strong linear forcing for ascent and cap will support a
   narrow line of storms, though the very strong deep-layer shear
   vectors/long hodographs oriented across the boundary, and increasing
   low-level buoyancy by mid-late afternoon, should allow some
   supercell structures within the band of storms.  Downward momentum
   transport by the convection will result in the potential for
   widespread wind damage with peak gusts potentially in the 75-100 mph
   range, and embedded circulations will be capable of producing a few
   fast-moving tornadoes (one or two of which could be strong) with
   both right-moving supercells and embedded QLCS mesovortices.

   Gradually weakening buoyancy with east/northeastward extent should
   result in a diminishing severe threat by 06z across WI.  The severe
   threat will also diminish with southward extent, where forcing for
   ascent will be weaker and the orientation of the flow will be much
   more parallel to the cold front tonight.

   ..Thompson/Kerr.. 12/15/2021

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