SPC AC 191946

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0246 PM CDT Tue Sep 19 2017

   Valid 192000Z - 201200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS THE
   EASTERN DAKOTAS AND WESTERN MN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST SD
   INTO NORTH-CENTRAL MN...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EXTREME
   NORTHEAST NE TO THE ARROWHEAD OF MN...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE SOUTHERN PLAINS...

   ...SUMMARY...
   Severe storms with a threat for a few tornadoes, numerous large hail
   and damaging wind events, a few of which should be significant, are
   expected across parts of the eastern Dakotas into western Minnesota
   from about 4 pm to 4 am CDT.

   ...Discussion...

   No significant changes to 1630z outlook are warranted.

   Pre-frontal warm advection convection is spreading across portions
   of ND well ahead of primary short-wave trough. While this activity
   may generate locally severe hail or wind gusts it appears stronger
   convection will evolve in its wake later this afternoon. Earlier
   thoughts regarding the evolution of severe thunderstorms remain.

   ..Darrow.. 09/19/2017

   .PREV DISCUSSION... /ISSUED 1123 AM CDT Tue Sep 19 2017/

   ...Synopsis...

   Satellite and RAP analysis data indicate several impulses moving
   through a broad synoptic upper trough that is situated over the
   northwestern states. A lead shortwave trough is currently moving
   northeast through eastern MT into western ND, and an upstream
   impulse will affect this region this afternoon.  

   ...Central and eastern Dakotas into Minnesota...

   Late this morning a cold front extends from a weak surface low
   through the western Dakotas into eastern WY preceded by a
   pre-frontal trough. A warm front stretches from central MN through
   southern and western ND. Low to mid 60s F boundary layer dewpoints
   will advect through pre-frontal warm sector beneath plume of steep
   lapse rates, sampled by 12Z RAOBS, contributing to a corridor of
   moderate instability with 1500-2500 J/kg MLCAPE. However, strong
   convective inhibition associated with the elevated mixed layer and
   widespread multi-layer clouds should delay more robust thunderstorm
   development until late afternoon. Mid-level convection embedded
   within multi-layer cloud plume will continue spreading through the
   warm sector with limited solar insolation. This and strong
   convective inhibition suggest most of the robust thunderstorm
   development will probably remain confined to the zone of stronger
   forcing along eastward-advancing cold front during the late
   afternoon. The low-level jet will strengthen within the exit region
   of an approaching mid-upper jet resulting in strong (45-55 kt)
   effective bulk shear supportive of organized severe storms. Mixed
   modes with both supercells and linear bowing segments are expected
   early in the storms evolution, with a gradual trend toward upscale
   linear growth during the evening. Tornadoes, very large hail and
   damaging wind are all possible. Overall tornado threat will depend
   to some degree on mode evolution and appears to be greatest in
   corridor from northeast South Dakota into southeast North Dakota.
   Later this evening into overnight storms will continue into the
   upper Mississippi Valley with damaging wind becoming the primary
   threat.

   ...West TX to southwest OK...

   The dryline will be the focus for isolated to scattered storm
   development late afternoon to early evening. Steep 700-500 mb lapse
   rates of 7-8 C/km will support moderate buoyancy with MLCAPE
   reaching 1500-2500 J/kg. While mid-level winds will be modest,
   veering of the wind profile with height should yield 25-30 kt
   effective shear, supporting a few transient supercell and multicell
   clusters. Isolated occurrences of large hail and damaging winds are
   possible before storms wane after sunset.

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