SPC AC 071630

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1130 AM CDT Sun Jun 07 2020

   Valid 071630Z - 081200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON INTO EARLY TONIGHT FROM CENTRAL SD TO EASTERN ND AND
   NORTHWESTERN MN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TODAY FROM
   NORTHEAST UT INTO THE NORTHERN HIGH PLAINS...AND ALONG THE GULF
   COAST....

   ...SUMMARY...
   Severe thunderstorms capable of producing very large hail,
   significant damaging gusts, and a few tornadoes are expected late
   this afternoon into early tonight from central South Dakota into
   eastern North Dakota and northwestern Minnesota.  More isolated
   severe storms will be possible farther southwest into Wyoming. 
   Isolated tornadoes may occur along the north central Gulf coast.

   ...Northern Plains late this afternoon into early tonight...
   A 989 mb surface cyclone in southern SK has occluded and will
   continue moving northward and slowly weakening in conjunction with a
   shortwave trough.  Embedded speed maxima will rotate northeastward
   around the eastern periphery of a deep, upstream midlevel trough
   over the interior Pacific Northwest.  This will contribute to lee
   cyclogenesis in the vicinity of the NE Panhandle this
   afternoon/evening along a surface trough.  East of the trough,
   boundary-layer dewpoints of 65-70 F are spreading northward from
   eastern NE to the eastern Dakotas, along and immediately west of a
   surface warm front.  Regional 12z soundings showed that the moist
   layer is a bit shallow, and will likely be subject to some vertical
   mixing/dilution this afternoon in the open warm sector, especially
   along the surface trough as afternoon temperatures reach or exceed
   90 F.

   The surface heating/mixing, and low-level convergence along the
   trough, will support surface-based thunderstorm development after
   21z from central SD into eastern ND.  The storm environment will be
   characterized by MLCAPE of 3000-4000 J/kg, midlevel lapse rates of
   7-8 C/km, and effective bulk shear of 40-50 kt, which will support
   supercells capable of producing very large hail.  The tornado threat
   will likely be focused with initial (more discrete) storm
   development along the surface trough, or with more isolated storms
   that form and interact with the warm front into northwest MN, where
   low-level moisture will be more sheltered from mixing and low-level
   shear will be greater.  The environment will also favor strong
   downdraft production, and deep-layer flow/shear aligned along the
   surface trough suggests that upscale growth into clusters/line
   segments is likely this evening, as damaging winds become the
   greater severe-weather threat.  

   West of the surface trough, residual low-level moisture and surface
   heating will be sufficient for at least weak SBCAPE and minimal
   convective inhibition.  A few high-based storms may form this
   afternoon from southeast MT into western SD, with storms
   subsequently spreading northeastward into ND this evening.  Though
   buoyancy will not be particularly large, long hodographs will
   support the potential for supercells with isolated large hail and
   damaging winds.

   ...Northeast UT into WY this afternoon...
   Thunderstorms are forming as of mid morning across north central UT,
   in a zone of ascent preceding an embedded speed max rotating
   east-northeastward from northern NV.  Despite somewhat limited
   moisture and initially cool surface temperatures, surface heating
   and relatively cool midlevel temperatures will allow for roughly 500
   J/kg of SBCAPE to develop from northeast UT across WY this
   afternoon.  Strong midlevel winds/long hodographs and steep
   low-level lapse rates, along with the weak SBCAPE, will support a
   threat for low-topped cells/clusters capable of producing damaging
   winds and marginally severe hail this afternoon/evening.  

   ...North central/northeast Gulf coast through tonight...
   Tropical storm Cristobal is forecast to make landfall and continue
   moving north-northwestward over southeast LA through early Monday. 
   Enhanced low-level flow east of the center (40-45 kt
   east-southeasterly winds per MOB VWP) will contribute to sufficient
   hodograph curvature/SRH for rotating storms.  The threat for a
   couple of tornadoes should be greater in the zone of overlap between
   the stronger low-level shear and the north edge of the mid-upper 70s
   dewpoints, where surface heating in a few cloud breaks will boost
   buoyancy inland this afternoon.  A low-end tornado threat will
   persist and spread slowly inland through tonight.

   Farther east, a differential heating zone/pseudo warm front will
   provide a focus for thunderstorms this afternoon across north FL. 
   Low-level shear will be strongest along and north of the boundary,
   and there will be some potential for organized/rotating storms along
   the north edge of the richer moisture/larger buoyancy.

   ..Thompson/Lyons.. 06/07/2020

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