SPC AC 060558

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1258 AM CDT Sat Jun 06 2020

   Valid 061200Z - 071200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR WESTERN AND
   CENTRAL SOUTH DAKOTA AND PARTS OF NORTHWEST NEBRASKA...

   ...SUMMARY...
   Severe thunderstorms are likely across parts of the northern Great
   Plains this afternoon into tonight.  Severe gusts, large to very
   large hail, and a couple of tornadoes are possible.  Severe gusts
   are also possible across the central Rockies during the day.

   ...Synopsis...
   A seasonably powerful mid-level trough is forecast to move quickly
   northeast from AZ across the central Rockies and into the western
   Dakotas late tonight.  Another mid-level trough is forecast to move
   southeastward across New England.  In the low levels, the
   aforementioned western U.S. trough will feature a developing surface
   low over the northern High Plains with a well-developed lee trough
   extending through the central High Plains. A warm front will advance
   north across the north-central Great Plains and extend across SD
   during the afternoon/evening.

   ...Black Hills and north-central states...
   Strengthening low-level flow will usher increasingly moist air from
   the central Great Plains northward into SD with an arc of 66-74 deg
   F dewpoints extending from southeast SD west-northwest into
   west-central SD by mid afternoon.  Strong heating will contribute to
   steepening low-level lapse rates beneath 7-8 deg C/km 700-500mb
   lapse rates.  Storm development will likely ensue in the more deeply
   mixed/hotter air across far eastern WY/NE Panhandle and adjacent SD.
   A mix mode of organizing bands growing upscale and discrete
   supercell activity are expected.  Large hail, severe gusts, and a
   few tornadoes are possible.  Farther east in southeast SD, lower
   storm coverage along the warm frontal zone is expected towards early
   evening.  Despite weaker mid- to high-level flow, large hodographs
   within a boundary layer characterized by 17-18 g/kg lowest 100mb
   mean mixing ratios, suggests the potential for a few tornadoes.  As
   the mid-level wave approaches during the evening, upscale growth
   will favor a transition to primarily a severe-gust threat spreading
   north and east during the evening and overnight as a LLJ
   strengthens.  

   ...Central Rockies...
   The early arrival of strong large-scale forcing for ascent will
   likely lead to storms developing by the late morning over southwest
   CO and southeast UT.  The mid-level flow increases markedly during
   the morning with long hodographs.  Model forecast soundings show
   adequate buoyancy for updraft rotation for a few of the stronger
   storms.  The intense wind fields forecast to overspread the region
   coupled with the organizing bands of thunderstorms, will facilitate
   the potential for severe gusts of 60-85 mph.  

   ...New England...
   Isolated to scattered thunderstorms are forecast to develop along a
   front (as early as midday) and are forecast to push southeast across
   the region during the afternoon.  Ample heating will steepen
   low-level lapse rates beneath gradually increasing westerly
   tropospheric flow.  Strong to severe multicells capable of
   marginally severe hail and damaging gusts in the 50-60 mph range are
   expected.  This activity will push east of the coast by the early
   evening.

   ...Mid-Atlantic states...
   The southern portion of the boundary forecast to move across New
   England will serve as a focus for thunderstorm development this
   afternoon.  Models indicate widely scattered thunderstorms by mid
   afternoon near the Chesapeake Bay vicinity.  Forecast soundings show
   around 35kt effective shear and moderate instability.  Isolated
   large hail/damaging gusts are possible with the stronger storms
   before this activity weakens during the evening as it shifts
   southward into southeast VA.

   ..Smith/Nauslar.. 06/06/2020

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