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Mesoscale Discussion 909
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   Mesoscale Discussion 0909
   NWS Storm Prediction Center Norman OK
   0351 PM CDT Thu Jun 10 2021

   Areas affected...Western South Dakota and western Nebraska

   Concerning...Severe potential...Watch likely 

   Valid 102051Z - 102245Z

   Probability of Watch Issuance...95 percent

   SUMMARY...Supercells are expected to develop along the lee trough
   later this afternoon. Very large hail, severe wind gusts, and a
   couple tornadoes are possible. With time, initial storms are likely
   to grow upscale into intense lines/bowing segments capable of
   significant wind gusts. A WW is likely within the next 2-3 hours.

   DISCUSSION...Cumulus clouds have begun to form along the MT/SD/WY
   border within the lee trough. Higher-based cumulus are also evident
   farther south in eastern Wyoming as well. Continued boundary-layer
   heating and mid-level height falls should continue to promote this
   development and eventual storm initiation later this afternoon. The
   19Z UNR sounding showed extreme surface-based buoyancy, though this
   was only supported by a very shallow layer of low-level moisture.
   Farther to the east, the 18Z LBF sounding showed a deeper moist
   layer with similar instability/buoyancy. This richer moisture is
   apparent on visible satellite where boundary-layer cumulus have
   develop in central Nebraska/South Dakota.

   Current visible satellite trends suggest that storm development is
   still likely a 2-4 hours away, which has been the general consensus
   of CAM guidance today. With 30-45 kts of effective shear orthogonal
   to the lee trough, initial storms will likely be supercellular and
   capable of very large hail (potentially 3+ inches) and severe wind
   gusts. Given the lower boundary layer RH towards the west, the
   tornado threat is not expected to be overly high, but a couple of
   tornadoes will be possible. If a storm can remain discrete into the
   evening, richer moisture to the east and a strengthening low-level
   jet, a brief window of greater tornado potential could develop.

   The main concern beyond the initial supercells is expected to be a
   threat for significant wind gusts along with linear/bowing segments
   that develop. Guidance has varied in the exact evolution of
   convection this evening, but one or two of intense squall lines are
   possible as storms move into central Nebraska/South Dakota.

   A WW is likely within the next 2-3 hours.

   ..Wendt/Guyer.. 06/10/2021

   ...Please see for graphic product...


   LAT...LON   40950381 42540444 44400486 45650459 45890282 45160116
               42270068 41050084 40240165 40170278 40950381 

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