Mesoscale Discussion 0605
   NWS Storm Prediction Center Norman OK
   0238 PM CDT Thu May 13 2021

   Areas affected...Parts of northern/western Nebraska and adjacent
   portions of southeastern Wyoming and southwestern South Dakota

   Concerning...Severe potential...Watch unlikely 

   Valid 131938Z - 132215Z

   Probability of Watch Issuance...20 percent

   SUMMARY...Strong thunderstorm development, possibly including an
   evolving cluster of storms, appears underway.  This may pose a risk
   for hail and strong surface gusts which could occasionally approach
   or exceed severe limits into early evening.  Due to the marginal
   anticipated risk, it is not clear that a severe weather watch is
   needed, but trends will be monitored.

   DISCUSSION...Boundary-layer destabilization is underway across
   central portions of the high plains, modest due to low moisture
   levels, but becoming maximized as far north as the Nebraska
   Panhandle vicinity.  This is generally north and east of a
   developing weak surface low, and along a strengthening zone of
   differential surface heating, which is becoming a focus for
   deepening convective development.  This is probably being supported
   by increasing large-scale ascent, largely due to low/mid-level warm
   advection, downstream of a low-amplitude mid-level perturbation
   digging within northwesterly flow across the northern Rockies.

   Deep-layer ambient mean flow only appears to be strengthening from
   20-30 kt, but associated vertical shear, aided by veering profiles
   in low to mid-levels, is probably becoming conditionally supportive
   of organized convection.  With mixed-layer CAPE, particularly around
   the Alliance, NE vicinity, as high as 500 J/kg, but up to around
   1000 J/kg for surface-based parcels, there appears potential for at
   least a few stronger storms capable of producing small to marginally
   severe hail.  

   Given the steep low-level lapse rates associated with the modestly
   deep, well mixed boundary-layer, locally strong surface gusts might
   be the more prominent convective hazard, aided by sub-cloud cooling
   associated with melting and evaporating precipitation.  It is
   possible that forcing associated with consolidating surface cold
   pools, coupled with the larger-scale ascent, could support an
   upscale growing, southeastward propagating cluster of storms by
   early evening.

   ..Kerr/Thompson.. 05/13/2021

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...LBF...UNR...CYS...

   LAT...LON   43560347 42309958 41460155 41340384 42450404 42920451
               43560347