SPC AC 241252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Thu Jun 24 2021

   Valid 241300Z - 251200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
   OF NORTHERN KANSAS...SOUTHERN NEBRASKA AND NORTHERN MISSOURI....

   ...SUMMARY...
   A corridor of severe weather (including destructive hail, severe
   wind and a few tornadoes) is possible today into tonight near the
   Kansas/Nebraska line, eastward over northern Missouri.

   ...Synopsis...
   In mid/upper levels, a series of shortwave troughs from the northern
   Rockies to the Great Lakes -- including a few that are convectively
   generated/enhanced -- will contribute to the formation of a
   positively tilted synoptic trough.  By 12Z, the broader 500-mb
   trough should extend from the northern High Plains across the Great
   Basin, connecting to a low near (or just offshore) from the SFO
   area.  To its east, a somewhat confluent, nearly zonal pattern will
   prevail across the central/northern Plains, Upper Midwest, and upper
   Great Lakes.  An MCV over IA, from an ongoing MCS over IA and
   northern MO, is expected to move eastward across eastern IA and
   northern IL today.  The associated perturbation should weaken as it
   crosses southern Lower MI, northern IN and northwest OH tonight.
   West of the synoptic trough, a northern-stream shortwave trough --
   apparent in moisture-channel imagery over the BC/AB Rockies -- will
   move east-southeastward to the MT/AB/SK border region by 00Z, then
   weaken and move more slowly southeastward over eastern MT tonight.  

   At the surface, the 11Z analysis showed a quasistationary frontal
   zone just offshore from the Carolinas, arching across southern GA,
   then becoming a weakening warm front over northern MS, northern AR
   and central MO.  The western limb of this boundary will become more
   diffuse today as it moves northeastward over the mid Mississippi
   Valley.  A cold front was drawn from northwestern ON to near the MN
   North Shore, through a weak low near MSP, across southeastern SD and
   central/southwestern NE to eastern CO.  By 00Z, that cold front
   should extend from WI southwestward across IA to a low over central
   KS, then to southeastern CO.  By 12Z, the low should reach
   central/eastern Upper MI and southeastern IA, becoming slow-moving
   to stationary southwestward over northwestern MO, central KS and the
   OK Panhandle.  A dryline initially near the KS/CO border, and over
   eastern NM, will mix eastward today to west-central/southwestern KS,
   the eastern TX Panhandle, and the lower Pecos Valley area of west
   TX.  Cyclogenesis and frontogenesis also are expected this afternoon
   and evening over eastern MT ahead of the northern-stream
   perturbation.

   ...Central Plains to mid Mississippi Valley...
   The ongoing MCS over southern IA and northern MO is moving into
   lower boundary-layer theta-e on the cool side of the diffuse warm
   front, but may produce isolated gusts near severe limits over the
   remainder of the morning as it encounters a marginally favorable but
   destabilizing boundary layer.  An outflow boundary -- trailing
   southwestward and westward from the complex across northern portions
   of KS and MO -- will decelerate this morning, then retreat northward
   this afternoon.  

   Scattered thunderstorms are expected to form this afternoon near the
   boundary, with isolated to scattered convection also occurring near
   the dryline and perhaps part of the nearly cold front.  Large to
   giant hail is possible in early-stage convective evolution, when
   supercells are most favored, with severe gusts and a few tornadoes
   also possible (especially near the boundary).  Clustering and
   upscale organization to one or more complexes should occur through
   the evening, concentrating the wind threat better. 

   Possible foci for initiation this afternoon and evening include the
   boundary, cold front and dryline/lee trough.  The outflow boundary
   will be the main focus for severe potential today.  In the absence
   of substantial cloud cover, the air mass on both side of the western
   and perhaps eastern portions of the boundary (KS and MO
   respectively) will destabilize substantially today amidst strong
   insolation and rich low-level moisture, with surface dew points
   commonly in the upper 60s and 70s F, under favorable midlevel lapse
   rates.  Modified model soundings for along and just north of the
   boundary indicate MLCAPE may reach 2500-4000 J/kg, while slightly
   weaker (but still moderate-strong) buoyancy occurs to its south,
   amidst stronger heating and deeper warm-sector mixing.

   Meanwhile, with some reinforcement from the front, the surface flow
   on the north side of the boundary will remain relatively backed,
   leading to enlarged shear and storm-relative flow vectors in the
   boundary layer.  Modest mid/upper winds will impede venting and
   likely contribute to messy/heavy-precip modes in fairly short order,
   but the backed winds north of the boundary still may yield around
   40-50-kt effective-shear magnitudes.  The extent of outflow-boundary
   return northward remains uncertain, and further mesoscale
   adjustments to the outlook area (including tornado probabilities
   tied largely to the vorticity/hodograph enhancement along and to its
   north) are possible. 

   ...Northern High Plains...
   Widely scattered thunderstorms are expected to develop this
   afternoon across portions of western through north-central/
   northeastern MT, offering the potential for isolated, marginally
   severe hail and gusts.  This activity should form as a plume of
   large-scale DCVA-related lift ahead of the shortwave trough
   superimposes with a diurnally destabilizing boundary layer with
   weakening MLCINH.  Also contributing will be increasing low-level
   convergence near the area of cyclogenesis.  Additional convection
   should develop over parts of eastern WY, western SD and northwestern
   NE, near the Black Hills and along a lee trough, with potential for
   isolated severe gusts for a few hours.  Across the northern Plains
   corridor, residual moisture (surface dew points 40s to 50s F) and
   steep low/middle-level lapse rates will contribute to peak MLCAPE in
   the 500-1000 J/kg range, atop deep/well-mixed subcloud layers. 
   Activity should diminish markedly this evening with near-surface
   cooling.

   ..Edwards/Broyles.. 06/24/2021

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