SPC AC 271256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0756 AM CDT Thu Jun 27 2024

   Valid 271300Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
   OF THE CENTRAL AND NORTHERN HIGH PLAINS...

   ...SUMMARY...
   The greatest severe-thunderstorm threat is over parts of the
   northern/central High Plains this afternoon and evening.  Gusts near
   80 mph, damaging hail greater than 2.5 inches and a few tornadoes
   are possible.

   ...Synopsis...
   The mid/upper-level pattern will feature a progressive northern
   stream, with troughs initially located over the inland Pacific
   Northwest and over the Northeast.  A series of phased vorticity
   maxima extend from the Northeastern trough southwestward across the
   central/southern Appalachians to parts of MS/LA in recent moisture-
   channel imagery.  This feature should shift eastward to the eastern
   Carolinas, southern GA and coastal MS by 00Z, becoming stationary
   tonight across GA and the Gulf Coast east of LA, but moving offshore
   from NC and SC.

   The Northwestern trough will move eastward astride the international
   border, with a closed 500-mb low forming and moving over extreme
   southern parts of AB/SK tonight.  By 12Z tomorrow, the trough should
   extend from the southern SK low across central MT to southern ID. 
   Though the flow will remain nearly zonal well to its southeast over
   the central High Plains, height falls are expected to extend as far
   south as parts of the OK/TX Panhandles.

   At the surface, the 11Z analysis showed a cold front across western
   MT, central ID and northern CA, and a warm front from near HVR
   across east-central MT, western/southern SD and northern NE. 
   Cyclogenesis will continue over MT through the day, shifting
   eastward.  By 00Z, the main low should reach the MT/ND border area
   roughly between GDV-DIK, with cold front southwestward across
   northwestern WY, southeastern ID and central NV.  A dryline should
   sharpen over the High Plains through the day, and by 00Z, extend
   from near the MT/SD line across eastern CO to the western TX
   Panhandle.  The warm front should extend from the low southeastward
   over the western Dakotas and central parts of NE/KS, becoming
   diffuse and fragmented farther southeast.  A lingering segment of a
   separate front should be quasistationary and act as a surface
   trough/convergence zone across southern parts of AL/GA and the
   central/eastern Carolinas.

   ...Great Plains...
   Scattered thunderstorms are expected to develop in multiple episodes
   over the northern/central Plains, probably leading off with
   cold-frontal initiation from late morning through afternoon across
   parts of western/central MT, where large-scale lift will be
   strongest with the approach of the northwestern trough aloft.  This
   activity -- which may arise from the same vertical-motion field now
   associated with convection in higher elevations of southwestern MT
   -- should move generally eastward over the Plains, with upscale
   organization possible.  Severe gusts (some exceeding 70 mph) and
   sporadic hail (some exceeding 2 inches diameter) will be the main
   concerns.  Subsequent development is expected closer to the surface
   low, dryline and warm front over the northeastern MT/western Dakotas
   area this afternoon/evening, also posing a threat for large to very
   large hail and severe gusts.  Tornado potential will be relatively
   maximized with any discrete or embedded supercell(s) moving into the
   plume of greatest low-level moisture and backing of surface flow
   near the low and warm front (east of the dryline).

   Deep-layer shear should strengthen throughout the day ahead of the
   cold front, across the central/northern High Plains, while heating
   gradually weakens MLCINH, and regional lift increases in relation to
   low-level cyclogenesis.  This will combine with a plume of favorable
   low-level moisture (dewpoints generally in the 60s F by mid/late
   afternoon) to support a corridor of MLCAPE in the 1000-2000 J/kg
   range near and just west of the warm front.  Effective-shear
   magnitudes in the 40-55-kt range (strongest on the northern areas)
   will support supercell potential, with longer hodographs west of the
   warm front favoring destructive-hail production.  One or more storm
   clusters may evolve and grow upscale into MCS activity this evening,
   further reinforcing the wind potential, with support from moisture
   transport and enhanced storm-relative flow of a 40-50-kt LLJ.

   Convection should be more isolated southward down the dryline and
   lee trough into the western KS/OK Panhandle region, with deep shear
   also weakening southward.  However, intense surface heating and
   somewhat high LCL will foster boundary-layer conditions suitable for
   at least isolated severe gusts and hail.  Some clustering and
   mesobeta-scale cold-pool aggregation may occur to maximize
   severe-wind potential for a few hours sub-regionally.

   ...UT and western parts of CO/WY...
   Widely scattered to scattered thunderstorms are expected to form
   atop a seasonally moist, well-heated and weakly capped boundary
   layer this afternoon, offering locally severe gusts, isolated large
   hail, and a marginal/conditional tornado threat.  This activity
   should develop as height falls spread over the area related to the
   Northwestern trough, along/ahead of the surface cold front.  Diurnal
   heating will reduce MLCINH substantially, especially in the presence
   of moisture characterized by 50s F surface dewpoints remaining in
   many basins/valleys through the heating mixing cycle, while
   orographic forcing also aids lift locally.  That moisture, combined
   with steep low/middle-level lapse rates, will support MLCAPE of
   500-1500 J/kg, atop favorable subcloud thermal profiles to support
   downdraft accelerations to severe levels.  Although environmental
   flow should be nearly unidirectional through most of the
   troposphere, terrain-forced backing will enhanced deep shear
   locally, with effective-shear magnitudes generally in the 25-40-kt
   range.  Mostly multicells and a few supercells are possible.

   ...Southeast...
   Widely scattered to scattered thunderstorms should develop this
   afternoon into early evening, from the NC Coastal Plain
   southwestward to southern portions of AL/GA.  Damaging to isolated
   severe gusts will be the main concern.  Activity should develop most
   densely near the remnant front/convergence zone as the mid/upper
   trough closely approaches, as well as along sea-breeze, outflow and
   differential-heating boundaries south/southeast of the front. 
   Despite weak midlevel lapse rates, a combination of strong diurnal
   heating and rich low-level moisture (with surface dewpoints commonly
   in the upper 60s to mid 70s F) should yield MLCAPE in the 1500-2000
   J/kg range.  Weak low/middle-level flow will limit vertical shear
   substantially, and convection should be multicellular in nature with
   some clustering and local cold-pool development possible.  Though
   moist-, subcloud layers nonetheless should be well-mixed, supporting
   strong/locally severe downdrafts.

   ..Edwards/Bunting.. 06/27/2024

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