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Forecast Discussion - Convective Outlook

   SPC AC 051250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Sun Jul 05 2020

   Valid 051300Z - 061200Z


   The greatest potential for severe thunderstorms should be this
   afternoon and evening, from northeastern Wyoming across parts of the
   Dakotas to northwestern Minnesota.

   In mid/upper levels, a belt of nearly zonal/west-southwest flow will
   extend obliquely across the northern border, from the Pacific
   Northwest to northwestern ON.  An embedded, positively tilted
   shortwave trough -- located initially over central/southwestern MT
   -- will move east-northeastward to southwestern MB, western ND and
   southeastern MT by 00Z, then weaken as it ejects across eastern MB

   Farther south, a broad, zonally elongated anticyclone will persist
   from NM, far west TX and northern MX westward across similar
   latitudes of the northeastern Pacific.  On its northeastern rim, a
   slow-moving shortwave trough over eastern CO and the TX/OK
   Panhandles, including an MCV now evident over the northeastern TX
   Panhandle -- should move slowly southeastward to southward.  A
   broad, weak cyclone will longer over the central Gulf Coast. 
   Related cyclonic flow and embedded, mostly convectively induced
   vorticity maxima will persist from the Ozarks across the
   northern/central/eastern Gulf to portions of FL/GA.

   A strong shortwave trough -- now apparent in moisture-channel
   imagery over central/northern QC -- is forecast to dig southeastward
   toward New England today, forming a closed 500-mb low that should
   pass close to the northern tip of ME tonight.  The low should reach
   NB by the end of the period, with trough southwestward across
   southern New England.

   At the surface, 11Z analysis showed a weak cold front across
   southwestern MB to northeastern WY, somewhat diffused behind an area
   of convective outflow that covers much of the western Dakotas. 
   After the front moves eastward today, the combined baroclinic zone
   should reach northeastern ND, central SD, and east-central/south-
   central WY by 00Z.  The front then should move somewhat farther
   southeastward but weaken through the remainder of the period.

   ...Northeastern WY to northwestern MN...
   Scattered thunderstorms are expected to develop this afternoon along
   and near the front over the Dakotas, moving into a very moist and
   unstable warm sector, while widely scattered thunderstorms form on
   the higher terrain of southern MT and northern WY and move generally
   eastward behind the front.  In both regimes, supercells and
   organized multicells will be possible, offering large hail and
   severe gusts.  Tornado potential is substantially more conditional
   and isolated, but cannot be ruled out. 

   In the eastern/central Dakotas, satellite imagery shows cloud cover
   from earlier convection is breaking up.  This trend should continue
   through the day, allowing areas of sustained insolation amidst
   surface dew points in the upper 60s to low 70s F ahead of the
   outflow/front.  This will contribute to strong instability, with
   peak preconvective MLCAPE reaching 3500-4500 J/kg.  Low- to
   middle-level flow will be modest, but convective organization should
   be aided by ventilating anvil-level winds, as 200-500-mb layer flow
   strengthens with the approach of the mid/upper trough.  Isolated
   very large/damaging hail cannot be ruled out relatively early in the
   convective cycle, before activity congeals into clusters; however,
   uncertainties on location and duration of relatively discrete modes
   mesoscale and duration uncertainties preclude a sig-10% hail area at
   this time.  

   Moisture will be more limited behind the front, but still sufficient
   amidst steep low/middle-level lapse rates to support surface-based
   convection away from the mountains.  Activity should move into a
   well-mixed and diurnally heated boundary layer supporting
   maintenance of hail/gusts to the surface.  The northeasterly flow
   behind the front will elongate hodographs and augment the deep
   shear, beneath stronger mid/upper-level winds than experienced by
   frontal convection farther east.  Forecast soundings show effective-
   shear magnitudes potentially reaching the 50-60-kt range.  When
   combined with MLCAPE in the 800-1500 J/kg range (locally/briefly
   higher), the environment is well within the parameter space
   supporting supercell mode. 

   ...Central/southern High Plains...
   Isolated to widely scattered thunderstorms may develop this
   afternoon over the higher terrain of northeastern NM and central CO.
   Additional development is possible over adjoining High Plains --
   particularly near outflow and differential-heating boundaries -- as
   diurnal heating and modest cooling aloft steepen lapse rates and
   weaken MLCINH near the shortwave trough and MCV.  Isolated severe
   gusts and hail are possible, though low/middle-level flow and shear
   will be weak, limiting overall organization of the threat.  Also, an
   extensive area of clouds, precip and outflow has spread across a
   large part of the southern High Plains, to west-central/northwest TX
   and the Permian Basin.  This feature is related to yesterday's
   convection over northeastern NM, southeastern CO, and the TX/OK
   Panhandles.  As a result, lower theta-e, longer diabatic-
   destabilization times, and somewhat poorer moisture quality will be
   available to convection forming and moving off the higher terrain

   ...New England...
   Widely scattered to scattered thunderstorms should develop this
   afternoon -- perhaps in a couple of episodes -- near a subtle
   surface trough and convergence line across the northern and western
   parts of the outlook area.  Isolated hail and damaging to marginally
   severe gusts should be the main concern.

   Though near-surface flow will be weak, MLCINH will be as well, with
   only subtle kinematic boundary-layer forcing needed for convective
   lift in concert with diurnal heating.  This will occur as the strong
   mid/upper shortwave approaches from QC, with associated cooling/
   destabilization aloft contributing to a favorable convective
   environment.  Sufficient low-level moisture is in lace to support a
   narrow corridor of 500-1000 J/kg MLCAPE in northern ME, increasing
   to 1000-1500 J/kg MLCAPE south-southwestward across the eastern
   remainder of New England.  Though low-level shear will be
   constrained by the weak speeds, veering with height will contribute
   to 30-40-kt effective-shear magnitudes, and approximately straight
   low/middle-level hodographs.  This indicates potential for a blend
   of multicell and somewhat-organized supercell modes.

   ..Edwards/Broyles.. 07/05/2020