SPC AC 131959

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0259 PM CDT Mon Jul 13 2020

   Valid 132000Z - 141200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS THE
   UPPER MIDWEST AND SOUTH-CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Severe thunderstorms are expected from the central High Plains into
   parts of the Upper Midwest this afternoon into tonight.  Severe
   gusts, possibly significant, large hail and a couple of tornadoes
   are the threats with the stronger storms.

   ...20Z Update...
   Existing convective outlook remains on track based on latest
   observational trends/short-term guidance.

   ..Guyer.. 07/13/2020

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Mon Jul 13 2020/

   ...Synopsis...
   The upper-air pattern across the CONUS will remain dominated by
   three primary features: 
   1.  An anticyclone centered over southern NM and far west TX,
   attached to a ridge extending from the Gulf Coast States across
   northern Baja to adjoining Pacific waters.
   2.  A synoptic cyclone centered initially over northeastern AB near
   Lake Athabasca, with trough southward across eastern AB then
   southwestward across portions of ID, eastern OR and northern CA. 
   The cyclone should migrate erratically eastward to northern SK
   through the period.  Meanwhile the primary/basal shortwave trough
   and vorticity banner -- now evident in moisture-channel imagery over
   northwestern MT and the MT/SK/AB border junction area -- will move
   east-northeastward to southeastern SK and eastern MT by 00Z.  This
   perturbation then should eject northeastward across southern and
   eastern MB overnight. 
   3. A lengthy, but not particularly intense trough, extending from
   Greenland across Labrador and eastern QC to the lower Great Lakes,
   north-central Appalachians, and Carolinas.  A shortwave trough --
   initially to its west near lake Huron -- will pivot southeastward to
   eastward across the Mid-Atlantic region to western New England by
   12Z tomorrow.  Associated weak large-scale lift, overlain with
   diurnal low-level destabilization, will contribute to general
   thunder potential over the Eastern Seaboard.  Diurnal convection
   over portions of the Northeast and Carolinas regions potentially
   will offer locally strong/isolated severe gusts, along/ahead of the
   cold front described below.

   Otherwise, the southern CONUS will remain the domain of MCVs in
   midlevels, including a particularly pronounced one evident in radar
   composites and satellite animations over northeastern OK.  This
   feature, and the associated shortwave trough, will move roughly
   southeastward across eastern OK and AR into tonight. 

   At the surface, a weak low was drawn at 14Z in the PWM/CON area,
   with wavy cold front across the lower Delaware Valley area to
   southeastern VA then over eastern/central KY, becoming diffuse
   northwestward from there.  Another cold front -- preceding the AB/SK
   cyclone and trough -- extended from south-central MB across the
   central Dakotas and through a low near MBG, to southeastern WY. 
   This boundary should proceed to northwestern MN, eastern SD and
   southwestern NE by 00Z, then to eastern/southern MN and eastern/
   southern NE by the end of the period. 

   ...Central Plains...
   No substantial changes appear warranted for this update.  Boundary-
   layer lift is expected to increase through the afternoon from a
   combination of strengthening lee troughing south of the front, the
   cold front itself, upslope flow to its north into the higher western
   terrain, and diurnal heating.  The related weakening of MLCINH, and
   moist advection into western parts of the outlook area, will foster
   scattered thunderstorm development this afternoon in roughly a
   north/south corridor.  Intense surface heating, steep boundary-layer
   lapse rates and intense vertical mixing will produce areas of DCAPE
   exceeding 1500 J/kg, beneath MLCAPE increasing from around 1000-1500
   J/kg near western fringes of the outlook areas to 2000-4000 J/kg
   where mixing is somewhat more subdued (and MLCINH greater) over
   central/southwestern NE and western KS. 

   After initial discrete to semi-discrete stages with a mixed
   wind/hail threat, convection should organize upscale in association
   with aggregating outflows.  One or more clusters of resulting
   cold-pool-driven convection then should accelerate eastward from the
   hotter/more-mixed air into richer low-level moisture, with forced
   ascent along its leading edge overcoming any EML-related increase in
   MLCINH with eastward extent.  The greatest associated/organized wind
   potential still appears to be over the 30% wind area, and
   hurricane-force/65+ kt gusts may occur from the most intense/surging
   segments and bowing clusters.  

   ...Upper Midwest...
   Scattered to potentially numerous thunderstorms are expected to form
   through the afternoon and sweep eastward across the region from
   eastern ND over portions of northern MN.  The main threat will
   become damaging wind, and an enhanced threat area for that hazard
   has been added to this outlook. 

   Early, somewhat elevated convection will move into a destabilizing
   boundary layer with increasing potential for organized, surface-
   based activity, including early supercell(S) and multicells evolving
   into a bowing MCS.  Large hail is possible with relatively discrete
   and early convection, along with a tornado or two in an environment
   of favorable low-level shear that also will extend some tornado
   potential eastward in QLCS mode. This scenario increasingly is
   supported by an array of both synoptic and convection-allowing
   guidance in the near term.  This also is consistent with the basic
   pattern evolution, as difluent yet strengthening mid/upper flow
   impinges on the area, and deep shear increases over the warm sector.
    
   Large-scale support and frontal lift should decrease with southward
   extent into eastern SD, indicating a relative minimum in organized
   severe potential compared to areas of denser, longer-lived
   convection to the north and southwest.  Still, the potential remains
   for isolated to scattered thunderstorms to pose at least a brief
   window for large hail and damaging wind along/ahead of the cold
   front. 

   ...Arklatex to southern LA...
   A mesoscale pocket of enhanced large-scale ascent, low/middle-level
   flow/shear, and low-level mass response for lift, will exist across
   the southern rim of the troughing related to the MCV as it moves
   slowly southeastward.  That portion of the forcing extending over
   and ahead of the antecedent outflow boundary will support renewed
   thunderstorm development this afternoon, in a richly moist and
   strongly diabatically heated air mass, with a corridor of 2000-4000
   J/kg MLCAPE over the outlook area along and west of an old surface
   trough/frontal zone.  The most intense associated convection can
   yield severe gusts/hail on a localized basis.

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