SPC AC 041241

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0741 AM CDT Mon May 04 2020

   Valid 041300Z - 051200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NEAR THE
   MISSOURI/KANSAS/OKLAHOMA BORDER JUNCTION ACROSS PARTS OF EASTERN
   OKLAHOMA...

   ...SUMMARY...
   Severe thunderstorms are forecast today in a loosely T-shaped area
   concentrated over parts of Oklahoma, Kansas, the Ozarks, and western
   Tennessee/Kentucky.  All forms of severe weather may occur
   (downdraft wind, hail, tornadoes).  Very large/damaging hail, over 2
   inches across, is possible from the western Ozarks across eastern
   Oklahoma.

   ...Synopsis...
   In mid/upper levels, the upper-air pattern is characterized by split
   flow around a temporary Rex pattern over northwestern North America,
   north of a nearly zonal southern-stream belt from the West Coast
   across the central/northern Plains, bending southeastward with
   approach to the Atlantic in deference to northeastern CONUS
   troughing.  A mid/upper-level speed max -- associated with a series
   of closely spaced shortwaves/vorticity maxima now over the central
   Plains and central Rockies -- will move/develop eastward across the
   central Plains to parts of MO today, and the lower Ohio Valley
   overnight.

   At the surface, 11Z analysis showed a weak surface low near GUY,
   with wavy warm front across northern OK to near FSM, then over
   central AR and northern MS.  A cold front was moving southeastward
   across southeastern CO and northwestern KS, reinforced by convective
   outflow from prior/overnight activity.  

   By 00Z, a new surface low should develop near the cold front/warm
   front intersection in the western MO/eastern KS area, though
   location uncertainty is considerable, given potential mesoscale
   convective factors.  The cold front then should extend across
   northeastern through south-central/southwestern OK, to the southern
   TX Panhandle and northeastern NM.  The cold front should overtake
   the dryline from north-south through the afternoon, so that their
   intersection is over southwestern OK by 00Z.  The warm front should
   reach a position from the low east-southeastward across the MO
   Ozarks to western TN, with its effective position potentially
   shifted southward locally by convective outflow. By 12Z, the low
   should reach the EVV area, with cold front to near a DYR-TXK-JCT-FST
   line. 

   ...Central/southern Plains, Ozarks to western TN/KY...
   Two primary episodes of severe will affect the broader outlook area,
   with some spatial overlap:

   1.  Evolution of ongoing convection over KS and the related,
   initially elevated UVV plume shifting southeastward across MO toward
   western portions of TN/KY through the day.  A severe-hail threat
   already has developed with elevated convection north of the surface
   warm front across portions of Kansas.  The wind potential will
   increase through the morning as more convection forms further south
   and east, atop a shallower and less-intense stable layer, with
   more-buoyant inflow.  Refer to SPC severe thunderstorm watch 161 and
   related mesoscale discussions for the near-term threats. 

   A conditionally greater wind potential may develop near the warm
   front, across parts of the Ozarks to western/mid TN this afternoon,
   strongly dependent upon sufficient cold-pool aggregation and
   organization in upshear convection across southern MO.  The surface
   warm-frontal zone from southeastern KS toward western TN/KY will
   possess backed near-surface winds, greater storm-relative boundary-
   layer flow, enlarged hodographs, and maximized low-level vorticity. 
   This also indicates a conditional corridor of relatively maximized
   supercell/tornado threat.  The greatest potential offsetting factor
   may be those driving the aforementioned wind threat (potential for
   cold-pool-driven, clustered or bowed storm mode).  For now, the wind
   probability and marginal tornado threat have been extended somewhat
   southeastward in accordance with these indications.

   2.  Afternoon supercell development along the cold front, with more
   isolated/transient multicell to marginal supercell convection
   possible along the dryline.  The frontal activity should offer
   greatest hail threat, both in coverage and size, with the potential
   for significant to giant hail 2-4 inches in diameter (locally/
   potentially larger), along with severe downdrafts.  Some tornado
   threat also can be expected from any mature, relatively discrete
   supercells that move into the richer moist layer of eastern
   OK/northwest AR/southwestern MO/southeastern KS.  

   Forecast soundings suggest higher LCL with southwestward extent
   along the front near projected convection-initiation period mid/late
   afternoon.  Very steep low/middle-level lapse rates exceeding 8 deg
   C/km -- characteristic of the EML -- will overlie a field of dew
   points in the mid 60s to low 70s F.  This should yield 3500-5500
   J/kg MLCAPE as frontal lift breaches a previously stout EML
   inversion.  Rapid to explosive thunderstorm development is possible,
   with extreme updraft speeds, and more than sufficient inflow-layer
   moisture for production of very large hail.  Effective-shear
   magnitudes may range from 45-55 kt around southeast Kansas to 35-45
   kt in southern OK.  Some southward, discrete propagation of storms
   may occur along the front in the southern OK/Red River region, given
   the orientation of the boundary with respect to ambient winds and
   thermal profiles.  Severe potential should diminish with eastward
   extent into the late evening/overnight hours.

   ..Edwards/Broyles.. 05/04/2020

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