SPC AC 081938

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0238 PM CDT Tue Sep 08 2020

   Valid 082000Z - 091200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF WEST TEXAS...AND ALONG A
   CORRIDOR FROM NORTHEASTERN OKLAHOMA THROUGH SOUTH CENTRAL MISSOURI
   AND ILLINOIS...

   ...SUMMARY...
   A few strong to locally severe thunderstorms could occur this
   afternoon and early evening across parts of west Texas, and from
   northeastern Oklahoma to central Illinois.

   ...20Z Outlook Update...
   Some mostly minor changes to categorical and probabilistic lines
   have been made, generally to account for the progression of synoptic
   and sub-synoptic features, and ongoing trends concerning
   destabilization.

   Across the south Atlantic Seaboard, the western periphery of a broad
   plume of seasonably high precipitable water is in the process of
   gradually advecting across and inland of coastal areas.  However,
   dry air associated with surface ridging extending along an axis to
   the east of the Appalachians is maintaining generally stable
   boundary-layer conditions across the Piedmont into the higher
   terrain.  Although forcing for ascent along westward advancing
   convective outflow has maintained a narrow line of weak thunderstorm
   activity across the North Carolina coastal plain, it seems probable
   that this activity will weaken as it encounters more stable air west
   of the Raleigh and Fayetteville NC areas.

   Across the Southwest, the categorical thunderstorm line has been
   adjust southeastward across parts of northern and central Arizona,
   ahead of the leading edge of the stronger mid-level cooling
   associated with the digging upstream closed low.

   ..Kerr.. 09/08/2020

   .PREV DISCUSSION... /ISSUED 1128 AM CDT Tue Sep 08 2020/

   ...Southern Plains the mid MS Valley this afternoon/evening...
   A midlevel trough is beginning to evolve into a closed low over UT,
   with only slow southward movement expected through early Wednesday. 
   East of the midlevel low, a cold air damming scenario is underway
   across the southern High Plains, while the cold front is moving more
   slowly to the south from southeastern KS into MO/IL.  Boundary-layer
   dewpoints in the mid 60s to lower 70s are present south of the front
   from west TX to MO, which will combine with afternoon temperatures
   of 85-90 F to result in moderate buoyancy (MLCAPE of 1500-2500 J/kg)
   and little convective inhibition.

   Scattered thunderstorm development is expected along and to the cool
   side of the front by mid-late afternoon.  Faster movement of the
   front across west TX and more southerly flow aloft both suggest that
   any frontal convection will quickly become elevated to the cool side
   of the boundary across west TX, aside from convection forming this
   afternoon over the higher terrain in southwest TX.  Slower frontal
   movement and weaker flow support convection remaining closer to the
   surface boundary from MO northeastward, with a transition area
   across OK.  Vertical shear in the warm sector will be relatively
   weak, which in combination with steep low-level lapse rates and
   DCAPE greater than 1000 J/kg will primarily favor multicell clusters
   capable of producing isolated downbursts from about 20-01z.

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