SPC AC 311252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Mon Aug 31 2020

   Valid 311300Z - 011200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   SOUTHERN PLAINS...OZARKS AND MID-SOUTH...

   ...SUMMARY...
   Severe thunderstorms are possible Monday over parts of the southern
   Plains, Ozarks and Mid-South.  Other strong storms may occur from
   the central Carolinas into western Virginia.

   ...Synopsis...
   A progressive and somewhat amplified northern-stream pattern will
   continue through the period, with greatest influence from two strong
   shortwave troughs:
   1.  A leading perturbation -- initially located over the eastern
   Dakotas near the MN line -- is forecast to eject across Lake
   Superior by 00Z then over James Bay overnight.
   2.  A subtle trough and upper speed max - now evident in
   moisture-channel imagery over southern BC and the interior Pacific
   Northwest -- will strengthen considerably as it moves southeastward
   over southern ID and far northern NV by 00Z.  By 12Z tomorrow, this
   trough should be located from southern WY across eastern UT to
   northwestern AZ.  In response, the flow aloft to its southeast, over
   the southern Plains and Ozarks, gradually should pivot from mostly
   westerly to mostly southwesterly through the period. 

   Farther south, a series of mostly low-amplitude vorticity lobes and
   shortwaves -- some convectively induced/enhanced -- will traverse
   that southern Plains/Ozarks flow field.  One is apparent in
   moisture-channel imagery over extreme western OK, while a somewhat
   larger perturbation extends meridionally across eastern AZ. 

   At the surface, 11Z analysis depicted a wavy/quasistationary frontal
   zone across northern NC into southwestern VA, then a warm front
   west-northwestward to a triple-point intersection with a cold front
   over southwestern IA. The cold front was drawn from there
   southwestward over northwestern OK and the TX Panhandle.  The cold
   front will proceed southeastward to eastern KS, central/southwestern
   OK and southeastern NM by 00Z, then decelerate.  By 12Z the front
   should extend from a weak low over southeastern KS to the Permian
   Basin region and southern NM. The Carolinas/southern Appalachians
   portion of the other frontal zone will move little. 

   ...MO to southern Plains...
   A complex convective evolution is likely through the period across
   the outlook area, which represents the spatial overlap of multiple
   episodes.  The ongoing activity across western AR may persist into
   midday with the threat for severe gusts and isolated hail across AR;
   see severe thunderstorm watch 477 and related mesoscale discussions
   for details.  Another complex may evolve upscale and move
   southeastward ahead of the front, from ongoing, initially nonsevere
   convection over northwestern MO, but that remains uncertain at this
   time, having been mostly poorly handled so far by
   convection-allowing and synoptic models.  

   This afternoon, widely scattered to scattered thunderstorms may
   develop along/ahead of the cold front as it proceeds southeastward
   into southeastern KS and northern to southwestern OK.  Areas where
   the front overtakes boundaries from earlier convection, and where it
   impinges on pockets of relatively undisturbed, well-heated, and
   high-theta-e boundary layer, will experience locally maximized lift
   with greatest diurnal severe wind/large- to very large-hail
   potential.  Though low-level lapse-rate and deep-CAPE fields may be
   somewhat fragmented across the area as a result of assorted
   convective/cloud-cover processes, rich moisture is expected away
   from deeper cold pools.  Peak MLCAPE in the 3000-4000 J/kg range is
   possible, with areas of 1500-2500 J/kg being common.  

   Though low-level flow generally will be weak, favorable mid/upper
   winds and a deep troposphere (thereby rendering a deep buoyant
   profile) will contribute to effective-shear magnitudes around 30-40
   kt over much of the area.  This environment will support organized
   multicells and a few messy/heavy-precip supercells, with additional
   upscale clustering and forward propagation of the resulting MCS
   possible.  Given the somewhat steep midlevel lapse rates and
   favorable shear aloft, marginal hail also may occur from cells
   developing in elevated warm-advection layers above cold pools.

   ...Carolinas/VA...
   A persistent low-level confluence/convergence zone should remain
   essentially in place across western portions of the Carolinas and
   western VA, supporting episodes of thunderstorms with the potential
   for sporadic damaging to marginally severe gusts.  A tornado cannot
   be ruled out, though this threat appears marginal/conditional.  An
   extensive area of clouds, precip and isolated embedded thunderstorms
   is ongoing across parts of the outlook in NC/VA/WV.  This activity
   will impede the pace and magnitude of surface heating through the
   day.  Favorable buoyancy is more in doubt with northward extent
   where low-level and deep shear (and also the stabilizing influence
   of the frontal zone) will be stronger.  In the preconvective warm
   sector, weaker low/middle-level winds will limit shear, but strong
   heating and richer low-level moisture will contribute to MLCAPE
   potentially reaching the 1500-2000 J/kg range.

   ..Edwards/Kerr.. 08/31/2020

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z