SPC AC 161957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Sun Aug 16 2020

   Valid 162000Z - 171200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE CENTRAL AND SOUTHERN PLAINS...

   ...SUMMARY...
   Scattered severe storms with large hail and damaging winds, some of
   which could be significant, will be most likely through this evening
   from eastern Nebraska to the central High Plains. Additional
   isolated severe storms may continue into this evening across the
   eastern Texas Panhandle as well.

   ...OK/TX Panhandle vicinity...

   The only change to the outlook at 20z is to extend the slight risk
   southward through the eastern TX Panhandle. A cluster of supercells
   is currently slowly shifting southward across the area. Strong
   instability coupled with steep lapse rates and modest effective
   shear should maintain this activity over the next few hours. Large
   hail and occasional strong gusts could accompany these storms. While
   large hail and strong gusts will be the main threat, a tornado is
   possible. Low level easterly flow is contributing to weak effective
   SRH around 50-100 m2/s2. Forecast RAP soundings also show curved,
   albeit small, low level hodographs. Furthermore, surface dewpoints
   in the upper 60s also will contribute to somewhat lower LCLs on the
   moist side of the surface trough/dryline, and a brief tornado cannot
   be ruled out. 

   Another round of storms could also move across portions of the TX
   Panhandle late this evening or overnight related to convection
   developing further north over eastern CO and western KS. If this
   activity remains organized as it moves into the Panhandle later this
   evening, some gusty wind potential will exist.

   ..Leitman.. 08/16/2020

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Sun Aug 16 2020/

   ...NE/KS/eastern CO...
   Ongoing elevated convection from eastern KS to northwest OK is
   expected to largely demarcate the southeast extent of the organized
   severe risk later today. A rich low-level moisture plume arcs around
   this activity from the TX/OK Panhandles northward into central NE.
   This plume resides beneath the eastern extent of steep mid-level
   lapse rates from an elevated mixed-layer around the eastern
   periphery of the stout anticyclone over the southern Great Basin.
   Strong to extreme buoyancy (MLCAPE of 3000-4500 J/kg) is expected by
   peak heating afternoon from south-central NE to western KS. 

   A weak reinforcing cold front sagging south across NE and a lee
   trough in eastern CO should be foci for scattered late afternoon
   thunderstorms. The region will lie in the gradient between stronger
   mid-level northwesterlies over the Upper Midwest and weak flow over
   the Rockies. Sufficient deep-layer shear should exist given the
   robust thermodynamic environment to support supercells, at least a
   couple of which should produce very large hail. Storms that form on
   the boundary in NE should develop south or southwestward through
   this evening, with some potential for upscale growth into a small
   MCS across western KS with a risk for isolated significant severe
   gusts. Further cell/cluster mergers may sustain an isolated severe
   wind/hail threat lingering through tonight.

   ...Southern Great Plains to Lower MS Valley...
   A plume of mid 70s surface dew points and hot temperatures warming
   well through the 90s will fuel an extremely unstable air mass across
   southern OK into the ArkLaMiss ahead of a weak cold front. While
   guidance varies greatly in the degree of convective development
   along the weakly convergent boundary this afternoon, scattered
   thunderstorms are possible. Deep-layer wind fields will be weak
   characterized by 10-20 kts at 500-mb in the late afternoon. But
   given the northerly component to the flow and the very large MLCAPE,
   the setup may yield loosely-organized multicell clusters with
   isolated damaging/severe outflow gusts and hail possible across a
   broad region of the south-central states.

   For further short-term discussion regarding the threat in OK, please
   see MCD 1508.

   ...Northern CA to OR...
   Scattered elevated thunderstorms within mid-level moisture advection
   around the southern Great Basin anticyclone are ongoing coastal
   central CA into the northern Sacramento Valley. A leading MCV
   appears to have developed and is advertised by morning model
   guidance to rapidly translate north across northern CA into central
   OR through this evening. The convection should eventually root into
   the boundary layer later this afternoon in response to steepening
   low-level lapse rates. The storms may produce isolated severe gusts
   given the steep lapse rate profiles and potential for downward
   momentum transfer of 35-50 kt mid-level southwesterlies. Meager
   buoyancy should be a limiting factor to a greater severe risk.

   ...Southern AZ...
   An MCV over the northern Gulf of CA in the wake of convection that
   decayed overnight in Sonora will translate west away from the region
   today. Latest water vapor imagery is suggestive of weak
   mid/upper-level subsidence upstream. Nevertheless, PW values
   initially from 1.25-1.5 inches as sampled in 12Z Tucson and Phoenix
   soundings should be sufficient for isolated late afternoon
   thunderstorms over the higher terrain. 10-20 kt mid-level easterlies
   around the southern Great Basin anticyclone should be adequate for a
   few cells and a brief cluster to spread west-southwest towards the
   lower deserts with an accompanying threat of isolated strong/severe
   outflow gusts.

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