SPC AC 301957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Sun Aug 30 2020

   Valid 302000Z - 311200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
   THE CENTRAL AND NORTHERN PLAINS AS WELL AS A PORTION OF THE SOUTHERN
   PLAINS...

   ...SUMMARY...
   Severe thunderstorms capable of very large hail, damaging winds, and
   a tornado or two remain possible from South Dakota into central
   Nebraska, mainly between 4 to 10 PM CDT. Other strong to severe
   storms are expected to develop over a portion of the Texas Panhandle
   later this afternoon and spread southeast into southwest Oklahoma
   and northwest Texas this evening.

   ...Discussion...

   Primary change to previous outlook has been to introduce a SLGT risk
   area for the southeast TX Panhandle into southwest OK and northwest
   TX. A weak low-amplitude shortwave trough over eastern NM will move
   through the southern High Plains this evening. The downstream
   atmosphere is destabilizing with MLCAPE from 1500-2000+ J/kg in
   vicinity of a diffuse east-west boundary. High-based storms will
   probably develop along dryline/lee trough feature in the more deeply
   mixed regime of eastern NM, then intensify as they spread east and
   intercept greater low-level moisture and instability. This region
   resides beneath belt of 30+ kt flow between 500 and 400 mb with
   35-40 kt effective bulk shear. Initial storms will likely be
   discrete with a couple of supercells possible before activity begins
   to consolidate into a small forward-propagating MCS this evening,
   supported by a strengthening low-level jet. Large hail and damaging
   wind are the primary threats.

   Elsewhere, the forecast appears on track with severe storms expected
   to develop over a portion of the central and northern Plains later
   this afternoon.

   ..Dial.. 08/30/2020

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Sun Aug 30 2020/

   ...Eastern Dakotas to northern KS...
   Primary change is to add a significant severe hail area for the most
   likely corridor of supercell initiation between 21-00Z. While
   coverage should remain rather isolated, magnitudes may reach
   baseball size. 

   A shortwave trough over the northern High Plains will progress into
   MN by 12Z Monday. The bulk of strong mid-level west-southwesterlies
   will remain displaced behind a pronounced surface cold front that
   should reach eastern ND to western NE by 21Z. A trough/dryline will
   extend south of this front from south-central SD into northwest KS. 

   A well-defined elevated mixed layer characterized by very steep
   mid-level lapse rates up to 9 C/km was sampled by 12Z area
   soundings. A narrow plume of low to mid 60s surface dew points are
   being advected north beneath the EML and will yield a corridor of
   moderately large MLCAPE from 2000-3000 J/kg, the northern extent of
   which should reach the ABR/HON vicinity. Convective initiation is
   expected around 21Z along the surface trough/dryline as MLCIN
   subsides with a low-level thermal ridge between the dryline and cold
   front. A veering wind profile with height will support 30-40 kt
   effective shear, supportive of at least a few supercells from
   eastern SD into central NE. Initial development will have the
   greatest opportunity to produce very large hail. But weakness in the
   mid to upper portion of the hodograph suggests a tendency for some
   clustering to occur during the evening. Clusters that attempt to
   spread east will encounter a more stable environment east of the
   relatively narrow buoyancy plume suggesting an abrupt demise of the
   severe threat into western MN/IA. Convection should linger longest
   tonight across southern NE into northern KS with an isolated severe
   wind/hail threat.

   ...Southern Great Plains to the TN Valley...
   A pair of MCVs from remnants of MCSs last night into this morning
   were located near southwest KY and west-central AR. These features
   should aid in scattered thunderstorms later this afternoon, mainly
   east of the MS river into the TN Valley. The overall environment in
   this area will be moderately unstable and modestly sheared,
   supporting a few multicell clusters capable of locally damaging
   winds.

   Farther west, isolated thunderstorms are expected near the juncture
   of the dryline and diffuse quasi-stationary front in the central to
   eastern TX Panhandle, with activity spreading east along the Red
   River this evening. An overall modest combination of deep-layer
   shear and mid/upper-level lapse rates should favor an isolated
   severe hail/wind risk. 

   A separate zone of low-level warm advection forced convection should
   develop overnight across AR. Rich tropical boundary-layer moisture
   returning north may compensate for weak nocturnal MLCIN and support
   surface-based effective inflow parcels. An enlarged low-level
   hodograph could foster a low probability risk for all hazards
   centered on central AR.

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