SPC AC 121249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Tue Oct 12 2021

   Valid 121300Z - 131200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS EVENING
   AND TONIGHT...FROM WESTERN/CENTRAL KANSAS TO WESTERN OKLAHOMA AND
   THE EASTERN TEXAS PANHANDLE...

   ...SUMMARY...
   Severe thunderstorm gusts, large hail and tornadoes are possible
   mainly this evening into tonight, across parts of the central and
   southern Plains.

   ...Synopsis...
   The dominant mid/upper-level feature for convective/severe potential
   today will be a synoptic-scale cyclone initially centered over the
   UT/AZ border region.  The associated 500-mb low is forecast to eject
   northeastward through the period, reaching northwestern/north-
   central CO by 00Z, with the trough across south-central/southeastern
   CO to northern/western NM.  By 12Z, the low aloft should move to
   near CDR, with 500-mb trough arching southeastward then
   southwestward over the central plains and southern High Plains.

   At the surface, 11Z analysis showed an area of low pressure over
   central/western CO and eastern UT, with cold front southward across
   western NM and southeastern AZ.  A dryline -- initially developing
   over the lower Pecos Valley and Permian Basin regions of west TX on
   the northwestern rim of substantial Gulf moisture return -- should
   sharpen from south to north today across the TX/OK Panhandles and
   into western KS.  This will occur as low-level moist advection
   occurs out of a source region of 60s to 70s F surface dew points
   initially located south of about an SJT-ACT-SHV line.

   Meanwhile, the primary surface low related to the cyclone aloft
   should reorganize/consolidate and deepen to the lee of the Rockies
   near BFF by 00Z. The cold front by then should arch across eastern
   CO, the western OK Panhandle, northwestern TX Panhandle, and
   southeastern NM.  The front will overtake the dryline rapidly from
   north-south thereafter, as the front advances and dryline retreats
   nocturnally.  By the end of the period, the low should be over the
   SD Badlands, becoming better aligned vertically with the cyclone
   center aloft.  The cold front should arc southeastward over east-
   central NE, south-central KS, western OK, and northwest through far
   west TX.

   ...Central/southern Plains...
   Thunderstorms should develop initially late this afternoon over the
   central High Plains, then as deep-layer forcing for ascent and
   low-level moisture each increase with southward extent after sunset,
   additional convection should form southward over more of western KS
   into the eastern TX Panhandle and western OK.  Supercells are
   possible in the first few hours of convective activity across a
   given mesobeta-scale area -- some discrete or nearly so with large
   to very large hail, severe gusts and tornadoes most probable in a
   corridor from western KS to the eastern Panhandles and western OK.

   A supercell or two may pose a threat for significant (EF2+ damage)
   tornadoes, in addition to very large hail.  This will be of greatest
   concern during a time window this evening defined by:
   1.  Warm-sector hodographs enlarging substantially with the
   influence of an LLJ strengthening to 50-65 kt,
   2.  Moist/warm advection near the surface maintaining surface-based
   effective inflow parcels to partly offset gradual diabatic cooling,
   and
   3.  Convection isn't yet solidly linear and lacking embedded
   supercells.
   Forecast soundings indicate such conditions at least briefly
   collocated with effective SRH of 300-500 J/kg and effective-shear
   magnitudes around 50-60 kt over the hatched areas on the graphics. 
   Even the eventual QLCS will be moving into a high-SRH environment
   over parts of KS and western OK, maintaining tornado potential into
   part of the overnight time frame.

   As the cold front surges out farther eastward and southeastward,
   impinging on increasing boundary-layer moisture, quick transition to
   quasi-linear mode is expected, with damaging gusts becoming the main
   severe mode, isolated hail still possible, and some continued
   tornado threat from embedded supercells, bows and QLCS mesovortices.
   The resulting convective band should outrun the supportive surface-
   based parcels in the moist/warm sector late tonight from north to
   south, as it approaches the Missouri Valley and eastern KS.  Extent
   of both evening linear backbuilding into TX is uncertain, as is
   potential dryline development south of about the latitude of (and
   east of) LBB, but at least marginal unconditional probabilities will
   be maintained for isolated potential.

   ...South TX...
   Some concern also exists for convection to develop in the high
   terrain (Serranias del Burro and vicinity) of northern Coahuila late
   this afternoon or early evening, and subsequently spread east-
   northeastward to northeastward across the adjoining Rio Grande
   Valley, into portions of south TX near severe levels.  At least
   isolated severe hail/gusts are possible in a CAPE/shear parameter
   space favorable for either supercells or upscale-expanding clusters.

   Thick mid/upper-level cloud cover should spread northeastward across
   this region today and especially this afternoon/evening, with
   increasing contribution to that moisture from eastern Pacific
   Hurricane Pamela.  While this should limit intensity of diurnal
   heating, a weak perturbation now apparent in moisture-channel
   imagery north/northeast of Pamela and northern Coahuila may become
   enhanced convectively over western/northern MX today.  This
   perturbation then may move northeastward into the southeastern
   fringe of the favorable mid/upper flow, supplying large-scale lift
   aloft.  Meanwhile, evening dryline retreat and related moist
   advection will lead to increasing low-level theta-e over higher
   terrain, along with weaker MLCINH and storm-initiation potential. 
   Forecast soundings suggest 2000-3000 J/kg preconvective MLCAPE and
   40-45 kt effective-shear magnitude.

   Since this threat still appears uncertain in terms of both coverage
   and storm mode, the unconditional probabilities are kept at marginal
   levels, but expanded to account for the possible spatial extent/
   penetration into TX.  If confidence increases in a relative
   concentration of severe threat with such a regime, an upgrade may be
   needed in a succeeding outlook.

   ..Edwards/Gleason.. 10/12/2021

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