SPC AC 121946

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0246 PM CDT Tue Oct 12 2021

   Valid 122000Z - 131200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS NORTHEAST TEXAS
   PANHANDLE...FAR NORTHWEST OKLAHOMA AND THE EASTERN OKLAHOMA
   PANHANDLE...AND SOUTHWEST KANSAS...

   ...SUMMARY...
   Scattered to numerous severe thunderstorms are expected to begin
   late this afternoon, peak this evening, and continue into tonight
   across parts of the central and southern Great Plains. Several
   tornadoes, some of which may be strong, along with destructive hail
   and wind are possible.

   ...Discussion...
   Forecast thinking from the previous outlook is on track regarding
   the overall scenario, and confidence has increased for several
   long-track supercells capable of a couple strong and perhaps an
   intense tornado this evening/tonight.  As a result, this outlook
   update is introducing a 15-percent tornado probability and an
   associated Moderate Risk for portions of the northeast TX Panhandle
   and far northwest OK, and areas to the north-northwest into
   southwest KS.  Storm-simulated model guidance continues to show
   quasi-discrete convective mode with storms that develop initially
   near the dryline and move east into richer low-level moisture and
   high SRH this evening.  Forecast soundings strongly favor discrete
   supercells and long-track, fast-moving supercells appear likely to
   traverse across the Moderate Risk.  Linear forcing and slightly
   drier/less unstable air will probably favor upscale growth into a
   band of storms in northwest KS and northeast CO, limiting the
   tornado risk.  Have adjusted the southeastern bound of the
   10-percent tornado area to be farther northwest and mainly be
   confined to western and northwestern OK.

   ..Smith.. 10/12/2021

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Tue Oct 12 2021/

   ...Central/southern Great Plains...
   Thunderstorms should initially develop during the late
   afternoon/early evening over the central High Plains. 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. Numerous supercells are expected, some discrete or
   nearly so with significantly large hail, severe gusts, and tornadoes
   most probable in a corridor from western KS to the eastern
   Panhandles and western OK.

   A couple supercells may pose a threat for significant tornadoes. 
   This will be of greatest concern during a time window around 01-05Z.
   Warm-sector hodographs will be enlarging substantially with the
   influence of a LLJ strengthening to 50-65 kt. The leading edge of
   the rich boundary-layer moisture plume (characterized by mid 60s
   surface dew points) has reached the Permian Basin/Big Country/Low
   Rolling Plains regions of west to northwest TX. This moisture plume
   will continue to advect north, likely spreading into southern KS by
   later this evening. Guidance has spatiotemporal timing differences
   which should be crucial to the degree of surface-based CAPE/CIN this
   evening into tonight within the most favored area for significant
   tornado potential. Recent RAP runs are consistently more moist,
   while the 12Z NAM is consistently slower and colder especially
   compared to the RAP and even the 00Z ECMWF. As such, NAM soundings
   indicate more neutral near-surface lapse rates until late evening
   and the overnight. However, 16Z surface observations suggest
   warmer/moister surface conditions are running ahead of schedule
   across southwest KS through the TX Panhandle. An upgrade to a
   tornado-driven Moderate Risk may be needed in later outlooks should
   this trend continue given the spread in guidance with regard to
   near-surface thermodynamics and longevity of discrete supercell
   structures.

   As the cold front surges out farther east and southeast, impinging
   on the increasing boundary-layer moisture, a transition to mostly
   linear mode is expected from north to south with damaging gusts
   becoming the main severe mode. Some continued tornado threat should
   persist overnight in parts of KS/OK from embedded supercells, bows,
   and QLCS mesovortices. The resulting convective band should outpace
   the supportive surface-based parcels in the moist/warm sector
   overnight 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...
   Isolated severe thunderstorms will be possible this afternoon into
   tonight, but mostly anticipated this evening. A series of weak
   perturbations emanating northeast from eastern Pacific Hurricane
   Pamela should aid in multiple bouts of convective potential through
   the period. While the low-level forcing mechanisms should remain
   nebulous and mid-level lapse rates will not be as steep as a typical
   severe setup, very rich boundary-layer moisture will remain
   prevalent and maintain a broad plume of 2000-3000 J/kg MLCAPE
   through dusk, before waning tonight. Effective bulk shear will be
   adequate for a conditional supercell risk, perhaps somewhat more
   favorable in the Edwards Plateau region this evening. If confidence
   increases in a relative concentration of severe threat within this
   regime, an upgrade may be needed in a succeeding outlook.

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