SPC AC 191244

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0744 AM CDT Tue Sep 19 2023

   Valid 191300Z - 201200Z

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

   ...SUMMARY...
   Severe thunderstorm gusts and large hail are possible today and
   tonight across portions of the southern Plains.

   ...Synopsis...
   In mid/upper levels, amplification of the pattern is forecast over
   parts of the northwestern and north-central CONUS later today into
   day-2, related substantially to a strong shortwave trough and
   embedded 500-mb cyclone now over the AK Panhandle and adjacent
   coastal waters.  This trough should dig southeastward to
   southwestern AB, southern BC and WA by 12Z tomorrow, while a
   downstream ridge builds across parts of the upper Mississippi Valley
   into ON.  As this occurs, initially zonal but increasingly difluent,
   westerly to northwesterly flow will prevail downstream to the
   trough's southeast, over the central/southern parts of the Rockies
   and Plains.

   Within the increasing difluence, a shortwave trough now over parts
   of western KS to the TX Panhandle will shift eastward to eastern
   portions of KS/OK by 00Z.  This feature may decelerate in the
   difluent regime tonight over the Ozarks to Arklatex.  Following
   that, just upstream, a smaller perturbation is evident in moisture-
   channel imagery over southeastern CO and northeastern NM.  This
   trough should track eastward to parts of western/central KS and
   northern/central OK through 00Z, then weaken eastward in favor of
   convectively driven vorticity processes, and perhaps merge with the
   lead wave late in the period.

   At the surface, 11Z analysis showed a cold front over south-central
   FL, becoming quasistationary over the north-central Gulf, and a warm
   front northwestward across southeast to north-central TX.  This
   boundary is expected to move slowly northward/northeastward over the
   east TX/eastern OK region today, while a dryline develops across
   western KS, western OK, northwest TX, to the Big Bend area.  A
   pseudo-warm-frontal zone of relatively backed near-surface flow may
   develop over parts of OK, in the western/modifying part of outflow
   air and behind morning convection, encouraged by isallobaric forcing
   related to weak cyclogenesis along the dryline.  There is
   uncertainty on this process -- depending on mesobeta-scale
   thermodynamic recovery and kinematic response behind morning
   convection.

   ...Southern Plains...
   Widely scattered to scattered showers and thunderstorms are ongoing
   across portions of northwest Texas, northward and northeastward over
   parts of OK, in a zone of large-scale ascent (warm advection and
   DCVA preceding the leading mid/upper perturbation.  This activity
   should remain largely nonsevere as it shifts eastward through the
   remainder of the morning, though an isolated, marginally severe
   instance of hail cannot be ruled out.

   Convective and severe potential should ramp up this afternoon as the
   boundary layer diurnally/diabatically destabilizes south of the
   morning clouds/convection (across parts of southern OK and north
   TX), southwest of it atop a strongly heated/mixed boundary layer
   near the dryline, and perhaps behind it over western/central OK late
   this afternoon.  The latter potential is more uncertain and
   conditional, given time needed for both direct heating/
   destabilization and warm advection in the recovering low-level air
   mass immediately preceding the second mid/upper-level perturbation.

   Where convection can develop over northwest TX and OK, especially
   near any dryline/outflow intersection, supercells are possible amid
   strong veering of winds with height in low/middle levels, yielding
   well-curved and favorably enlarged hodographs.  Development appears
   best focused near the dryline over the southwestern OK/northwest TX
   area, with sustained heating and MLCINH erosion likely, but less
   low-level shear.  Conversely, coverage is more uncertain in and
   north of the backed-flow regime, though hodographs will be more
   favorable.  As such, too much mesoscale uncertainty lingers to
   assign a more-specific, focused area of higher unconditional
   probabilities at this time.

   Forecast soundings near the trailing dryline depict small 0-1-km SRH
   and shear due to uniformity of winds in that layer, however, 200-400
   J/kg effective SRH and 40-45-kt effective-shear magnitudes should be
   common (with larger values northward).  A corridor of mid 50s to
   lower 60s F surface dewpoints -- even after diurnal mixing -- should
   combine with steep low/middle-level lapse rates to yield 1500-2500
   J/kg MLCAPE, in a northward-narrowing moist-sector corridor.  In
   addition to severe downdrafts encouraged by the deep mixed layer,
   isolated significant (2+ inch) hail appears possible given the
   supercell-favoring shear, the shape/depth of the buoyant profile,
   sounding analogs, and peak size given by 2D hail models applied to
   progged soundings.  Some evening/nocturnal clustering of convection
   also is possible, locally boosting wind potential for a couple
   hours.  However, given aforementioned mesoscale uncertainties, great
   spread understandably appears among convection-allowing guidance on
   where and how intensely any such wind threat will organize.

   ..Edwards.. 09/19/2023

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