SPC AC 201246

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0746 AM CDT Wed Sep 20 2023

   Valid 201300Z - 211200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   EASTERN OKLAHOMA...WESTERN ARKANSAS AND NORTHEAST TEXAS...

   ...SUMMARY...
   Severe hail and gusts will be possible across parts of eastern
   Oklahoma, western Arkansas and northeast Texas late this afternoon
   and evening.

   ...Synopsis...
   In mid/upper levels, a strong shortwave trough -- now apparent in
   moisture-channel imagery over southern BC and northern WA -- is
   forecast to strengthen to a closed cyclone today and dig south-
   southeastward.  By 00Z, the associated 500-mb low should be near
   PDT, along a trough extending from northern CA to central/southern
   AB.  By the end of the period, the low aloft will deepen and
   decelerate over eastern OR, while associated height falls and
   cyclonic flow spread over most of the CONUS from the Rockies
   westward.  That expansive cyclonic pattern will absorb an initially
   separate southern-stream cyclone now over the south-central CA
   coastline.  By 12Z, this feature should be a rapidly weakening
   shortwave trough ejecting northeastward over the Four Corners
   region.

   Downstream from these processes, ridging aloft will shift eastward
   from the southern/central Rockies to the southern/central Great
   Plains States, with an anticyclonically bent, upper-level,
   subtropical jet core shifting eastward across NM, the southern High
   Plains, parts of north TX and OK, and the central Gulf Coast region.
   To its north, and east of the mid/upper ridge, strongly difluent
   flow will exist over parts of the central Plains, OK, and the
   Mississippi Valley.  Embedded in that regime, radar and satellite
   imagery depict a well-defined MCV over southern MO, with shortwave
   trough southwestward across eastern OK to northeast TX.  This
   feature should move slowly northeastward to eastward, extending from
   the STL area southward over parts of the Mid-South by 00Z, while a
   smaller vorticity lobe just upstream blends with the leading
   perturbation.  A subtle shortwave trough now over parts of CO and
   northern NM will cross the south-central High Plains today, with its
   weak southern fringes over central OK by 00Z.  This trough --
   perhaps with some convective vorticity enhancement on the southern
   end -- should reach eastern KS, southwestern MO, and central AR by
   12Z.

   The 11Z surface analysis showed a wavy, diffuse warm front over the
   Arklatex region and southern OK, to a low between SPS-CDS, with a
   slow-moving cold front southwestward from there over southern NM. 
   The low should weaken through the day as lee troughing intensifies
   farther west across the central/southern High Plains, where a
   dryline also should settle by early/mid afternoon.  The frontal zone
   east of the low will be reinforced near the Red River today, by
   outflow and differential heating related to morning clouds/precip.

   ...OK/AR/northeast TX...
   Ongoing nonsevere convection and associated cloud cover over eastern
   OK and northeast TX are expected to shift eastward across southeast
   OK, AR and parts of northeast TX through early afternoon, in step
   with the progress of the leading shortwave trough.  Although
   isolated, marginally severe hail cannot be ruled out with this
   activity, given favorable elevated deep-shear profiles and buoyancy,
   organized severe potential appears very low.

   Thunderstorms are possible this afternoon behind that activity, with
   potential for isolated large hail and strong-severe gusts.  However,
   both spatial and temporal offsets of most-favorable parameters make
   severe potential marginal and conditional.  In a regime of northwest
   flow aloft and backed near-surface winds in the moist, diurnally
   destabilizing boundary layer, strong low/middle-level veering with
   height is expected.  Deep shear, and especially cloud-layer shear
   beneath the north rim of the subtropical jet, will be favorable. 
   However weakening low/middle-level speeds from midday through
   afternoon will foster shrinking (though still supercell-supporting)
   hodographs.  Low-level shear will strengthen again this evening over
   the outlook area as a LLJ develops, conterminous with nocturnal
   cooling/stabilization of the boundary layer.  The most heated/
   unstable airmass, and weakest MLCINH, may be over southern parts of
   the outlook area, where any convection that can develop along
   residual boundaries may move south-southeastward to southward over
   northeast TX, and into 1000-1500 J/kg MLCAPE.

   Heating and a narrow corridor of similar or perhaps slightly
   stronger destabilization are possible near the western rim of the
   outlook area; however, height rises aloft and capping will limit
   initiation.  This, along with concerns over magnitude and duration
   of airmass recovery behind ongoing areas of clouds/convection, cast
   considerable uncertainty on afternoon convective coverage,
   especially near and north of I-40.  Overnight, elevated thunderstorm
   potential (with isolated hail possible) exists amid steep midlevel
   lapse rates atop the old outflow plume, across parts of eastern OK
   and western AR.

   ..Edwards/Gleason.. 09/20/2023

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