SPC AC 210551

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1251 AM CDT Thu Sep 21 2023

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   CENTRAL NEBRASKA...FAR NORTHERN KANSAS...SOUTHERN OKLAHOMA AND NORTH
   TEXAS...

   ...SUMMARY...
   Thunderstorms, associated with large hail and wind damage, will be
   possible this afternoon and evening across parts of the southern and
   central Plains. An isolated tornado threat, along with a potential
   for very large hail, may develop in parts of southern and central
   Nebraska.

   ...Central Plains...
   A shortwave ridge will move eastward into the mid Missouri Valley
   today, as southwest flow becomes established at mid-levels across
   much of the central Plains. At the surface, a low will deepen in
   northeast Colorado as flow remains backed to the southeast across
   much of the central Plains. A moist airmass will be in place across
   Kansas and Nebraska where surface dewpoints will mostly be in the
   lower to mid 60s F. As surface temperatures warm, a
   southeast-to-northwest corridor of moderate instability is forecast
   to develop from central Kansas to southwest Nebraska. Convective
   initiation will be most likely on the northern end of this corridor,
   ahead of a distinct mid-level vorticity maximum. A cluster of storms
   is expected to organize and move east-northeastward across central
   Nebraska during the late afternoon and early evening.

   RAP forecast soundings by late afternoon along the instability axis
   near McCook, Nebraska have MLCAPE near 2500 J/kg, 0-6 km shear
   around 40 knots and 700-500 mb lapse rates between 8 and 8.5 C/km.
   This will be favorable for supercells with large hail and wind
   damage. The more intense updrafts may be able to produce hailstones
   greater than 2 inches in diameter. In addition, 0-3 storm-relative
   helicity is forecast to be between 300 and 400 m2/s2 during the late
   afternoon, suggesting a tornado threat will be possible. The severe
   threat is expected to persist into the early to mid evening, as the
   low-level jet gradually strengthens across the region.

   Further to the south into parts of western Kansas, convective
   initiation is expected to remain more isolated due to weaker
   large-scale ascent. Although the environment will be slightly less
   favorable further south, any cell that can become sustained could
   have potential for hail and marginally severe gusts.

   ...Southern Plains/Ark-La-Tex...
   Southwesterly flow at mid-levels will be in place today from the
   Desert Southwest into the southern High Plains. A shortwave trough
   is forecast to move eastward across the southern Plains. At the
   surface, a moist airmass will remain from parts of central and east
   Texas extending northward into Oklahoma. As surface temperatures
   warm, scattered thunderstorms are expected to form and move eastward
   across Oklahoma and north Texas. Multiple small clusters of
   thunderstorms will likely become organized in the early to mid
   evening, moving eastward towards the Ark-La-Tex.

   Warming surface temperatures during the day, along with dewpoints in
   the lower to mid 60s F, will result in moderate instability across
   much of north Texas and Oklahoma. MLCAPE is forecast to peak in the
   2000 to 2500 J/kg range over much of this airmass by late afternoon.
   This, combined with about 40 knots of 0-6 km shear, evident on
   forecast soundings, will likely be favorable for supercell
   development. Supercells will have potential for isolated large hail
   and wind damage.

   Further west into parts of the eastern New Mexico and west Texas,
   some models suggest that moderate instability will be in place by
   late afternoon. Although thunderstorm development is expected to
   remain isolated, moderate deep-layer shear and steep mid-level lapse
   rates could support a marginal severe threat during the late
   afternoon and early evening.

   ..Broyles/Bentley.. 09/21/2023

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