SPC AC 181630

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1130 AM CDT Tue May 18 2021

   Valid 181630Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS
   SOUTH-CENTRAL/EAST TEXAS TO SOUTHEAST OKLAHOMA/ARKLATEX...

   ...SUMMARY...
   Severe thunderstorms, including the risk for large hail, damaging
   winds, and a couple of tornadoes are expected mainly across
   south-central and east Texas toward the ArkLaTex region this
   afternoon through evening.

   ...Central/east Texas to ArkLaTex region...
   An upper low centered over New Mexico/southern Colorado will
   continue to shift slowly east-southeastward toward the southern High
   Plains. A lingering storm cluster and related outflow persists
   across the Edwards Plateau late this morning, with the outflow
   boundary extending roughly west/east near and south of I-10 to the
   south of the Sonora/Junction areas, and then northeastward toward
   the Hill Country and I-35 corridor of central/north-central Texas.
   With considerable convective overturning across much of
   west-central/northwest into western north Texas last evening/night,
   low-level moisture is maximized at midday across South Texas as well
   as the eastern third of Texas into Louisiana/Arkansas.

   The air mass to the south/east of this modifying boundary will
   remain moist (generally mid 60s F surface dewpoints) and is expected
   to become at least moderately unstable with negligible CINH by
   mid-afternoon. Late-morning visible satellite imagery already shows
   ample cloud breaks to the south/east of the composite outflow.
   Upwards of 2000-3000 J/kg MLCAPE is plausible across
   south-central/east-central/northeast Texas by peak heating.
   Low-level/deep-layer shear will be maximized in proximity to the
   effective boundary and support a mixed mode of supercells and
   organized linear bands/bowing segments capable of damaging winds,
   some hail, as well as a tornado risk. With aid of a strengthening
   low-level jet/warm conveyor toward/after sunset, one or more
   east/northeastward-propagating linear clusters of storms may evolve
   by early evening across east/southeast Texas and the ArkLaTex
   vicinity with a damaging wind and some tornado risk continuing
   through late evening, and possibly part of the overnight.

   ...West/northwest Texas and eastern New Mexico...
   The low-level moist axis, characterized by moderate moisture,
   persists east of the dryline, particularly in closer proximity to
   the Texas portion of the Rio Grande (sampled by 12Z Del Rio observed
   sounding). While low-level moisture and buoyancy will be more modest
   with northward extent (1000-1500 J/kg MLCAPE at most), some
   stronger/locally severe semi-low-topped thunderstorms are expected
   to develop near a surface trough from far eastern New Mexico into
   the Texas/Oklahoma Panhandles and Texas South Plains. Isolated large
   hail should be the most probable severe risk, although some
   severe-caliber winds could occur. A brief tornado (probably via
   non-supercell processes) could occur in vicinity of the surface
   boundary across the northeast New Mexico and west/northwest Texas
   border vicinity.

   ...Gulf Coast including parts of LA/MS/AL...
   Within a moist environment, enhanced low-level flow appears to
   persist via an MCV near/east of a decaying cluster of storms. This
   scenario may support a brief tornado risk this afternoon through
   early evening.

   ..Guyer/Kerr.. 05/18/2021

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