SPC AC 280553

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1253 AM CDT Wed Apr 28 2021

   Valid 281200Z - 291200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE SOUTHERN PLAINS AND LOWER GREAT LAKES REGION...

   ...SUMMARY...
   Severe thunderstorms are possible across parts of the southern
   Plains this afternoon and evening. Other severe storms may develop
   in parts of the lower Great Lakes region. The stronger thunderstorms
   may produce large hail and wind damage. A tornado threat may also
   develop in the southern Plains.

   ...Southern Plains...
   An upper-level low will move eastward from southeast Arizona into
   southwest New Mexico today. Ahead of the system, mid-level flow will
   be southwesterly across much of the southern Plains. At the surface,
   a moist airmass will be in place across much of Texas and Oklahoma.
   Thunderstorms will move east-northeastward across the northern edge
   of the moist airmass this morning and will lay out a west-southwest
   to east-northeast thermal gradient from northwest Texas into central
   Oklahoma. Moderate instability is forecast to develop south of this
   corridor where MLCAPE values are expected to reach the 1500 to 2500
   J/kg range by afternoon. The strongest instability will likely
   develop from northwest Texas into southwest Oklahoma where
   surface-based initiation should take place during the mid to late
   afternoon.

   RAP and NAM forecast soundings late this afternoon near Wichita
   Falls, Texas have supercell wind profiles, with directional shear in
   the low to mid-levels and 0-6 km shear near 60 kt. Supercells should
   be capable of producing large hail and wind damage. A tornado threat
   may also develop during the late afternoon and early evening as the
   low-level jet strengthens. The severe threat is expected to move
   northeastward across central and northeastern Oklahoma during
   evening where a threat for large hail and wind damage will continue.

   Further to the south into southwest Texas, strong instability is
   expected to develop, with MLCAPE values reaching the 2000 to 3000
   J/kg range. Convection should develop in the higher terrain of
   northern Mexico and move eastward into the Del Rio, Texas area
   during the early evening. The strong instability combined with 60 to
   70 kt of deep-layer shear should be favorable for large hail, with
   some hailstones greater than 2 inches in diameter possible. A
   tornado threat may also develop with supercells that move across
   southwest Texas this evening.

   ...Lower Great Lakes...
   A shortwave trough will move eastward across the Great Lakes region
   today. To the southeast of the shortwave trough, a moist airmass
   will be in place from parts of lower Great Lakes region
   southeastward into the central Appalachians. As surface temperatures
   warm today, moderate instability should develop across much of Ohio
   and Pennsylvania. Thunderstorms are forecast to initiate to the east
   of Lake Erie and move east-southward across eastern Ohio and western
   Pennsylvania during the late afternoon and early evening. NAM
   forecast soundings near Pittsburgh, Pennsylvania at 00Z/Thursday
   have MLCAPE reaching 1200 J/kg with 0-6 km shear in the 45 to 50 kt
   range. This combined with 700-500 mb lapse rates near 7.0 C/km will
   likely support supercells with storms that remain discrete. Isolated
   large hail and wind damage could occur with supercells. A
   wind-damage threat also appears likely to develop with any line
   segments that can become organized.

   ...Mid Mississippi Valley...
   An anticyclonic mid-level flow pattern will be in place today across
   much of the east-central U.S. At the surface, a moist airmass will
   be located from the Ozarks eastward into the Ohio Valley. Several
   thunderstorm clusters will likely be ongoing at the start of the
   period on the northern edge of the moist airmass. These clusters
   will move east-northeastward and stabilize the airmass.  Areas that
   are not affected by morning convection should heat up sufficiently,
   allowing for pockets of instability to develop by afternoon.
   Thunderstorms that move east-northeastward near these pockets of
   instability may have a marginal severe threat. Strong wind gusts and
   hail would be possible. Instability should remain weak, keeping any
   severe threat marginal.

   ..Broyles/Moore.. 04/28/2021

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