SPC AC 040625

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1225 AM CST Sat Dec 04 2021

   Valid 051200Z - 061200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   COASTAL/EAST TEXAS TO THE MID-SOUTH AND LOWER OHIO VALLEY...

   ...SUMMARY...
   Isolated strong to severe thunderstorms appear possible mainly
   Sunday evening and overnight from parts of coastal/east Texas to the
   Mid-South and lower Ohio River Valley.

   ...Synopsis...
   An upper trough initially over the north-central CONUS will likely
   undergo substantial amplification on Sunday as it digs
   east-southeastward across much of the Midwest and Great Lakes. A
   weak upper low over the Baja California vicinity is forecast to move
   little through the period. Still, low-amplitude perturbations within
   the southern branch of the mid-level westerlies downstream from the
   upper low may also aid the amplification of the large-scale upper
   trough over the lower/mid MS Valley by Sunday night. A surface low
   initially over the Dakotas Sunday morning will develop eastward
   towards the Upper Great Lakes by late Sunday night. An attendant
   cold front is forecast to sweep east-southeastward over much of the
   Plains, Midwest, and mid MS Valley through the period.

   ...Coastal/East Texas to the Mid-South and Lower Ohio River
   Valley...
   Modest low-level moisture emanating from the western Gulf of Mexico
   should return northward through the day across much of the southern
   Plains, lower MS Valley, Mid-South, and perhaps a small part of the
   lower OH Valley ahead of the advancing cold front. Various model
   forecast soundings across this region suggest this moisture will
   likely remain fairly shallow away from the Gulf Coast. Residual
   low-level capping will probably tend to inhibit robust convective
   development through much of the day across the warm sector.

   Still, thunderstorm initiation looks increasingly likely by Sunday
   evening along or just ahead of the front from parts of eastern OK to
   the Mid-South vicinity as ascent associated from the amplifying
   upper trough overspreads these areas. Regardless, MLCAPE should
   remain fairly weak, generally in the 250-1000 J/kg range, with mid
   to upper 50s and lower 60s surface dewpoints prevalent. But, both
   low-level and deep-layer shear are forecast to strengthen quickly
   Sunday evening and through the overnight hours with the approach of
   the upper trough. This should foster some convective organization,
   and isolated strong to damaging winds may occur with thunderstorms
   that form along the front and quickly become linear while moving
   east-southeastward. Given the expected strength of the
   boundary-layer flow and favorable low-level shear as a southwesterly
   low-level jet strengthens through the evening, a tornado or two also
   appears possible. The lack of stronger forecast instability
   currently limits confidence in the potential for a more robust
   severe threat.

   There also appears to be some chance for open warm-sector
   development from parts of east/coastal TX and southeastern OK into
   AR and western LA Sunday evening/night. Steepened mid-level lapse
   rates and slightly greater low-level moisture across these areas
   should support MUCAPE up to 1000-2000 J/kg. Isolated severe hail may
   occur with any discrete storms that can form given the strong
   effective bulk shear, in addition to gusty downdraft winds and
   perhaps a tornado. The convection that develops farther north along
   the front should outpace the modest low-level moisture return late
   Sunday night into early Monday morning, especially with eastward
   extent across the Mid-South into the TN and OH Valleys. Accordingly,
   these thunderstorms should gradually weaken in this time frame.

   ..Gleason.. 12/04/2021

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