SPC AC 281741

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1241 PM CDT Fri Apr 28 2017

   Valid 291200Z - 301200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
   TEXAS AND EASTERN OKLAHOMA NORTHEAST INTO THE ARKLATEX AND OZARKS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM CENTRAL TEXAS
   NORTHEAST INTO THE MIDDLE MISSISSIPPI AND OHIO VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK...

   ...SUMMARY...
   Severe thunderstorms are expected from parts of central Texas
   northeastward to portions of the Ohio Valley Saturday into Saturday
   night. These storms will be capable of large hail, damaging winds,
   and a few tornadoes.

   ...Synopsis...
   Midday Friday water-vapor imagery shows an amplifying mid-level
   trough over the western U.S. while a ridge builds downstream over
   the Southeast coast.  A mid-level low will develop over the Four
   Corners and move east into the TX Panhandle/western OK by early
   Sunday morning.  In the low levels, a surface low over the southern
   Plains will slowly move northeast into the Ozarks while a stationary
   frontal zone ---location likely modulated by thunderstorm outflow
   prior to and during the day 2 period--- is draped east-northeast
   from central OK through MO and into the OH Valley.  An arctic cold
   front will push southward into the Permian Basin during the morning
   before it and a Pacific cold front combine and accelerate southeast
   across TX and into the lower Sabine Valley by daybreak Sunday
   coincident with the upper trough pivoting east.  

   ...TX and OK into the ArkLaTex...
   Strong low-level southerly flow will advect a maritime tropical
   airmass ---featuring lowest 100-mb mean mixing ratios (at or above
   16 g/kg) and translating to 70-74 degree F surface dewpoints--- in
   much of the warm sector across the ArkLaTex region and northeastward
   into northern portions of the lower MS Valley.  Ongoing
   thunderstorms are likely across portions of OK near the terminus of
   a 60-kt LLJ as the cold front impinges on the warm sector.  Isolated
   severe hail and severe gusts are possible with this initial
   activity.  An elevated mixed layer and capping inversion will allow
   a very unstable boundary layer (MLCAPE 2500-4000 J/kg) to develop
   south of convective outflow and cloud debris across southeast OK/AR
   and into eastern and central portions of TX.  Models indicate storm
   development may gradually zipper southwest into north-central TX
   during the afternoon in association with a couple of storm clusters.
   It appears the more substantial severe risk will be timed with the
   eastward ejection of a strong mid-level vorticity maximum into the
   southern Plains during the late afternoon and through the overnight.
    Less probable and more uncertain is the potential development for
   quasi-discrete storms developing ahead of the an extensive MCS
   forecast to develop on the front.  Any sustained and robust
   supercells that manage to develop in the moisture-rich environment
   with a sizable hodograph may be associated with a risk for a
   tornado.  As the mid-level wave pivots into the region,
   strengthening effective shear profiles from 30-kt to 45-60 kt will
   become more favorable a mature squall line pushing eastward across
   this region and moving into AR and LA late.  The threat for severe
   gusts and associated wind damage will become more widespread
   primarily after dark centered on the ArkLaTex region.

   ...MO eastward into the OH Valley...
   Several strong to severe thunderstorm clusters are forecast Saturday
   morning in the vicinity of the frontal zone from eastern KS/MO into
   the OH Valley.  Hail and damaging winds will be the primary threats
   with the early-day activity.  Additional storms are forecast owing
   to strong low-level warm/moist advection on the nose of a
   south-central U.S. LLJ.  Forecast soundings show moderate buoyancy
   developing within an area characterized as supportive of organized
   storm structures.  Weak upper-level flow will tend to result in
   messy storm modes and a probable wavy squall line capable of
   primarily of a damaging-wind threat and perhaps a tornado risk with
   more pronounced mesovortices across MO eastward into the lower OH
   Valley.  Farther east, models suggest sparse storm coverage from OH
   east into PA after morning storms but isolated severe storms are
   possible during the afternoon/early evening owing to a conditionally
   supportive buoyancy/shear setup.

   ..Smith.. 04/28/2017

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