SPC AC 281250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Sun May 28 2017

   Valid 281300Z - 291200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   SOUTH TEXAS TO PARTS OF CENTRAL ALABAMA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM
   CENTRAL/EASTERN NORTH CAROLINA TO SOUTHEASTERN LOWER MICHIGAN...

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

   ...SUMMARY...
   Severe thunderstorms will be possible from portions of
   southern/central Texas northeastward to the lower Great Lakes and
   Carolinas today.  With these storms, the primary threats will be
   damaging winds and large hail, although a tornado cannot be ruled
   out.

   ...Synopsis...
   Dominant features in the upper-air pattern today will include a
   synoptic-scale cyclone digging southeastward from the MB/ON border
   area toward Lake Superior, and longwave ridge from western MX across
   the Great Basin to the Northwest Territories and beyond.  A
   positively tilted shortwave trough was apparent in moisture-channel
   imagery south of the MB/ON cyclone, from MN to central KS and
   northern NM.  By 00Z, this feature is forecast to move to Lower MI,
   central IL, southern MO, and northern OK.  This perturbation should
   weaken and accelerate eastward thereafter, its vorticity pattern
   fracturing and becoming more nebulous. 

   At the surface, the 11Z analysis showed a warm front over central OH
   to a weak low over north-central IN, then cold front southwestward
   across the Ozarks, southeastern OK, the Edwards Plateau and far west
   TX.  The low should move slowly east-northeastward across
   northwestern OH through 00Z, with the front reaching southern IN,
   central AR, and south-central TX.  By 12Z, the front should be
   located over western NY, western TN and southeast TX, becoming weak
   and quasistationary along much of that stretch.  A prefrontal,
   composite outflow boundary from several convective systems is very
   well-defined this morning from northern GA across central parts of
   MS/AL, northern LA, and parts of east TX.  This boundary should
   decelerate during the next few hours as reinforcing convection
   weakens and moves away.  The boundary may retreat slightly northward
   on the mesoscale through late afternoon.

   ...Lower Mississippi Valley to south TX...
   Considerable stabilization has occurred north of an aggregate of
   outflow boundaries from northern GA to northern LA that may move
   somewhat further south before stalling late morning into midday. 
   Trailing precip/clouds are expected to linger through much of the
   morning, greatly impeding diurnal destabilization with northward
   extent from the outflow edge.  As such, affected parts of the
   earlier outlook area have been reconfigured.  

   Scattered thunderstorms are expected to develop this afternoon near
   the cold front and residual outflow/differential-heating boundaries.
    The main threats will be damaging wind and sporadic large hail. 
   The boundary layer south of the front and outflows will be
   characterized by rich moisture, with mean mixing ratios of 17-19
   g/kg and surface dew points generally in the mid 70s F.  Midlevel
   lapse rates will steepen with westward extent into a progressively
   better-defined EML, combining with strong surface heating this
   afternoon to yield MLCAPE ranging from around 2500 J/kg over
   central/southern AL to around 4500 J/kg in parts of south TX.  Flow
   just south of the combined front/outflow zone, surface and aloft,
   will be nearly parallel to the combined boundary, contributing to
   quick evolution from initial multicellular/marginally supercellular
   phases into a messy, quasi-linear mode with embedded severe
   clusters.  The veered surface winds also will temper both low-level
   and deep shear, with forecast soundings showing effective-shear
   magnitudes less than 35 kt.  As such, multicell modes should
   dominate.  The severe threat will become more fragmented through the
   evening and diminish with time as the near-surface layer slowly
   cools/stabilizes.

   ...Eastern NC to Great Lakes...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon across this swath, offering occasional damaging gusts
   and large hail.  

   Activity will be supported by residual low-level moisture ahead of
   the surface cold front, lift from the front and possibly prefrontal
   convergence lines, large-scale DCVA/destabilization aloft ahead of
   the approaching shortwave trough (in the Great Lakes region), and
   elimination of most MLCINH via surface diabatic heating.  Lapse
   rates aloft will increase with northwestward extent under cooler
   mid/upper level temps, but boundary-layer lapse rates and moisture
   will be greater over and east of the Piedmont.  MLCAPE should peak
   between 500-1000 J/kg on the Great Lakes lobe of the outlook, amidst
   modest effective-shear magnitudes generally less than 35 kt, and
   nearly straight hodographs.  Farther southeast into NC, 1000-2000
   J/kg MLCAPE should prevail, locally higher, with surface dew points
   commonly in the mid 60s to low 70s F.

   ..Edwards/Broyles.. 05/28/2017

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