SPC AC 040830

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0230 AM CST Sat Mar 04 2017

   Valid 061200Z - 071200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   THE UPPER/MIDDLE MISSISSIPPI VALLEY TO THE OZARKS AND ARK-LA-TEX...

   ...SUMMARY...
   Isolated severe storms will be possible from Monday evening through
   early Tuesday morning from parts of the upper and middle Mississippi
   Valley southwestward to the Ozarks and the Ark-La-Tex.

   ...Synopsis...
   The general synoptic evolution remains fairly consistent compared to
   previous day's forecasts, with a deep mid-upper trough expected to
   progress eastward from the Rockies to the central/northern Plains. 
   An associated surface cyclone is progged to move northeastward from
   the eastern Dakotas to western Ontario and deepen, as a trailing
   surface cold front translates eastward across the upper MS Valley,
   and southeastward into the Ozarks/southern Plains.  The cold front
   should be the main focus for thunderstorm development late Monday
   afternoon through Monday night and Tuesday morning.

   ...Upper MS Valley to the Ozarks and Ark-La-Tex late Monday...
   The 00z deterministic runs of the ECMWF, GFS, and NAM show variation
   in the cold front placement ranging from about 100 miles across
   IA/MN, and about 50 miles east-west variation in the dryline
   placement around I-35 in OK valid at 07/00z.  Most of the convection
   should form initially to the northeast of the cold front/dryline
   triple point in Kansas, where deep-layer forcing for ascent will be
   stronger in proximity to the mid-upper jet core and height falls. 
   However, low-level moisture and buoyancy will weaken with
   northeastward extent along the front, such that linear forcing for
   ascent and a narrow buoyancy corridor should favor a band of
   convection along the front.  The relatively weak buoyancy but strong
   vertical shear environment will tend to favor isolated damaging
   gusts as the main risk.

   Farther south, the potential range in low-level moisture and surface
   warming is substantial within the moist sector into OK.  As is
   typical, the NAM represents the high end with MLCAPE near 2000 J/kg
   with surface temperatures in the mid 70s and dewpoints in the mid
   60s, while the operational GFS/ECMWF show more subdued warm-sector
   buoyancy (MLCAPE of 500-1000 J/kg) based on a somewhat cooler/drier
   boundary layer.  Forcing for ascent will be relatively shallow/weak
   along the dryline, given the relatively veered flow as low as 850
   mb, and passage of the midlevel trough/synoptic ascent well to the
   north.  Thus, storm initiation along the dryline is uncertain,
   though the environment should support a conditional supercell risk. 
   Otherwise, the primary convective risk is expected to develop
   southward overnight as the cold front overtakes the remnant dryline,
   while the east edge of any severe risk will be delineated by the
   extension of marginal buoyancy.  With questions about warm-sector
   buoyancy, frontal position and convective mode to the north, and
   storm coverage to the south along the dryline, will opt to introduce
   a broad 5%/marginal risk.

   ..Thompson.. 03/04/2017

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