SPC AC 150558

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1258 AM CDT Sat Apr 15 2017

   Valid 151200Z - 161200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS EXTENDING FROM THE
   UPPER MIDWEST/CENTRAL PLAINS TO THE SOUTHERN HIGH PLAINS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS EXTENDING FROM
   THE UPPER MIDWEST/CENTRAL PLAINS TO THE SOUTHERN HIGH PLAINS...

   ...SUMMARY...
   Isolated to scattered severe thunderstorms producing large hail and
   damaging winds are forecast this afternoon and evening from portions
   of the southern High Plains into the central Plains and middle/upper
   Mississippi Valley.

   ...Synopsis...
   A shortwave trough embedded within a large-scale upper trough/low
   over SK/MB will move eastward across the northern Rockies/Plains
   through this evening, reaching the Upper Midwest by the end of the
   period. Modest mid-level height falls associated with the shortwave
   trough will overspread the central Plains and Upper Midwest through
   the afternoon, while farther south across the southern High Plains
   large-scale ascent will remain more nebulous. At the surface, a cold
   front will continue to progress southeastward across the
   northern/central Plains and Upper Midwest, while a dryline mixes
   eastward across the TX/OK Panhandles and west TX by late afternoon.

   ...Upper Midwest to the Southern High Plains...
   The surface cold front and dryline will be the primary forcing
   mechanisms that will focus severe weather chances today and tonight.
   A cluster of elevated thunderstorms may be ongoing at the beginning
   of the period across parts of SD into the Upper Midwest in
   association with a low-level jet. These thunderstorms should track
   northeastward through the early afternoon, posing an isolated hail
   and gusty wind threat across WI. Convection should initiate along
   the cold front from the Upper Midwest to central Plains by late
   afternoon as it encounters an increasingly moist and unstable
   airmass. Low-level moisture will likely be in place ahead of the
   cold front across this region, as surface dewpoints increase into
   the at least the low 60s in a narrow northeast to southwest corridor
   from southwestern WI to central KS. Moderate to locally strong
   instability is forecast to develop along/ahead of the front (MLCAPE
   1500-2000 J/kg), as steep mid-level lapse rates overspread this
   region from the southwest. Mid-level winds and related 0-6 km bulk
   shear are forecast to be strongest across the Upper Midwest and
   upper MS Valley closer to the previously mentioned shortwave trough.
   This combination of instability and shear will support organized
   updrafts along the cold front, and large hail may occur with any
   initial supercell structures. Eventually, storms will probably grow
   upscale into line segments owing to the linear forcing of the front,
   with damaging winds becoming the predominant threat by early
   evening.

   Farther south along the dryline, convection will likely be isolated
   in nature due to nebulous large-scale forcing for ascent. Still, a
   couple of thunderstorms should initiate in the late afternoon
   along/just east of the dryline, which should mix eastward across the
   TX/OK Panhandles and west TX. Instability will be quite strong east
   of the dryline (MLCAPE 2000-3000 J/kg) owing to plentiful diurnal
   heating, likely low to mid 60s surface dewpoints, and very steep
   mid-level lapse rates associated with an EML. Although mid-level
   winds will be weaker compared to locations farther north, a veering
   wind profile in low to mid levels and effective bulk shear values of
   30-35 kt will be adequate for supercell structures, and large hail
   will be the main threat with any storm that can form, in addition to
   some damaging straight-line winds. Very large hail (2+ inches in
   diameter) may occur owing to the strong instability forecast to be
   present. An isolated tornado may also occur in the early evening as
   a southerly low-level jet strengthens slightly. The prospect for any
   convection becomes more conditional with southwestward extent along
   the dryline into west TX, with large hail/damaging winds still
   possible, but on an even more isolated basis compared to the eastern
   TX Panhandle/western OK.

   ..Gleason/Mosier.. 04/15/2017

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