SPC AC 150535

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1235 AM CDT Fri May 15 2020

   Valid 161200Z - 171200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SATURDAY INTO
   SATURDAY NIGHT ACROSS PARTS OF EASTERN AND SOUTHERN TEXAS AND PARTS
   OF THE LOWER MISSOURI AND MIDDLE MISSISSIPPI VALLEYS...

   ...SUMMARY...
   Strong thunderstorms may impact parts of eastern and southern Texas
   early Saturday, and parts of the lower Missouri and middle
   Mississippi Valleys Saturday afternoon and evening, accompanied by
   at least some risk for severe weather.

   ...Synopsis...
   Amplification is underway within the westerlies across the
   mid-latitude Pacific, and this appears likely to continue through
   this period, with increasing amplification within branching
   downstream flow across the U.S. also forecast.  This will include
   the continuing evolution of a broad and deep mid-level closed low
   over the eastern Pacific, which may begin digging into the U.S.
   Pacific coast by late Saturday night.  Downstream of this feature,
   mid-level ridging is forecast to build across the Intermountain West
   into the Rockies.  

   Farther east, models indicate that an evolving short wave trough
   within one branch will migrate from the lee of the northern Rockies
   through the mid/upper Mississippi Valley during this period.  This
   may be accompanied by modest surface cyclogenesis along a stalled
   frontal zone across northeastern Kansas/southeastern Nebraska into
   southeastern Iowa.

   At the same time, initially broad troughing within a branch to the
   south may gradually consolidate over eastern Texas.  This may
   include an embedded developing mid-level low and associated surface
   low.

   The spread within and among the various model output concerning
   these synoptic and associated sub-synoptic developments over the
   Plains into Mississippi Valley vicinity remain sizable, however. 
   And the extent and intensity of potential associated convective
   development remains somewhat uncertain.

   ...Eastern and southern Texas into western Louisiana...
   Preceding the southern branch perturbation, seasonably high moisture
   content (including surface dew points around 70f), beneath steep
   mid-level lapse rates, is expected to contribute to large
   mixed-layer CAPE of 2000-3000+ J/kg.  Although deep-layer flow and
   shear will likely be initially rather modest to weak, models
   generally suggest that convection initiating off the higher terrain
   of Mexico today will spread into central Texas by tonight, with
   considerable upscale growth and perhaps mergers with other
   convection.  One or two large clusters of storms, with significant
   and progressive surface cold pools, may still be ongoing at 12Z
   Saturday, with perhaps forcing aloft contributing to renewed
   convective development and upscale growth to the northeast and/or
   south during the day Saturday.

   Much uncertainty remains concerning the details of the convective
   evolution, with a considerable amount of guidance suggesting that
   the initial storm cluster may be in the process of advancing across
   Texas coastal areas as early as 12Z Saturday, while other model
   output suggests activity may still lag to the west at that time. 
   Regardless, thermodynamic profiles in the pre-convective environment
   appear conducive to the development of surface cold pools and rear
   inflow capable of producing strong surface gusts.  And the
   environment may remain conducive to storms capable of producing
   severe hail and potential for strong wind gusts in renewed
   convective development during the day.

   ...Lower Missouri and middle Mississippi Valleys...
   Convective potential remains uncertain.  However, models suggest
   that low-level moisture return and boundary-layer destabilization
   (including CAPE up to 1000-2000 J/Kg) into the vicinity of the
   surface front and developing surface low could contribute to an
   environment at least marginally conducive to organized
   thunderstorms.  Lapse rates may remain relatively weak, but
   deep-layer shear may become sufficient for isolated supercells
   and/or organizing clusters capable of producing severe hail and
   strong surface gusts Saturday afternoon and evening.

   ..Kerr.. 05/15/2020

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