SPC AC 250551

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1251 AM CDT Fri May 25 2018

   Valid 251200Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
   THE UPPER MISSISSIPPI VALLEY AND SOUTHERN PLAINS AREAS...

   ...SUMMARY...
   A few severe thunderstorms are possible later today into the evening
   over parts of the upper Mississippi Valley and the southern Plains.
   A few strong to severe storms are also possible over the interior
   Northwest and northern Maine.

   ...Synopsis...

   Synoptic regime will change little today with an omega block
   reflective of the large-scale pattern. A shortwave trough currently
   over the northern Plains is in the process of cresting an upper
   ridge and this feature will continue through the upper MS Valley and
   Great Lakes. Farther west, an cutoff upper low will advance very
   slowly east through CA into the Great Basin region. A cold front
   should extend from eastern MN southwestward into western KS, while a
   dryline sharpens across west TX and will extend north into southwest
   KS where it will intersect the cold/stationary front. 

   ...Upper Mississippi Valley region...

   In wake of any early morning convection, abundant warming of the
   boundary layer with low to mid 60s F dewpoints and modest mid-level
   lapse rates will result in moderate instability with 1500-2000 J/kg
   MLCAPE. A vorticity maximum embedded within the upper trough will
   overtake the cold front, and storms should develop along and ahead
   of this boundary as the atmosphere destabilizes during the
   afternoon. While vertical wind profiles and weak shear suggest
   storms will remain multicellular in character,  low-level lapse
   rates should steepen sufficiently to support a threat of strong
   downdrafts, and storms may eventually consolidate into line segments
   before weakening during the evening.

   ...Western Kansas through Oklahoma and north Texas...

   Middle to upper 60s F dewpoints are in place across the central and
   southern Plains warm sector, and plume of steep mid-level lapse
   rates will spread east along the western periphery of the richer
   low-level moisture contributing to moderate to strong instability
   (2000-3000 J/kg MLCAPE). Some uncertainty still exists regarding
   convective initiation and evolution, but storms currently moving
   through southern KS may leave an outflow boundary as they continue
   east and southeast through the early morning. The intersection of
   this boundary with the dryline and the dryline/cold front
   intersection in KS may serve as foci for storms to initiate as the
   boundary layer destabilizes this afternoon. Winds aloft are
   relatively modest, but veering profiles from the surface through 500
   mb will promote 30 kt 0-6 km shear suggesting most storms will be
   multicellular in character but some marginal supercell structures
   are also possible, especially with storm/outflow boundary
   interactions. Tendency may be for activity to congeal into an MCS
   and continue southeast during the late afternoon and evening.
   Damaging wind and large hail may accompany the stronger storms. 

   ...Interior Northwest...

   Belt of stronger winds aloft will evolve across the interior
   Northwest as upper low moves inland. As diabatic heating commences,
   deeply mixed boundary layers and marginal instability will
   characterize the thermodynamic environment. Storms developing in
   this regime will become capable of producing a few downburst winds
   and small to marginally severe hail during the afternoon and early
   evening.

   ...Northern Maine...

   A weak cold front will move into northern Maine, and the atmosphere
   will become at least marginally unstable in vicinity of this
   boundary during the afternoon. Weak impulses embedded within the
   northwest flow regime and 35-40 kt effective shear suggest a few
   storms could organized and produce locally strong wind gusts and
   marginally severe hail during the day.

   ..Dial/Marsh.. 05/25/2018

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