SPC AC 130455

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1155 PM CDT Sat May 12 2018

   Valid 141200Z - 151200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS MONDAY AFTERNOON
   AND EVENING ACROSS THE LOWER MISSOURI/MIDDLE MISSISSIPPI VALLEY
   REGION AND ADJACENT CENTRAL PLAINS...

   ...SUMMARY...
   Severe thunderstorms, accompanied by the risk for large hail and
   potentially damaging wind gusts, are possible Monday afternoon and
   evening across parts of lower Missouri/middle Mississippi Valley
   region and adjacent central Plains.  Other strong storms may be
   accompanied by at least some severe risk across parts of the Ohio
   Valley into Mid Atlantic region, and across the southern high
   Plains.

   ...Synopsis...
   Blocking near/just inland of the U.S. Pacific coast will maintain a
   split in the westerlies emanating from the Pacific through this
   period.  Models do continue to indicate considerable weakening of a
   broad closed low centered over the Great Basin, but broad southern
   branch troughing is forecast to persist across much of the Southwest
   and southern Rockies.  At the same time, broad downstream ridging
   appears likely to hold firm across the central Plains and
   middle/lower Missouri Valley into the Ohio Valley.  This is more
   unclear across the upper Ohio Valley into northern Mid Atlantic
   region, where some suppression is possible, as a vigorous short wave
   impulse emanating from the Arctic latitudes digs in phase with
   northern branch, across and southeast of Hudson Bay.  In lower
   latitudes, a mid-level low may continue to evolve within the
   subtropical westerlies, over the northeastern Gulf of Mexico.

   In lower levels, the Arctic impulse likely will be accompanied by a
   significant cold front which may approach the upper Great Lakes
   region by late Monday night.  Farther south, a remnant frontal zone,
   reinforced in areas by convective outflow, is expected to remain
   generally quasi-stationary.  This probably will roughly align with
   the northern periphery of the southern branch ridging, along which a
   lingering plume of elevated mixed-layer air may continue to
   contribute to steep lower/mid tropospheric lapse rates.  It appears
   that this environment, coupled with lower/mid 60s+ surface dew
   points, will contribute to corridors of moderately large CAPE
   near/south of the front and along/east of the southern Plains
   dryline by peak heating Monday.  Given this destabilization, it
   still appears that deep layer shear, while not strong, may be
   marginally sufficient to support potential for organizing storm
   clusters.  However, this may largely depend on forcing associated
   with subtle perturbations progressing through the larger-scale
   anticyclonic flow, which generally still remains uncertain.

   ...Lower Missouri/middle Mississippi Valleys/central Plains...
   It seems, with at least some increasing confidence, that a low
   amplitude mid-level wave (accompanied by 30-40 kt flow in the
   700-500 mb layer) will emerge from the Southwestern troughing and
   progress across the region favorably timed with peak boundary layer
   destabilization.  This may include CAPE on the order of 3000+ J/kg. 
   As large-scale forcing for ascent aids the initiation of storms,
   multiple thunderstorm clusters may evolve and perhaps eventually
   merge, while tending to propagate eastward and southeastward through
   Monday evening.  This activity probably will pose a severe hail
   risk, at least initially, before the risk for strong surface gusts
   becomes more prominent as activity grows upscale.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:   2%     - Marginal
   Wind:     15%     - Slight
   Hail:     15%     - Slight

   ..Kerr.. 05/13/2018

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