SPC AC 210455

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1155 PM CDT Wed May 20 2020

   Valid 221200Z - 231200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FRIDAY THROUGH
   FRIDAY NIGHT ACROSS SOUTHERN PORTIONS OF THE CENTRAL PLAINS INTO
   PARTS OF THE OZARK PLATEAU AND LOWER MISSOURI VALLEY...AND ACROSS
   PARTS OF THE SOUTHERN APPALACHIANS INTO THE SOUTHERN MID ATLANTIC
   COAST...

   ...SUMMARY...
   There appears at least some risk for severe thunderstorm development
   Friday into Friday night across southern portions of the central
   Plains into parts of the Ozark Plateau and lower Missouri Valley, as
   well as across parts of the southern Appalachians into southern Mid
   Atlantic coast region.

   ...Synopsis...
   Large-scale mid-level troughing will remain prominent across much of
   western North America Friday through Friday night.  One significant
   embedded short wave impulse is forecast to dig southeast of the
   Oregon Cascades, contributing to some amplification of the
   larger-scale troughing over the U.S. Intermountain West, before it
   turns eastward across the Great Basin late in the period.

   Downstream, large-scale ridging will continue to evolve east of the
   U.S. Rockies and Canadian Prairies.  However, a modest perturbation,
   within a weak branch of westerlies emanating from the Southwest, is
   forecast to migrate northeast of the southern high plains through
   the lower Missouri Valley, before short wave ridging builds in its
   wake across the high plains.

   In response to the progression of this latter feature, and the
   continued weakening of a ridge axis initially across the upper Great
   Lakes into northern Mid Atlantic/southern New England coast
   vicinity, the remnants of a cut-off low appear likely to become
   increasingly progressive across the middle Ohio Valley into Mid
   Atlantic coast vicinity.

   In lower levels, surface troughing likely will remain focused across
   the high plains, where it is forecast to continue to deepen through
   the period. However, higher moisture content will generally advect
   in a plume off to the east, toward the Mississippi Valley, ahead of
   the perturbation emerging from the Southwest, and another weakening
   impulse accelerating north of the central Canadian/U.S. border area.


   ...Southern Plains into Lower Missouri Valley/Ozark Plateau...
   Mid 60s to lower 70s F surface dew points are forecast to advect
   north of the Red River through the Missouri/Kansas border vicinity,
   ahead of the northeastward migrating short wave trough.  Beneath
   steep mid-level lapse rates associated with elevated mixed-layer
   air, this appears likely to contribute to moderate to large
   mixed-layer CAPE (1000-3000+ J/kg) by Friday afternoon.  Coupled
   with a belt of modest (30-40+ kt) southwesterly to westerly
   mid-level flow accompanying the mid-level impulse, the environment
   is expected to become at least conditionally supportive of organized
   severe storm development.  However, largely due to sizable spread
   evident among the output of the various models, severe probabilities
   are being maintained at 5 percent for now, but a more focused area
   of higher severe probabilities appears possible in later outlooks
   for this period.

   Convective cloud cover and precipitation left over from an overnight
   cluster of storms may be ongoing early in the period, and its
   influence remains unclear.  Some model output has suggested the
   potential for re-intensification of this activity across parts of
   Oklahoma, as boundary-layer warming weakens inhibition.  

   Otherwise, a cyclonic vorticity center within the mid-level trough
   may provide a focus for late afternoon and evening storm development
   across parts of eastern Kansas into western Missouri.  Barring
   complications from early day convection, this could include isolated
   supercells.  

   The latest deterministic NAM, more so than other model output,
   suggests that the dryline, in the immediate wake of the southern
   periphery of the mid-level trough, may become a focus for intense
   thunderstorm development by late Friday afternoon.  This could
   include isolated supercell development and an upscale growing
   cluster of storms Friday evening.

   ...Southern Appalachians into southern Mid Atlantic coast...
   Primary uncertainty concerns the extent of boundary-layer
   destabilization, as low-level moisture begins to return northward. 
   However, CAPE increasing in excess of 1000 J/kg appears possible,
   and a belt of 30-40 kt flow around 500 mb, on the southern periphery
   of the increasingly progressive mid-level low, could support
   organizing clusters of storms Friday afternoon and evening.  This
   may pose a risk for potentially damaging wind gusts, and perhaps
   some hail.

   ..Kerr.. 05/21/2020

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