SPC AC 211731

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1231 PM CDT Tue Apr 21 2020

   Valid 221200Z - 231200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE WEDNESDAY
   AFTERNOON INTO WEDNESDAY NIGHT ACROSS PARTS OF SOUTHEASTERN
   OKLAHOMA...MUCH OF NORTHEASTERN TEXAS...SOUTHWESTERN ARKANSAS AND
   NORTHWESTERN LOUISIANA...

   ...SUMMARY...
   Severe thunderstorms are expected to develop east of the Interstate
   35 corridor of southern Oklahoma into north central Texas by late
   Wednesday afternoon, before spreading southeastward across the
   Ark-La-Tex and Piney Woods Wednesday evening, into the lower
   Mississippi Valley by Thursday morning.  This will be accompanied by
   a risk for severe hail, a few tornadoes, and potentially damaging
   wind gusts.

   ...Synopsis...
   While mid-level ridging may begin to build across the eastern
   Pacific, downstream flow across the eastern U.S. into the western
   Atlantic may trend more zonal during this period.  This is expected
   to occur as a strong short wave trough within a distinct northern
   branch of mid-latitude westerlies pivots east-northeast of the New
   England coast.  As an upstream perturbation rounds the crest of the
   eastern Pacific ridging, and digs into the Pacific Northwest, a
   fairly significant southern stream impulse is forecast to turn east
   of the southern Rockies, toward to the middle/lower Mississippi
   Valley Wednesday through Wednesday night.

   The southern stream perturbation appears likely to maintain a
   positive-tilt orientation, but models continue to suggest that it
   will be accompanied by a slowly deepening surface cyclone, from the
   southern Texas Panhandle vicinity early Wednesday morning into the
   Mid South by late Wednesday night.  

   Initially cool/dry and stable boundary layer air, reinforced by a
   cold front associated with the lead northern branch impulse, likely
   will be entrenched as far south and west as northern Florida and the
   northeastern Gulf of Mexico into the lower Mississippi Valley at 12Z
   Wednesday.  Some modification may already be underway across parts
   of the southeastern Plains, but a more established return flow is
   expected to be ongoing from across the lower Rio Grande Valley into
   the vicinity of the developing cyclone.  And this moisture return
   may gradually broaden eastward within an evolving warm sector across
   the lower Mississippi Valley by late Wednesday night.

   The moisture return and associated destabilization, coincident with
   large-scale forcing for ascent and strengthening deep-layer wind
   fields and vertical shear, probably will support increasing severe
   weather potential across the southern Plains into the lower
   Mississippi Valley.

   ...Southern Plains...
   Some uncertainties do linger, which could impact convective
   evolution and associated severe weather potential.  This includes
   the potential for fairly strong capping associated with elevated
   mixed-layer air, which may initially suppress convective
   development, particularly along a sharpening dryline to the south of
   the surface cyclone.  This may also impact areas east of the
   dryline, across much of eastern Texas, but considerable model output
   appears to suggest that large-scale forcing for ascent, likely aided
   by lower/mid tropospheric warm advection, may support increasing
   convective development by midday across this region, before
   spreading toward the lower Mississippi Valley.  This activity could
   be accompanied by some risk for severe hail, but probably would also
   reinforce stable boundary layer conditions.

   A consensus of model guidance does suggest that substantive
   boundary-layer destabilization through much of the day Wednesday may
   be initially confined to a fairly narrow corridor along the dryline,
   roughly along/east of the Interstate 35 corridor of central/southern
   Oklahoma into north central Texas by late afternoon.  However,
   mixed-layer CAPE of 1000-2000+ J/kg, in the presence of strong
   deep-layer shear is expected to provide a conducive environment to
   discrete supercell development.

   Aided strengthening wind fields to 40-50+ kt in the 850-700 mb
   layer, contributing to sizable clockwise curved low-level
   hodographs, the supercells may be accompanied by increasing risk for
   a few tornadoes, some strong, while spreading eastward into early
   evening.

   Thereafter, continued gradual low-level moisture advection and
   boundary layer destabilization ahead of the convective activity,
   coupled with mid-level cooling, may support the evolution of an
   upscale growing convective system with a transition to increasing
   potential for damaging wind gust, along with a continuing risk for
   tornadoes.  This is expected to overspread the Ark-La-Tex region and
   Piney Woods of Texas through late Wednesday evening, before
   progressing eastward into the lower Missisippi Valley overnight.

   ..Kerr.. 04/21/2020

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