SPC AC 221253

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0753 AM CDT Fri May 22 2020

   Valid 221300Z - 231200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
   OF NORTH-CENTRAL/NORTHEAST TEXAS AND SOUTHEASTERN OKLAHOMA...

   ...SUMMARY...
   Severe storms are expected today from north Texas into the Ozarks.

   ...Synopsis...
   In mid/upper levels, a longwave trough will persist over the western
   CONUS, while a blocking pattern breaks down in the East.  The latter
   will be related to height falls across the Great Plains states,
   shifting to the Mississippi Valley, contributing to the
   northeastward ejection of a formerly cut-off low now near CVG.  The
   associated cyclone will fill gradually as it moves northeastward up
   the Ohio valley, past PIT around 06Z, then eastward over the
   southern PA/MD area.

   To the west, as a cyclone over central AB weakens and shifts
   northeastward, a strong shortwave trough now over the interior
   Northwest will dig east-southeastward.  This perturbation should
   become a closed cyclone over southern ID and northwestern UT around
   the end of the period.  A shortwave trough -- now evident in
   moisture-channel imagery over the central/southern High Plains --
   will move east-northeastward across KS and northern OK, phasing with
   MCVs produced by an ongoing MCS over portions of OK/KS.  The
   combined perturbation may develop a closed 500-mb low over IA by 12Z
   tomorrow. 

   At the surface, a low was analyzed over sotuhern VA, with weak,
   slow-moving cold front over eastern NC and coastal SC, becoming
   quasistationary over central GA.  This part of the boundary should
   become more diffuse and retreat northward over GA and the Carolinas
   through the day.  The warm-frontal segment -- analyzed across
   central AL, northern MS, southern AR and eastern OK, will be
   overtaken over much of its OK extent (and perhaps some of AR) by an
   extensive outflow boundary from an MCS now mostly covering southern/
   eastern OK.  A dryline -- analyzed initially from the TX Big Bend
   region north-northeastward to just west of CDS, will mix eastward
   today to parts of northwest/west-central TX, to northern Coahuila. 
   The dryline and outflow boundary may intersect this afternoon near
   the Red River over northwest TX. 

   ...North TX/Arklatex/Ozarks...
   Except for an initially separate multicellular cluster over south-
   central OK, near its southern flank, an extensive MCS that has
   produced several measured severe gusts in central/northern OK
   appears to be undergoing a general weakening trend while moving
   east-southeastward astride the warm-frontal zone.  See SPC watch 201
   and related mesoscale discussions for near-term details. 

   The major influence on the convective/severe potential today will be
   the MCS' outflow boundary.  This complex has progressed farther/
   faster than indicated by most progs used for the previous outlook
   cycle, and it still bears a substantial pressure/theta-e
   perturbation.  This introduces more uncertainty regarding the
   ultimate stalling position of the boundary, and therefore associated
   severe probabilities, which still may need more shifting as
   mesoscale trends warrant.  Nonetheless, the boundary still should
   focus the bulk of afternoon severe convective development, even if
   more likely farther south near the Red River in north TX and
   southeastern OK.  This repositioning would displace the most robust
   convection farther from stronger midlevel westerlies related to the
   shortwave trough, in turn marginalizing deep shear somewhat.

   Scattered thunderstorms are expected to form this afternoon along
   the boundary, perhaps very rapidly, given the strong instability
   expected on its south side.  Convection should be isolated to widely
   scattered farther southwest near the dryline.  The moist sector
   across southern OK, north-central/northeast TX, and AR south of the
   warm front will be characterized largely by surface dew points in
   the upper 60s to low 70s F.  Where these preconvective conditions
   persist, steep midlevel lapse rates and a deep troposphere will
   contribute to peak afternoon MLCAPE in the 3500-4500 J/kg, locally
   higher.  The thermodynamic profiles favoring very intense updrafts,
   combined with effective-shear magnitudes around 30-35 kt and
   localized boundary-layer vorticity enhancement along the boundary,
   indicate a mix of multicell and heavy-precip supercell modes, with
   at least broken quasi-linear clustering ultimately occurring.  Until
   the mode gets too messy, considerable hail is likely, some of it
   significant/ destructive in nature, along with severe gusts. 
   Storm-scale and boundary interactions still support a threat for a
   few tornadoes as well.  Isolated severe hail and gusts also will be
   possible from convection near the dryline. 

   Pockets of somewhat drier and more readily mixed, lower-buoyancy air
   may linger over portions of southern OK and western north TX,
   advected from a convectively processed air mass now southwest
   through west of the Metroplex, where mid-50s to low-60s F surface
   dew points have been observed for a few hours.  If maintained, this
   air mass may affect severe potential over parts of northwest/north-
   central TX, by making convection more high-based and outflow-
   dominant.  

   ...GA/Carolinas/eastern TN...
   Isolated severe gusts and hail are possible across the outlook area
   through this afternoon.  An MCV and associated midlevel shortwave
   trough -- currently associated with nonsevere convection across
   portions of the Mid-South region into northern AL and mid TN, will
   move eastward today.  The associated field of large-scale ascent
   will impinge on a boundary layer that will be destabilizing from
   south-north across the outlook area, from a combination of diabatic
   heating and theta-e advection.  Scattered thunderstorms should move
   eastward from the higher terrain of southeastern TN/northern GA,
   into an airmass with around 1000-1500 J/kg MLCAPE.  Additional
   development may occur in a weakly capped warm sector over southern
   parts of the outlook area.  Deep shear should increase as the
   mid/upper low pivots northeastward and the MCV/shortwave approaches,
   with effective-shear magnitudes in the 30-40 kt range.  A mix of
   multicell and at least isolated supercell modes are possible.

   ..Edwards/Smith.. 05/22/2020

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