SPC AC 091251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Fri Apr 09 2021

   Valid 091300Z - 101200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   SOUTHERN ARKANSAS...NORTHERN LOUISIANA...CENTRAL MISSISSIPPI...AND
   WESTERN ALABAMA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SEPARATELY OVER
   PARTS OF SOUTHERN VIRGINIA AND NORTHERN NORTH CAROLINA...

   ...SUMMARY...
   Multiple episodes of severe thunderstorms are expected today from
   parts of eastern Oklahoma and north-central/northeast Texas, across
   the Mid-South and Mississippi Delta regions, to parts of Alabama and
   Georgia.  This includes a threat for a swath of severe wind from 
   Arkansas across the lower Mississippi Valley.  Large, damaging hail
   and tornadoes also are possible.

   ...Synopsis...
   In mid/upper levels, a split-flow pattern will be maintained over
   the central CONUS, as one cyclone ejects out of a broader-scale
   trough causing the split, and another takes its place.  The leading
   cyclone -- initially centered near LSE -- is forecast to weaken
   gradually to an open-wave trough and head northeastward over Upper
   MI and Lake Superior overnight.  Upstream, a strong shortwave trough
   was apparent in moisture-channel imagery from eastern MT southward
   to central CO.  This feature will dig southeastward across the
   central Plains today and this evening, before pivoting eastward
   tonight as a closed cyclone, across parts of the lower Missouri
   Valley.  By 12Z, the associated 500-mb low should be located near
   the KS/MO line south of MKC.

   Farther south, a broad area of strong cyclonic flow aloft will
   spread eastward across the outlook area, with embedded, closely
   spaced perturbations now evident over southwestern/central NM and
   the TX Panhandle.  These perturbations will move across OK and south
   TX today.  Tonight, they should reach parts of TN/MS/LA, likely
   blending into convectively generated vorticity fields.  The broad
   left-exit region of a cyclonically curved, 250-mb subtropical jet,
   also exhibiting pronounced difluence, also will shift eastward over
   much of central/north TX and LA today and tonight toward the Delta
   region.

   At the surface, 11Z analysis showed an occluded, elongated area of
   low pressure across northwestern WI to northern IA.  An occluded
   front was drawn from the northern part across Lake Superior and
   Lower MI, with weakening/quasistationary front across western OH,
   eastern portions of TN/KY, and northern AL.  A warm front extended
   across Lake Erie, eastern WV, and southwestern/southern VA near the
   NC line.  This boundary will move slowly northward over southern/
   central VA today.  A cold front extended across southern IA,
   northwestern MO, central/western KS, and southeastern CO.  A
   marine/warm front extended from a low over northwest TX across
   north-central TX, central LA, and southern portions of MS/AL. This
   boundary will move northward across southern /eastern OK, AR, and
   central MS/AL through the day.

   The northern low should consolidate in form but continue to fill,
   and move northeastward over ON.  Meanwhile, a second low is expected
   to develop along the front today near the dryline intersection,
   perhaps evolving out of the initial/prefrontal low over northwest
   TX.  The resulting low should deepen as the next mid/upper cyclone
   approaches, and move northeastward to near STL by 12Z tomorrow.  By
   that time, the cold front should extend from the low southward down
   the Mississippi Valley to near BTR, then southwestward across
   mid/upper TX coastal waters to deep south TX.

   ...Southern Plains to Arklatex/Mid-South/Delta regions...
   Episodes of severe thunderstorms are expected today into this
   evening between eastern OK, north TX and Mississippi, resulting in
   large (locally very large) hail, severe gusts and several tornadoes.

   This is a complex scenario with an assortment of mesoscale
   possibilities more diverse than the number of models and ensemble
   members available.  Progs somewhat inconsistently depict early
   initiation and evolution of the convection across this region,
   whether near the front in OK (a very plausible consensus theme), on
   the dryline (more isolated/conditional due to strong capping), and
   in the warm sector (highly variable).  Multiple episodes of
   strong-severe thunderstorms may move over any given spot through
   tonight.  This outlook represents both:
   1.  The potential for overlapping areas of severe at different times
   through the day and evening as different areas of convection evolve,
   but also:
   2.  A scenario that most progs reasonably do converge upon despite
   early differences:  a dominant QLCS/MCS evolving out of convection
   initiating in OK and/or northeast TX, then moving east-southeastward
   to southeastward over the Arklatex to Delta regions, organizing a
   well-developed cold pool, and producing a swath of severe wind.

   Whether or not this event will qualify as a derecho is a semantic
   exercise, and likely regulated by mesoscale convective/boundary
   processes yet to become apparent.  Regardless, hurricane-force
   convective gusts are a threat with any such complex, in the Enhanced
   and Moderate areas.  Also, because both the probable QLCS and any
   preceding supercell(s) will encounter an environment of moderate-
   strong buoyancy, steep midlevel lapse rates around the eastern rim
   of an EML, enlarged low-level hodographs, and favorable vertical
   shear, tornadoes also are expected.  Peak preconvective MLCAPE
   across the warm sector commonly should be in the 2000-3000 J/kg
   range, amidst 40-55-kt effective-shear magnitudes.  As one or more
   MCSs plow cold pools into this environment, they should be
   maintained southeastward toward southern MS/AL tonight, continuing a
   wind-damage and sporadic tornado threat.

   ...Central Gulf Coast to AL/GA this morning/afternoon...
   Widely scattered to scattered thunderstorms should develop from
   mid/late morning through this afternoon, with a threat for sporadic
   large hail and damaging gusts.  A conditional, localized tornado
   threat also may develop.  For near-term coverage, please see SPC
   mesoscale discussion 338.

   Midlevel lapse rates will be less than in the more EML-influenced
   air mass of the Plains/Delta area.  Still, rich low-level moisture
   spreading inland, combined with areas of diurnal heating (tempered
   in pace by mid/upper cloud cover from upstream convection), will
   weaken MLCINH and yield favorable surface-based buoyancy. 
   Meanwhile, low-level shear will be maximized near the marine frontal
   zone, though weak absolute speeds will limit hodograph size
   somewhat.  Modified 12Z RAOBs and forecast soundings from midday
   through afternoon indicate around 1500-2000 J/kg MLCAPE is possible,
   with 35-45 kt effective-shear magnitudes. Effective SRH will range
   from around 200-250 J/kg on the front to around 100-1-50 J/kg
   coastward in the warm sector.  With upper support meager in this
   area, but also modest MLCINH, convective coverage along and south of
   the front is uncertain.

   ...VA/NC...
   Widely scattered to scattered thunderstorms should form this
   afternoon over higher terrain near the eastern WV Panhandle and VA
   Blue Ridge, with additional convection developing later in the
   afternoon and evening southward across portions of western NC.  This
   activity is expected to move eastward across much of NC and VA,
   offering isolated large hail and the potential for damaging to
   marginally severe gusts.  A tornado cannot be ruled out, especially
   with any convection that can interact with the front, where weak but
   also backed surface winds will enhanced low-level hodograph size and
   boundary-layer bulk shear.  Modest low/middle-level winds will limit
   deep shear, with effective-shear magnitudes generally 20-35 kt in
   most areas, perhaps around 40 kt along the front.  As such, mostly
   multicell modes are expected, with isolated, transient supercell
   character possible near the front.

   Convection in the northern parts of the outlook area will be tied
   more strictly to orographic processes, and will have a much narrower
   zone of favorable instability to access, north of the warm front. 
   That convection should weaken within a short distance eastward of
   their initial flare-up.  However, activity across central/southern
   VA into central NC should persist well into the evening, perhaps as
   far eastward as the Tidewater areas from Hampton roads toward the
   northern sounds of NC, before weakening in the ongoing nocturnal/
   boundary-layer stabilization process.  Surface dew points mid 50s to
   low 60s F, beneath steep low/middle-level lapse rates, will support
   pockets of 1000-1500 J/kg MLCAPE along and south of the front, atop
   a well-mixed afternoon boundary layer supporting potential for
   strong/isolated severe gusts and hail to reach the surface.

   ..Edwards/Broyles.. 04/09/2021

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