SPC AC 281250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Fri Apr 28 2023

   Valid 281300Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   NORTH...CENTRAL AND SOUTH TEXAS...

   ...SUMMARY...
   Destructive hail (some at least as large as baseballs), thunderstorm
   gusts potentially exceeding 75 mph, and a couple tornadoes are
   possible this afternoon and evening from north Texas southwestward
   to parts of the Rio Grande Valley.

   ...Synopsis...
   The large-scale pattern over North America will remain characterized
   by high-amplitude ridging just inland from the Pacific Coast, and a
   longwave trough over east-central portions of the U.S. and Canada. 
   In the latter's broad cyclonic flow field, four main shortwave
   perturbations will affect convective potential this period:
   1.  An ejecting trough -- apparent in moisture-channel imagery from
   Lower MI across eastern parts of KY/TN, and forecast to weaken
   gradually before it reaches the Lower Great Lakes and Mid-Atlantic
   regions around 12Z tomorrow.
   2.  A compact shortwave trough -- initially located over the central
   Rockies -- which will amplify considerably as it follows an arc
   southeastward across the central/southern High Plains today.  By the
   end of the period, the trough should extend from  northeastern OK
   southwestward across the DRT area and Coahuila, with a closed 500-mb
   low developing near DAL.
   3.  A northern-stream perturbation -- now located near the northern
   MB/SK border, and projected to dig southeastward to the eastern
   Dakotas/western MN region by 12Z.  Near 12Z or early day-2, it
   should merge with...
   4.  A foregoing, slow-moving trough now over the Dakotas.  As
   large-scale ascent aloft related to this feature overspreads a plume
   of modest low-level moisture in parts of the upper Mississippi
   Valley, isolated to widely scattered thunderstorms may occur this
   afternoon and evening.

   At the surface, 11Z analysis showed a wavy warm front across
   eastern/central NC, central SC and central GA, forecast to move
   northward somewhat today ahead of an occluded/cold front analyzed
   from OH across eastern parts of KY/TN, to central MS.  The latter
   boundary became quasistationary and poorly defined, through
   previously precip-cooled air, westward to a low near DYS.  A
   separate cold front was drawn across central KS, western OK,
   northwest TX, to near INK, then over extreme southern NM.  Through
   this evening this cold front will overtake a dryline initially drawn
   across central TX to near DRT.  By 00Z, this cold front should
   extend from southeastern IA to southeastern OK, then across central
   TX to near DRT.  By 12Z, the cold front should reach central/
   southern AR, where a frontal-wave low may form ahead of the
   mid/upper perturbation, then south-southwestward across parts of
   western LA to the northwestern Gulf.

   ...TX...
   This afternoon, severe hail is expected across parts of north-
   central and central TX, with some very large/destructive hail
   possible (3+ inches).  A couple tornadoes also may occur.  From late
   afternoon through the evening, expansive convective growth should
   result in a severe-wind threat, with some significant (65+ kt) gusts
   possible.

   Scattered thunderstorms -- including a few supercells -- are
   expected to develop this afternoon along/ahead of the front. 
   Greatest initial convective coverage and intensity are expected in a
   corridor of relatively maximized convergence and backed surface
   winds between the low and the northern rim of the "just in time"
   returning moist plume.  Given the observed translation of the cold
   front -- which has been consistently faster than in most progs --
   ideal moisture return appears limited to the area from around DFW
   southward before clouds, precip and convection truncate its poleward
   advance.  As such, outlook lines have been shunted southward
   somewhat, but with some potential left near the Red River to account
   for hail potential from elevated thunderstorms.  The Metroplex
   appears again to be in the gradient of the severe threat, with
   increases substantially with southward extent across the area.

   Though near-surface winds may be weak, pronounced veering with
   height and large 2-3-km hodographs are expected, amidst 50-60-kt
   effective-shear vectors.  Diurnal heating, low/middle-level lapse
   rates greater than 8 deg C/km, and surface dewpoints in the 60s F,
   will drive peak MLCAPE into the 2000-3000 J/kg range in the ACT-AUS
   corridor, and even larger (perhaps approaching 4000 J/kg) over parts
   of south TX where heating/moisture are greater (but vertical shear
   somewhat weaker).  Hail models applied to forecast soundings
   consistently yield 3+ inch max diameter, with some 4+ inch analogs. 
   With a well-mixed boundary layer expected in areas receiving a few
   hours of heating, severe downdrafts are possible from supercells as
   well as from the subsequent upscale clustering expected over
   central/south TX.

   Convection should develop farther south near the front across
   central/south TX, and perhaps over the eastern slopes of the
   Serranias del Burro in Coahuila (with the latter potentially
   aggregating upscale and moving eastward into TX).  This upscale
   growth should lead to one or two organized, bowing MCSs, potentially
   merging, with enough cold-pool development (amid increasing
   mid/upper winds) to drive a substantial severe-wind threat over
   parts of central and south TX.  With the warm sector being narrow in
   the north and larger near I-10, the wind threat should last longer
   over southern areas this evening, and wind probabilities accordingly
   have been expanded somewhat in that region.

   ...FL/GA...
   An outflow boundary from the prior day's convection has settled
   across southernmost mainland Fl and the upper keys/Florida Bay
   region, arching northwestward toward a slowly weakening/shrinking
   MCS over the east-central Gulf.  That weakening trend should
   continue through the remainder of the morning as what's left of the
   complex encounters further convectively processed boundary-layer
   air.  Clouds and perhaps a dissipating precip area from that
   activity will limit diurnal destabilization across much of west-
   central/southwestern FL today.

   However, enough heating is possible to enable an East Coast
   sea-breeze boundary from the MIA area northward, as well as north of
   the MCS cloud plume over parts of northern FL and southern GA.  In
   those areas, widely scattered to scattered thunderstorms should form
   this afternoon, supported by low-level lift along outflow,
   differential-heating and sea-breeze boundaries.  Though midlevel
   temperatures will not be as cool as the past few days, marginal
   shear on the southern rim of the ejecting midlevel wave's influence
   may support a blend of organized multicell and transient supercell
   structures, with attendant risk of isolated severe hail/gusts.

   ...Mid-Atlantic region to Upper Ohio Valley...
   Multiple rounds of convection are expected across this region --
   mainly involving early thunderstorms near the warm front over parts
   of the Carolinas, then afternoon convection from NC into the upper
   Ohio Valley nearer to the remnant cold core region of the ejecting
   mid/upper trough.  Isolated large hail and a few damaging to severe
   gusts are possible.  Buoyancy will increase through the day behind
   the early warm-frontal activity, as both diurnal heating and
   midlevel cooling occur, amid modest but sufficient boundary-layer
   moisture.  With the mid/upper trough deamplifying, effective-shear
   magnitudes generally should remain below 35 kt, but a few organized
   multicells and transient supercells near the occluded/cold front
   should offer localized strong downdrafts and marginally severe hail.

   ..Edwards/Mosier.. 04/28/2023

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