SPC AC 031254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0754 AM CDT Sat Jun 03 2023

   Valid 031300Z - 041200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   SOUTH TEXAS IN AND NEAR THE RIO GRANDE VALLEY...

   ...SUMMARY...
   Scattered thunderstorms with strong-severe gusts may move out of
   northern Mexico tonight and across parts of deep south Texas. 
   Isolated strong to locally severe thunderstorms are possible today 
   across parts of the southern Plains and north-central Appalachians
   vicinity.

   ...Synopsis...
   In mid/upper levels, net retrogression of the messy, central North
   American omega block is forecast, with the 500-mb high shifting
   westward from northwestern ON to southern MB.  Slow-moving mean
   troughing characteristically will flank the anticyclone to the
   southwest (Intermountain West and southern High Plains) and
   southeast (East Coast to eastern Gulf).  As a part of the eastern
   mean trough, a compact cyclone -- now developing from a strong
   shortwave trough over northern ME -- will dig southward and
   strengthen, with its center over or just east of Cape Cod by 12Z
   tomorrow.  Farther west, assorted vorticity maxima should meander
   erratically through the troughing between the northern Great Basin
   and the southern High Plains.  A subtle perturbation now over the
   southern Sonora/Sinaloa region of northwest MX should move
   east-northeastward toward the mountainous terrain of northern
   Coahuila through the day.

   At the surface, a cold front was apparent at 11Z over eastern and
   southern NY.  The overland part should shift southwestward across
   parts of the coastal and interior Mid-Atlantic through the period as
   the mid/upper cyclone to its northeast organizes and moves
   southward.  Farther southwest a dryline was drawn from a weak low
   near ROW southward across far west TX, the Big Bend area, and
   south-southeastward over the western slopes of the Serranias del
   Burro and nearby ranges in Coahuila.  This dryline should mix
   slightly eastward through the day, but leave favorable moisture over
   higher elevations, near a dewpoint axis now drawn from south of BRO
   northwestward between LRD and Monterrey, to eastern parts of the Big
   Bend area of TX.  An outflow boundary from prior/overnight
   convection was drawn near CLL-VCT-COT-DRT to eastern Brewster County
   TX.

   ...Deep south TX, southern Plains...
   Scattered thunderstorms should develop this afternoon over northern
   Coahuila, including the Serranias del Burro and nearby elevated
   terrain, and perhaps farther north across the international border
   into the lowest Pecos River region of TX.  Large hail and severe
   gusts are possible in the early stages of the event, transitioning
   to a dominant severe-wind threat.  Significant hail cannot be ruled
   out on the TX side, but is more conditional than over nearby parts
   of MX where certainty is greater for sustained supercell(s).

   The primary forcing mechanisms for this convection should include
   the western limb of the remnant boundary, and even more crucially,
   strong diurnal heating of higher terrain amidst an upslope component
   of a very moist boundary layer.  The modified 12Z DRT RAOB and model
   soundings suggest a corridor of 3000-4000 J/kg MLCAPE is possible
   this afternoon near the surface moist axis, which should move little
   through the day.  40-50 kt effective-shear magnitudes and long
   hodographs are possible, beneath the northern fringes of the
   subtropical jet.

   Early-stage supercells and organized multicells over MX may
   aggregate, forming a cold-pool-driven, forward-propagating MCS with
   embedded bow/LEWP formations and perhaps lingering/embedded
   supercell(s).  Given the ambient pattern, and positioning of the
   theta-e gradient related to the remnant outflow boundary, such a
   complex would move southeastward  and obliquely across the Rio
   Grande Valley (RGV), this evening into the overnight parts of the
   period.  Convection should be maintained by forced ascent of a
   favorably moist airmass undisturbed by the prior day's MCS.  Even
   with some nocturnal near-surface cooling ahead of the activity
   (slowed somewhat by the moist/high-theta-e conditions), a remnant
   diurnal mixed layer below the weak EML inversion should support
   maintenance of damaging gusts to the surface.

   The outlook still appears somewhat conditional; however, the
   favorable environment combined with a preponderance of synoptic-
   scale and CAM guidance indicating this potential compels an upgrade
   at this time.  An even more concentrated/intense corridor of wind
   potential may organize across some part of northeastern Coahuila
   into the lower RGV region, but that level of mesobeta-scale threat
   is still to uncertain to warrant even larger unconditional
   probabilities this soon.

   Otherwise, a more randomized, poorly defined, lower-moisture, but
   also weakly capped environment will exist across most of the
   southern Plains, with several lingering mesoscale outflow/
   differential-heating boundaries potentially concentrating afternoon/
   early-evening convection locally.  Isolated large hail and strong-
   severe gusts would be the main concerns, amid weak deep-layer wind
   profiles and lack of substantial shear.

   ...North-central Appalachians and vicinity...
   Widely scattered, predominantly afternoon thunderstorms are expected
   to form along/ahead of the front, atop a favorably moist, yet weakly
   capped and well-mixed boundary layer destabilized by diurnal
   heating.  This activity should move generally southwestward across
   the outlook area, offering damaging gusts approaching severe limits,
   and small to marginally severe hail cannot be ruled out. Deep-layer
   wind profiles and shear in the warm sector are forecast to remain
   weak, with the strongest height gradient aloft relegated  above the
   post-frontal sector in the Northeast.  Despite the weak shear and
   modest midlevel lapse rates, however, a diurnally heated and
   well-mixed boundary layer will support potential for localized
   damaging downdrafts.  Peak preconvective MLCAPE near 1000 J/kg is
   possible, with 500-1000 J/kg DCAPE.  The threat should diminish
   sharply after dark as the near-surface layer cools/stabilizes, and
   buoyancy weakens.

   ...FL Keys...
   Through the period, NHC forecasts marginal T.S. Arlene -- already
   small and rather skeletal in convective structure -- to move
   southeastward and degenerate to a depression, then remnant low.  As
   such, local wind profiles/hodographs and low-level shear are
   expected to remain too weak to support an eastern-semicircle tornado
   risk that would affect the Keys, even as what is left of the system
   approaches northwestern Cuba.  Please see latest NHC advisories for
   details on the expected track and intensity of Arlene.

   ..Edwards/Smith.. 06/03/2023

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