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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