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SPC May 9, 2024 1300 UTC Day 1 Convective Outlook

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SPC 1300Z Day 1 Outlook
Day 1 Outlook Image
Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0823 AM CDT Thu May 09 2024

Valid 091300Z - 101200Z



Very large hail with multiple supercells appears probable across
parts of north and central Texas into the ArkLaTex vicinity this
afternoon and evening -- including parts of the Dallas/Ft. Worth
Metroplex.  A broader corridor of severe hail and damaging-wind
potential will extend from east Texas into the lower Mississippi
Valley and Southeast.

In mid/upper levels, an extraordinarily long, more than continent-
spanning, positively tilted trough was apparent in moisture-channel
imagery.  The trough extended from an Atlantic cyclone south-
southeast of Greenland, across Newfoundland/southern Labrador and
southern parts of QC/ON, the Upper Great Lakes, upper Mississippi
Valley, NE, the central Rockies, southern Great Basin, central CA,
and west-southwestward over the Pacific.  On the western segment of
the trough, a remnant cyclone near FSD/YKN will dissipate shortly,
while a closed vortex develops over UT and slowly retrogrades
through the period.  Meanwhile, weak shortwave troughs now over
parts of MO/IA/IL will move eastward across the Ohio Valley region,
in advance of a larger, amplifying shortwave trough now over Lake
Superior and northeastern MN.  A weak, but still potentially
influential southern stream shortwave will move east-northeastward
from southwest to central TX today.  Picking up some convective
vorticity and related amplification, this perturbation should
accelerate eastward and reach parts of AL by 12Z tomorrow.

At the surface, 11Z analysis showed a diffuse frontal-wave low over
central IL, with quasistationary to locally warm front eastward
across central/southern OH to northern WV.  A cold front extended
from the low across southern IL to eastern AR, becoming wavy/
quasistationary through a weak low south of ABI, then cold again
into the Big Bend region.  This front, overall, will sag southward
across TX through the period, but hang up for much of the day ahead
of the low and around the DFW/SEP region, and across the Edwards
Plateau.  The Ohio Valley low is expected to move eastward to near
the southwestern corner of PA by 00Z, then move/redevelop eastward
across VA to near the Delmarva Atlantic Coast by the end of the

...North, central and east TX...
At least a few supercells are expected to erupt in a high-
instability environment this afternoon, along and southeast of the
front.  This activity will be capable of large hail -- some of it
potentially 3+ inches in diameter -- as well as locally severe gusts
and perhaps a tornado or two.  The threat area includes the DFW
Metroplex -- unfortunately, no stranger to swaths of destructive
hail -- and areas southward/eastward into the Hill Country, I-35/45
corridors and Piney Woods.

The airmass across the region begins the day quite moist, with
surface dewpoints commonly in the low/mid 70s just southeast of the
front, and mid/upper 70s over the Gulf Coastal Plain.  Steep
midlevel lapse rates, related to an EML, were sampled by 12Z RAOBs
at MAF and FWD.  This airmass aloft will remain over the region,
with some subtle/reinforcing large-scale ascent possible ahead of
the southern-stream perturbation.  Meanwhile, the boundary layer
should destabilize amidst strong diurnal heating, causing the cap to
weaken considerably through early/mid afternoon.  Development is
possible as soon as late morning to midday near the front, over the
Edwards Plateau, and midday to mid afternoon farther north near the
low.  By then, 3500-4500 J/kg MLCAPE should be common, locally near
5000 J/kg.  Low-level flow should remain weak, but with enough
easterly component southeast of the front to foster strong veering
with height and long (sometimes nearly straight) hodographs.  This
will support splitting storms, with both left- and right-moving
supercells offering the threat for significant (2+ inch) hail. 
Sustained storms could produce 3-4-inch hailstones, based on
historic analogs and a 2D hail model applied to forecast soundings.

With a deep troposphere and abundant inflow-layer moisture, deeply
precip-loaded downdrafts are possible, with some midlevel momentum
augmentation and related severe-gust potential.  The wind threat
could evolve upscale and start forward-propagating eastward into LA
wherever early cells can aggregate into clusters.  The supercell
tornado threat is conditional, mainly dependent on local storm-scale
processes such as mergers and boundary interactions.  QLCS tornadoes
are also possible with any MCS.

...Lower Mississippi Valley to southern Atlantic Coast...
Ongoing, organized areas of thunderstorms over the Southeast,
interacting with and focused astride baroclinic/instability
gradients from outflow boundaries, will continue to offer damaging
wind, a few tornadoes, and isolated severe hail through at least
midday as they move across parts of northern MS, AL, territory
eastward/southeastward toward the GA/SC coast, and perhaps northern
FL.  For near-term concerns, refer to tornado watches 215-216 and
related mesoscale discussions.

The related outflow/baroclinic gradient should serve as a focus for
forward propagation, and perhaps initiation, of thunderstorms this
afternoon into tonight.  Surface dewpoints commonly in the 70s F and
strong diurnal heating south and west of the outflow boundaries will
contribute to peak afternoon MLCAPE of 3500-4500 J/kg, only slowly
diminishing through the evening and overnight hours.  Given the very
richly moist and favorable unstable environment in place, beneath
parallel-oriented midlevel flow, one or two organized MCSs may
result.  A conditional derecho potential exists, depending on timing
and location of upscale organization within this corridor.  If a
complex moves out of east TX, or develops far enough west in MS/LA
and is sustained all the way across the remainder of the Gulf Coast
States tonight, either readily could qualify.

One factor that may limit overall wind potential is the gradual
nocturnal stabilizing of the boundary layer.  However, sufficient
cold-pool organization, rear-inflow-jet development and forced
ascent of the still-favorably moist air may produce enough of a
vertical circulation throughout an MCS to overcome the nocturnal
influence.  At this time, too many mesoscale uncertainties remain to
insert an unconditional 45%/MDT wind corridor, but that may need to
be done in a succeeding outlook if convective guidance comes into
better agreement and/or mesoscale trends warrant.  A few tornadoes
also may occur, especially near the outflow boundaries where
low-level vorticity/shear will be maximized.

...Mid/upper Ohio Valley to Tidewater...
Scattered to widely scattered thunderstorms are possible today
across parts of the Ohio Valley region, with damaging to isolated
severe gusts and marginally severe hail possible.  Activity should
occur in an environment characterized by steepening midlevel lapse
rates ahead of the ejecting MO/IA/IL perturbations, modest
convergence near and ahead of the surface low, adequate near-surface
moisture with dewpoints in the 50s to mid 60s, and modest diurnal
heating.  MLCAPE up to the 1000-1500 J/kg range is possible.  Though
upper flow will be strong, nearly unidirectional profiles and lack
of greater midlevel winds should keep effective-shear magnitudes in
the 30-40 kt range over much of the area.

Farther east/southeast, cool/stable air north of a damming/preceding
frontal zone across VA and the Delmarva should limit the northern
end of severe potential.  Destabilization and available moisture
will be substantially limited today in the nominal warm sector,
between the damming front and a great deal of MCS/outflow activity
to the south over the Carolinas/GA.  Therefore, while thunderstorms
are possible this afternoon near the max temperature hours, and deep
shear may be adequate for a few organized cells, severe potential
appears isolated and marginal at best.  As such, severe
probabilities over much of the region have been reduced.

..Edwards.. 05/09/2024

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