SPC Aug 4, 2024 1300 UTC Day 1 Convective Outlook

[NorthGeorgiaWX]

SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0754 AM CDT Sun Aug 04 2024

Valid 041300Z - 051200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR
TROPICAL-CYCLONE TORNADOES...OVER PARTS OF FLORIDA AND EXTREME
SOUTHERN GEORGIA...

...SUMMARY...
Tropical Storm Debby may produce a few tornadoes over parts of
Florida and extreme southern Georgia through early tomorrow morning
as it intensifies.

...Synopsis...
A persistent, mid/upper-level anticyclone will remain centered over
the Four Corners region through the period, but with deamplification
of the ridge to its north.  This will occur as a long-lived
shortwave trough -- that emanated from the subtropical easterlies a
few days ago and is located over the interior Northwest -- pivots
eastward through the mean-ridge position across the northern
Rockies.  By 12Z tomorrow, this perturbation should reach north-
central/northeastern MT.  Downstream, a belt of relatively enhanced,
slightly difluent northwesterly flow will persist over the northern
Plains and Upper Midwest, maintained by a large cyclone centered
over northeastern Canada.

Farther downstream, a persistent, positively tilted, mid/upper-level
trough was apparent in moisture-channel imagery from western/central
NY southwestward over the central/southern Appalachians to portions
of AL.  This feature is expected to deamplify through the period,
with the middle-northern part ejecting off the Mid-Atlantic and New
England Coasts by about 06Z.  Though weakening, the southern part of
this trough will contribute to the continuing recurvature of what
now is Tropical Storm Debbie, per NHC discussions.

At the surface, 11Z analysis showed a low over the northern shore of
Lake Ontario, with a weak, slow-moving cold front across central NY,
eastern PA, to northern/central VA.  This boundary should move
slowly eastward while continuing to weaken through the period. 
Another, wavy, quasistationary frontal zone was analyzed from the
same low across northwestern OH, southern Lake Michigan, southern
MN, to another low near YKN, then across southwestern SD to
southeastern MT.  This boundary will oscillate on the mesoscale
through the period in response to weak perturbations aloft, and
proximal convective processes.

...FL, southern GA...
Tropical Storm Debby continues to intensify in accordance with NHC
forecasts, and is expected to strengthen further to a hurricane near
the end of the period and just prior to landfall.  [Refer to latest
NHC advisories for track and intensity forecasts for Debby, as well
as tropical-cyclone watches and warnings.  See SPC mesoscale
discussion 1812 for coverage of marginal/near-term tornado potential
very near the west-central FL Coast.]  As Debby strengthens today
into this evening:
1.  Expansion of the peripheral wind fields will provide
supercell-favorable low-level shear and hodograph size, and
2.  The diurnal-heating cycle should destabilize moisture-rich
inland areas of peninsular FL and remove MLCINH, while
3.  Low-level convergence/lift will strengthen in step with system
intensification.

Those factors should enlarge the outer convective pattern eastward
over more of the landmass.  Such development may backbuild into
parts of south FL, but the greater probability for mature supercells
will be in the "slight-risk (5% tornado) area over western/central
FL today, spreading north toward the area near the FL/GA line
overnight.  Late in the period into early day 2, as the system
approaches landfall and precip increases across interior northern FL
and southern GA, an overland, low-level baroclinic zone should
develop and strengthen.  This feature -- with relatively stable,
rain-cooled air to its poleward side -- should act as a fairly
sharp, buoyancy-limiting boundary for the northern edge of the
tornado threat.  However, a looser tornado-probability gradient
remains for now in the outlook lines, since the boundary's specific
location will depend strongly on mesobeta-scale convective/precip
geometry and density yet to be determined, within and just off the
northern rim of Debby's circulation.

...Southeastern/south-central MT and vicinity...
Widely scattered to scattered thunderstorms are expected to develop
over the higher terrain of southwestern/south-central MT this
afternoon, offering a few strong-severe gusts and isolated/marginal
hail.  Steepening of low/middle-level lapse rates and concordant
erosion of MLCINH should occur as:
1.  DCVA and accompanying large-scale ascent /destabilization aloft
spread into the area ahead of the shortwave trough, and
2.  Diurnal heating acts on higher elevations amidst marginal but
sufficient low-level moisture, with lower-elevation surface
dewpoints remaining in the mid 40s to lower 50s through the mixing
cycle.
Though low-level windspeeds will be modest, strong veering with
height north of the front will underlie strengthening mid/upper
winds to contribute to areas of 35-45-kt effective-shear magnitudes
in support of storm organization (and at least brief supercell
characteristics for any discrete activity).  In general, an upscale
aggregation of high-based convection should move eastward over the
outlook area, and over a deep/well-mixed subcloud layer, to support
gust potential.  Activity should weaken considerably during the
evening as the foregoing boundary layer stabilizes.

...Upper Midwest/upper Mississippi Valley...
A corridor of favorable moisture (dewpoints commonly in the 60s,
locally topping 70 F) and diurnal heating may support late-afternoon
development near the front, with isolated severe hail/gusts
conditionally possible.  However, strong capping, on the
northeastern rim of a well-developed EML, may preclude this
scenario.  More confidence exists in later development (evening) in
a regime of increasingly focused low-level warm advection and lift
over eastern SD and southwestern MN, shifting east-southeastward
across the outlook area into tonight.  Steep midlevel lapse rates
will support 1500-2500 J/kg MUCAPE, with around 35-45-kt
effective-shear magnitudes.  Clustering of cells may promote wind
potential, with isolated severe gusts potentially penetrating the
near-surface stable layer, and occasional hail near severe limits
also possible.

...Northeast CONUS...
Widely scattered to scattered thunderstorms are forecast to develop
along/ahead of the weak/leading front and ejecting shortwave trough
through the afternoon, with isolated damaging to marginally severe
gusts being the primary concern.  Marginally severe hail may occur
in the most intense cores, especially over northern parts of the
outlook area under somewhat colder temperatures aloft.  This
activity should occur in a regime of diurnally minimized MLCINH and
rich low-level moisture, with surface dewpoints commonly in the
upper 60s to low 70s F.  This will offset modest mid/upper-level
lapse rates enough to yield peak/preconvective MLCAPE in the
1000-1500 J/kg range (locally/briefly higher).  Low-middle-level
flow should be modest, keeping hodographs small, but with strong
upper/anvil-level winds to aid in multicellular storm organization.

..Edwards/Grams.. 08/04/2024

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