SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0758 AM CDT Sat Aug 03 2024
Valid 031300Z - 041200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
MINNESOTA AND SOUTH DAKOTA...AND FROM THE CAROLINAS PIEDMONT TO THE
DELAWARE VALLEY...
...SUMMARY...
The greatest severe-thunderstorm threat areas are over parts of
Minnesota and South Dakota (for severe hail and gusts) and from the
Carolinas Piedmont to the Delaware Valley (gusts).
...Synopsis...
In mid/upper levels, mean ridging will hold over the West, anchored
by a slow-moving high over the Four Corners area. However, the high
and surrounding anticyclone will weaken slightly through the period,
as a series of shortwaves contributes to height falls around its
western, northern and eastern fringes. One of these is an MCV and
associated trough over western KS and extreme eastern CO, forecast
to move slowly southward through the day. With additional
convective vorticity augmentation, this feature should proceed
southward over parts of the TX Panhandle and northeastern NM
tonight.
In the northern stream, a field of gradual height falls and difluent
mid/upper flow is expected to spread across much of the Upper
Midwest and upper Mississippi Valley through tonight. This will
occur without substantial/embedded perturbations over the CONUS, but
with several upstream Canadian shortwaves approaching the
international border. Downstream, a positively tilted trough was
evident in moisture-channel imagery from southern ON across Lake
Erie, OH, central KY and parts of the Tennessee Valley region. This
trough is expected to deamplify gradually and become more positively
tilted, as the northern part ejects with an eastward motion
component faster than the rest. By 00Z, the trough should extend
near a RME-PIT-CSG line. By 12Z, only slight eastward axial shift
is expected to near BTV-BKW-MCN, with continued gradual weakening.
Still, this trough should maintain enough identity to contribute to
recurvature of the developing tropical cyclone over the eastern Gulf
by tomorrow, as discussed by NHC.
At the surface, 11Z analysis showed a low over southeastern ON, with
weak cold front southwestward cross OH, central KY, northwestern MS,
southern AR, and north TX. This boundary should move slowly
eastward across the up er Ohio Valley and central Appalachians,
decelerating over the inland Mid-Atlantic and southern Appalachians
through the period, while drifting southward into central TX and
weakening. Another low was drawn over north-central ND, with cold
front across southwestern ND and south-central/southeastern MT.
This low should move/redevelop southeastward to southeastern MN
through tonight, with the cold front reaching southeastern SD, the
NE Sandhills, and north-eastern WY by 12Z. A warm front --
initially drawn from the low south-southeastward over the FSD/SUX
area, should shift diffusely northeastward today over northern IA
and southern MN.
...Upper Midwest...
Isolated to scattered thunderstorms may develop this afternoon
along/ahead of the front and near the low, over portions of central
MN into eastern SD. Some of this activity may become supercells,
with large to very large hail and locally severe gusts possible,
Some upscale clustering then may occur, shifting the relative severe
threat toward gust potential with southward extent. By mid/late
afternoon, mid/upper 60s surface dewpoints -- near the warm front
and westward ahead of the cold front -- will underlie very steep
low/middle-level lapse rates related to the northeastern fringes of
an EML. This will contribute to large buoyancy, with 3500-4500 J/kg
MLCAPE in several forecast soundings over the outlook area. Modest
midlevel winds limit effective-shear magnitudes to 30-40 kt, but
with enough low-level hodograph curvature to support supercell
potential. Hail models and analog data reasonably suggest
significant (2-4-inch) hail possible in any sustained supercell(s).
The area of greatest convective/severe potential will be bound by
the cold front on the north, decreasing and less-favorable low-level
theta-e through the warm-frontal zone on the east, strengthening
capping on the west, and lack of lift in the free warm sector on the
south. Considerable uncertainties remain mainly with the strength
and positioning of three of those bounds (excepting the front).
This likely is the reason that synoptic models and CAMs guidance
since 00Z have high spread in convective timing and extent, aside
from typical biases and differences in model physics. Later
initiation (stronger western capping) may yield lower coverage
skewed eastward (into MN) with less upscale-growth potential, and
vice versa. Cold-pool depth/strength (if one can organize from
early convection) will play a major role on how far south the threat
extends away from the front and into the warm sector. Given the
uncertainties and conditionalities, little change is justified to
the unconditional outlook area at this time, but mesoscale
diagnostic trends and later guidance may compel substantial changes,
especially if capping appears stronger than expected.
...Mid-Atlantic and vicinity...
Scattered to locally numerous thunderstorms are expected to form
from the Blue Ridge vicinity eastward across the Piedmont, spreading
into the Atlantic Coastal Plain with time. Scattered damaging winds
(trees/wires, etc.) and at least a few severe gusts are possible.
Though the mid/upper trough will be slowly weakening, associated
large-scale ascent still should be more than sufficient to support
an extensive corridor of convection, in the presence of diurnally
minimized MLCINH, steep boundary-layer lapse rates, convergence
along/ahead of the front, and rich moisture. Surface dewpoints
commonly in the low 70s will persist across most of the region,
combining with diurnal heating and offsetting modest mid/upper-level
lapse rates enough to yield MLCAPE around 2000-3000 J/kg over the
Carolinas Piedmont region, transitioning to 1000-2000 J/kg in the
northern Mid-Atlantic. The area of greatest convective
concentration (in and near the "slight risk") should reside beneath
a channel of 25-35-kt southwesterly effective-shear vectors,
supporting multicellular organization. The well-mixed subcloud
layer will foster strong to sporadic severe gusts over the region.
...FL...
NHC forecasts Tropical Depression 4 to cross west-central Cuba
obliquely today then turn more northward over the eastern Gulf,
becoming a tropical storm tonight. The peripheral eastern
semicircle of the circulation will pass across the FL Keys, then
spread up the southwestern FL Gulf Coast, potentially with favorable
low-level shear/hodographs for supercells extending inland a couple
counties. With the system still in early organizational stages for
much of this period, along with uncertainties on both size of
favorable wind field and character of outermost mesoscale convective
structure, the unconditional day-1 tornado threat is held at
"marginal" levels for the time being. Refer to NHC advisories for
latest tropical watches/warnings and track/intensity forecasts for
this system.
...South-central High Plains...
Widely scattered to scattered, high-based thunderstorms are forecast
to develop this afternoon -- perhaps starting as early as midday
near the MCV -- and probably later near the outflow boundary. This
activity should develop atop an environment of diurnally weakened
MLCINH, amidst strong heating and mainly 50s F surface dewpoints
following mixing. Thunderstorms should move mainly southward across
the outlook area, with some southwestward translational shift
possible for any convection that aggregates into a cold-pool-
building cluster. A well-mixed subcloud layer with "inverted-v"
thermodynamic profile is expected, contributing to potential for
downdraft acceleration and at least isolated severe gusts. The
concentration of severe wind on the mesobeta scale will depend on
upscale growth and cold-pool organization. Despite modest deep-
layer windspeeds, strong veering with height and some gradient-flow
enhancement from the MCV will contribute to 30-40-kt effective-shear
magnitudes, supporting potential for a supercell or two and isolated
hail potential as well, in early stages of the convective cycle.
..Edwards/Grams.. 08/03/2024
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