SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Fri Jul 12 2024
Valid 121300Z - 131200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE OZARKS
TO LOWER OHIO VALLEY...AND OVER THE NORTHERN AND CENTRAL GREAT
PLAINS...
...SUMMARY...
Isolated severe wind and hail will be possible this afternoon from
the Ozarks to lower Ohio Valley, and over the northern and central
Great Plains from late afternoon into tonight.
...Synopsis...
Deamplification of the northern-stream pattern is forecast in mid/
upper levels, as a persistent trough over the Great Lakes and
vicinity moves eastward and weakens substantially. A height
weakness will continue to extend from that area southwestward across
the mid Mississippi Valley, Arklatex, and south TX, with several
embedded/slow-moving vorticity lobes. Meanwhile, a broad area of
difluent, zonal to northwesterly flow should continue over the
central/northern Plains region, northeast of a prominent anticyclone
shifting slowly eastward across the Great Basin and Four Corners
regions.
At the surface, the 1Z analysis showed weak lows over northeastern
MT and southwestern SD, along a quasistationary to slow-moving warm
front drawn from southern AB to western SD, central/southeastern NE,
central MO, and southern IL. The slightly better-defined MT low
should move slowly eastward to northwestern ND by early evening,
while the frontal zone to its southeast drifts eastward and
northward over central SD, eastern NE, northeastern KS, and central/
northern MO. A lee trough -- initially drawn from western SD to
eastern CO and the TX Panhandle -- should meander erratically near
its present position today, with some adjustment by falling
pressures related to strong surface heating.
...Northern/central Plains...
Isolated to widely scattered thunderstorms are moving southeastward
over portions of southeastern SD to central N this morning, in a
zone of weak DCVA aloft superimposed on a broader area of low-level
warm/moist advection above the surface. While an isolated,
marginally severe hail report cannot be ruled out in another couple
hours, the overall decreasing trend should continue as the LLJ
likewise weakens through the remainder of the morning.
Isolated to scattered, high-based thunderstorms are expected to
develop over western parts of the outlook area near the lee trough
today, within a broad area of strong diurnal heating. Isolated
development also may occur farther east near the warm front and
greater low-level moisture, though weaker lift and stronger MLCINH
are expected in that corridor than near the lee trough. Activity
west of the front will move atop a deeply well-mixed boundary layer
suitable for strong-severe downdrafts. Surface dewpoints mixing
well down into the 40s to mid 50s will result in decreasing MLCAPE
as MLCINH also weakens, with around 500-1000 J/kg expected in the
most-probable area for storm formation. Buoyancy will increase
eastward as mixing weakens, but capping strengthens, with a corridor
of 60s F dewpoints and 2000-3000 J/kg MLCAPE along and just east of
the front. Considerable uncertainty exists how much of the lee-
trough activity can aggregate cold pools and potentially utilize
forced ascent eastward into the greater moisture, for longer and
better organization.
Outside the lee trough, forcing for convective-scale lift appears
nebulous and poorly focused across the region, with considerable
uncertainty remaining on preferred potential corridor(s) for greater
severe concentration. One possibility is closer to a northern-
stream mid/upper trough north of the international border, but still
with uncertainty on the southern extent into the CONUS of any
resulting MCS. These uncertainties are reflected in a wide range of
convective distributions and intensities across the various
convection-allowing guidance, as well as implied from precip fields
in the deterministic synoptic models and their ensembles. As such,
a broad marginal area is maintained for this outlook cycle, with
embedded upgrade(s) possible if mesoscale trends and 12Z/later
guidance come into clearer focus.
...Ozarks to lower Ohio Valley...
Isolated to widely scattered, non-severe thunderstorms are ongoing
over parts of southern MO, northwestern AR and extreme eastern OK,
in a field of low-level WAA and weak DCVA. A related increase in
convective coverage and intensity is possible farther east and
southeast across portions of this region as the foregoing boundary
layer diurnally destabilizes, with marginally severe gusts or hail
possible in the strongest cells.
One of the vorticity lobes embedded in the height weakness aloft,
over eastern KS to central/eastern MO, is being enhanced by an MCV
from prior/overnight convection now over southwestern MO. This
feature will move slowly eastward today and may contribute to
relatively maximized thunderstorm coverage in an increasingly
unstable airmass from the Ozarks to lower Ohio Valley this
afternoon. Rich low-level moisture (surface dewpoints commonly
upper 60s to low 70s F) and diurnal heating should offset modest
mid/upper-level lapse rates to yield peak MLCAPE in the 2000-3000
J/kg range. Weak low/middle-level flow and modest shear should
limit organization enough that severe potential should remain
isolated and poorly organized.
..Edwards/Broyles.. 07/12/2024
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