SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0756 AM CDT Thu Aug 01 2024
Valid 011300Z - 021200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE MIDWEST AND OHIO VALLEY...
...SUMMARY...
Severe thunderstorms will be possible today across parts of the
Midwest and Ohio Valley, with a threat of damaging winds, isolated
hail, and a tornado or two.
...Synopsis...
In mid/upper levels, extensive subtropical ridging is apparent from
the northern Bahamas across FL, the north-central/northeastern Gulf,
central/west TX, to the Four Corners region, bending northwestward
into a large-scale ridge over the Great Basin and northern Rockies.
This ridging will consolidate around a developing high over the Four
Corners, while weakening over the Southeast/Gulf Coast in response
to northern-stream trough amplification across the Great Lakes. The
main factor in the latter is a strong shortwave trough -- apparent
in moisture-channel imagery over parts of northern/western MN and
western IA, with an embedded 500-mb low over west-central MN. The
trough/low should dig southeastward through the period, with the low
reaching WI by 00Z and trough south-southwestward across MO. By
12Z, the trough should extend from Lake Huron through the low (then
near southern Lake Michigan) to near STL.
The surface analysis at 11Z showed a primary synoptic low over
western Lake Superior, with cold front southwestward across western
IA to central/southwestern KS, the TX Panhandle and northeastern NM.
The KS portion of the front may advance far enough southeastward
today so that its baroclinicity will blend with that of a northward-
drifting, prefrontal outflow boundary now over central/northwestern
OK. Otherwise, the low should move/redevelop southeastward to Lower
MI through the period, while the trailing front advances down the
upper/mid Mississippi Valley to the Ozarks.
...Parts of the Midwest/Ohio Valley...
One or more clusters of strong-severe thunderstorms are possible
between the Ohio Valley and Great Lakes this afternoon into evening,
offering damaging to severe gusts, isolated large hail and a
marginal/conditional tornado risk. A mesoscale corridor of more-
concentrated wind potential may evolve on the region, but still
appears too conditional for greater probabilities at this time.
That is because uncertainty continues regarding the relative
influence on the main afternoon/evening severe potential of:
1. The ongoing remnant MCS across portions of IL/IN, and the
related pressure/theta-e perturbation as it moves eastward over
parts of IN/OH and encounters destabilizing, favorably moist
boundary layer from midday into early afternoon, and
2. Development either side of the trailing outflow boundary across
portions of central IN into southern IL, and especially its northern
extent into heated/modified outflow air from the previous MCS.
A corridor along and south of the outflow boundary, and east of the
continuing MCS, should be characterized by upper 60s to mid 70s F
surface dewpoints. This will combine with strong surface heating to
yield peak/preconvective MLCAPE in the 2500-3500 J/kg range, beneath
difluent yet strengthening mid/upper winds ahead of the shortwave
trough. Deep-layer shear will weaken with eastward extent, but
should be favorable for supercells in the first few hours of the
main convective cycle. Ultimately, one or two dominant, forward-
propagating MCSs may evolve out of either scenario numbered above,
with a swath of damaging to severe gusts possible.
...Upper Mississippi Valley to parts of the Upper Great Lakes...
Closer to the mid/upper low -- across WI and vicinity -- widely
scattered to scattered mainly afternoon-developing thunderstorms are
possible in an arc moving mostly eastward over the region. This
close to the low/trough, deep shear will be relatively weak,
limiting overall convective organization. However, mid/upper-level
DCVA/cooling, and related steepening of deep-layer lapse rates in
conjunction with diurnal heating, will overlie favorable pockets of
low-level moisture, with surface dewpoints commonly in the upper 60s
to lower 70s F. As such, 1500-2500 J/kg of MLCAPE is expected in a
"top-heavy" buoyancy profile (with largest LIs in the 500-300-mb
layer). Pulse-severe hail and gusts will be the main concerns. The
threat should weaken rapidly after sunset.
...Northeastern NM to Ozarks...
The combined outflow/frontal baroclinic zone cross this corridor, as
well as the Raton Mesa and eastern parts of the southern Rockies,
should focus isolated to widely scattered afternoon thunderstorms.
Some of this convection may persist through the evening where
upscale clustering can occur, but in general, should diminish after
dark. Isolated severe gusts are the main concern. The most intense
cores may bring down locally severe hail as well, despite very high
ambient freezing levels.
The kinematic environment along the boundary will be characterized
by strong veering of winds with height, but weak absolute speeds,
rendering largely 25-35-kt effective-shear magnitudes and slow
motions for those thunderstorms not accelerated by local to
mesobeta-scale cold-pool processes. A well-mixed and deep boundary
layer showing "inverted-V" profiles on forecast soundings should
develop through the afternoon, with temperatures commonly over 100F
in elevations below about 4500 ft, and along and south of the
boundary. This will support DCAPE of 1000-2000 J/kg beneath similar
MLCAPE values. Just north of the front, somewhat less-intense
heating/mixing, but richer moisture, will yield an axis of 2000-3500
J/kg MLCAPE near and largely parallel to the KS/OK line. A mix of
wet and dry downburst characteristics therefore is possible across
this corridor.
...Mid Atlantic to GA...
A broad swath of the Eastern Seaboard from the Mid-Atlantic region
to parts of GA, and the central/southern Appalachians, will
experience isolated to scattered thunderstorms today. Strong
diurnal heating and rich boundary-layer moisture will increase
buoyancy and erode MLCINH, beneath broadly difluent mid/upper-level
flow. Large-scale vertical motion will be weak (upward or downward)
over the region, with the bulk of lift coming from residual outflow/
differential-heating boundaries, terrain features, and perhaps
sea-breeze boundaries. Veering of winds with height is likely
beneath modest mid/upper westerly to northwesterly flow, but lack of
greater low/middle-level speeds will limit deep shear. A few
multicellular clusters are possible within this regime, offering
locally concentrated damaging-wind potential, with isolated gusts
near severe limits possible in the most intense, water-loaded cores.
..Edwards/Grams.. 08/01/2024
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