SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0257 PM CDT Fri Aug 15 2025
Valid 152000Z - 161200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THOUGH TONIGHT
FROM THE UPPER MISSISSIPPI VALLEY INTO SOUTH DAKOTA...
...SUMMARY...
Scattered strong to severe storms with damaging winds and large hail
are likely through tonight from the Upper Mississippi Valley into
South Dakota.
...Upper MS Valley through tonight...
Convection is ongoing in a cluster across north central IA, with
clear evidence of cold pool development and a bowing structure. The
storms will likely continue to spread eastward/southeastward through
the afternoon while encountering a very unstable environment
characterized by surface temperatures in the upper 80s and
boundary-layer dewpoints in the low-mid 70s. Cluster is expected to
be maintained given deep-layer shear vectors near 30 kt (oriented
west-northwest to east-southeast), and steep midlevel lapse rates
will support the potential for occasional damaging/severe outflow
gusts and isolated large hail with the strongest embedded updrafts.
Otherwise, a strengthening low-level warm advection regime will
develop tonight into MN along a slow-moving front, and atop the cold
pool from the ongoing IA storms. Elevated buoyancy will remain
large, with the potential to support additional severe storm
development with occasional large hail and damaging/severe outflow
gusts.
...NE/SD this evening into tonight...
High-based convection is expected to form this afternoon from
southeast WY into the western NE Panhandle and southwest SD. The
initial storms will be capable of producing a few severe outflow
gusts in an environment with deep inverted-V profiles and modest
buoyancy. Though surface convective inhibition will increase this
evening with diurnal cooling, storms could grow into a cluster
through outflow interactions, while moving into richer low-level
moisture and larger buoyancy with eastward extent (along and south
of a baroclinic zone across SD).
..Thompson.. 08/15/2025
.PREV DISCUSSION... /ISSUED 1130 AM CDT Fri Aug 15 2025/
...MN/IA/WI...
Recent satellite imagery shows a pair of convectively generated
vorticity maxima moving across the northern/central Plains, with the
northernmost vorticity max over the eastern ND/SD border and the
southernmost vorticity max over north-central NE. Showers and
thunderstorms have persisted near these features over the last
several hours, but the strongest cells in both of these clusters
have recently shown a weakening trend. A moist and buoyant airmass,
characterized by dewpoints in the upper 60s/low 70s, exists
downstream and redevelopment is possible ahead of both of these
vorticity maxima. How that redevelopment evolves is uncertain, with
recent guidance offering varied solutions regarding overall coverage
and evolution.
Redevelopment appears most probable with the southern vorticity
maxima, given the ample moist and buoyancy that exists downstream.
This development should grow upscale quickly, with the resulting
convective cluster then tracking southeastward. Recent guidance
continues to trend farther south, so the wind probabilities were
extended southward to cover this potential. Damaging gusts appear
like the most likely threat through the evening. Current progression
of the northern vorticity maximum takes it into a less buoyant
airmass across central MN and northern WI, and redevelopment
along/ahead of this disturbance appears less likely.
Depending on the strength of the IA cluster, additional development
is possible in its wake amid a strengthening low-level jet and
associated warm-air advection over its outflow. Steep mid-level
lapse rates should still be in place, supporting strong updrafts and
large hail with these elevated storms.
Lastly, a decaying convective line may reach the southwestern
MN/northwestern IA vicinity late in the period. Given the potential
for preceding storms, the amount of buoyancy that will still be in
place remains is uncertain. However, if thunderstorm coverage is low
or most storms develop farther east, enough buoyancy could remain to
support strong updrafts capable of damaging gusts and/or hail within
the convective line or any preceding more cellular storms.
..NE/SD this afternoon through tonight...
Strong heating and ample mid-level moisture will result in scattered
high-based thunderstorm development across eastern WY and western
NE/SD by late afternoon. Isolated hail and strong gusts are possible
with this initial activity. This activity will then track
east-northeastward through the early evening into a progressively
more moist and unstable air mass across NE and SD. The combination
of improving thermodynamic environment and an intensifying nocturnal
low-level jet will likely aid in thunderstorm intensification and
upscale growth through the evening across northern NE and central
SD. Guidance continues to vary significantly with the strength of
this MCS, but general consensus would suggest the threat for strong
to severe gusts will persist across SD tonight. There is at least a
low probability for the development of a well-organized MCS that
could produce significant severe gusts. However, that probability is
currently too low to introduce an unconditional sig wind area.
...TX Coastal Plain into the Hill Country and Southeast TX...
A maturing tropical disturbance is near south TX, with associated
outflow currently spreading northward across the TX Coastal Plain.
Limited lightning has been noted with in the short-lived cores that
have been developing along this outflow. This process of brief
intensification along this outflow is expected throughout the day,
with an overall increase in thunderstorm coverage anticipated. A
modest increase in low to mid-level flow is expected along the
eastern periphery of this system, spreading northward into central
and southeast TX. Even so, shear will remain weak, with a
disorganized multicellular pulse storm mode anticipated. Given the
very moist profiles (i.e. PW values over 2"), a few water-loaded
downbursts are possible, but should be isolated enough to preclude
the need for severe probabilities.
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