Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0244 PM CDT Sat Sep 20 2025
Valid 202000Z - 211200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE SOUTHERN PLAINS...
...SUMMARY...
Scattered strong to locally severe thunderstorms may impact parts of
the south-central Plains this afternoon and evening, accompanied by
an isolated threat for strong to severe wind gusts and marginally
severe hail.
...20z...
...Oklahoma...
The primary update for this forecast was a slight northward
expansion of severe probabilities into far southern KS to encompass
the placement of a weak surface front analyzed in recent surface
observations. This boundary may act as a foci for thunderstorm
development later this evening and likely delimits the northern
extent of appreciable buoyancy/severe threat. Additionally, a
targeted corridor of 2% tornado probabilities was introduced to
parts of northeastern OK and far southeast KS. A residual outflow
boundary is noted in surface observations with temperatures on the
cool side of the boundary increasing into the low/mid 80s and
dewpoints remaining in the upper 60s. RAP mesoanalysis estimates
suggest MLCAPE has increased to around 2000 J/kg along this boundary
with decreasing inhibition, and VWP observations from KINX (Tulsa,
OK) sample 0-1 km SRH on the order of 125 m2/s2. This low-level SRH
and low through the lowest 3 km is notably stronger than depicted by
morning/early afternoon model guidance, and is sufficient for at
least a brief tornado threat (though this threat is conditional on
semi-discrete thunderstorm development on/near the boundary).
Elsewhere, the forecast largely remains on track. Latest GOES
imagery and lightning data show initial thunderstorm development
from the OK/TX Panhandles into southwest OK. Thunderstorm coverage
should steadily increase through the evening across western to
central OK. Regional VWP observations continue to show meager
deep-layer wind shear, which implies that convection will largely
remain only loosely organized. While instances of large hail are
possible, strong heating/boundary-layer mixing will promote sporadic
strong to severe gusts this afternoon/evening (see MCD #2106 for
additional details).
...Illinois...
Consideration was given to introducing 5% hail probabilities across
central to northeast IL. Thunderstorms across this region have
periodically intensified to near severe limits based on MRMS hail
and vertical ice metrics. However, weak shear has limited storm
longevity with most convective cores persisting for only around
10-20 minutes. This trend should continue through early evening, but
the overall duration and coverage is expected to remain sufficiently
limited to warrant broader risk probabilities.
..Moore.. 09/20/2025
.PREV DISCUSSION... /ISSUED 1115 AM CDT Sat Sep 20 2025/
...South-Central Plains...
Visible-satellite imagery late this morning shows a couple of small
thunderstorm clusters over parts of the Ozarks into northeast OK and
mostly clear skies farther west and southwest over OK into the TX
Panhandle. Surface analysis places a zone of outflow extending from
west-central AR northwestward into north-central OK. The primary
front arcs from the OK Panhandle vicinity northeastward into
central/eastern KS. A moist and moderately unstable airmass is
forecast by mid-late afternoon across the Marginal Risk.
A relatively non-descript upper pattern, north of a flattened upper
ridge, features a couple of very minor perturbations slowly moving
east across the central High Plains into the mid MS Valley, to the
south of more amplified disturbances over the upper MS Valley and
ND. Weak capping and the aforementioned seasonably moist airmass
will favor isolated to scattered thunderstorms developing near the
front and outflow late this afternoon into the evening. Diurnal
steepening of low-level lapse rates may yield an isolated risk for
localized severe gusts and perhaps marginally severe hail. This
activity will likely diminish by mid evening owing to the loss of
heating.
...Upper Midwest/Great Lakes...
An upper trough centered near the IA/WI/MN/IL border will continue
to slowly migrate east across WI/northern IL today. Similar to
areas farther southwest over the south-central Plains, a seasonably
moist will undergo some heating in areas void of thicker cloud cover
and aid in 1000 J/kg MLCAPE developing over WI according to forecast
soundings. Weak 0-6 km shear (i.e., < 20 kt) should temper updraft
organization and limit storm intensity. Nonetheless, a couple of
strong gusts or small hail may occur with the more vigorous storms
this afternoon.
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