Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0252 PM CDT Tue Aug 18 2026
Valid 182000Z - 191200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PORTIONS OF THE
UPPER MISSISSIPPI VALLEY INTO THE GREAT LAKES...ACROSS PORTIONS OF
THE PLAINS...AND ACROSS PARTS OF THE NORTHERN AND CENTRAL HIGH
PLAINS...
...SUMMARY...
Thunderstorms with severe wind gusts and isolated large hail will be
possible this afternoon and evening from the central Plains into the
lower Missouri Valley, as well as the Upper Midwest. Other storms
capable of severe wind gusts and hail will also be possible from the
Northern Rockies into the central High Plains this afternoon into
tonight.
...20z Update...
The primary adjustment for this update was the introduction of
additional 15% wind probabilities across southern Montana into
northern Wyoming and far western South Dakota. Latest GOES imagery
and lightning trends show deepening convection within the higher
terrain of southern to central MT. An approaching shortwave trough
will promote additional ascent and east/southeastward propagation
off the terrain through late afternoon and early evening. As this
occurs, thunderstorms will spread into a warm and deeply-mixed
environment featuring 0-3 km lapse rates on the order of 8-9 C/km
and sufficient buoyancy to sustain convection. Given this favorable
thermodynamic profile and ample shear across the region (sampled by
18 UTC RAOBs from RIW and UNR), the potential for severe winds
appears higher than previously anticipated. Recent HRRR/RRFS
time-lagged ensemble guidance appears to be capturing this trend and
depicts higher severe wind potential than earlier solutions.
...Southeast Wyoming into Colorado and Nebraska...
The early stages of deepening convection is noted near the Casper,
WY area ahead of a weak mid-level disturbance and where daytime
heating has resulted in surface temperatures climbing into the low
80s. While this initial activity will likely remain sub-severe, the
potential for intensification should steadily increase as it spreads
southeast into a more moist air mass. Similarly, cumulus is noted
across north-central CO on the western fringe of the higher-quality
moisture. Guidance continues to show considerable spread regarding
convective coverage and evolution across both regions, but given
these observed trends and a fairly supportive environment downstream
of emerging convection, expanding hail and wind risk probabilities
appeared prudent.
...Northeast Kansas/western Missouri...
Latest guidance continues to show additional thunderstorm
development later tonight within a warm advection regime across
northeast KS into west/northwest MO. However, there has been a
notable northward trend in recent HRRR/RRFS solutions compared to
12z guidance. 15% hail and wind probabilities have been adjusted
northward to account for this trend.
..Moore.. 08/18/2026
.PREV DISCUSSION... /ISSUED 1212 PM CDT Tue Aug 18 2026/
...Central Plains into Lower Missouri Valley...
Scattered weakening thunderstorms continue across northeast
Kansas/southeast Nebraska into northern Missouri, reinforcing a
convective boundary that extends across the region. Thunderstorm
redevelopment/intensification is conditionally possible through the
diurnal heating cycle near the boundary, which would carry with it
the potential for at least isolated hail/damaging winds.
A more certain scenario is expected this evening, when thunderstorm
development/re-invigoration is more probable as the low-level jet
increases. Steep lapse rates and moderate vertical shear support the
potential for thunderstorms capable of large hail during the
initially more cellular mode. These storms will likely cluster
quickly, with a trend towards upscale growth into a
forward-propagating MCS with an increasing damaging wind potential
as it spreads toward/across Missouri.
...Northern Rockies through the Central High Plains...
A shortwave trough will progress southeastward from British
Columbia/Alberta into the northern Rockies. Thunderstorm development
is anticipated across the northern Rockies as this wave interacts
with the destabilized and buoyant airmass across the region.
Moderate westerly flow aloft with support the potential for more
organized storm structures and the development of several line
segments. These widely scattered thunderstorms are forecast to
progress east-southeastward this afternoon/evening, with additional
development possible from Wyoming into the central High Plains. The
moderate westerly flow aloft, high storm bases, and steep low-level
lapse rates support the potential for strong to severe wind gusts
and possibly some increased hail potential through evening/early
overnight as these storms progress east-southeastward toward the
richer reservoir of moisture/buoyancy across the central High
Plains, including western Nebraska/northeast Colorado/northwest
Kansas.
...Upper Midwest including portions of Iowa/Wisconsin/Michigan...
Low-level moisture is expected to increase across the Midwest ahead
of an approaching cold front, resulting in moderate buoyancy by late
afternoon. Near-frontal convergence will influence convective
initiation, with the combination of moderate buoyancy and seasonally
strong westerly flow aloft supporting strong to severe storms
including some initial/semi-discrete supercells, which could yield
large hail. A transition into linear segments is expected by
evening, with damaging wind gusts becoming the more prevalent severe
risk as storms spread generally southeastward.
...Mississippi/Alabama to northern Florida...
Scattered thunderstorm development is expected this afternoon,
particularly across portions of Mississippi into western/southern
Alabama and northern Florida. These storms will develop amidst a
very moist and unstable environment. Vertical shear will be weak,
with an outflow-dominant/multicellular mode expected. Some
clustering into one or more line segments appears probable,
resulting in scattered thunderstorm coverage. Steep low-level lapse
rates and relatively high cloud bases support the potential for
strong downdrafts capable of locally damaging winds.
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