573
ACUS01 KWNS 311929
SWODY1
SPC AC 311928

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0228 PM CDT Mon Aug 31 2026

Valid 312000Z - 011200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE UPPER MIDWEST/GREAT LAKES INTO WESTERN NEW YORK...THE CENTRAL
PLAINS...AND LOWER COLORADO RIVER VALLEY INTO THE GREAT BASIN...

...SUMMARY...
Isolated strong to severe thunderstorms may occur this afternoon
into tonight across parts of the Upper Midwest/Great Lakes into
western New York, the central Plains, and lower Colorado River
Valley into the Great Basin. Severe hail and strong/damaging winds
should be the main threats.

...20z Update...
Minor adjustments were made to remove the Marginal Risk across
portions of northern Arizona/far southern Nevada where lingering
precipitation/cloud cover is limiting instability and drier
mid-level air is filtering in from the west. Otherwise, the forecast
remains on track.  See previous discussion below for more
information.

..Thornton.. 08/31/2026

.PREV DISCUSSION... /ISSUED 1126 AM CDT Mon Aug 31 2026/

...Upper Midwest/Great Lakes into Western New York...
A closed mid/upper-level low over the northern Plains and
Saskatchewan/Manitoba will move generally eastward today, with a
mid-level anticyclone remaining centered over the mid MS
Valley/Mid-South through the period. Between these two features, a
belt of enhanced westerly mid-level winds will overspread parts of
the Upper Midwest and Great Lakes. A rather moist and strongly
unstable airmass is expected to be in place this afternoon across
portions of WI/MI ahead of a weak front. While there are still some
concerns about convective initiation and overall thunderstorm
coverage, it appears that a weak mid-level perturbation over eastern
NE/western IA late this morning will track northeastward towards WI
by late afternoon/early evening. This feature may aid the
development of isolated to widely scattered thunderstorms across
parts of WI into the U.P. of MI.

Given strong to very strong instability and moderate to strong
deep-layer shear forecast, widely spaced supercells capable of
producing large hail appear possible late this afternoon into the
evening across these areas, if convection develops. Considered
greater hail probabilities to account for this scenario, but warm
low-level temperatures (around 850 mb) and mixed signals in
high-resolution guidance cast enough uncertainty on coverage to
defer upgrading with this update. Thunderstorms may also spread from
Lower MI into western NY tonight into early Tuesday morning.
Increasing low-level moisture and related instability may support an
isolated threat for damaging winds with this activity, even though
lapse rates aloft should remain modest.

...Central Plains...
High-based thunderstorms should develop this afternoon across the
higher terrain of the central Rockies/Front Range, and spread
east-northeastward across the adjacent High Plains through the
evening. While low-level moisture/instability will remain limited
across eastern CO and vicinity, steepened low/mid-level lapse rates
amid a very well-mixed boundary layer may still support isolated
strong to severe downdraft winds with this activity. Occasional hail
may also occur with the strongest cores. Thunderstorm coverage
farther east across NE along/near a weak front remains uncertain.
But, any convection which can develop and be sustained across this
area may also pose an isolated severe threat.

...Lower Colorado River Valley into the Great Basin...
A mid-level shortwave trough over CA will move northeastward across
NV today and aid thunderstorm development as the boundary layer
warms into this afternoon across parts of the lower CO River Valley
and Great Basin. Some enhancement to the mid-level flow is forecast,
which should support sustained updrafts and possibly a few transient
supercells, primarily across far eastern NV, southwest UT, and
northwest AZ. Isolated severe hail could occur with these cells.
But, strong to severe wind gusts will probably become the primary
risk with time given steep low-level lapse rates and tendency for
convection to cluster through the late afternoon/early evening.

...Coastal/Southeast Texas into Southern Louisiana...
Reference NHC forecasts/advisories for the latest information on PTC
Five over the northern Gulf. Latest model guidance continues to
suggest that low-level easterly flow on the northern side of this
system will only modestly strengthen before landfall tomorrow. As
such, the degree of low-level shear appears too limited to warrant
low severe wind or tornado probabilities at this time.

$$