899
ACUS02 KWNS 211711
SWODY2
SPC AC 211709

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1209 PM CDT Thu Aug 21 2025

Valid 221200Z - 231200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE NORTHERN/CENTRAL HIGH PLAINS TO THE UPPER MIDWEST...AND OVER
SOUTHWEST AZ AND FAR SOUTHEAST CA...

...SUMMARY...
Strong to locally severe thunderstorms are possible Friday from
parts of the central and northern High Plains into the Upper
Midwest, and across southwest Arizona into far southeast California.

...Northern/Central High Plains to Upper Midwest...

An upper cyclone centered over Manitoba Friday morning and its
attendant shortwave trough extending into the northern Plains will
develop southeast into Ontario and Upper Midwest through early
Saturday. At the surface, a trailing cold front will spread
east/southeast across the region, extending from northeast MN into
southwest NE/northeast CO by 00z, before continuing on to portions
of the U.P. of MI to southwest KS/southeast CO by Saturday morning.

Ahead of the front, seasonal boundary layer moisture will be in
place, and pockets of weak to moderate destabilization are forecast.
However, lapse rates are generally expected to remain modest across
the Upper Midwest, which may limit updraft intensity somewhat.
Additionally, remnant convection from the Day 1/Thu period may be
ongoing across parts of MN at the beginning of the period. The
extent of this activity and associated cloud cover may temper
stronger heating/destabilization as well. Nevertheless, as
large-scale ascent increasing with the approach of the upper trough
and along the surface cold front, isolated strong to severe storms
may develop during the afternoon and evening, posing a risk for
strong gusts and marginal hail.

Further southwest into the High Plains, the post-frontal upslope
flow regime will likely result in isolated thunderstorm development
across parts of western WY/CO, with convection spreading into parts
of SD/NE. Steeper lapse rates and a deeper, mixed boundary layer may
support strong/severe gusts and isolated hail.

...Southwest AZ into far southeast CA...

A surge of monsoonal moisture is expected over the region on Friday
amid steep lapse rates and hot boundary layer conditions. This
should support modest destabilization, with strong DCAPE near 1500
J/kg noted in forecast soundings. Around 20-35 kt east/northeast
mid/upper flow atop weak westerly low-level winds will foster around
30-40 kt effective shear magnitudes. Initial thunderstorm activity
is expected to develop over higher terrain. Most CAMs guidance and
HREF calibrated/ML guidance suggest sufficient clustering and
surging outflow will move east/southeast across southwest AZ/far
southeast CA. Given the deeply mixed boundary layer/steep lapse
rates, have introduced a Marginal risk (level 1 of 5) for severe
wind gust potential across the lower deserts/Lower CO Valley
vicinity.

..Leitman.. 08/21/2025

$$