332
FXUS65 KRIW 301814
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1214 PM MDT Sat Aug 30 2025

.KEY MESSAGES...

- Shower and thunderstorm chances this afternoon. The greatest
  chances (50%-70%) are across Johnson County, including the
  east slopes of the Bighorns. Locally heavy rain is possible.

- A drier trend Sunday through Tuesday of next week.
  Temperatures will be cool to seasonal with highs in the upper
  70s to upper 80s. A cold front could bring highs down to the
  60s and 70s on Wednesday and Thursday.

&&

.UPDATE...
Issued at 1213 PM MDT Sat Aug 30 2025

No major changes to the forecast with this update. Today features
the last bits of moisture from the recent monsoonal pattern. The 12z
weather balloon flight had a precipitable water (PWAT) value of 0.70
inches, which continues the downward trend over the past few days;
the peak was this past Wednesday. Hi-res models have isolated
showers and storms along and east of the Divide this afternoon and
evening. Coverage will be less than previous days. A deepening
trough to the east will bring more widespread showers and storms to
Johnson County, and to a lesser extent Natrona County. Chances here
are higher than elsewhere, generally 50 to 70 percent. With decent
moisture still present, moderate to heavy rain is possible,
especially along the eastern Bighorn Mountains from north of Kaycee
to around Buffalo. Showers may linger through about midnight.

&&

.DISCUSSION...
Issued at 118 AM MDT Sat Aug 30 2025

Synopsis...Precipitation is expected once again today, with drier
conditions returning Sunday. An upper level ridge will build over
the Great Basin into the Northern Rockies today, helping to push the
moist airmass that has been in place this week out of the area. The
Cowboy State will then be sandwiched between this ridge and a strong
longwave trough. This trough will encompass much of the eastern half
of the CONUS, including the Great Plains. As such, the forecast for
the middle of next week could become highly variable in the next few
days as models come into better agreement. At this point, it is
looking like a tale of two Divides where it will be warm with
chances for precipitation west of the Divide and seasonal and dry
east of the Divide Wednesday and Thursday.

Discussion...Showers and embedded thunderstorms over southern
portions of the Bighorn Basin into the Wind River Basin will
continue to move toward the southeast through the rest of the
morning, ending as they move into western and southern portions of
Natrona County after sunrise. Lightning will be the main threat.

Showers and thunderstorms are expected once again today, with
Johnson and Natrona counties having the better chances. This will be
due to a remnant low that continues to linger over the Black Hills
region. There is a possibility for brief heavy rain occurring
with storms over the east slopes of the Bighorns this
afternoon. Additional showers and thunderstorms will develop off
the east slopes of the Absaroka and Wind River Mountains and
move over the Bighorn and Wind River Basins into the evening.
Precipitation is expected to end across the area by midnight.
Otherwise, temperatures will be seasonal today and winds
relatively light. Gusts up to 25 mph will be possible through
the afternoon.

Conditions are expected to be mostly dry through Tuesday, with
temperatures ranging from the upper 70s to upper 80s. A cold front
looks to move across areas east of the Divide by Wednesday. Models
are in disagreement on timing and strength, with the GFS being the
(typically) faster solution and bringing it through Tuesday night.
The ECMWF has had more run-to-run consistency and brings it through
on Wednesday. Additionally, it is the more colder solution and drops
700mb temperatures to 0C to 2C. Consequently, the main storm system
will bring cold and windy conditions across the Northern Plains and
the Upper Midwest from Tuesday night into Friday (at least). The
influence of the ridge to our west will determine how much of this
system will impact the CWA. This ridge will eventually move over the
region Thursday and Friday, as that storm system moves over eastern
portions of the CONUS and Canada.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1145 AM MDT Sat Aug 30 2025

VFR conditions prevail over the next 24 hours. Isolated showers
and thunderstorms are possible (20% to 40% chance) east of the
Continental Divide this afternoon and evening. KBYG, KCPR, and
the Powder River Basin have the highest chances of seeing a
nearby storm. Overall winds will be light today area-wide, but
gusty outflow winds of 20 to 30 kt are likely in the immediate
vicinity of any thunderstorms.

Skies clear overnight, but given decreasing atmospheric
moisture, fog is not expected at any TAF terminals. Light winds,
mostly clear skies, and VFR conditions prevail again tomorrow
(Sunday), with fewer showers and storms.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Wittmann
DISCUSSION...LaVoie
AVIATION...VandenBoogart