998
FXUS65 KRIW 310007
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
607 PM MDT Sat May 30 2026

.KEY MESSAGES...

- A weather system moving through the region keeps it cool and
  mostly cloudy conditions tonight.

- Tonight`s precipitation will be focused over the mountains and
  western valleys, but basins in central and northern Wyoming
  will see hit- or- miss scattered showers as well.

- Accumulating snow is likely in the higher elevation mountains
  tonight through Sunday, mainly above 10000 to 11000 feet.

- After the current weather system exits Sunday night, dry
  conditions prevail with temperatures gradually warming up over
  the first half of the work week.

&&

.UPDATE...
Issued at 915 AM MDT Sat May 30 2026

No major changes to the forecast today after looking through
12Z high-resolution model guidance. My esteemed colleague
articulated the evolution of the current system well in the
Discussion section below, and this overall thinking holds.

Highlighting a few items here: Central Wyoming basins look to
remain mostly dry during the daytime hours today, with only
isolated showers and perhaps a few thunderstorms, while most of
today`s moisture falls over southwestern Wyoming and over the
mountains in the form of snow above about 10000 ft. 12Z model
guidance highlights the Bighorn Mountains, Johnson County, and
the greater Powder River Basin as having decent (80%) chances
for more numerous showers and overnight thunderstorms as the
core of the upper low rotates through that area. Thunderstorms
are not expected to be strong, but are notable for being
outside of their usual 2 PM to 7 PM diurnally normal occurrence
window, thanks largely to cold air aloft creating some
instability without the help of solar surface heating. For the
system as a whole, forecast precipitation amounts have trended
down slightly, but many locations will still receive a pleasant
late Spring soak.

&&

.DISCUSSION...
Issued at 144 AM MDT Sat May 30 2026

Showers will continue to increase early this morning as the upper
level low approaches, mainly occurring over western and southern
portions. A sfc reflection of the low will develop near La
Barge/Farson by mid-morning and track to the northeast over the
Divide and into the Wind River Basin through the rest of the morning
into the early afternoon. Showers will rotate to the northwest
during this time, with showers being roughly confined along and west
of a Cody-to-Rock Springs line. Dewpoints will drop into the middle
30s over Natrona County, eastern portions of Fremont County, far
southern portions of the Bighorn Basin and Johnson County by 21Z as
a result of southerly winds shunting the available moisture
northward. Most areas east of the Divide are likely to stay dry
during the afternoon. Northern portions of the Bighorn Basin and
Johnson County will be the exceptions, with Johnson County having
the higher chances. A boundary will be in place that could provide a
trigger for any shower/storm to develop in this area, but will
quickly move northward due to the aforementioned southerly winds
pushing the boundary northward. The potential for any strong storm
will be in this area, with forecasted CAPE values of 1000-1500 J/kg
and LI values < -6. Shear will not be strong, so instability will be
the main driver for any severe storm. Otherwise, isolated to
scattered thunderstorms will be embedded within the aforementioned
shower activity over the west this afternoon.

The upper level low will slowly track over the Cowboy State through
the overnight hours tonight, reaching the northeast corner of the
state by 12Z Sunday. Showers and will continue over western portions
and Sweetwater County early in the evening, with additional showers
developing over the Bighorn and Wind River Basins after 06Z on the
backside of the low. Showers will increasingly become confined over
northern portions after 12Z Sunday as the upper low continues its
progression over eastern MT/the western Dakotas through the day.
Winds will increase across the CWA Sunday, as the gradient tightens
in the wake of the departing low. Gusts of 25 to 35 mph will occur
through much of the day, quickly decreasing through the evening.

Rainfall amounts over 0.10" from today through Sunday are still
forecasted to occur along and north of a Kemmerer-to-Riverton-
to-Kaycee line, with far southwestern portions of Sweetwater
County included. The heaviest amounts of 0.75 to 1" are still
expected over northern and northwestern portions of the CWA.
Flooding or rain-on-snow is not expected from this event, as
most snow in the 9000 to 10000ft range (SnoTel sites) is gone
already.

Isolated showers and thunderstorms will be possible Monday, as a
shortwave moves over the Cowboy State. Winds aloft will be
southwesterly as a trough develops over the Northern Rockies from
the remnants of today`s upper low. Most of the convection looks to
develop over the western mountains between 19Z and 21Z and quickly
push eastward over areas east of the Divide and Sweetwater County.
Additional showers will move over northwestern and northern portions
late in the afternoon into the evening, as a second shortwave
rotates through the base of the trough. Tuesday looks to stay mainly
dry, with west-southwest winds aloft keeping any potential
convection over far eastern portions of the state. Showers and
thunderstorms will be possible again Wednesday and Thursday, with
dry conditions returning for Friday. Temperatures will be above
normal these days, with highs in the upper 70s into the 80s.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 517 PM MDT Sat May 30 2026

Scattered showers and thunderstorms will be ongoing across much of
Wyoming from the start of the TAF period into the overnight hours.
There is higher confidence (40 to 60 percent) that KJAC, KCOD, and
KPNA will have shower and/or thunderstorm potential through at least
the first half of the TAF period. Shower and thunderstorm potential
should largely come to an end, at the latest, by 05Z/06Z at KRKS,
KRIW, KLND, and KBPI. There is a 30 percent chance of light rain
moving into KRIW, KBPI, and KLND overnight. Rain showers arrive at
KWRL around sunrise Sunday and persist the remainder of the period.
Thunderstorms are possible (30 percent) again late in the TAF period
at KCPR, KCOD, and KWRL. MVFR to IFR flight conditions are possible
in any shower or thunderstorm.

Expect wind to range from 5kts to 15kts this TAF period, with wind
gusts of 20kts to 30kts at most terminals. The exception terminals
are KWRL and KJAC, where frequent gusts are not forecast. However,
gusty and erratic winds are possible in and around any thunderstorm.
Mountain obscurations will be common through at least 18Z,
especially around showers and thunderstorms.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

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

&&

$$

UPDATE...VandenBoogart
DISCUSSION...LaVoie
AVIATION...Gerhardt