816
FXUS65 KRIW 042300
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
400 PM MST Thu Dec 4 2025

.KEY MESSAGES...

- Breezy to windy condition across parts of the state today with
  light snow increasing in coverage over western WY this
  evening.

- The first significant snowfall for the mountains of western WY
  is on final approach. Periods of light to moderate snowfall
  will be possible tonight through Saturday afternoon across
  the lower elevations with moderate to heavy snow expected for
  the western mountains.

- Widespread breezy to gusty winds are expected Friday afternoon
  through the overnight hours. Gusts of 25 to 35 mph will be
  common across the Wind Corridor with wind prone locations
  seeing gusts around 50 mph.

- The upcoming week looks to see mild and dry conditions return
  to eastern WY with active weather possibly continuing across
  the west.

&&

.UPDATE...
Issued at 108 PM MST Thu Dec 4 2025

The overall forecast remains for the most part on track with only a
few slight adjustments. Firstly, temperatures today will be near
seasonable west of the Divide and on the warmer side of
seasonable east of the Divide. Highs will range in the mid to
upper 30s with some locations nearing 40 east of the Divide,
temperatures west of the Divide will be cooler mainly around the
upper 20s. Gusty winds have arrived at some locations this
afternoon with gusts of 40 or more mph already occurring at
Cody and Rock Springs. The upper Wind River Basin is also seeing
some gusty winds currently with gusts of 30 to 40 mph. These
winds are forecast to weaken slightly overnight but still remain
windy through the overnight into Friday.

As for the next few days the main focus continues to be the incoming
system which will bring rounds of moderate to heavy snow across
western WY. The previous discussion still does a good job of
explaining the setup along with the timing of the system. Overall
the forecast remains mostly unchanged with the only deviation being
a slight shift in snowfall amounts. Western mountains are still
expected to see anywhere from 10 to 20 inches with the higher
elevations seeing 2 to 3 feet. As more hi-res models become
available it seems there has been a slight increase in snowfall
amounts across the Bighorns. Amounts look to have increased in
coverage with much of the range north of Powder River Pass
forecast to see 10 to 18 inches now. The other shift is over
lower elevations of western WY this includes Jackson Hole, Star
Valley, southern Lincoln County, and the upper Green River
Basin. These locations have seen snowfall amounts drop slightly
compared to overnight. A likely reasoning for this is the warmer
temperatures that will be present Friday and Saturday.
Forecasted temperatures at times may be flirting with the
freezing mark with portions of Jackson Hole possible getting
into the mid 30s. However, even with these slightly warmer
temperatures, precipitation is still expected to fall as snow
but accumulation may be hindered at times. Storm total amounts
for these lower elevations look to range from 5 to 10 inches
with lesser amounts of 1 to 4 inches nearby Pinedale and over
southern Lincoln County. Winds increase during the
afternoon/evening Friday across much of the state. This increase
in winds combined with snowfall will likely make travel very
difficult over portions of western WY Friday night through
Saturday morning. Besides western WY, winds are forecast to
increase east of the Divide with widespread gusts of 35 to 45
mph along the Wind Corridor from Casper to Rock Springs. An
isolated snow shower or two may be possible Friday night through
Saturday morning with most expected to remain dry. No
additional winter highlights have been issued or changed.

&&

.DISCUSSION...
Issued at 343 AM MST Thu Dec 4 2025

10Z water vapor imagery shows strong north-northwest flow aloft with
ridging over the Pacific and a broad trough to the east. An embedded
weak disturbance is entering the northwest portions of the state.
This will provide weak support for orographically-induced light snow
chances (20%-40%) across higher elevations of Yellowstone National
Park and the Teton Mountains through just after sunrise. Some breezy
20 to 30 mph winds will also be present from Rock Springs to Casper
as the weak disturbance traverses the state through the morning.
Seasonal temperatures are forecast today ahead of the incoming
weather system.

By the early afternoon, the first "wave" of this multi-day snow
across the west and widespread gusty winds will approach. This first
wave, currently (10Z) moving onshore over the Pacific Northwest,
will mostly be a wind maker across the Absarokas (including the Cody
area) , upper Wind River Basin, and across the Wind Corridor. This
wind will be a result of a tightening pressure gradient ahead of the
second "wave" and associated cold front later tonight. Gusty winds
of 30 to 35 mph will be common across the aforementioned locations,
with isolated gusts of 50 mph possible. Snow with this first wave
will be confined to western Wyoming. Snow amounts this afternoon
(from 11AM to 5PM) are forecast to be up to 0.5 inch across Jackson
and Star Valleys and much of Yellowstone. The Tetons and high
elevations of Yellowstone could see around 2 inches during this time
frame.

The second wave begins around sunset tonight as upper level
divergence is enhanced over western Wyoming, thanks to being in the
left exit region of a 125kt-150kt upper level jet. With abundant
moisture filtering into the region and enhanced lift, due to
orographics and upper level divergence, moderate to heavy snow is
expected from sunset tonight through early Friday morning for the
western mountains. During this time frame, snow amounts across the
Teton, Gros Ventres, Salt River, and Wyoming Mountain Ranges and
high elevations of Yellowstone range from 6 to 12 inches. As for the
western valleys and low elevations of Yellowstone, snow amounts
overnight tonight generally range from 1 to 3 inches.

A lull in moderate to heavy snow is currently forecast between
sunrise and midday Friday as the upper level jet shifts east over
Wyoming. However, light snow will likely (90% chance) be falling
during this time. Once the 125kt-150kt jet is overhead, the third
wave begins early Friday afternoon and continues through Saturday
morning. During this time, moderate to heavy snow is forecast across
most of western Wyoming. Snow totals across the Teton, Gros Ventres,
Salt River, and Wyoming Mountain Ranges range from 1 to 2 feet with
the highest elevations seeing 3 feet. Thus, the Winter Storm Watch
has been upgraded to a Winter Storm Warning beginning 5PM tonight
and continuing through 2PM Saturday. Travel over Teton, Togwotee,
and Salt River Passes could become very difficult, especially during
both "waves".

Winter Weather Advisories have also been issued for the Upper Green
River Basin, the western Wind River Mountains, the Absarokas,
Yellowstone, and Jackson and Star Valleys. Total snow amounts of at
least 7 inches (80-90% chance) are forecast for Jackson and Star
Valleys, Yellowstone, and the Upper Green River Basin. For these
locations, the probabilities of at least 8 inches ranges from 40% to
80% over a 40+ hour period so opted only for Winter Weather
Advisories at this time. Higher elevations of Yellowstone
(Pitchstone Plateau) could see upwards of 2 feet, however, because
these areas are largely untraveled, kept with the Advisory. Similar
thinking swayed the thinking for the western Wind River MOuntains,
the Absarokas, and the western Bighorns, with snow amounts between 8
and 12 inches. Locally higher amounts are possible (70%) across the
highest elevations of these mountain ranges. It is important to note
that gusty winds look to accompany the moderate to heavy snow,
leading to reduced visibilities.

With the strong upper level jet moving overhead Friday afternoon,
winds begin to ramp up across the Wind Corridor and the Absarokas
with frequent 25 mph to 35 mph gusts. This will likely (80-90%
chance) translate to frequent gusts of at least 35 to 40 mph.
Wind prone locations, such as South Pass and Muddy Gap, could
potentially (50-80% chance) see gusts around 50 mph. Gusty 30 to
40 mph drainage winds across the upper Wind River Basin look to
develop (90% chance) late Friday into Saturday as well. As the
jet moves overhead, 700mb winds of 50kts to 60kts are forecast
to develop over southern Wyoming, the Absarokas and the Upper
Wind River Basin late Friday evening into the overnight hours.
There is a 50% chance for wind gusts 60+ mph to occur in these
areas as a result of the jet max moving overhead and mixing
these stronger winds to the surface. Similar conditions look to
develop through the rest of the Wind River Basin and northern
Johnson County between 09Z and 12Z Saturday. These potentially
high winds could last through Saturday morning, before gradually
decreasing after 21Z Saturday.

The upper-level jet begins to weaken and shift south during the day
on Saturday, bringing an end to the impacts of the winter system.
Ensemble guidance is in consensus that the northwest flow will
persist Sunday into the early parts of next week, allowing for light
snow to chances (30-80%) continue across the west. Elsewhere, dry
conditions and a warm up to above normal temperatures is also
looking more favorable.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 400 PM MST Thu Dec 4 2025

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.

More persistent snow is beginning to move into western Wyoming late
this afternoon. This has brought and will continue to bring IFR
conditions at KJAC this evening through most of Friday morning.
Confidence is lower on snow making it farther east, though there
will be occasional MVFR ceilings at KPNA and potentially KBPI
overnight. KRKS will continue to see a west wind through the period
with gusts generally over 20 knots. Conditions are expected to
improve at KJAC after 17Z Friday, though another push of snow is
expected late in the period. Frequent mountain obscurations are
expected through the period.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.

Snow will have a more difficult time moving east of the Divide. The
primary impact from this system will be strong wind at KCOD. Gusts
will peak over 40 knots through this evening, dropping only slightly
overnight. Otherwise, terminals will remain VFR through most of
tonight. Cloud decks will lower around 12Z Friday, with MVFR
ceilings expected at KCOD, KCPR, and KWRL through the end of the
period. KCPR will have the best chance of light snow after sunrise
Friday and have included a PROB30 group to account for this.

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...
Winter Weather Advisory until 2 PM MST Saturday for WYZ001-002-
008-013-014-023-025.

Winter Storm Warning until 2 PM MST Saturday for WYZ012-024.

&&

$$

UPDATE...Dziewaltowski
DISCUSSION...Gerhardt
AVIATION...Myers