509
FXUS65 KRIW 032016
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
116 PM MST Wed Dec 3 2025

.KEY MESSAGES...

- Dry and pleasant early December day across the Cowboy State
  today.

- The first significant snowfall for the mountains of western WY
  looks to be on the horizon with snow arriving as early as
  Thursday afternoon/evening.

- The heaviest snowfall looks to occur Friday night through
  Saturday morning with winds increasing at the same time which
  may lead to difficult driving conditions due to reduced
  visibilities and blow/drifting snow.

- Dry conditions and warming temperatures are expected for
  locations east of the Divide through the weekend. A few
  isolated shower cannot be ruled out at times.

&&

.UPDATE...
Issued at 110 PM MST Wed Dec 3 2025

Many locations saw a fresh coating of snow this morning. Amounts
ranged from 1 to 2 inches over central WY with higher amounts of 3
to 4 inches along the Lander Foothills. Other areas such as the
Bighorn Basin, the Green River Basin, and Jackson Hole saw anywhere
from a trace to a few inches. Skies are expected to gradually clear
out through the afternoon. Highs today will range in the upper 20s
to low 30s. Overnight lows are forecast to be chilly with freshly
fallen snow and mostly clear skies leading to temperatures in the
single digits to low teens. As mentioned in the forecast discussion
winds will increase across portions of the state late this evening
through Thursday. No major changes are expected compared to what was
previously discussed in terms of the winds.

Attention now turns to the potential for the first significant
snowfall of the season across western WY. The setup remains mostly
unchanged and this update will be focusing mainly on the impacts.
Snowfall looks to begin Thursday afternoon with periods of moderate
to heavy snowfall occurring through Saturday evening. The bulk
of the snow will remain over the mountains for much of Thursday
and Friday. However a few light snow showers may make their way
into the western valleys of Jackson Hole and Star Valley. Snow
accumulations for lower elevations may be around a few inches by
the time Friday evening rolls around. There are some signs for
a brief lull in snow during the day Friday but recent trends
have shown this lull not being as prolonged as originally
thought. The heaviest snowfall is expected to arrive by the
afternoon/evening hours Friday through the morning hours on
Saturday. Snowfall rates of 1 to 2 inches look likely over the
higher elevation mountains. Lower elevations should see snowfall
increase as well around this time with accumulating snow likely
to begin and persist through Saturday morning. Winds of 20 to
30 mph will be possible during this heavy period of snowfall.
Due to this blowing/drifting snow may occur along with reduced
visibilities leading to overall difficult driving conditions
for western mountain passes and lower elevations. Probabilistic
guidance shows the higher elevations of the Tetons, Gros Ventre,
Wyoming, and Salt River Ranges seeing a 70-90% for over a foot
with a 30-60% for two feet or more across the higher peaks.
Lower elevation valleys such as Jackson Hole and Star Valley
look to have a 60-80% for 8 inches or more. At this time Winter
Storm Watches have been issued for the Tetons, Gros Ventre,
Wyoming, and Salt River Ranges. This watch starts Thursday
evening through Saturday afternoon with a high likelihood of
being upgraded to Warnings. Other locations such as the lower
elevation valleys, YNP, and the Absarokas will not see any
highlights at this time. However, there is a good chance for
Advisory level criteria to be met so highlights for these areas
may come within the next 24 hours.

&&

.DISCUSSION...
Issued at 129 AM MST Wed Dec 3 2025

Snow continues to fall over central portions shortly after midnight
this morning, as it progresses southward. This snow will end over
the next few hours as drier air begins to settle into the area from
the north. Snow will primarily be over before sunrise this morning,
but a flurry or two cannot be ruled out through the rest of the
morning. Wind gusts of 30 to 40 mph will occur across Sweetwater and
southern Lincoln counties this morning, as the cold front continues
to track across these areas. Blowing snow will be a concern for the
morning commute across I-80, US-191 and South Pass/Hwy 28. These
winds will decrease through the morning after sunrise, with light
winds across the CWA today. Temperatures will be 5 to 15 degrees
colder today, as clouds gradually scatter out through the day.
Needless to say, it will be cold again tonight with lows in the
single digits and teens across the area. Fog will be possible over
Yellowstone and could infiltrate into the Jackson Valley from the
north like it did Monday morning. Gusty winds will develop tonight
over the east slopes of the Absaroka and Bighorn Mountains after
midnight, due to a tightening 700mb temperatures gradient. Gusts of
30 to 40 mph will occur, with locally higher gusts around 50 mph
near Clark. Similar winds with gusts of 25 to 35 mph will occur near
Dubois, Crowheart and the Casper area. These winds will continue
through the rest of the morning Thursday.

A large ridge over the EPAC and a Hudson Bay upper low will remain
in place through the rest of the week and be the primary drivers for
our weather for the next several days. A northwest flow pattern will
be in place over the region as a result. The PFJ will round the top
of the ridge tonight into Thursday, bringing the next surge of
Pacific moisture. This will bring light snow to northwestern
portions during the day, with increasing chances over far western
portions Thursday night. This will be the start of a multi-day snow
event for western portions of the CWA. A break in the snow looks to
occur Friday afternoon, before the next round quickly returns by
Friday evening. There is a good chance for steady snow to then occur
across the west from Friday night to early Sunday morning. Initial
snowfall amounts (from Thursday afternoon to Saturday night) range
between 1 to 2 feet in the western mountains and 8 to 12 inches in
the Jackson and Star Valleys. Again, this will be spread out over a
3-day period with the heaviest snowfall occurring Friday night.
Details for areas east of the Divide and southern portions are not
as clear. There is a chance for light snow over these areas Thursday
night and Friday night, but confidence is low for any real impacts
at this point. However, gusty winds do look likely especially
between Kemmerer and Casper. Gusts of 25 to 35 mph Thursday
afternoon and evening and similar speeds returning Friday afternoon.
Winds of 30 to 45 mph are possible Friday night, as the stronger
storm system moves over through the area. The east slopes of the
Absaroka, Bighorn and Wind River Mountains will have similar
conditions. The potential for high winds (wind gusts 60+ mph) will
have to be monitored, as models are forecasting 700mb winds of 50+ kt
occurring over southern portions, and areas east of the Divide (the
Cody Foothills, the Wind River Basin, and Johnson/Natrona counties).

Additional snow chances over northwestern portions will be possible
Sunday through Tuesday, as the ridge flattens. The pattern looks to
remain active next week and are making up for the warm and
relatively quiet November.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1009 AM MST Wed Dec 3 2025

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

Mid-level clouds should clear through the day, with VFR conditions
through the period. Winds will be relatively light and diurnally
driven today into tonight. Clouds begin to increase late tonight and
Thursday morning ahead of the next weather system. Current models
hold off snow chances until the very end, or after, the period. At
KJAC, a few models show snow chances arriving as early as late
Thursday morning, but the consensus is until at least 21Z, where
there is a PROB30 group. Chances increase further after the period.

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

VFR conditions prevail through the period as low to mid-level clouds
thin through the early afternoon. Winds remain relatively light and
diurnally driven through this evening. An approaching weather
system brings increasing winds and mid-to-high-level clouds starting
tonight. Strongest winds will initially be seen over the mountains,
so wind shear is possible at KCOD starting around 02Z. Elsewhere,
winds increase towards the end of the period or shortly after.

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 Storm Watch from Thursday afternoon through Saturday
afternoon for WYZ012-024.

&&

$$

UPDATE...Dziewaltowski
DISCUSSION...LaVoie
AVIATION...Wittmann