587
FXUS65 KRIW 070446
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
946 PM MST Sat Dec 6 2025

.KEY MESSAGES...

- Moist, northwest flow aloft will persist into the early
  morning hours Sunday allowing for continued light snow showers
  in the western mountains and in and around the Bighorn Range.

- After a quieter morning Sunday, another round of increased
  snowfall rates arrives for the western Wyoming mountains
  beginning early Sunday afternoon. The snow continues through
  Sunday night with a 40 percent chance of new snowfall topping
  six inches by midday Monday.

- Above-normal temperatures and windy conditions are expected
  across the area for much of the upcoming week. An active
  pattern will keep near-continuous snow chances for the western
  mountains during this time.

&&

.UPDATE...
Issued at 117 PM MST Sat Dec 6 2025

Snowfall rates are beginning to decrease across western Wyoming
early this afternoon as moisture and jet support dwindle. There has
been little change in additional forecast accumulation rates through
today, and most of the highlights will expire at 2 PM. We`ll keep
the Winter Storm Warning going across the western slopes of the
Bighorns through sunset. Lighter snow will then continue through
tonight across the same areas. One change we`re seeing in the short
term is an uptick in snow potential across western Wyoming on
Sunday. While midlevel heights will generally rise across the
state, a quick shortwave will combine with warm air and moisture
advection up the Snake River Plain to produce a period of
moderate snowfall Sunday afternoon and night. As usual this will
be most noted in the Tetons where guidance is now showing
around a 70% chance of another 8+ inches; surrounding west-
facing slopes will also be favored to see 6+ inches during this
time. A short Winter Weather Advisory may be considered for the
Tetons given the potential for travel impacts over Teton Pass
Sunday afternoon and night.

Otherwise, the gusty west wind seen across the area will decrease
through the afternoon. However, wind will not shut off completely
given the active northwesterly flow and another building pressure
gradient for Sunday. This will keep breezy conditions for most
through the weekend.

&&

.DISCUSSION...
Issued at 247 AM MST Sat Dec 6 2025

A strong, 120kt to 130kt upper level jet continues to advect
moisture rich air into the Intermountain West early this morning.
Wyoming is in the left-exit region of this upper level jet, a region
where upper level divergence and lift is maximized. These favorable
upper level dynamics combined with abundant moisture (above the
climatological 90th percentile) will continue support periods of
moderate to heavy snowfall across western Wyoming and the western
Bighorn Mountains today. Thus, no changes have been made to the
Winter Weather Advisories and Winter Storm Warnings. Additional
snowfall amounts over the Teton and Gros Ventre Mountains and the
Salt River and Wyoming Ranges range from 5 to 12 inches, with higher
elevations seeing up to 20 inches, through the early afternoon. The
snow show for the western Bighorns is expected to continue with
possible (60-80% chance) snowfall rates of 1"/hour from 5AM to noon
today. These high snowfall rates are partially due to favorable
upsloping across the western Bighorns. Total snow accumulations of 8
to 15 inches are likely (80% chance) across the western Bighorns
through sunset today. Periods of snow are also possible (30-50%
chance) across the eastern Bighorn Basin through sunset.

Additional snow accumulations across Jackson and Star Valleys,
Yellowstone National Park, and the Upper Green River Basin range
from 1 to 4 inches, with locally higher amounts across foothill
areas and higher elevations of Yellowstone. Although these amounts
may seem minimal, webcams across these areas show snow covered roads
and reduced visibilities due to gusty winds. Because these conditions
are expected to continue through the morning, the Winter Weather
Advisories remain in effect. The western Wind River and Absaroka
Mountains have an 60-80% chance of 8 to 15 inches through the
morning. For all of western Wyoming, the periods of moderate to
heavy snow should end by late morning/early afternoon as drier air
begins to move in and support aloft weakens slightly (upper jet
reduces speed to 80kts-100kts).

The other story this morning are gusty winds across much of the
area. 2AM SPC Mesoscale Analysis page shows winds of 40 to 50kts
across the much of Wyoming. These winds are look to somewhat be
translating down to the surface with observations showing gusts of
25 to 45 mph, with isolated gusts above 50 mph in wind prone areas.
These 700mb winds are forecasted to increase to 50 to 65kts behind a
weak boundary through the early afternoon. This means winds at the
surface are forecast to gust 40 to 50 mph, with wind prone locations
seeing gusts of or above 60 mph during this time. No wind highlights
have been added given where Wyoming is in proximity to the upper
level jet and a weak diurnal inversion is just strong enough to
hinder full mixing. By early afternoon, mean 700mb winds decrease to
25 to 35kts across the area as the upper level jet weakens. By this
time, mixing will allow these winds to translate to the surface so
expect a gusty conditions through the afternoon and overnight hours,
especially across the Wind River Basin, the Absarokas, and the
southern half of Wyoming.

There is good consensus that ridging will begin to shift into the
region on Sunday and cut off the widespread abundant moisture that
we have been seeing the past few days. Snow chances (40-80%) on
Sunday will be confined to western Wyoming given favorable
orographics. Elsewhere, a mild day can be expected with slightly
above normal temperatures. A cold front is on track to traverse the
area Sunday night into Monday with snow chances (60-90%) across the
west. However, this cold front will do the opposite of cool us down.
Mean 700mb temperatures are currently forecast between 0C and -5C
Monday through at least Thursday. This reflects widespread high
temperatures between 40 and 50 degrees. This is 10 to 20 degrees
above normal for mid December! A weather system looks to move
through the region Wednesday and with warm temperatures this
means some areas would see rain as opposed to snow. As for wind,
Tuesday through Thursday look to also be gusty with early
signal of a 130kt upper level jet overhead. However, these
temperatures and winds are subject to change given we are 3 to 6
days out.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 946 PM MST Sat Dec 6 2025

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

A passing jet streak aided lingering light snow and MVFR conditions
in northwest flow aloft much of Saturday evening at KJAC and
even at KPNA. However, VFR conditions greet the start of the
period and continue through most of Sunday morning. The next
batch of moisture reaches far west Wyoming during the midday
hours Sunday and begins lowering cloud decks and producing light
snow. MVFR conditions and light snow return to KJAC around
20Z/Sunday with conditions deteriorating to IFR between
00Z-03Z/Monday. KPNA and KBPI see ceilings lower to low-end
VFR during this timeframe. Mountain tops frequently obscured
until 18Z/Sunday, then becoming obscured by 21Z across the far
west mountain ranges. Ongoing west-southwest surface wind
15-25kts at KRKS ratchets up a bit between 18Z-23Z/Sunday, but
for the most part speeds remain in this range through the
period. Afternoon southerly wind around 10kts to prevail at
KJAC, while other terminals are 8kts or less.

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

Mid-level moisture and a passing jet streak responsible for
persistent snow bands through much of Saturday evening are both
beginning to shift away from the region. This leads to decreasing
cloud cover and a loss of banded snow by around 09Z/Sunday.
Terminals to remain VFR throughout the period with mainly high cloud
cover during the daylight hours Sunday. Lee enhanced cloud cover
will occur east of the mountains Sunday afternoon. Gusty west-
southwest wind 14-28kts increases at KCPR and KCOD between 15Z-
18Z/Sunday and continues through the day. The wind should diminish
at KCOD early Sunday evening, but persist at KCPR through at least
the end of the forecast period. Mountain tops frequently obscured
until 15Z/Sunday.

&&

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

&&

$$

UPDATE...Myers
DISCUSSION...Gerhardt
AVIATION...CNJ