338
FXUS65 KRIW 052024
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
124 PM MST Fri Dec 5 2025

.KEY MESSAGES...

- Moderate to heavy snowfall will return to western WY this
  evening persisting through Saturday afternoon.

- A Winter Storm Warning has been issued for the western
  Bighorns due to increased snowfall amounts tonight through
  Saturday evening.

- Strong gusty winds are expected to develop this evening
  becoming widespread overnight through Saturday morning.

- Mild and dry temperatures are forecast to return to locations
  east of the Divide through the middle of next week. Western WY
  looks to have another couple chances for additional snow
  accumulations.

&&

.UPDATE...
Issued at 118 PM MST Fri Dec 5 2025

Not much to add regarding the forecast for the short term.
Temperatures today will be mild across the state with highs in the
upper 30s east of the Divide and low 30s west of the Divide. The
main focus over the next 36+ hours will be periods of snow across
western WY and portions of northern WY along with strong gusty winds
for nearly all the state. Winds will continue to increase this
afternoon/evening as a rather potent jet streak moves over the
region. 700mb winds are relatively strong especially across southern
and northern WY where models show winds of 60 to 70 knots tonight
through Saturday morning. However, these winds do look to struggle
to translate down to the surface. Hi-res models show near High Wind
Criteria at the surface mainly over southern WY and portions of the
Upper Wind River Basin and Cody Foothills. Due to this no wind
highlights are to be issued at this time.

Focusing back on snow not much has changed for western WY with
snow expected to increase in intensity by the evening. Moderate
to heavy periods of snow will settle in over the west through
the overnight into Saturday morning. Snowfall rates are forecast
to exceed 1 inch an hour at times mainly across the higher
elevations of the Teton, Wyoming, and Salt River Ranges. The one
noteworthy deviation compared to the previous discussion is a
pretty large jump in QPF and snow across the central and
northern Bighorns. Likely due to more favorable orographic
dynamics moving into the region overnight. Additional snow
amounts through Saturday evening over the Bighorns have now
increased to 10 to 18 inches with lesser amounts for southern
portions. There will likely be periods of moderate to heavy
snowfall with snowfall rates exceeding an inch per hour at
times. Due to this the Winter Weather Advisory has been upgraded
to a Warning to account for this jump in snow amounts. An
additional note is the western foothills of the Bighorns where
some banding of snow may setup tonight through Saturday evening.
An inch or two of snow over the next 24 hours is definitely
possible especially farther east of Greybull. Snow will
dissipate over much of the state late Saturday with some light
snow showers lingering across the west and Bighorns. The longer
range forecast has not changed much with increased chances for
gusty winds, mild temperatures, and dry conditions returning to
locations east of the Divide. West of the Divide looks to see
more chances for additional snowfall periodically through the
week.

&&

.DISCUSSION...
Issued at 343 AM MST Fri Dec 5 2025

Northwest flow aloft, evident on 10Z water vapor imagery, will
continue through late Saturday night with multiple embedded
disturbances. These disturbances will be the drivers of the
meaningful snow across western Wyoming and the widespread gusty
winds. The snow will be first up for discussion. Moderate snow will
continue to fall across western Wyoming and portions of eastern
Sweetwater County through sunrise this morning as a vorticity maxima
exits the region. However, there is high confidence (90%+) in snow
lightening in intensity across western Wyoming between 8AM and 2PM
local today. This "break" in moderate to heavy snowfall is a result
of a shortwave aloft bringing in slightly drier air due to its
origin over western Canada. Snow is forecast to continue to fall due
to Wyoming being in the right entrance region of an approaching
strong upper level jet. Snow amounts during this lighter snowfall
period will be limited. There is a 40% to 60% chance of at least 1
inch across Jackson and Star Valleys and much of Yellowstone during
this 6 hour period. Amounts across the western mountains are
forecast to be between 3 and 8 inches, with up to 12 inches across
the highest elevations.

There is high confidence (80-90%) in moderate to heavy snowfall
beginning this afternoon (2-3PM local) across western Wyoming and
the west Bighorns. This uptick in snow intensity occurs as the upper
level jet moves overhead and another, more potent shortwave brings
abundant moisture into the region. Expect the greatest snowfall
accumulations to occur between 2-3PM today and sunrise Saturday.
Snowfall amounts during this timeframe across the Teton and Gros
Ventre Mountains and the Salt River and Wyoming Ranges range from 10
to 15 inches, with 2 feet over the highest elevations. Snowfall
amounts across Yellowstone, the Absarokas, the western Bighorns, and
west Wind River Mountains will likely be anywhere from 4 to 15
inches during the same time frame. The highest elevations of the
Absarokas and the Wind River Mountains will likely (80%) see amounts
closer to that 15 inch mark. As for Jackson and Star Valleys and the
Upper Green River Basin, snow accumulations from 2-3PM today through
sunrise Saturday are generally between 2 and 6 inches. Despite the
moderate to heavy snowfall expected, these lower accumulations are
largely due to temperatures being near to a few degrees above
freezing (60-90% chance).

Snowfall will decrease in intensity Saturday morning as slightly
drier air begins to move in and support aloft weakens. Nonetheless,
snow will still be falling during the day Saturday with a few tenths
on an inch to 2 inches for the western Valleys, Upper Green River
Basin, and most of Yellowstone. The Absarokas, west Wind River
Mountains, and the west Bighorns will see 2 to 5 inches of snow (20%
chance of greater than 5 inches). The Tetons and Gros Ventre
Mountains and the Salt River and Wyoming Ranges will likely (80%)
see anywhere from 4 to 8 inches of additional snow during the day on
Saturday. This puts the three-day snow totals for these locations
between 12 and 20 inches with higher elevations seeing 2 to 3 feet.

Now to the wind. A tightening pressure gradient during the daylight
hours today will favor gusty 20 to 30 winds across southern Wyoming
and the Absarokas. However, as the aforementioned upper level jet
with speeds of 120 to 135kts moves over Wyoming after sunset today,
gusty winds increase across much the area. There is good consensus
that 700mb winds of 50 kts will be over the western mountains,
Sweetwater County, the Wind River Basin, the eastern Bighorn Basin,
and northern Johnson County after 10PM tonight through Saturday
morning. 700mb wind speeds of this magnitude would normally
translate to wind gusts of 60 mph (High Wind Criteria). However,
there is currently a 50/50 shot of this occurring in these
locations, especially Sweetwater County. The uncertainty is due to
Wyoming being in the proximity of the right entrance region of the
upper level jet (upward motion as opposed to downward motion) and a
diurnal inversion limiting the amount of wind making it to the
surface. However, it will still be windy overnight tonight with a
60% to 80% chance of gusts between 40 and 50 mph. The greatest area
of concern for gusts around 60 mph is Sweetwater County, where there
is only a 30% chance of 60 mph gusts. Given the uncertainty, no wind
highlights have been issued at this time. By mid-morning Saturday,
the upper-level jet, although weaker (around 100 kts), will be
focused over southern portions with associated 700mb winds around
30kts. This will keep much of southern Wyoming gusty on Saturday.

There is good agreement among model guidance that drier air will
begin to move over Wyoming Sunday morning with overall height rises.
This will keep snow chances (40-50%) confined to the western
Wyoming with dry conditions elsewhere. There is also agreement that
a weak cold front will pass through early Monday bringing additional
precipitation chances (80%) for the west. However, this front will
not impact temperatures for early next week as above normal
temperatures are looking likely (80 to 90%) for mid December.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1114 AM MST Fri Dec 5 2025

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

A winter storm will continue to impact terminals across western
Wyoming. KJAC will begin the period VFR/MVFR before another push of
persistent snow moves in around 22Z. IFR conditions are then
expected to persist at KJAC through around 08Z Saturday before an
improvement to MVFR with lingering snow through the end of the
period. KBPI and KPNA will also see periods of MVFR/IFR tonight.
KRKS should miss out on the worst of the snow but should still see
MVFR impacts. All terminals will see an accompanying gusty wind,
especially at KRKS which will see gusts to 40 knots after 06Z.

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

Wind will be the primary aviation concern east of the Divide as most
of the snow stays west. The strongest wind will come overnight
tonight through much of the rest of the period, most notable at KCPR
which will see gusts over 30 knots. Some snow will make it east of
the Divide, leading to occasional impacts at or near terminals with
the best chances at KCOD, KCPR, and KWRL after 04Z.

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-
013-014-023-025.

Winter Storm Warning until 5 PM MST Saturday for WYZ008.

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

&&

$$

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