707
FXUS65 KRIW 051815
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1115 AM MST Fri Dec 5 2025

.KEY MESSAGES...

-  Light snow continues through the early afternoon across the
   west with light accumulations. Moderate to heavy snow begins
   after 2-3PM across western Wyoming, including Jackson and
   Star Valleys, and the western Bighorns. Travel could become
   hazardous.

-  Widespread breezy to gusty winds this afternoon and
   increasing this evening and overnight across much of the
   area. Wind gusts of 40 mph (60-80% chance) after sunset
   across central and southern portions, with wind prone
   locations seeing gusts above 50 mph.

-  Mild, above normal temperatures are looking more favorable
   (80%), especially across eastern Wyoming Sunday through mid
   next week. An active pattern looks to continue across western
   Wyoming through next 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-
008-013-014-023-025.

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

&&

$$

DISCUSSION...Gerhardt
AVIATION...Myers