127
FXUS65 KRIW 072020
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
120 PM MST Sun Dec 7 2025

.KEY MESSAGES...

- Snowfall rates will increase across the western mountains this
  afternoon leading to moderate accumulations overnight,
  especially in the Tetons. Winter travel conditions are
  possible over mountain passes.

- Wind will be elevated through much of the upcoming week. The strongest
  wind is expected Monday night through Wednesday night across
  the Wind Corridor from southwestern Wyoming through Natrona
  County.

- An active pattern will keep near-continuous snow chances for
  the western mountains through the week. Generally above-normal
  temperatures are expected elsewhere.

&&

.UPDATE...
Issued at 117 PM MST Sun Dec 7 2025

Snow is beginning to pick up again across western Wyoming as the
next wave arrives. The only change made to the forecast this
afternoon was a little more snow accumulation for the Tetons given
latest model guidance. The heaviest snow will fall this evening
through the pre-dawn hours before rates drop off by sunrise. Further
ahead, we will likely be issuing a High Wind Watch shortly for the
extended period of strong wind starting Monday night as noted
below.

&&

.DISCUSSION...
Issued at 335 AM MST Sun Dec 7 2025

I have wanted to be a meteorologist since I was in 1st grade (thank
you Blizzard of 1978 in Massachusetts). However, at times I gave
thought at what other careers I would have liked. One of them was
engineering, more specifically mechanical engineering. And this
relates to the forecast. Any engineer or even mechanic can tell you,
one of the problems with moving parts if that, the more you have,
the more problems that can occur. And this is the problem with the
forecast this early morning, this forecast has more moving parts
than a Rube Goldberg machine, making the bust potential higher than
normal and details difficult to hash out, especially further out.

As for now, we still have a few showers over the area, especially
over Togwotee Pass, where it seems to snow every time someone
sneezes. But, most areas are in a lull right now. This won`t last
though, as the next wave approaches from the west. This one looks to
largely impact the western mountains. The most impacted area will be
the Tetons, where most locations have at least a 2 out of 3 chance
of 6 inches or more of new snow though tonight. With this, we have
issued a Winter Weather Advisory starting at 11 am today, although
the steadiest snow would be tonight when the trough axis moves
through. Other locations have generally have less than a 1 in 2
chance. Southwestern Yellowstone has a greater than 1 in 2 chance
of 6 inches as well, but seeing how the park is closed right now
and the only impact would be to wildlife, no highlights here. As
for East of the Divide, things look relatively uneventful, just
some breezes in the usual locations as downsloping flow occurs.
Temperatures here will average 5 to 10 degrees above normal.

Monday also looks relatively quiet as we will again be in between
systems, with only with only some orographic snow showers in western
Wyoming. However, wind will be increasing as the pressure gradient
really begins to tighten. And this may be the greatest concern for
the middle of the week, especially in central and southern Wyoming.
We will tackle these two concerns separately.

We will start with wind. Some of the telltale 50 knot winds at 700
millibars begin to show up as early as Monday night across the
favored southwesterly, prefrontal locations, like Rock Springs,
Casper and in the lee of the Absarokas. Many of these locations have
at least a 3 out of 4 chance of wind gusts past 50 mph starting
later Monday night, and some have a 1 in 2 chance of wind gusts past
60 mph. I would say there is at least a 1 in 2 chance of some sort
of High Wind Watch being needed starting at this time. Tuesday also
looks like a blustery day as the next system moves toward the area.

The time of concern may be Tuesday night through Wednesday night.
Most modeling shows an 130 knot jet dropping across Wyoming, with
the southern half of the state in the favored right front quadrant
of the jet, enhancing downward momentum. This is when the 700
millibar winds really peak as well, with the GFS showing these winds
as high as 80 knots. Extended MOS guidance shows sustained winds
over 40 knots Wednesday and Wednesday night. In the probabilistic
realm of the ensembles, much of the southern third of the state as
at least a 3 in 5 chance of wind gusts past 60 mph with even some
areas near Green River having a 1 in 2 chance of wind gusts over 70
mph. So, even for this normally windy area, this could be a notable
period of high wind, mainly for how long it could last (into
Thursday as well). As was said earlier, expect some wind highlights
sometime today or at the latest Monday morning for this period
before the wind finally decreased Friday. One mention of caution
though, the models, especially the GFS, can be a bit bullish on
wind this far out, so this is not 100 percent. However, chances
are fairly high of some sort of high wind during this period.

Now for the snowy part of the system. This is where there are more
moving parts and uncertainty increases significantly. There is still
a fairly large spread in guidance. For one, the aforementioned snow
looks to be more dynamically forced with the jet energy. This leads
to a more banded snow that is hard to nail down 24 hours out and
especially so 4 to 5 days out. Many of the western mountains have at
least a 1 in 2 chance of a foot or more of new snow, with some
orographic enhancement from the westerly flow. Some additional
highlights look fairly likely there (at least a 1 in 2 chance)
starting Tuesday night. The uncertainty increases exponentially East
of the Divide. The deterministic models have a lot of different
solutions in regards to timing, as much as a 24 hour difference.
Placement of the heaviest snow is also uncertain. The greatest
chance of greater than 3 inches of snow for the 48 hours ending
Friday morning would be across northern Wyoming. The reasoning for
this would be the area is in the left front quadrant of the jet,
which will enhance upper level divergence. But pinpointing where
this is almost impossible this far out. So, it looks like a rather
active period of weather across western and central Wyoming for the
second week of December.

As for temperatures, it looks rather mild ahead of the front,
especially Tuesday and Wednesday when strong downsloping flow  may
bring highs in the 50s East of the Divide. Things cool off behind
the front Thursday. However, with the Pacific origin of the air,
temperatures would only fall back to seasonal normals, with no
Arctic air expected over the next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1009 AM MST Sun Dec 7 2025

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

The next weather system brings increasing clouds through today. For
KJAC, snow begins in the mountains late morning, with the first
flakes falling at KJAC as early as 18z, but more likely around 20z
or 21z. Snow then continues into tonight, with snow rates increasing
around 02z. Latest models lighten snow by around 10z, and dropping
snow chances below 30 percent by 14z. During snow, expect MVFR to
IFR conditions due to cloud ceilings and snowfall visibility drops.

Elsewhere, snow chances are much less through the period, with KPNA
being the next most likely place to see snow. Conditions here should
improve around sunrise Monday.

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

An incoming weather system from the west brings increasing mid-to-
high-level clouds through today. No precipitation is expected,
though a stray snow shower could (15% chance or less) move off the
mountains and impact a place like KCOD or KLND briefly. The main
weather impact continues to be westerly winds, which persists today
into tonight.

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 5 AM MST Monday for WYZ012.

&&

$$

UPDATE...Myers
DISCUSSION...Hattings
AVIATION...Wittmann