798
FXUS65 KRIW 071045
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
345 AM MST Sun Dec 7 2025

.KEY MESSAGES...

- Another period of light to moderate snow moves into the
  western mountains today and tonight.

- A prolonged period of snow looks likely across the west though
  much of the week, although there will be breaks from time to
  time.

- A prolonged period of strong to high wind looks likely (a
  greater than 3 in 5 chance) from Tuesday through Thursday,
  with the strongest wind expected in southern Wyoming.

&&

.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 /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...
Winter Weather Advisory from 11 AM this morning to 5 AM MST
Monday for WYZ012.

&&

$$

DISCUSSION...Hattings
AVIATION...CNJ