172
FXUS65 KRIW 081122
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
422 AM MST Mon Dec 8 2025

.KEY MESSAGES...

- Snow will spread into the north mountains tonight through
  Wednesday, with the highest amounts in the Absarokas.

- Strong to high wind is likely (greater than a 9 or 10 chance)
  in much of central and southern Wyoming starting tonight, with
  the strongest wind in Sweetwater County.

- Above normal temperatures will continue for the next few days.

&&

.DISCUSSION...
Issued at 306 AM MST Mon Dec 8 2025

Sometimes I think the weather is like a rubber band. Basically, you
can have a long stretch of quiet weather, but each day builds up
tension in the band. Eventually, it ends up snapping. And this is
the case over the next few days. Much of November was very quiet and
tranquil weather wise. And we are now in one of the more active
periods we have had in a while. And a rather complex one, with most
threats including wind and snow. In a bit of a change in format, we
will hit each one of these individually followed by a general
outlook of aspects of the forecast that are less impactful as well
as the weather past Wednesday night.

SNOW DISCUSSION...We continue to have some mainly light snow falling
over the western mountains as I write this around 2 am. It is
beginning to let up in the Tetons, so we will likely expire the
advisory around 5 am. There should then be a lull in the snow for
around 12 to 24 hours before the next round of snow moves in from
north to south as a boundary sags southward and a potent 140 knot
jet sags southward toward the area, enhancing upper level
divergence. The highest confidence for heavy snow continue to be in
the Absarokas where there will be some orographic enhancement to go
along with the jet energy. Many locations here have greater than a 9
in 10 chance of a foot of new snow with some of the southern
portions of the range having nearly a 1 in 2 chance of 2 feet or
more. This, combined with some strong wind, looks to be the most
impacted area. So, we have issued a Winter Storm Warning here
starting tonight and continuing through Wednesday afternoon.
Southern portions of Yellowstone Park also have a greater than 3 out
of 5 chance of a foot of new snow, but lesser amounts elsewhere. As
a result, we had more questions about a warning here, but with the
strong wind we went ahead and issued and also to match up with our
neighbors to the north. Northern portions of the Park will likely
only see advisory amounts though.

Further south there are more questions, as the dynamics are not as
favorable here with the jet being further away. Nevertheless, a
decent portion of the Tetons, Wind Rivers and Salt and Wyoming Range
have at least a 1 in 2 chance of a foot of new snow. I`m not
completely convinced yet though. Since the bulk of the snow here
would not fall until at least Tuesday, we have elected to go with
watches and let future shifts decide between warnings, advisories or
even nothing. As for the Bighorns, there is at least a 3 out 4
chance of greater than 6 inches of new snow, but the chance of a
foot is generally less than 1 out of 3. Any impactful snow would
hold off until Tuesday afternoon or evening, so we still have time
to decide on any highlights here.

As for the lower elevations, impacts look minimal. Warmer air will
try to push into the area, especially on Wednesday. With 700
millibar temperatures expected to rise as high as minus 2 by
Wednesday, snow levels may rise as high as 7500 feet, keeping the
valleys a rain / snow mix and even all rain at times. Chances
of 3 inches of snow or more are basically zero because of this.
As for the lower elevations East of the Divide, there could be
some light snow in the Big Horn Basin and Johnson County. But,
with the warmer air moving in, especially on Wednesday, any
accumulation would be negligible.

One caveat is that there is still some spread in the details of
timing of the periods of heaviest snow. The deterministic runs have
a lot of differences as well, with the European much wetter than the
GFS for example.

WIND DISCUSSION...We are already starting to see some gusty wind
develop in places like Casper this morning. There has been a bit of
a change in that the jet has moved a bit further north, and this
brought some of the strongest wind gusts down a bit. However, we
still have a good set up for strong wind much of southern and
central Wyoming in the right front quadrant of the jet, a tight
pressure gradient and strong winds at 700 millibars. The strongest
700 millibar winds are now down to around 60 to 65 knots as opposed
to the 80 knots yesterday, guidance does tend to be a bit bold a few
days out. Nevertheless, we still have a very good chance of high
wind across much of the area. Much of the southwestern wind corridor
from Kemmerer to Rock Springs to Casper has at least a 3 in 4 chance
of wind gusts past 55 mph sometime between Monday night and
Wednesday night. With this, we have upgraded the High Wind Watch
to warning for all of the zones starting at 11 pm tonight. The
most prolonged high wind will be across southern Lincoln and
Sweetwater County, this includes Interstate 80 as well. In Natrona
County, Tuesday looks like the most impactful day with a greater
than 4 out of 5 chance of wind gusts past 55 mph. Chances drop
a bit on Wednesday, but not enough confidence to drop it for
this day.

There are two other areas of concern. One is in northern Johnson
County, which could see a brief period of high wind Wednesday
afternoon as flow turns northwest, with some areas having a greater
than 1 in 2 chance of wind gusts past 50 mph. The other is the Upper
Wind River Basin, which has a similar set up. Again, this would
likely not be until Tuesday, so there is still time to decide. Gusty
winds are possible elsewhere, but changes of high wind are 1 out of
4 or less.

REST OF THE FORECAST...It will be fairly mild outside of the
mountains, with some areas seeing highs into the 40s and even the
50s, especially on Wednesday as flow becomes southwesterly. We are
also watching another piece of energy that could bring some
precipitation to northern Wyoming later Thursday into Friday. There
is a large spread in guidance though so confidence remains rather
low. Looking ahead to next weekend, at this point to looks largely
dry with a gusty wind and above normal temperatures, but confidence
remains rather low.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 417 AM MST Mon Dec 8 2025

Snow and low clouds starting to lift around KJAC currently, with
continued improvements through the morning. Main aviation issue
today will be gusty winds across the state. Most areas will be
gusting around 20kts, but KRKS and KCPR will have frequent gusts
above 30kts. Winds will stay elevated overnight with the
approach of the next system, and increasing at all sites. The
next weather system will begin to spread some light snow into
KJAC again after sunset, with better chances waiting until well
after 06Z/09

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 Storm Warning from 5 PM this afternoon to 5 PM MST
Wednesday for WYZ001-002.

Winter Weather Advisory until 5 AM MST early this morning for
WYZ012.

Winter Storm Watch from late tonight through Wednesday
afternoon for WYZ012-014-015-024.

High Wind Warning from 11 PM this evening to 5 AM MST Thursday
for WYZ019-020-022-027>030.

&&

$$

DISCUSSION...Hattings
AVIATION...Straub