646
FXUS65 KRIW 152329
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
429 PM MST Mon Dec 15 2025

.KEY MESSAGES...

- Dry and warm today, with areas of elevated fire weather conditions.
  Gusty winds (30 mph or greater) occur for the Wind Corridor
  from South Pass to Casper, and the eastern Absaroka, Wind
  River, and Bighorn Mountains.

- Light snow occurs over the western mountains starting tonight, continuing
  through Tuesday. Rain and rain/snow mix for the western
  valleys. Gusty winds also occur Tuesday and Tuesday night.

- A widespread high wind event occurs Wednesday. Periods of
  heavy snow over the western mountains will create winter
  travel impacts for mountain passes.

&&

.UPDATE...
Issued at 131 PM MST Mon Dec 15 2025

Weather highlights have been issued for the Tuesday/Wednesday
weather system incoming. The main message continues to be wind, with
widespread westerly gusts of at least 30 to 40 mph, and many areas
expecting over 50 mph gusts. Moderate to heavy mountain snow
Wednesday will also increase impacts over the western mountains,
especially for the mountain passes.

For the forecast itself, not too many notable changes were made.
Have increased winds slightly across the area Wednesday to be better
in line with anticipated dynamics shown between various models.
Currently forecasted winds Wednesday evening into Thursday morning
may drop off too early, as post-frontal winds may keep gusts going
through the night. Will need to evaluate this for future updates.

&&

.DISCUSSION...
Issued at 138 AM MST Mon Dec 15 2025

An active weather pattern will begin today and continue through the
week, as a broad ridge that was in place Sunday flattens and a more
zonal flow pattern develops. Conditions today will be somewhat
benign for most areas as a result, however there are a few areas
that will have gusty winds once again this afternoon. Otherwise,
conditions will be dry today with above normal temperatures
(especially east of the Divide). Concerns for fire weather will also
be in play this afternoon where winds will be gusting over 25mph. RH
values will be below 20%, approaching 13% (15% is a critical
threshold), across most of the forecast area, but especially east of
the Divide. Areas from southern Fremont County to southern Johnson
County (including the southern end of the Bighorns), the Upper Wind
River Basin and the Cody Foothills should be wary with any kind of
burning even though it is December. The record temperatures from
last week, coupled with a dry start to the winter season, make
conditions conducive for possible erratic fire behavior. Take the
usual precautions if planning any burning today. There will be just
enough midlevel (700mb-500mb) moisture to produce virga showers/snow
flurries this evening over northwestern portions, the central basins
and Johnson County as a weak shortwave moves over the area.

Winds will be increasing over the east slopes of the Absaroka,
Bighorn and Wind River Mountains after 06Z (11pm Monday) Tuesday due
to a tightening 700mb temperature gradient and 700mb winds
approaching 50kt. Similar conditions will also develop over the
western mountains during this time. These winds look to impact areas
along Hwy 120 near Cody and Clark, as well as the Upper Wind River
Basin near Dubois and Crowheart. Wind gusts over 50mph are likely by
12Z as these winds downslope off the mountains and into the lower
elevations. There is a 50% chance for gusts of 60-70mph to occur,
especially near Cody and Clark. These winds will continue through
the rest of the morning Tuesday, gradually decreasing but remaining
gusty through the afternoon with gusts of 35 to 50mph and
spreading through the Wind River Basin. Wind gusts of 35 to
50mph will also develop over the Wind Corridor east of the
Divide by 12Z, spreading across the south by midday. Light snow
will be possible over the western mountains from early Tuesday
morning through Tuesday evening, due to westerly upslope flow.
Snowfall amounts up to 3 inches is expected, with locally higher
amounts closer to 5 inches possible in the higher elevations of
the Tetons. Most of this snow will occur in the overnight/early
morning hours. A rain/snow mix to rain is likely in the Jackson
and Star Valleys by 12Z Tuesday, continuing through the day and
into the evening. Precipitation is likely to end sometime
Tuesday night before returning again Wednesday with the
approaching storm.

Wind gusts of 30 to 40 mph will continue over the Wind Corridor and
the mountains Tuesday night as the next storm system approaches.
These winds will increase through the overnight hours, with 700mb
winds of 50kt occurring across much of the CWA by 12Z Wednesday. A
leeside trough over eastern WY will strengthen during this time, irt
the approaching system/cold front and tightening the pressure
gradient. Wind gusts up to 60 mph will be possible by this time
at wind prone locations, like the south side of Casper, Clark
and even South Pass. These winds will continue to increase
through the rest of the morning, with much of the forecast area
experiencing wind gusts of 35 to 45 mph. Areas east of the
Divide have a 50 to 80 percent chance for wind gusts over 55
mph, with areas west of the Divide having a 30 to 60 percent
chance. Snow will be increasing over the western mountains
Tuesday night as well, with moderate to heavy snowfall possible
Wednesday morning. Forecasted snowfall amounts are in the
Advisory range (between 6 and 12 inches), with snow tapering off
by the evening. The timing of the cold front remains in flux
and could be entering northern portions by early afternoon, but
it could lag and not reach northern portions until sunset
Wednesday evening. This timing will determine whether the
stronger winds continue through the afternoon, or begin to
decrease as they switch to a more northwesterly direction behind
the front. Winds will still be strong behind the front, but are
not likely to be gusting to 70 mph and will be decreasing
through the overnight hours into Thursday morning.

Conditions will be dry for the first half of Thursday, with gusty
winds continuing, especially over southern portions and the Absaroka
and Wind River Mountains. Another round of snow will return to
western portions by late afternoon, with strong to high winds
returning through the overnight hours Thursday night, as another
storm system approaches the area. Similar conditions from Wednesday
look likely to return with this storm. with snow over the west and
widespread strong to high winds through Friday night.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 425 PM MST Mon Dec 15 2025

There are several impactful aviation weather items of interest
over the next 24 hours.

The most impactful, but lowest confidence (20 to 30%), is the
chance for a brief period of freezing rain at KJAC Tuesday
morning. The most recent high resolution weather models show a
bit more moisture coming through, and given surface
temperatures forecast to be near or just below freezing, there
could be a brief period where melted-as-it-fell-snow (rain) is
able to refreeze on contact with surfaces. If surface
temperatures remain above freezing overnight, this threat will
be largely mitigated, so pay close attention to near-surface-
air-temperatures and physical-surface temperatures during the
early AM hours.

Another area of interest is KCOD, where winds overnight could be
light, or they could be very gusty, depending on whether the
cold air near the surface (the inversion) is thick enough to
prevent mixing down of the higher momentum air about a thousand
feet above the surface. For now, have leaned with prevailing
breezy conditions, with a TEMPO group for part of the overnight
which shows some stronger gusts reaching the surface. Regardless,
it is worth preparing for some turbulence near the surface
given the very strong winds just a couple thousand feet AGL.

The final near-term note is for KBPI and KPNA, where a brief
period of late morning melting-snow-mix is possible. See TAF
timing for details.

Stronger and more widespread gusty winds are forecast Tuesday
night into the day Wednesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Tuesday night through Wednesday
evening for WYZ001-002-012-014-015-024.

High Wind Warning from 1 AM Tuesday to 1 AM MST Wednesday for
WYZ002.

High Wind Warning from 1 AM Tuesday to 11 PM MST Wednesday for
WYZ003.

High Wind Watch from late Tuesday night through Wednesday
evening for WYZ011-016-025>030.

High Wind Watch from late Tuesday night through Wednesday
evening for WYZ019-020-022.

&&

$$

UPDATE...Wittmann
DISCUSSION...LaVoie
AVIATION...VandenBoogart