638
FXUS65 KRIW 200928
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
228 AM MST Sat Dec 20 2025

.KEY MESSAGES...

- After a brief lull this morning, wind gusts of 25 to 45 mph
  will return across much of the area this afternoon.

- Snow returns to western portions tonight, as the start of a
  multi-day heavy snow event. Snow expected in the western
  valleys to start, but will change over to rain by Sunday
  afternoon. Wind gusts up to 30 mph will occur with the snow
  Sunday, adding to the already difficult travel conditions over
  Teton and Togwotee Passes. These winds will increase to 40 mph
  Sunday night. Snowfall amounts of 18 to 36 inches expected
  through Monday night.

- Above normal, possibly record high, temperatures, and windy
  conditions Monday through Christmas Eve.

&&

.DISCUSSION...
Issued at 210 AM MST Sat Dec 20 2025

As of 07Z, the cold front has reached Casper and Riverton and is
expected to reach the Divide in the next few hours. Temperatures at
these locations have dropped below freezing, with bands of snow
occurring over southern portions of the Bighorn Basin and Johnson
County, Natrona County along the US-20/26 Corridor and off the
southern end of the Wind River Mountains. Snow will also continue to
fall over the Salt River/Wyoming Ranges and the Wind River Mountains
through the morning hours with an additional 2 to 4 inches expected.
Most of this will fall before sunrise. Rain showers are also
expected along the I-80 Corridor shortly, pushing south of the
Interstate by 10Z. Temperatures are currently in the upper 30s/lower
40s and are not expected to drop below freezing until 12Z. Winds
continue to decrease behind the front, with gusts of 25 to 45 mph
continuing across southern portions. These winds will gradually
decrease by sunrise this morning, with the east slopes of the Wind
River Mountains continuing to gust 45 to 55 mph. This will be a
brief lull, as west-northwest winds increase to gusts of 25 to 45
mph by midday. These winds will occur for most areas, except eastern
portions of the Bighorn Basin and northern Johnson County.
Temperatures will be closer to seasonal levels, but still slightly
above normal for this time of December.

Precipitation chances will increase over western portions after 06Z
Sunday (11pm tonight), as a plume of Pacific moisture moves over the
region. This will be part of an Atmospheric River moving onshore
over northern CA today and be a multi-day event. Precipitation will
likely start as snow in the Jackson/Star Valleys tonight, changing
over to rain by the afternoon. Snow levels will range between 7000
and 8000 ft over the western mountains by 18Z Sunday, with moderate
to heavy snow through the day and Sunday night. Precipitable water
values will be close to record values with this event. This, coupled
with favorable west-southwest upslope flow, will lead to snowfall
amounts of 18 to 36 inches through Monday night. The bulk of this
will occur through Sunday night, with snow becoming light during the
day Monday. Wind gusts up to 30 mph will also occur with the snow,
adding to the travel hazard over Teton and Togwotee Passes. One of
the challenges with the forecast will be if temperatures drop below
freezing in the western valleys Sunday night.

Elsewhere, it will be a windy day again Sunday with southwest winds
gusting 25 to 35 mph across southern portions and the Wind Corridor.
Exceptions will be portions of Natrona County and southern Fremont
County, where gusts of 35 to 45 mph will be common. Locally higher
gusts up to 60 mph will be possible over South Pass and Hwy 258 on
the south side of Casper. There is a possibility for high wind
Sunday night into Monday, as 700mb winds increase to 50kt over the
Cody Foothills, South Pass and Natrona County. A cold front will
move over areas east of the Divide Monday. No precipitation is
expected either ahead or behind the front.

The impacts from the Atmospheric River will continue to wane
Tuesday, as a ridge begins to build over the region. This will lead
to another day of above normal temperatures (possibly record high
temps), with highs in the 40s and 50s. Any precipitation will be
confined to far northwestern portions, as the ridge shunts most of
the moisture to the north and west. Dry, warm and windy conditions
are likely Christmas Eve. Another storm system will impact the CWA
on Christmas, with western portions having the best chance for a
White Christmas. There is the potential that this will be another
windy day, as the flow aloft will be southwesterly.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1037 PM MST Fri Dec 19 2025

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

MVFR to IFR conditions are expected at times through the first
couple of hours of the period before the system pulls away. Wind
will follow a similar trend, generally decreasing through the
overnight hours. VFR conditions are expected at all terminals
between 10-12Z Saturday. Wind will increase Saturday afternoon with
gusts of 20 to 30 knots through sunset. Light snow over surrounding
peaks will keep frequent terrain obscuration through Saturday
morning. KJAC will begin to see a lowering cloud deck by 02Z Sunday
as the next wave approaches.

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

A cold front will continue to drop south through the area. KCPR is
likely to see IFR conditions as this front arrives, improving after
10Z. Other terminals may see brief MVFR to IFR conditions as the
front pushes through, though confidence is largely too low to
include many impacts. Prevailing VFR conditions are expected by 12Z
as the system departs. Wind will continue to decrease through the
overnight hours, though will increase again for Saturday afternoon.

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 until 5 AM MST early this morning for
WYZ001-002-012-014-015-024.

Winter Weather Advisory until 5 AM MST early this morning for
WYZ013-023-025.

&&

$$

DISCUSSION...LaVoie
AVIATION...Myers