257
FXUS65 KRIW 100430
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
930 PM MST Tue Dec 9 2025

.KEY MESSAGES...

- Moderate snowfall will continue across the northwest and
  northern WY higher elevations overnight and into much of
  Wednesday, with periods of lighter precipitation and periods
  of heavier snowfall. Total snow accumulation through Thursday
  will reach up to 2 feet in some locations in the northwest
  mountains. Warmer valley temperatures will inhibit most
  snowfall accumulations. Winter Storm Warnings and Advisories
  continue through late Wednesday afternoon.

- Strong and gusty winds continue across much of the state
  today, with High Wind Warnings in effect for much of southern
  and central Wyoming. Wind gusts over 50 mph are also occurring
  in the higher elevations of the western and northwest WY
  mountains.

- Temperatures remain 10 to 20 degrees above normal through
  Thursday in the foothills and lower elevations, with some
  cooler air moving into northern WY Friday and Saturday.

&&

.UPDATE...
Issued at 102 PM MST Tue Dec 9 2025

The very energetic west-northwest moist flow pattern continues
over Wyoming this afternoon into Wednesday. A weak shortwave
trough in the flow is currently moving from central Montana into
northern WY enhancing snow/rain more over northwest and northern
WY this afternoon as higher surface pressure follows the trough
southward. Gusty north-northwest winds are expected across
northern WY through about 8 PM. With precipitation continuing
over the higher terrain, snow/rain should also push into the
northeast Bighorn Basin and Johnson County from about 3-6 PM.
Have opted to not issue a winter weather advisory or high wind
warning for northern Johnson County given the short duration of
the snowfall and winds in the 20-30 mph range with gusts 35-40
mph. This trough will push the mid-level moist WNW flow
southward back to mid-western WY overnight to bring light to
moderate snow back to the western WY mountains while it
decreases over the northern WY mountains. Another embedded
shortwave trough in this pattern then approaches after midnight
tonight, increasing precipitation over northwest WY, which will
continue over the winter storm warning area through mid- to
late-afternoon Wednesday. This trough will also increase mid-
level flow to 40-50 knots, which will increase winds over the
area again on Wednesday (after somewhat of a lull overnight).
Conditions appear to be increasing for strong downslope winds
along the Absarokas and Wind Rivers Thursday morning as well in
advance of some colder air pushing into northern WY Friday and
Saturday.

Have increased the winds in the forecast over the higher terrain
of the western mountains per feedback from higher elevation
observation sites and the avalanche center. QPF and snow
forecast are still consistent with previous forecasts.

&&

.DISCUSSION...
Issued at 145 AM MST Tue Dec 9 2025

The next passing shortwave energy is beginning to impact the NW
higher elevations this morning, and the overall flow the next
several days continues to be favorable for continued snow
accumulation. Between orographic lifting and the passing wave,
accumulations of 2 to 3 feet are becoming more likely for the
Tetons and Absarokas, with 6 to 12 inches for remaining higher
mountain ranges and the greater YNP region. Current variety of
winter warnings and advisories are well positioned, so no
adjustments needed at this time. In a usual winter, temperatures
would be cold enough to help get some of this precip into the
lower valleys, but not this year. Temperatures today will be in
the 40s for the western valleys, and in the low 50s for the
lower elevations east of the Divide, so snowfall will be locked
into the mountains for the remainder of the week.

The core of the low-level jet streak will be passing by just to
the north today, and with temperatures in the 50s, a large
chunk of this wind energy will mix to the sfc during the
afternoon. With H7 winds at 70kts as they pass by, peak sfc wind
gusts of 50 to 60 mph are likely for southern WY stretching NE
into the Casper vicinity. The only question is winds across
northern Johnson County late this afternoon as the cold front
barrels through. While winds sfc and aloft are aligned, models
are not really bringing winds up until the system gets a little
farther east. Will monitor this if High Wind Warnings are needed
there.

Beyond this, the H5 ridge along the Rockies doesn`t move much
through the weekend, deflecting weather systems to the north and
east, and keeping the unusually warm temperatures in place.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 930 PM MST Tue Dec 9 2025

VFR conditions ongoing at all locations and throughout the
period expected. Low ceilings but VFR for JAC/COD/CPR with light
rain/snow mix overnight ending between 12-15Z. Mid level
ceilings there after at all locations, more upper level for
LND/RIW/RKS further south. Gusty winds once again remaining
elevated at RKS overnight peaking up to 40kts after 11Z with a
resurgent low level jet towards sunrise Wednesday morning. All
other locations will see 25-30kt gusts after 17-18Z through
00-01Z diminishing some after sunset but remaining elevated for
RKS/CPR/JAC, as well as COD/RIW with the persistent west to
northwest funneling flow. Precipitation chances will wane come
Thursday morning in the next TAF cycle as this ongoing system
exits to the east.

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 PM MST Wednesday for WYZ001-002-
012.

Winter Weather Advisory until 5 PM MST Wednesday for WYZ008-
014-015-024.

High Wind Warning until 5 AM MST Thursday for WYZ019-020-022-
027>030.

&&

$$

UPDATE...McDonald
DISCUSSION...Straub
AVIATION...Lowe