896
FXUS65 KRIW 142327
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
527 PM MDT Tue Oct 14 2025

.KEY MESSAGES...

- Isolated showers this afternoon increasing in coverage
  overnight, especially across western Wyoming mountains and
  valleys.

- Widespread showers Wednesday with a few embedded thunderstorms
  (20% chance) during the afternoon. Afternoon winds will be
  breezy, with 30 to 45 mph gusts for southern and central
  Wyoming.

- Snow is likely in the mountains starting Wednesday evening, as
  snow levels fall from around 9500 feet Wednesday evening to
  around 7000 feet by sunrise Thursday. Winter Weather
  Advisories in effect for those in the backcountry.

- Widespread sub-freezing temperatures (about 5 degrees below
  seasonal normals) for all basins Friday and Saturday mornings.

- Drier weather forecast for the weekend, with temperatures
  warming to above normal by Sunday.

&&

.UPDATE...
Issued at 135 PM MDT Tue Oct 14 2025

The previous forecast remains largely on track. The main change
this afternoon was to update precipitation amounts using the
latest higher-resolution forecast model guidance, which is now
beginning to cover much of the next weather system. This trough
arrives tomorrow and lingers through Thursday night. Along with
the higher confidence in at least some winter impacts for both
mountain passes and those using the backcountry, have issued
Winter Weather Advisories starting around sunset tomorrow
(Wednesday). Snow amounts are not expected to be extreme with
this system, but those above 7500 ft could see at least light
snow accumulations by Thursday night. The highest elevations,
including high passes such as Togwotee, are forecast to see over
6 inch accumulations over a 24-hour period between Wednesday
evening and Thursday evening. Advisories end late Thursday
evening when the majority of precipitation exits the region.

Models continue to show modest instability Wednesday afternoon,
supportive of isolated thunderstorms (20% chance). The strong
pressure gradient ahead of the surface low will also lead to
gusty afternoon winds, with 30 to 45 mph gusts across southern
and central Wyoming. These could be locally enhanced by any
nearby shower or thunderstorm. Winds decrease quickly after
sunset.

&&

.DISCUSSION...
Issued at 316 AM MDT Tue Oct 14 2025

The overall framework of the forecast remains intact this morning,
although there are a few tweaks to the details. We do have a few
areas of showers with a bit of embedded lightning moving north
across the area as I write this. On the balance though, today looks
like the least active day of the next three. Most models are in
agreement in showers becoming less numerous as the shortwave
bringing these showers moves away to the north. There will still be
some around, but most areas should end up mostly dry most of the
time. The most numerous showers still look to be across western
Wyoming. And with the southerly flow, temperatures should be a bit
warmer than yesterday, especially East of the Divide where there
should be increasing amounts of sunshine through the day.

Things begin to become somewhat more active later tonight and into
Wednesday as the upper level low, now located near the California
coast, begins to move onshore and toward the Great Basin.
Precipitation still largely looks more intermittent Tuesday night
into Wednesday morning, with more widespread showers moving in
during the afternoon hours. The question yesterday was the track
of the upper level low as it moved over Wyoming. Model guidance
has come into better agreement as to the timing of the low,
with it crossing from Wednesday night through Thursday
afternoon. There is also somewhat better agreement in the timing
of the ending of the precipitation, with most ending by
Thursday night. However...

There is still some question to the ultimate track of the low. We
still have about an 150 to 200 mile difference in this as it moves
over Wyoming, exiting anywhere from Powell to Newcastle. This
track will still favor western Wyoming with the highest
precipitation regardless of the track. We will discuss amounts
in the next paragraph. There is more uncertainty East of the
Divide though, as portions of the area would likely end up in
the dry slot of the low, limiting amounts. And there continues
to be a spread in the models as to where this would set up. The
best chance would be across southern Wyoming, but how far north
and west the dry slot can get is still up for debate. So,
confidence is fairly high on a decent amount of QPF in western
Wyoming, but confidence drops substantially further east.

Now our thoughts on precipitation amounts. The highest amounts still
look to be across northwestern Wyoming, where there is nearly a 100
percent chance of a quarter of an inch or more of QPF, with a
greater than 1 in 2 chance of over three quarters of an inch of QPF.
Most areas East of the Divide have at least a 2 in 3 chance of over
a quarter of an inch of QPF. However, the chance of a half an inch
is generally less than 1 out of 2. And now on to the colder form of
QPF, how much snow may fall. Through Wednesday morning, amounts look
to remain small, since southerly flow should keep snow levels fairly
high, with 700 millibar temperatures of 1 celsius keeping snow
levels above 9000 feet. As the low passes and an associated cold
front moves through, 700 millibar temperatures fall to minus 3, snow
levels should lower to around 7000 feet. As for highlights, it still
looks like borderline advisory for now. The NBM ensembles give
around a 1 in 2 chance of 6 inches of snow or more in portions
of the Tetons, Absarokas and Wind Rivers, but this is mainly
above 9000 feet. The chance of a foot or more is at most 1 out
of 4 and restricted mainly to the southern Absarokas where
impacts are slim to none. We still have time to decide on
highlights since impacts to roads would likely hold off until
after sunset on Wednesday afternoon. As for the wind concern,
there will be some gusty wind on Wednesday but the chance of
gusts over 40 mph is at most 1 out of 3 and largely in
unpopulated areas.

The extended looks uncertain as well, especially Friday night and
Saturday. Some models show one last shortwave / cold front moving
into the area from Montana, bringing some showers with it.
Other guidance have mainly dry conditions. There is agreement
for Sunday though, with ridging bringing dry and mild conditions
to most of the area.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 527 PM MDT Tue Oct 14 2025

MVFR ceilings are expected (60-80% confidence) to filter into KCOD,
KWRL, and KCPR overnight. Recent trends have decreased the chance
for MVFR ceilings at KJAC, KRIW, and KLND to around 20-30%, so
mention has been removed at these terminals while maintaining
FL035 groups. At KRKS and KCPR, gusty surface wind has been
quickly diminishing as sunset Tuesday approaches. Gusty
southerly surface wind between 15-30kts begin between 16Z-19Z
at KRKS, KPNA, and KLND as the pressure gradient tightens.

Scattered showers spread north across western Wyoming after
sunset Tuesday, with impacts possible at KJAC and KPNA as early
as 02Z and at KCOD around 04Z. Showers and a few thunderstorms
generally spread to all other terminals Wednesday afternoon.
However, confidence in direct impacts and timing is around 30%,
so PROB30 groups reflect shower/thunderstorm chances. MVFR
flight conditions could accompany any convection.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on smoke, icing, and turbulence forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM Wednesday to 11 PM MDT
Thursday for WYZ002-012-014-015-024.

&&

$$

UPDATE...VandenBoogart
DISCUSSION...Hattings
AVIATION...Gerhardt/CNJ