908
FXUS65 KRIW 040133
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
733 PM MDT Fri Oct 3 2025

.KEY MESSAGES...

- Rain continues to spread across the area late tonight and
  Saturday, with the highest amounts in northern and western
  portions of Wyoming.

- Turning much colder Saturday afternoon and Sunday, with
  moderate to heavy mountain snow, especially Saturday night
  through Sunday afternoon.

&&

.UPDATE...
Issued at 105 PM MDT Fri Oct 3 2025

With this update, the overall message for the next few days remains
the same: rain chances increasing late today, spreading across the
area through this weekend, with moderate to heavy mountain snow.

Latest hi-res model data is showing a greater fetch of moisture over
the northern Salt/Wyoming Ranges into the Tetons and Absarokas
Saturday morning. Temperatures should be too warm for this to be
mostly snow (snow levels 9800 feet and higher). Liquid precipitation
amounts have therefore increased a little across this swath of area.
As colder air begins to filter in Saturday afternoon, snow levels
will drop and widespread mountain snow increases (snow levels around
9000 feet, and 8000 feet filtering into northern WY). Focus of
precipitation then shifts from far western Wyoming (Tetons) to
northern Wyoming (Absarokas, Bighorns, and Wind Rivers). Heaviest
mountain snow (8000 feet and above) should occur Saturday evening
through Sunday afternoon as the coldest surge of air moves in. There
may be times when snow levels get as low as 6000 to 6500 feet, which
could bring some flakes to western valleys, like in Jackson. There
is some uncertainty with 700mb temperatures between the GFS and
Euro, so snow levels could be a little different than currently
forecasted. The GFS has minus 4C as far south as South Pass area,
while the Euro keeps it around Thermopolis and north. The colder
solution would favor more snow over the southern Absaroka Range and
the Wind River Range, with a few flakes even possible (15%) for the
Lander area. Snow decreases Sunday evening into early Monday
morning.

For total snow amounts, there have been only minor adjustments. The
Absarokas, Bighorns, and eastern Wind Rivers should see the most
snow above 9000 feet, generally 8 to 12 inches, with up to 18 inches
at the very highest elevations/peaks; between 8000 and 9000 feet, a
trace to 8 inches. Snowflakes could be seen as low as 6000 feet, but
there should not be any impacts below 8000 feet. Winter Weather
Advisories have been posted for mountain locations Saturday
afternoon through Sunday afternoon.

&&

.DISCUSSION...
Issued at 257 AM MDT Fri Oct 3 2025

Right now, things are quiet across western and central Wyoming, the
area served by the humble Riverton weather abode. However, this will
be changing, as it is looking increasingly likely that we will see
our first taste of mid to late autumn weather, which would be
classified as winter across much of the United States, but we are
one of lucky (or unlucky ones depending on your perspective) to see
it first.

It likely won`t be for the first 24 hours, as we have one more day
of concerns of things that are more indicative of summer than
winter. This will be another warm day ahead of the approaching cold
fronts (yes there will be two cold fronts) with highs in the 80s
across some of the warmer locations. The main concern West of the
Divide is thunderstorms, and possibly a few strong ones. The
reasoning for and against stronger storms is the same as yesterday.
The reasons for the storms will be a warm afternoon, with
precipitation holding off until late in the day. There will also be
good upper level divergence with a jet streak moving over the area.
There will also be decent speed and directional shear. However, CAPE
is not that high, maxing out at only around 600 J/Kg and lifted
indices are only around minus 1 in the afternoon. We still think
there will be a few stronger storms, but should be isolated. The
threat of stronger storms should end by around sunset. Meanwhile,
East of the Divide the concern shifts to fire weather. This will be
mainly from Rock Springs through Casper across the southwestern wind
corridor as the pressure gradient tightens. Most of this area has at
least a 4 out of 5 chance of wind gusts over 40 mph. The chance of
wind gusts over 55 mph is at most 1 out of 4 though and this is
mainly confined to areas with no roads and as a a result little
impact. So, no wind highlights will be issued. As for humidity, it
still looks like few places will be see 3 hours of values at or
under 15 percent. The NBM ensemble mean still looks to range
anywhere from 17 to 20 percent for low humidity. So, no fire weather
highlights will be issued this morning.

Steadiest precipitation will begin to spread into the west tonight.
Latest guidance has slowed progress though, so we lowered POPS
somewhat East of the Divide. Many areas may end up being largely dry
through most of Friday night as a result. Through the night,
precipitation should remain largely rain except the highest
elevations as 700 millibar temperatures largely remain at or above 2
degrees celsius.

The first cold front and low will move across the area Saturday, and
this looks like the wettest period. Although again, 700 millibar
temperatures remain fairly mild through the most of the day, not
falling below 0 until after around 20Z across western Wyoming. And
again, this should keep things largely wet except for the highest
elevations. This will change after the low moves to the northeast
and the second cold front drops in from Montana. This one is from
Canada, unlike the first one which is Pacific in origin. And this is
where temperatures will begin to drop, especially Saturday night. By
sunrise Sunday, the 700 millibar temperatures should range anywhere
from minus 2 in southern Wyoming to as cold as minus 5 in northern
Wyoming, putting snow levels roughly from 7500 feet to as low as
6000 feet, respectively. The mountains will see snow at this point,
with the worst travel conditions likely during Saturday night. If
there is a saving grace here, it is that the deepest moisture will
be east of the area by this point, and that should limit snowfall
amounts somewhat. With the Canadian origin of the second front,
moisture is more limited. There should still be decent snowfall
though. The northern and central mountains all have at least a 4 out
of 5 chance of over 6 inches of snow above 8000 feet. The chance of
a foot or more of snow is less than 1 out of 2 though except for the
highest elevations though. We will keep the Winter Storm Watches
going for now though in coordination with surrounding offices. It
looks more like advisory level snow to us this morning, but the day
crew can make the final decision.

And even in areas that don`t get snow, the change in the weather
will be a slap in the face. Some locations will see high
temperatures on Sunday as much as 35 to even 40 degrees colder than
on Friday. Add in a blustery northwest wind on the back side of the
low, with a greater than 1 in 2 chance of gusts across northern
Wyoming of gusts over 40 mph, it will not be pleasant Saturday night
or Sunday. And with the colder air coming in, locations as low as
5500 feet in northern Wyoming could see some snowflakes, including
places like Cody and Meeteetse later Saturday night or Sunday. As
for precipitation amounts, most of the area north of an Kemmerer to
Casper line has at least a 3 out of 5 chance of a quarter inch of
QPF. The highest amounts will be across northern Wyoming again, with
a greater than 1 in 2 chance of over a half an inch of QPF. Flooding
concerns still look minimal though, since this looks more like a
steady rain and with recent mainly dry weather there is a lot of
room in the soil.

Precipitation should come to an end for the most part later Sunday
night. And that brings us to another concern, the chance for a
killing frost or freeze Sunday night. Ensemble guidance gives at
least a 1 in 2 chance of temperatures under 32 degrees Sunday night
for most of the area. The one concern here is if clouds hang around,
it could stay a bit warmer. There is another chance on Monday night,
maybe better with a clear sky. No highlights yet but we will watch
it.

More seasonal weather returns by Tuesday. There is more model
disagreement for midweek with chances for showers though. Given the
concerns in the shorter term, we will leave this for another
day.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 430 PM MDT Fri Oct 3 2025

An active TAF period is expected as a longwave trough approaches.
Scattered showers and thunderstorms across western Wyoming will
continue through the afternoon, with some of this activity spreading
east of the Divide as well as it moves northeastward. KCPR and KRKS
will see a gusty southwest wind through the afternoon, decreasing by
sunset. Precipitation chances continue mainly across western and
northern Wyoming overnight. Ceilings lower for most terminals
tonight through Saturday morning as the associated low begins moving
into Wyoming. Rain and mountain snow will then become more
widespread, with KJAC seeing MVFR conditions by 15Z. Near MVFR to
MVFR ceilings will then be seen for several terminals through
Saturday afternoon. Frequent mountain obscurations are expected
through the period.

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

&&

.FIRE WEATHER...
Issued at 257 AM MDT Fri Oct 3 2025

An approaching cold front will bring gusty to strong wind
today, with the strongest wind between Rock Springs and Casper.
Gusts past 40 mph are likely at times. Although relative
humidity will fall into the teens at times East of the Divide,
it should remain above 15 percent for the most part. Elevated
fire weather is a near certainty in the afternoon though. Cooler
and wet weather will improve conditions on Saturday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from noon Saturday to 6 PM MDT Sunday
for WYZ002-012-015.

Winter Weather Advisory from 6 PM Saturday to 6 PM MDT Sunday
for WYZ008-009.

&&

$$

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...Myers
FIRE WEATHER...Hattings