257
FXUS65 KRIW 110349
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
949 PM MDT Sat Oct 10 2026

.KEY MESSAGES...

- Scattered rain showers continue into the overnight hours,
  with gusty outflow winds up to 40 to 50 mph the main hazard.
  The heaviest precipitation looks to arrive Sunday through
  Monday, with locally heavy rain possible, especially for the
  western mountains.

- Cooler temperatures move in starting Sunday. Several inches of
  snow are likely (a greater than 1 in 2 chance) in the
  mountains above 10000 feet.

- Strong gusty winds (50+ mph) may be possible late Sunday into
  Monday, mainly across central and southern WY from Casper to
  Rock Springs.

- Periods of showers will continue through the weekend, although
  details of rainfall amount and placement of heaviest rain
  remains uncertain, especially from Tuesday and beyond.

&&

.UPDATE...
Issued at 1158 AM MDT Sat Oct 10 2026

A few things to update regarding the near term forecast as models
have begun to come into better agreement. Starting with the
remainder of Saturday, temperatures are looking to be on the warmer
side especially east of the Divide. Southwesterly winds have already
begun to warm temperatures into the 70s across many locations east
of the Divide. Highs are forecast to get into the upper 70s to low
80s east of the Divide with the Bighorn Basin being the cooler
location. West of the Divide, highs look to get into the mid 50s to
low 60s. Winds will be gusty throughout the day with speeds of 15 to
25 mph and periodic gusts of 30 to 40 mph. Weaker winds are expected
over northern and western WY. The focus now turns towards the start
of a prolonged period of unsettled weather as multiple systems
interact and move through the region over the next few days.

Showers have already begun to develop and move into portions of
western WY this morning. Due to some breaks in the cloud cover a few
thunderstorms cannot be ruled out this afternoon and evening. The
best chances look to be across western and southwestern WY where the
best dynamics are likely to occur. The biggest concern with any
thunderstorms would be strong gusty outflow winds, some small hail
and moderate to heavy rainfall. As mentioned earlier there are a few
adjustments to the forecast, as a result of hi-res models starting
to ingest data. The first is less precipitation amounts east of the
Divide due to downsloping and a shift farther west in the storm
track. This shift will lead to heavier precipitation remaining over
western WY and only periodic round of showers east of the Divide.
Any prevailing period of precipitation is looking less likely east
of the Divide, which will lead to lower rainfall amounts. The other
forecast adjustment is temperatures which have trended warmer not
only across lower elevations but the higher elevations as well. 700
mb temperatures are warmer than they were only 24 hours ago with
values now at 2 to 5C which would keep snow levels above 10,000 feet
and possibly higher. Northern portions of the Absarokas and YNP may
see colder temperatures leading to better chances for accumulating
snowfall across the highest elevations. As for the lower elevations,
the shift in temperatures mainly impacts the Bighorn Basin as models
show the coldest sfc lvl temperatures struggling to make it far
south. While cold temperatures near freezing are still possible for
the Bighorn Basin, they seem to be delayed further into Sunday night
and Monday morning. The degree of cold is also less with lows
forecast to not be as drastic as thought only a day ago. Due to this
uncertainty, any freeze related products are going to be held off
for now. The best chances look to be Sunday night into Monday
morning with much of the basin having a 30-60% chance for lows of
32F or less. These warmer temperatures have also led to lower
chances for any wintry precipitation mixing in but that
occurring in some isolated locations till cannot be ruled out.
The last adjustment is increasing winds especially for Sunday
night into Monday, mainly across portions of central and
southern WY. Models are showing a rather potent jet with 700 mb
winds of 50 to 65 knots move across the Wind Corridor from
Casper to Rock Springs. Probabilistic guidance is showing a
large swath of 30-50% for wind gusts exceeding 50 mph Sunday
night through much of Monday. So this will be worth monitoring
as there could be the need for some high wind highlights if
winds are forecast to gust this high.

&&

.DISCUSSION...
Issued at 250 AM MDT Sat Oct 10 2026

Being a member of Gen X, I was the first generation to watch Sesame
Street when I was a wee lad many eons ago. And at the end, right
before the closing credits, they always said the episode was
sponsored by two letters. Well, this Area Forecast Discussion is
sponsored by the letters T and U. T stands for transition, as we
move from the warm and dry pattern into a cooler and wetter one. And
U is for uncertainty, as are still a fair amount of questions with
the forecast, especially in regards to the snowfall potential at the
lower elevations and details of the forecast toward the middle and
end of next week.

The T for transition will be today. We had a few showers yesterday,
and radar shows more showers, generally of the light variety moving
from Idaho and Utah with the first hits of moisture. The players in
the forecast remain the same, a cold front dropping in from Montana,
an upper level low and associated trough dropping in from the
Pacific Northwest and Tropical Storm Rachel. The cold front should
remain largely north of the border today, so we will likely have one
more warm day today, maybe the warmest for the rest of the calendar
year. Some warmer spots will likely see another high in the 90s. We
will also have continued gusty wind in the favored southwest flow
locations from Rock Springs to southern Johnson County.  As for
showers, moisture from Rachel will increase through the day, drawn
northward by the trough. The best chance of showers and storms will
be largely West of the Divide today as downsloping flow should limit
shower East of the Divide through the daylight hours. There will
still be some, but not as numerous as West of the Divide. And there
will likely be some thunderstorms today, as there is decent CAPE for
October, up to 300 J/Kg with lifted indices down to minus 2.
Coverage of showers looks to increase tonight as deeper moisture
moves northward and some jet energy moves into the area, increasing
upper level divergence.

The wettest day still looks to be Sunday, as the deepest moisture
from Rachel moves across the area. Reasoning today is fairly similar
to yesterday. There will be locally heavy rain. Precipitable water
values could be as high as 0.75 inches, which is close to 200
percent of the mean and could be a daily record for Sunday. The jet
will also be nearby, keeping upper level divergence around. Again,
the placement of the heaviest precipitation will be across western
Wyoming, with decreasing chances and amounts further to the east. I
will discuss potential amounts later in the discussion. I did give
some consideration to flood watches, but decided against them for
now for the following reasons. One, it has been very dry over the
past couple of weeks, so there is decent amount of room in the soil
to soak up some of the rain. Two, the cold front will be further
south today, and instability will be limited. So, precipitation
looks more stratiform than convective, which should limit
precipitation rates somewhat. Three, some of the highest elevations
may be seeing snow at this point, limiting runoff. I will discuss
snowfall mounts later in the discussion as well. And four, most
deterministic models and a majority of the ensemble members show the
best QPF remaining west of Wyoming. The area I would be most
concerned about would be the western slopes of the Wind River Range
with the upslope flow, the Tetons as well as Yellowstone Park. We
still have time to decide though as the heaviest rain wouldn`t be
until Sunday, with most guidance zeroing in between noon and
late Sunday night, with some decrease after that as the deepest
moisture from Rachel moves away to the north and east.

By Monday, the dominant feature will be the Upper Level which by
this point will begin to turn eastward toward the Rockies. Most
guidance shows lighter precipitation Monday afternoon, and most
confined to areas West of the Divide by that time, with only
lighter amounts East, Most will also be confined to southern
Wyoming, with northern Wyoming largely dry.

We will now discuss precipitation amounts, I will reference from
Saturday night through Monday, when the most rain is expected to
fall. The vast majority of the area has at least a 1 in 2 chance of
a quarter of an inch or precipitation or more, with the smallest
chance south and east of a Rock Springs to Casper line. Chances of
an inch or more (greater than 1 in 4 chance) are largely confined
the northwestern Wyoming as well as the Bighorns where there will be
an upslope component. The areas with a greater than 2 inches of
precipitation, which is where the best chance of excessive rainfall
would be is in western Yellowstone Park, the Tetons and the Wind
River Range. However, some of this precipitation over the higher
elevations will likely fall in the form of snow, especially Sunday
through Monday when the cooler air moves out of Montana. Additional
rainfall will occur Monday night but amounts look lighter at this
time.

And now we will focus on the frozen form of precipitation, snow.
With warmer air remaining over the area today, any snow would be
restricted to the highest elevations, above 11000 feet for the most
part. Cooler air begins to ooze into the area Saturday night and
especially Sunday, lowering snow levels. Levels bottom out late
Sunday into Sunday night. Across Central Wyoming, with 700 millibar
temperatures bottoming out between plus 1 to plus 3, this should
keep snow levels above 9000 feet, above most passes, Togwotee Pass
could see some wet snow, but any impacts should be minimal. As for
highlights, we have held off for now. Looking at the NBM ensemble 72
hour snowfall through 6 am on Tuesday the areas that have a greater
than 1 in 2 chance of six inches or more are largely restricted to
portions of the Absarokas and the Wind River Range, largely above
10000 to 11000 feet. Portions of the Wind River Range will almost
certainly get at least a foot of snow, and possibly over two feet of
new snow. However, this would only be at the highest elevations
where impacts would be very limited. We will let the day shift take
another look. In the lower elevations, the main area of concern
would be northern Park County, where 700 millibar temperatures will
be the coolest. The question that remains is will there be enough
dynamic cooling to cool temperatures at the surface enough.
Ensembles only give a 1 in 5 chance of an inch or more here, but
model performance in this area is poor and we still have a decent
setup with the overrunning moisture and the jet nearby. It wouldn`t
be until Sunday or Sunday night so there is still time to take
another look but this has to be watched.

And following this is where the U for uncertainty comes in. And it
has to do with the progress of the upper level low, as guidance has
vastly different solutions with it. Tuesday has some agreement with
showery weather during the day and possibly more at night as deeper
moisture moves toward the area. Following this is were there is a
large amount of divergence. The GFS as well as some ensemble members
pick up the upper level low and move it quickly across the state
getting kicked by a trough moving across the US-Candian border,
bringing a wet Wednesday then moving it out of the area for later in
the week for a return to drier weather. The European has the upper
level low missing the connection and getting blocked by a building
ridge to the west, cutting the low off over the Great Basin and
keeping potentially unsettled weather into the end of the week.
Confidence is very low on which will happen at this time though. So,
there it is a (very long) Area Forecast Discussion brought to you
by the letters T and U.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 949 PM MDT Sat Oct 10 2026

A large weather system with plenty of moisture will impact the area
over the next 24 hours and beyond. A cold front will push south
through the area tonight stalling along the Continental Divide and
then retreating northward into the Bighorn Basin Sunday morning. All
terminals will drop to MVFR due to low CIGS, reduced VIS from rain,
or a combination of both.

A relative lull in rain coverage will occur Sunday afternoon before
more convective showers move into southern portions of the area late
afternoon into Sunday evening. There will be some isolated storms as
well Sunday afternoon and evening. This lull may lead to slightly
improved conditions for much of Sunday afternoon for most terminals.
KCOD will drop to IFR CIGS by around 08Z and remain IFR through much
of Sunday afternoon. It may improve to low MVFR at KCOD late Sunday
afternoon through Sunday evening, but CIGS will be right around
FL010. Precipitation should remain rain at all terminals through the
TAF period.

Wind will be quite breezy, especially Sunday afternoon and evening.
As stronger wind moves in aloft wind may increase more Sunday
evening into Sunday night. For northern terminals (KCOD, KWRL) wind
will remain northerly with the cold front south of the terminals.
For other terminals wind will generally be from the south through
the TAF period. Frequent mountain obscuration through the TAF
period.

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...
None.

&&

$$

UPDATE...Dziewaltowski
DISCUSSION...Hattings
AVIATION...Rowe