063
FXUS65 KRIW 102202
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
402 PM MDT Fri Oct 10 2025

.KEY MESSAGES...

- Widely scattered showers with an isolated embedded
  thunderstorm this afternoon, mainly west of the Divide and
  across Johnson and Natrona Counties.

- Widespread rain showers and isolated thunderstorms Saturday,
  primarily along and west of the Divide. Periods of heavy and
  continuous rainfall could bring isolated instances (up to 15%
  chance) of flash flooding across west and northwest Wyoming.

- Colder air filters in behind a cold front Saturday night into
  Sunday with mountain snow and the potential for wet
  accumulating snow across western valleys.

- Temperatures 15 to 20 degrees cooler Sunday and early next
  week with isolated precipitation chances (30%) along and west
  of the Divide.

&&

.UPDATE...
Issued at 1225 PM MDT Fri Oct 10 2025

The forecast largely remains on track today with near to above
normal temperatures and widely scattered rain showers and isolated
embedded thunderstorms. The primary change made this forecast
package is a slight increase in precipitation chances (30% to 40%)
across eastern Johnson and Natrona Counties this afternoon. A co-
located region of saturated mid-levels and a 25-30kt 700mb wind
maximum will be over the aforementioned locations as an upper-level
vorticity maximum traverses the area. These stronger winds aloft
will also create some breezy winds across the Wind Corridor this
afternoon and evening. Embedded thunderstorm remain across southwest
Wyoming this afternoon given its proximity to the left exit region
of the upper-level jet. Precipitation chances were also
increased/maintained longer across the western portions of Wind
River Mountains through the overnight hours given southwest flow
favoring upslope induced rainfall.

Outside of these changes, the potent cold front remains on track to
approach the state from the west Saturday afternoon with rain shower
and thunderstorm chances ahead of the front. Periods of heavy,
continuous rainfall brings a small flash flooding concern (15%
chance) for portions of western and northwestern Wyoming Saturday,
thanks to PWATs above the 90th percentile and continuous convective
initiation potential over higher terrain. Thus, portions of western
and northwestern Wyoming are highlighted in a Day 2 Marginal for
excessive rainfall.

As mentioned in the discussion, precipitation chances spread chances
spread eastward along the front Saturday night and early Sunday
morning. Cold air quickly filters in behind the cold front and will
lead to a rain to snow transition, especially across the western
mountains. The current forecast reflects a quick transition, however,
if the cold air takes a bit longer to filter in, current snow
amounts could too be high. Given the uncertainties, opting to hold
off on any winter highlights at this time.

&&

.DISCUSSION...
Issued at 241 AM MDT Fri Oct 10 2025

Another day of warm above normal temperatures across the Cowboy
State. Highs today will be in the low to mid 60s west of the Divide
and low to mid 70s east of the Divide. The period of dry and quiet
weather we saw for the first half of the week looks to come to an
end today. Tropical moisture begins to funnel across the region
today, leading to chances for showers and thunderstorms throughout
the day. The first round of precipitation will move across the state
this morning with showers developing across southwestern WY around
0700 MDT. The best chances (30-60%) for precipitation is expected to
be across southern and western WY with lesser chances (20-40%) over
central and northern WY. A second round of precipitation but this
time more convectively driven will be possible during the afternoon
with southwestern WY having the best chances (20-40%) for seeing any
showers or storms. Overall, CAMs have lowered precipitation across
the state for today with the bulk of the moisture not arriving until
later in the day on Saturday.

As mentioned earlier, the main event looks to start Saturday
with the bulk of tropical moisture entering the region at this
time. The trough that is currently digging across the PACNW will
also begin to enter the area Saturday. This will begin to
introduce stronger southwesterly/westerly flow, which should
enhance dynamics across favorable locations such as western WY.
Showers and maybe an isolated thunderstorm will be possible
Saturday morning. However, a cold front will begin to near the
state by the afternoon. Ahead of the front, storm initiation is
likely to begin as it moves west to east through the afternoon
and evening. Western WY will have the best chances (20-40%) for
seeing storms develop with the main limiting factor being the
amount of daytime heating that can occur. Either way the
stronger storms will be capable of producing strong gusty winds,
hail, and heavy downpours. These showers and storms will
continue to spread east into central portions of the state
through the afternoon and evening. A potent upper-level jet is
expected to move over the region along with the frontal passage.
700mb winds may be quite strong in some locations, with models
currently highlighting the Wind Corridor from Rock Springs to
Casper forecasted to range from 50 to 60 knots, leading to some
strong gusts of 35 to 45 mph at the surface. The timing for
these winds look to be late Saturday into the early morning
hours Sunday.

Focusing back over western WY, the frontal passage will be the
start of a period of moderate to heavy precipitation. Favorable
westerly/southwesterly flow will crank up during the evening
Saturday and morning hours Sunday. Upsloping will occur across
the western mountain ranges such as the Tetons, Salt/Wyoming
Ranges, Gros Ventres, the higher elevations of YNP such as
Pitchstone Plateau and the western slopes of the Wind River and
Absaroka Mountains. Initially, temperatures look to be too warm
for snow but as colder air fills in behind the frontal passage
a transition over to snow is expected by Saturday evening. Cold
700mb temperatures of minus 6C to minus 8C move into western WY
and gradually spread east throughout Sunday. Snow levels will
quickly drop over western WY Saturday night into early Sunday
morning going from 10,000 feet to less than 6,000 feet in a
matter of hours. This surge of cold air may lead to a transition
over from cold rain to wet snow in locations such as the Star
and Jackson Valleys Sunday morning. Snow accumulation will be
possible for western valleys, but due to warm ground
temperatures it may be limited. Currently, there looks to be a
20-40% chance for 1 inch of snow accumulation along western
valleys. As for higher elevations, snowfall amounts look to be
nearing Advisory amounts with northern ranges such as the
Tetons, Gros Ventre, and portions of the higher elevations in
YNP having around a (40-60%) chance of seeing 6 or more inches
of snow. So at this time no highlights will be issued, but they
may need to be issued during the day Friday or Saturday night.

A brief lull in activity looks to occur Monday into Tuesday. Cold
temperatures may be possible as the trough moves out the area Sunday
night into Monday morning. There are early indications of widespread
sub freezing morning lows possible for Monday. Western WY may see
temperatures in the low 20s and possibly upper teens with lows
around the upper 20s east of the Divide. Still being a few days out
this may be subject to change. It is worth keeping an eye on
especially with some areas still not seeing a hard freeze yet.
Chilly temperatures will persist across the region with highs likely
in the 50s for Monday and Tuesday. The upcoming week is looking
messy and active with the potential for another disturbance
moving in from Canada for the middle of next week. Timing and
impacts are still very uncertain at this point and a better idea
of what to expect wont happen for another few days.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 359 PM MDT Fri Oct 10 2025

General clearing continues through the evening behind the showers
from this morning and afternoon from west to east, with generally
prevailing light winds.

Saturday morning, the next wave of moisture moves into the region.
Impacts are generally expected to be limited and focused west of the
Divide (KBPI/KJAC/KPNA/KRKS Terminals) through the morning,
spreading east of the Divide into the afternoon. Occasional MVFR
conditions are possible (20% chance) at west of the Divide terminals
Saturday with the shower activity. Locally heavy rain and small hail
are possible with the showers and thunderstorms as well.

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...
None.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Dziewaltowski
AVIATION...Hensley