650
FXUS65 KRIW 140258
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
858 PM MDT Mon Oct 13 2025

.KEY MESSAGES...

- Patchy Freezing Drizzle is possible tonight along the Cody
  Foothills.

- Increasing rain showers pushing in from the south to southwest
  overnight but isolated in nature. Best chances will be along
  and west of the Continental Divide.

- An unsettled and wetter than normal pattern is expected
  starting Tuesday night and continuing through at least
  Thursday, but details on timing and amounts of precipitation
  are uncertain.

&&

.UPDATE...
Issued at 1230 PM MDT Mon Oct 13 2025

WV/IR imagery continues to show the dry southwest flow aloft in
between the two systems that has and will impact the CWA. Dry
conditions and seasonable temperatures remain for the rest of
today through sunset. However, increasing rain showers are
expected to push out of northeast Utah spreading up through
Sweetwater County after sunset and to the Winds northeastward.
These will be isolated in nature and most will remain dry as
expected into Tuesday morning.

Dry conditions expected by mid morning Tuesday ahead of the
next storm system that will begin to impact western portions of
the state after sunset towards midnight with some mountain snow
and lower elevation rain showers overnight into Wednesday. Some
uncertainty with timing and amounts but overall the track of the
low seems on par with the more global models. The Hi-res clarity
is a bit unsettled but until proven otherwise, the NAMNest will
be the go to for the shorter term forecast. Highest snow amounts
in the higher terrain of the Tetons with likely affects to Teton
Pass with 60-70% probability of accumulations over 6 inches.
Lower valleys should remain rain with limited chances for light
accumulations come Wednesday morning 10-20% of up to an inch.

Otherwise, expect the system to exit east Thursday with cooler
temperatures. Mainly dry and warming trend Friday and into the
weekend.

&&

.DISCUSSION...
Issued at 223 AM MDT Mon Oct 13 2025

As I have mentioned in some of my past discussions, I am a native of
the northeast, New England to be exact. And back there, it is now
fair season. And one of the most popular rides at a fair is the
carousel, also called a merry-go-round. And what happens in
this ride? Wooden horses or other animals rotate around it. And
this is similar to the weather over the next four days or so.
With the central part of the carousel being an upper level low
now spinning over the northwest, with the horses being a few
shortwaves that will be passing through the day before the main
low moves through the area.

Most areas should start of dry today. There could be a few showers
in eastern Sweetwater County, but nothing of consequence. The first
horse, AKA the first surge of moisture and shortwave, will move
in from the south late today but especially tonight and bring
the first chance of showers to the area, with an emphasis on
southern and western Wyoming. Instability is somewhat limited,
but lifted indices do drop down to around 0 late in the day so
we have allowed for some isolated (1 out of 6 chance) thunder
across western and southern Wyoming. Rainfall amounts look
relatively minor at this point. There is at most a 1 out 5
chance of a quarter of an inch of QPF through 6 am Tuesday, with
most areas less than 1 out 10. Snow levels generally look to
remain at 8000 feet through the period, so any impacts from snow
look limited.

Tuesday generally looks like a lull in the activity as we will be in
between waves. There will still be a few showers across the west,
but this looks to be the least active period. Areas East of the
Divide look largely dry though. And, with flow turning southward as
the low starts moving eastward in California and toward the
Great Basin, temperatures will begin to rise again, back to
near to somewhat above normal temperatures.

The second horse, or shortwave will move toward the area on Tuesday
night, increasing chances of showers across mainly western Wyoming
at first and spreading eastward through the night. Confidence
in the forecast details begins to drop past this point though.
This is where the models begin to diverge on the exact track of
the main upper level low and the speed it moves through, with as
much as a 24 hour spread in timing. We do have a fairly high
confidence (about a 4 out of 5 chance) of at least a 1 in 3
chance of showers across most of the area, with the best chance
of higher QPF in the west. However, placement and timing of the
heaviest precipitation remains in flux. Probabilistic ensemble
guidance gives around a 2 out of 3 chance of a half an inch of
QPF from Tuesday morning through Wednesday night across the
west, with lesser chances further east. The general gradient is
highest in the north and west, with lesser amounts further south
and east.

Now for precipitation type. With southerly flow ahead of the low,
850 temperatures generally remain above 0 through Wednesday morning,
keeping snow levels above 8500 feet. Levels will drop later as the
low moves through and the flow turns west to northwesterly,
possibly down to 6500 feet if things set up right. Again, exact
timing depends on the speed of the low. As for amounts, ensemble
guidance gives greater than a 1 in 2 chance of 6 inches or more
across the western mountains for the 72 hours from Tuesday
morning through Thursday night. But again, amounts and timing of
the changeover depend on the exact speed and placement of the
low.

One more factor to consider is wind, possibly strong wind. The best
chance for this would be Wednesday, as an 120 knot jet core will be
moving over Wyoming. The most likely spot for this looks to be
southern Wyoming. This will be especially so in eastern Sweetwater
County, where the right front quadrant of the jet will enhance
downward momentum. The NAM MOS is giving sustained wind of 34
knots Wednesday afternoon. However, other guidance is not as
impressed, with ensemble guidance giving only a 1 out of 5
chance of wind gusts past 50 mph. It also looks like 700
millibar winds only top out at around 40 knots, when I would
like to see 50 knots for high wind. So, it appears unlikely at
this time. It will be rather breezy across many areas,
especially the eastern half of the area.

Conditions should begin to improve by Friday, although there could
still be some wrap around showers across northern Wyoming. Things
are uncertain for Saturday as well, as another wave may bring some
showers to northern Wyoming. As for temperatures through the period,
given the Pacific origin of the air masses, should average fairly
close to seasonal normals for the latter portion of the week.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 451 PM MDT Mon Oct 13 2025

MVFR ceilings/clouds have continued much of the afternoon at KCOD
and KWRL. KWRL has a higher chance (20%-40%) of clouds
dissipating/VFR conditions this evening compared to KCOD. However,
IFR to MVFR ceilings look likely (60%-80%) again tonight at both
terminals. Clouds are expected to dissipate Tuesday afternoon.

At all other terminals, VFR clouds will gradually increase through
the TAF period with isolated precipitation chances (30%) spreading
from south to north across the area. Timing of precipitation has
continued to slow since the last TAF issuance with chances generally
during the overnight hours. Confidence in terminal impacts due to
precipitation is low (30%) so have maintained PROB30 groups at this
time. There is a 20% to 30% chance in MVFR flight conditions
accompanying any showers. There is also 40% chance in MVFR ceilings
at KRIW and KLND between 13Z and 17Z Tuesday morning. All other
terminals, not including KCOD and KWRL, have a 15% to 30% chance of
MVFR ceilings during this same time frame.

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.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Lowe
DISCUSSION...Hattings
AVIATION...Gerhardt