542
FXUS65 KRIW 160433
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1033 PM MDT Wed Oct 15 2025

.KEY MESSAGES...

- Isolated thunderstorms across central Wyoming gradually
  decrease by early Thursday morning. Brief heavy rain and small
  hail are possible with the strongest storms.

- An area of low pressure tracks from Utah tonight and across
  Wyoming Thursday. Cooler temperatures and widespread
  precipitation occur as this system moves through the state.

- Snow falls in the mountains through Thursday night, with the
  heaviest snow falling between 4 AM and 12 PM Thursday. Winter
  weather travel is likely across mountain passes. Snow levels
  could be as low 6500 feet across southwest Wyoming Thursday
  morning, though most accumulating snow falls above 8500 feet.

&&

.UPDATE...
Issued at 1223 PM MDT Wed Oct 15 2025

Not much change with the overall forecast through the next 48 hours
or so. With the low clouds over the Bighorn Basin and Johnson County
this morning, have lowered high temperatures around 5 degrees, to
the mid-to-upper 50s. The cold front is still on track to move
through tonight, lowering snow levels. Snow amounts for the western
mountains have come up slightly, but are still only around 8 to 11
inches at the highest peaks of the Teton, Wind River, and Salt River
Ranges. The highest peaks of the Bighorns (above 9500 feet) could
see up to about 9 inches, but this is too localized for any
highlights. The Bighorn mountain passes (Granite and Powder River)
should see 3 to 5 inches. Otherwise, mountains should generally see
up to 8 or 9 inches. Snow levels should lower enough for some light
snow over the lower elevations west of the Divide, including Big
Piney, Pinedale, Kemmerer, and Rock Springs. Flakes are possible in
Star and Jackson Valleys. Lowest snow levels should occur Thursday
morning, being 6700 to 7000 feet.

&&

.DISCUSSION...
Issued at 253 AM MDT Wed Oct 15 2025

The question of the day continues to be what will happen with the
upper level low that is now swirling over California and will move
toward and over the Cowboy State over the next 36 to 48 hours. And
the good news is that there is more agreement in the model guidance
this morning, although there are some differences in the exact track
that will make a difference in potential precipitation amounts (try
saying those three words fast five times, a bit of a tongue
twister). Well, let us dive in.

We currently have a few areas of mainly light showers moving
northward across the area. A couple of thunderstorms are also noted,
but nothing strong and these are mainly in Idaho. Today looks like
it will be inverse of yesterday, when we had more shower coverage in
the morning and then decreasing in the afternoon. Showers should
become more numerous in the afternoon as the low gets closer to the
area. And this is where we still have some uncertainty with the
track of the low. Some guidance, like European model, show the low
moving further south, which would bring a better chance of showers
across central Wyoming this afternoon and tonight. Others, like the
GFS, bring the low around 75 miles further north, and this could
bring a dry slot into central Wyoming, keeping things largely dry.
For now, we kept some POPS in the area but chances are generally less
than 1 out of 3 in any given area before better chances move in
later Wednesday night as the low makes a closer approach. The most
numerous showers will be across western Wyoming though. And, there
could even be a few stronger thunderstorms. CAPE is rather limited,
only around 300 J/Kg at the most. However, lifted indices drop to
around minus 3 in the afternoon across portions of Johnson and
Natrona County. There will be decent upper level divergence and
some direction shear as well. Nothing widespread is expected but
there could be a few more feisty storms. The Storm Prediction
Center agrees and has issued a marginal risk for Natrona County.

The other concern is snow. Few problems are expected through the
daylight hours today, as snow levels should remain above 8500 feet
and even above this still warm ground and borderline temperatures
should melt snow on roads. A cold front will press across the area
tonight and drop snow levels to around 7000 to 7500 feet by Thursday
morning and possibly as low as 6500 feet in some areas East of the
Divide as 700 millibar temperatures may fall as low as minus 4 to
minus 5. As for the advisories, they still look reasonably so few
changes will be made this morning. The areas with at least a 1 out
of 2 chance of 6 inches of snow or more are covered by the
advisories for the most part. I do have a small concern that some
portions of the Bighorn range may receive 6 inches or more but these
would be the higher peaks and therefore impacts would be limited.
The steadiest precipitation still looks to fall in the 18 hour
period from late tonight through Thursday afternoon, with showers
later Thursday as the low moves away to the south and east. As for
total precipitation, most areas north of a Kemmerer to Casper line
have at least a 1 out of 2 chance of a quarter of an inch or greater
over the next 24 hours. The highest amounts still look to be in
northwestern Wyoming, with a least a 1 out of 2 chance of greater
than three quarters of an inch of QPF over the same time period.

Most guidance is showing Friday as a dry day with ridging over
the area. Another fast moving cold front will move in from the
north on Saturday. There continues to be differences in guidance
in regards to shower chances though. For now, we leaned toward
the drier end since the front is of continental origin and
doesn`t have a lot of moisture to work with. Drier and warmer
weather then returns on Sunday.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1031 PM MDT Wed Oct 15 2025

300mb 100kt jet and diffluence aloft has been driving ongoing
convection Wednesday evening ahead of a tightly wound upper low over
Utah. Thunderstorms through the early morning hours Thursday will
generally be confined to Natrona and Johnson Counties. Otherwise,
scattered weaker convection spreads south-to-north across the
forecast area overnight. IFR/MVFR ceilings remain across the Bighorn
Basin and from KCPR to KBYG.

IFR/MVFR conditions become more widespread early Thursday morning as
the closed low moves northeast and begins to cross the region. The
best chance of precipitation is still between 10Z-18Z/Thursday.
Expect the IFR/MVFR ceilings to infiltrate KRIW and KLND between 08Z-
12Z/Thursday behind northerly surface flow. At KRKS, additional lift
provided by the closed low should provide a period of more intense
precipitation, likely in the form of snow, Thursday morning leading
to IFR/MVFR conditions. Gusty west to northwest surface wind 15-
30kts arrives in southwest Wyoming around midday Thursday allowing
for conditions to dry out and improve, especially at KRKS.
Similarly, west-southwest surface wind pushes into KCPR and erodes
the lower ceilings as VFR returns around 18Z-20Z/Thursday. However,
there is the potential (30 percent) that the wind does not
materialize as early and pesky lower ceilings persist. The general
trend between 23Z/Thursday and 03Z/Friday will be toward drier and
developing VFR from west-to-east. Mountain tops obscured across the
Bighorn Basin throughout the forecast period, while mountain tops
are obscured across the remainder of the area.

&&

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

&&

$$

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...CNJ