236
FXUS65 KRIW 120351
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
951 PM MDT Sat Oct 11 2025

.KEY MESSAGES...

- Scattered rain showers and thunderstorms will continue
  overnight. Storms may produce heavy rain at times.

- A cold front treks across the area tonight and will bring
  much colder temperatures. After sunset, this will transition
  rain to snow across western mountains and potentially wet
  slushy snow in western Valleys.

- Winds increase tonight ahead of and along the cold front with
  widespread 35 to 50 mph winds. Locally higher gusts to 60 mph
  are possible in wind prone locations.

- Sub-freezing temperatures ranging from the teens across
  western valleys and the Green River Basin to the 20s elsewhere
  Sunday night/Monday morning. The eastern Bighorn Basin and
  northern Johnson County will likely see their first sub-
  freezing temperatures so a Freeze Watch is in effect.

&&

.UPDATE...
Issued at 1230 PM MDT Sat Oct 11 2025

The forecast remains on track with unsettled weather over the next
24 hours. Current (18Z) satellite water vapor imagery shows the
moisture associated with Hurricane Priscilla`s remnants now over
Wyoming. This high moisture content is co-located with an upper-
level disturbance/vorticity maximum. This is producing scattered to
widespread rain showers across Sweetwater, Natrona, and eastern
Fremont Counties. These showers are still expected to spread
northeast through the early afternoon and gradually diminish in
coverage as the upper-level disturbance is pushed east out of
Wyoming by a strong upper jet. As mentioned in the discussion,
western and northwestern Wyoming are within the right entrance
region of this 70kt to 90kt jet maximum, a favorable region for
shower and thunderstorm development and maintenance. Forecast
soundings in this area show long, skinny CAPE (values ranging from
250 J/kg to 700 J/kg), saturated low and mid levels, and PWATs above
the 90th percentile. This suggests that any shower or thunderstorm
will be capable of heavy rainfall, with the potential for localized
flash flooding due to multiple showers going over the same location.
This threat continues until around 6PM/00Z tonight.

Two other features are also evident on water vapor imagery - a
region of unsaturated air and a deep upper trough with associated
Pacific moisture. The drier air separates the previously mentioned
moisture from Hurricane Pricilla`s remnants and the saturated trough
over the northwest CONUS. As of 18z, this unsaturated air is roughly
located from the southwest corner of Wyoming to Dubois and will
shift east through the afternoon. Sunshine within this area will
allow for the atmosphere to somewhat destabilize through the
afternoon, resulting in diurnally driven widely scattered showers
and thunderstorms, mainly across southwest and southern Wyoming this
afternoon. A few stronger storms cannot be ruled out with the
primary threats being hail and gusty winds up to 60mph.

The deep upper-level trough and the associated potent cold front
will push into Wyoming late tonight. Rain showers will push into the
central basins as the cold front nears. Rain is still forecast to
transition over to snow across the western mountains as the front
ushers in much colder air and drop snow levels. Accumulating snow
amounts above 6 inches (80% chance) are largely confined to the
highest elevations of the western mountains and northern Wind River
Mountains. The cold front will cut off some of the available
moisture and given the greatest amounts are largely forecast across
the highest elevations, no winter highlights have been issued at
this time. The wind forecast remains on track with gusty 25 to 50
mph gusts with the passage of the cold front thanks to a tightening
pressure gradient and strong jet aloft. The rest of the forecast is
on track for Sunday with much colder temperatures and precipitation
chances confined to northern Wyoming.

&&

.DISCUSSION...
Issued at 133 AM MDT Sat Oct 11 2025

Relatively quiet across the CWA as of 05Z, with most of clouds from
earlier Friday streaming further east over southeastern portions of
the state and the Central Plains. Additional clouds/moisture
associated with the remnants of Hurricane Priscilla are currently
over UT and are expected to reach far southern portions of the
forecast area by 13Z. Showers and embedded thunderstorms will
continue to move toward the northeast through the rest of the
morning and into the afternoon. Locally heavy rain will be the main
threat as PWATs with this airmass will be up to 0.9". For reference,
0.68" is the max values and 0.51" is in the 90th percentile.
Obviously, this is well above normal for this time of year. These
showers will continue to move to the northeast through the afternoon
and exit/end over Johnson County by late afternoon. The second area
of focus will be over western portions, as a storm system approaches
the area. Showers will begin to develop over these areas by 15Z,
with a 50-80% for showers and thunderstorms expected after 18Z.
These storms will be a bit stronger as they will be more dynamically-
driven, due to the proximity of the cold front over eastern ID and
the right exit region of the PFJ. Additionally, CAPE values of 200-
600 J/kg and LI`s around minus 2 will be in place. Even though PWATs
will be around 0.50" in this area due to the more Pacific origin of
the moisture, locally heavy rain and possible localized flash
flooding will be the main threats. Small hail also cannot be ruled
out. These showers and storms will move over areas near Cody and
Rock Springs late in the afternoon. Precipitation will begin to fill
in across the west after sunset this evening, while at the same time
showers and embedded thunderstorms ahead of the cold front move over
the central basins. Snow levels will be quickly dropping over the
western mountains during this time, from 10,000/11,000 ft to about
6000 ft by midnight. Travelers over Togwotee, Teton and Salt Passes
should be prepared for rain changing over to snow early in the
evening. This trend is expected between 03Z and 06Z (9pm to
midnight) in the Jackson and Star Valleys.

The focus will shift to wind tonight, as most of the precipitation
will be exiting/ending across the CWA after 06Z. The cold front will
quickly make its way from one end of the forecast area to the other
(west to east) between 00Z and 12Z Sunday. As such, the strength of
the front and the tightening of the sfc pressure gradient will lead
to gusts of 35 to 50 mph to occur with the passage of the cold
front. Wind prone locations like Clark and Hwy 258 on the south side
of Casper could have gusts up to 60 mph. The focus for these winds
will shift to the east slopes of the Absaroka and Wind River
Mountains, the Bighorns, as well as areas from Jeffrey City to
southern Johnson County Sunday morning. Gusts up to 45 mph will be
common in these areas, with locally higher gusts up to 55 mph
possible. These winds will gradually decrease Sunday afternoon, as
the main storm system moves over western ND. Any precipitation
Sunday will be confined to the northern mountains and Yellowstone,
with chances decreasing through the afternoon. Cold conditions will
be in place Sunday night in the wake of this storm, with widespread
readings in the 20s and the colder spots west of the Divide dropping
into the teens. A Freeze Watch will be issued for eastern portions
of the Bighorn Basin and northern Johnson County as a result.

Dry conditions will be in place Sunday night into Monday as a ridge
builds over the region. This will be irt a strong closed upper low
from British Columbia digging south just off the West Coast. This
will lead to increased southerly/southwesterly flow across the
Cowboy State and gusty winds across the Wind Corridor (Rock Springs
to Casper) as well as southern Lincoln County. Chances for showers
will begin to increase across the south Monday night, spreading
northward across areas west of the Divide, as moisture increases
across the area once again. This trend will continue Tuesday as
the upper low begins to move onshore over northern CA, with
western portions of the CWA having the best chances for
precipitation. Snow levels over the western mountains will rise
to 8500 to 9500 ft by this time. Gusty winds will continue over
the Wind Corridor, the southwest flow pattern remains in place.
The storm system will continue to approach the region Wednesday,
with precipitation chances becoming a bit more widespread.
However, western portions will continue to have the better
chances. Forecast confidence begins to drop by Wednesday, mainly
due to timing issues, but expect cooler/wetter conditions
Wednesday through Friday. The confidence in timing could be
applied as early as Monday, as this forecasted system will be
the fourth such storm in as many weeks. Each of these storms had
been forecasted to move over the area too fast, so would expect
the forecast to slow this system down.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 950 PM MDT Sat Oct 11 2025

As the weather system pushes through tonight so will a strong cold
front. Along this cold front scattered rain showers and isolated
storms will make their way from west to east across the area tonight
into Sunday morning. Most terminals will remain VFR through the TAF
period, excluding KCOD, KJAC, and KWRL. Chances are about 30% that
conditions will drop to MVFR/IFR at the aforementioned terminals,
thus PROB30 groups have been added for them. All rain will dissipate
or exit the area by 18Z Sunday.

The jet stream is strong overhead with this weather system, so that
means wind will continue to be breezy to strong tonight into Sunday.
Ahead of the cold front wind will be from the south/southwest and
behind it wind will be from the west/northwest. Wind will gradually
drop Sunday as the weather system and associated jet exit the area
to the east. Wind gusts will generally be in the 20-30 knot range
tonight into Sunday afternoon, with higher gusts around 40 knots
possible at KRKS and KCPR.

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...
Freeze Watch from late Sunday night through Monday morning for
WYZ004-006-010.

&&

$$

UPDATE...Gerhardt
DISCUSSION...LaVoie
AVIATION...Rowe