Lake Isabella Trail, Wyoming Weather Forecast Discussion

131
FXUS65 KRIW 222257
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
457 PM MDT Mon Sep 22 2025
.KEY MESSAGES...
- Widespread rain showers continue, especially along and east of
  the Divide, through the rest of the day. Lingering showers
  across the Wind River Basin and Natrona County, as well as
  eastern portions of Sweetwater County are expected into
  Tuesday morning.
- Freezing temperatures are expected across the western valleys
  Tuesday morning, including southern Lincoln County near
  Cokeville. There is a 95% chance of temperatures at or below
  32F and a 85% chance of temperatures below 28F for those
  valleys.
- Dense fog will also be possible in the western valleys (20%
  chance) as well as across Yellowstone (40% chance) Tuesday
  morning.
- Drier and warmer weather returns Tuesday and will continue
  through the rest of the week, with high temperatures around 8
  to 15 degrees above normal by Thursday.
&&
.UPDATE...
Issued at 217 PM MDT Mon Sep 22 2025
A wide band of moderate to heavy precipitation has set up between
Beaver Rim and Worland, spreading throughout much of the Wind River
Basin and southern Bighorn Basin. Notably, all models have seen a
significant uptick in QPF amounts since the overnight runs. Forecast
rainfall amounts are upwards of an inch along the east Wind River
Mountain Foothills and over 0.5 inches across most of the Wind River
Basin. This is nearly double the values that were expected as of the
overnight update. The uptick is likely due to better resolution in
the hi-res models of the aforementioned heavy rain band. Upslope
conditions behind this morning`s frontal passage are also helping to
boost the QPF amounts. Models have also pushed the focus of
precipitation farther west across central Wind River Basin and
Bighorn Basin (previous runs showed a greater focus across the
Bighorn Mountains and Natrona County). The heaviest rain will start
to taper off towards sunset this evening, but lingering showers are
expected to persist across the upslope favored regions mentioned
above as well as portions of Natrona County and eastern Sweetwater
County through at least sunrise Tuesday, possibly through midday
Tuesday.
Across western WY, skies will quickly clear after sunset. Though
those western valleys did not receive much precipitation, clearing
behind the front, with WAA moving in, we can expect fog to be
possible in the western valleys and across the lower elevations of
Yellowstone Tuesday morning. With little rainfall, available
moisture at the surface, chances for fog have decreased, but still
looking at a 20% chance across the aforementioned locations. Freezing
temperatures also remain likely (95%) for western valleys tonight.
Freeze Watches have been upgraded to Warnings and are in effect from
5 AM through 10 AM Tuesday. Most of the locations under the Freeze
Warnings, as well as across the Green River Basin (seasonal end of
growing season has already occurred, hence no Freeze Warnings) have
a 80 to 90% chance of temperatures less than 28F (hard freeze
criteria) tonight.
Beyond Tuesday morning, the forecast looks to remain on track, with
a general warming and drying trend through the end of the week.
Warmest temperatures look to occur Thursday, with temperatures 10 to
15 degrees above normal for most locations.
&&
.DISCUSSION...
Issued at 131 AM MDT Mon Sep 22 2025
Isolated showers, mainly over western portions of the CWA, will
begin to fill in through 12Z this morning as a trough and cold front
approach the Cowboy State. Flashes of lightning will be possible
over northwestern portions, as this activity moves to the east-
northeast. The cold front will reach far northern portions by 09Z,
quickly pushing southward and reaching the Casper area by 13Z and
Riverton/Lander by 15Z. Most of the precipitation will be focused
over western and northern portions through the morning, shifting to
areas east of the Divide after 18Z as the trough moves over western
WY. A mid-level low will develop off the southern end of the Wind
River Mountains by this time, moving toward the Muddy Gap area
through the afternoon and southward toward Rawlins by 00Z. This will
cause the precipitation to become more focused on the northern end
of the low over eastern Fremont/western Natrona County through the
overnight hours. This area is still forecasted to receive the bulk
of the precipitation today and tonight, with amounts ranging between
0.6" to 0.9". Locally heavy rain will be possible in these and
adjacent areas, as PWAT values will be near 0.8", which is the
highest value measured for September 22nd. Speaking of which, the
autumn equinox will occur at 12:19pm and it will certainly feel like
the start of fall. Temperatures today will be 10 to 20 degrees
colder and as much as 25 degrees colder over northern Johnson
County. Wind gusts up to 25 mph will occur for most locations today,
with notable exceptions. Gusts of 35 to 45 mph will occur across
northern portions of the Bighorn Basin and Johnson County this
morning behind the cold front. These winds will gradually subside in
the Bighorn Basin late in the afternoon, but gusts of 25 to 35 mph
will continue across Johnson County through the night. Similar winds
will occur across Natrona County into Sweetwater County during this
time as the front continues its southward progression and the
midlevel low moves south. Any lingering precipitation will be over
southern portions of Natrona County and eastern Sweetwater County,
as the upper level low reaches northern CO.
Any fog Tuesday morning would likely be in the western valleys and
Yellowstone, as these areas will likely have clear/mostly clear
skies tonight/Tuesday morning. Additionally, will issue a Freeze
Watch for these areas as well as southern Lincoln County.
Temperatures will drop below 30 degrees as a result of the cold
front and clear skies.
Conditions will be mainly dry Tuesday, as clouds gradually recede to
the south across Johnson, Natrona, Fremont and Sweetwater counties
through the day and clear skies in place by sunset. Winds will
remain gusty across Sweetwater County through Tuesday morning, as
the sfc pressure gradient will stay tight due to the proximity of
the low. Temperatures will be seasonal, except over southern
portions (as a result of the cloud cover). High pressure will
dominate the pattern through at least Friday, so expect dry
conditions and temperatures returning above normal by Thursday. Weak
showers could move over far southern portions of Sweetwater County
by Saturday due to a closed, upper level (nearly stationary) low
over the Desert Southwest. Extended models keep the PFJ well to the
north through the weekend, so no significant weather features are
expected.
&&
.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 452 PM MDT Mon Sep 22 2025
Showers and thunderstorms will continue through the next 12 to
18 hours as the trough sweeps through. Breaks are already
starting to show around KJAC, which should see an end to precip
once daytime heating is done, with quick cloud clearing through
the night. Drier air filters into KBPI/KPNA this evening, with
rain ending there by 06Z. Remaining sites continue with lighter
rain through 12Z. KCOD/KWRL/KRIW/KLND should start to thin out
at that time, with gradually clearing by 18Z. KCPR/KRKS hold
with some rain and low clouds in the vicinity through 18Z before
improvements finally move into those sites in the afternoon. By
00Z/24, all sites will be SKC with light winds.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Warning from 5 AM to 10 AM MDT Tuesday for WYZ013-023-
027.
&&
$$
UPDATE...Hensley
DISCUSSION...LaVoie
AVIATION...Straub

NWS RIW Office Area Forecast Discussion