Midwest, Wyoming Weather Forecast Discussion

036
FXUS65 KRIW 221702
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
1102 AM MDT Mon Sep 22 2025
.KEY MESSAGES...
- Shower and thunderstorm chances will be widespread across the
  forecast area today, becoming focused east of the Divide in
  the afternoon/evening.
- Wind gusts of 35 to 45 mph will occur across northern portions
  of the Bighorn Basin and Johnson County will occur this
  morning.
- Freezing temperatures are expected across the western valleys
  Tuesday morning, including southern Lincoln County near
  Cokeville. Dense fog will also be possible in the western
  valleys as well as across Yellowstone.
- Drier and warmer weather returns Tuesday, with above normal
  temperatures in place by Thursday.
&&
.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 /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1102 AM MDT Mon Sep 22 2025
A weather system is in the process of diving south through the area
this morning. This weather system will bring rain and storm chances
to the area through tonight, with most precipitation ending around
sunrise Tuesday morning. Conditions will drop to MVFR/IFR at times
as low clouds and rain develops and passes through the area. Every
terminal will have impacts from rain or storms and for exact timing
please refer to the TAFs. Low clouds may linger Tuesday morning, but
will begin to dissipate late in the morning.
A cold front is already dropping into the area from the north, which
is resulting in a north wind of 10-20 knots, with gusts up to 30
knots from the Bighorn Basin down into Natrona County. This cold
front will continue pushing south through the afternoon and reach
KRKS this evening around 02Z. For most terminals wind will decrease
below 10 knots after sunset, this evening.
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 tonight through Tuesday morning for
WYZ013-023-027.
&&
$$
DISCUSSION...LaVoie
AVIATION...Rowe

NWS RIW Office Area Forecast Discussion