511
FXUS65 KRIW 211517
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
917 AM MDT Mon Sep 21 2026

.KEY MESSAGES...

- Fog across western and northern Wyoming gradually dissipates
  through 11 AM. Otherwise expect seasonal temperatures with
  highs in the 60s and low 70s and plenty of sunshine.

- Apart from shower and thunderstorm chances (30% to 70%) on
  Tuesday afternoon and evening, the week is forecast to be dry
  with above average temperatures by Thursday.

&&

.DISCUSSION...
Issued at 130 AM MDT Mon Sep 21 2026

Diminishing cloud cover across the CWA in wake of the weekend`s
storm system that brought much needed and ample precipitation,
especially east of the Divide. IR shows warming cloud tops as
the aforementioned system makes its way eastward across the
upper Great Plains to the Great Lakes stretching southward all
the way to the Gulf of America. WV shows the main finger of the
PFJ well to the north across Canada pushing cyclonically around
an upper level low over the Gulf of Alaska. Another weak low
near California will be the next system to bring in storm
activity for Tuesday afternoon and evening with its slow
progression east and northeastward.

With the clear skies west of the Divide, expect fog development
over some of the area. Jackson hole looks to be the best chance
for a heavier impact with the surrounding lakes and recent
rainfall. Points further east of the Divide, the cloud cover
looks to hinder much of the area but possible (10-20%) around
the Casper area but confidence low at this point in time
dependent on cloud cover by Monday morning sunrise. Beyond that,
expect light winds and seasonable temperatures into the
afternoon with tranquil weather conditions to be had heading
into the next system for later Tuesday.

The aforementioned closed low near California will become more
of an open and progressive wave with its northeastward movement
through the upper Rockies Tuesday afternoon and evening.
Scattered rain showers and isolated thunderstorms likely
(40-70%) for much of the area west of the Divide, with lower
confidence to the east (20-40%). The strongest PVA anomalies
look to occur across Sweetwater County through the upper Green
River Basin for the strongest storms to occur. The likely
impacts will be that of heavy rainfall and gusty outflow winds.
Hi-res models are pushing the 36 hour limit on its output but
it tends to be close enough at this point in time. The Winds
will likely block much impact to the east, but strong winds will
be the main factor pushing down into the Wind River Basin
towards sunset Tuesday evening.

Beyond Tuesday`s storm and shower activity, expect induce
ridging to build in through the Rockies that will bring above
average temperatures for the remainder of the work week. Winds
will be relatively tame for the week as well with no indications
of fire weather concerns at this point in time. The next system
looks to push in by Friday night into the weekend as the
aforementioned GOA low extends down into the Pacific Northwest
as the PFJ dips into the continental US. Early indications of
model runs keep the main impacts north into Montana but still
affects to be had in the CWA with precipitation and more
seasonable temperatures for the weekend and into the following
week.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 430 AM MDT Mon Sep 21 2026

VFR conditions expected for most locations to start the period.
Possible fog development for JAC/CPR to start the period that
will bring MVFR ceilings (~30%) through 15-16Z before lifting to
VFR for the remainder of the period. Otherwise, expect passing
high clouds and relatively light winds 10kts or less at all TAF
sites for the entirety of the period. Dry conditions with no
other weather elements expected at this time.

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.

&&

$$

DISCUSSION...Lowe
AVIATION...Lowe