540
FXUS65 KRIW 210520
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1020 PM MST Thu Nov 20 2025

.KEY MESSAGES...

- Areas of fog are expected again overnight, with localized
  dense fog at times. The main impact locations would be along
  Interstate 25, and across Lincoln and Sweetwater Counties.

- Temperatures will remain 5 to 10 degrees above normal through
  the weekend with dry conditions.

- Confidence increases regarding a pattern change for early next
  week, with cooler temperatures and more active weather looking
  likely by Tuesday.

&&

.UPDATE...
Issued at 1113 AM MST Thu Nov 20 2025

Fog that developed across many of the basins this morning continues
to scatter out and improve. Several areas of fog and low clouds are
of note: Across the western WY valleys and upper Green River Basin
developed initially as a low stratus deck, as this deck scattered
towards sunrise, surface fog filled in, but as of 1100L, continues
to scatter out. Across the Wind River Basin, basin fog developed
from Boysen Reservoir, spreading across the central part of the
basin after sunrise. Fog continues to spread but thin and slowly
scatter out as of 1100L. The other area of fog was across eastern
Natrona and Johnson Counties, generally along and east of I-25.
That fog has continued to retreat to the northeast through the
morning with only a small patch lingering around Casper and
along northeast Johnson County east of I-25 as of 1100L, which
again, continues to improve and should scatter out completely by
1200L.

After the fog is gone this afternoon, there are few to no additional
weather impacts expected through the weekend. A few light occasional
showers and snow showers may be possible across the southwest this
afternoon, and to a lesser extent, tomorrow afternoon, but little to
no accumulation or impacts are expected. Fog is possible again
tomorrow morning, but confidence is low (20%). The rest of the
forecast remains on track through the weekend, with continued above
normal temperatures being the main thing to note.

&&

.DISCUSSION...
Issued at 237 AM MST Thu Nov 20 2025

A slight shift in the forecast compared to the previous discussion
from yesterday. The main difference is increased coverage and
chances for showers across portions of central and southern WY
through the rest of the morning. The best chances (20-40%) will
remain over western WY with lesser chances (10-30%) for portions of
central and southern WY. This change in the forecast is due to a
slightly farther north track of a weakening upper level low moving
out of the southwestern CONUS at this time. This shift in the track
will also lead to increased chances for showers early Friday morning
through the afternoon. Parts of southern and central WY look to have
the best chances (20-40%) for periodic showers Friday. Overall, the
showers today and Friday will have minor impacts. The greatest
impacts would be in the higher elevations of central and
western mountain ranges where a coating to an inch of snow may
be possible. Another thing to monitor this morning will be
patchy areas of fog. The locations with the best chances
(20-40%) of seeing fog are western valleys such as Star Valley,
Jackson Hole and portions of the Upper Green River Basin. Other
locations include central WY such as the Wind River Basin along
with parts of Natrona County. Any areas of fog that do develop
this morning should gradually dissipate through the morning into
the early afternoon today.

Other aspects of the near term forecast remain mostly unchanged.
Temperatures still are forecast to sit at or slightly above
seasonable values. Highs through the weekend range from the low to
mid 40s west of the Divide and low to mid 50s east of the Divide.
Winds look to be light through this period with maybe a slight
breeze in wind prone locations. As for precipitation, after Friday
high pressure tries to build in over the region, leading to mainly
quiet and dry conditions through the weekend.

The upcoming Thanksgiving week forecast is looking quite tricky
with lots of moving parts and little consensus among model
guidance. The first half of the week will see another
disturbance shift south and east out of the PACNW. This
disturbance looks to have cold air associated with it and the
track will determine just how cold it may get. Models differ
with some having the axis digging south leading to colder air
here. Other models have it further east leading to the coldest
temperatures east of the area. The timing of this cold air would
be around Tuesday into Wednesday with some chilly morning lows
possible these days. Another aspect with this disturbance will
be whether or not we see any widespread precipitation. Similar
to the degree of cold air, the track of this disturbance will
lead to differing impacts in terms of precipitation. The second
half of the week sees the active pattern continue with another
potential disturbance moving into the area. However, just like
the first half of the week, the track of this disturbance will
lead to greatly differing impacts. Being nearly a week out there
is even greater uncertainty regarding what we can expect here
in the Cowboy State.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1013 PM MST Thu Nov 20 2025

Moisture is slightly more limited tonight versus last night,
which will limit fog formation. Most TAF sites will see minimal
fog, mainly at KBPI/KPNA and at KCPR, but any impacts will be
more brief than last night. There is an outside chance that fog
off of Boysen Reservoir may reach KRIW around sunrise, but any
fog here will be shallow and patchy. Other airport sites at
KBYG, KEMM, and KAFO are already seeing fog in the vicinity, and
will continue through the rest of the night. KRKS is
fluctuating due to passing light rain/drizzle, which will
continue through the night. After sunrise, any remaining low
clouds will thin out except for KRKS/KEMM, which will hold on
through around noon before lifting. Minimal cloud cover remains
through the rest of the day and into Friday night.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Hensley
DISCUSSION...Dziewaltowski
AVIATION...Straub