542
FXUS65 KRIW 052151
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
351 PM MDT Sat Sep 5 2026

.KEY MESSAGES...

- Humidity below 20 percent and southwest wind gusting 20 to 30
  mph will contribute to elevated fire weather conditions
  through sunset.

- A weather system will return showers and thunderstorms to the
  area from Sunday afternoon through Monday morning. The best
  chances for precipitation will be west of the Divide (50-70%
  chance of at least 0.25").

- Cooler and windy on Monday with widespread gusts of 30 to 50 mph
  during the afternoon.

&&

.UPDATE...
Issued at 1248 PM MDT Sat Sep 5 2026

Little changes were made to the short term forecast today. Elevated
fire weather conditions remain the primary weather concern through
sunset given a gusty southwesterly wind and humidity falling to the
12 to 20 percent range. The weather change to end the weekend as
advertised below is on track; a plume of monsoonal moisture and a
vorticity lobe to provide forcing will arrive Sunday. Showers will
move into southwestern Wyoming by early afternoon, moving north
through Sunday night.

&&

.DISCUSSION...
Issued at 143 AM MDT Sat Sep 5 2026

We have reached the unofficial end of summer, Labor Day weekend. It
is a celebration of American workers and their contribution to
America. Not all workers are off this weekend (I am writing this
discussion after all), but I am thankful to have a job that I love.
In contrast to some of the jobs I had when I was younger, a lot were
hot, dirty and sometimes dangerous.  And for those fortunate enough
to have this long holiday weekend, at least some of the weekend
should be nice for outdoor activities.

Today would be the best day, as it will be a typical summer day.
Although a day more typical of July then early September.
Temperatures will average around 10 degrees above normal. Most
locations under 5000 feet will have at least a 1 in 2 chance of high
temperatures of 90 degrees or over. There will also be a few more
clouds than yesterday, but it should still balance at most partly
sunny. It will also be almost completely dry. There is one small
wrinkle though, there will be a bit more moisture moving toward
eastern Wyoming as the ridge retrogrades to the southern Plains.
Just enough moisture may creep toward Casper Mountain for a stray
shower, but the chance is 10 percent or less. The main concern today
will be fire weather. With the ridge moving closer, there should be
less of a pressure gradient, and wind should be somewhat lighter.
Humidity will also be somewhat higher. And areas that could see
critical relative humidity (under 15 percent), this is where wind
should be lightest. So, we will not issue any Red Flag Warnings
today. However, we will still see elevated to near critical fire
weather for most of the area. But any critical fire weather would be
isolated. Like previous nights, wind should decrease after sunset.
And tonight, a weak, emphasis on weak, cold front may slip into
northern Wyoming. With the dry air in place, the only impact would
be a shift in wind to the north.

Moisture increases more on Sunday. We will be between an upper level
low approaching from the northwest and a ridge to the southeast. And
these positions should allow for some moisture to be channeled from
Marie, the tropical system in the eastern Pacific. The moisture will
take its time getting here though. Almost all areas should be dry
through the early afternoon and other then southwestern Wyoming,
almost all locations will have a mainly dry Sunday during the
daylight hours. And this will bring another very warm day,
although a couple of degrees cooler with some more cloud cover. The
best chance of showers would be at night as the deepest
moisture moves over the area, with some guidance showing as much
as 1 to 1.25 inches of precipitable water Sunday evening. This
will be combined with some extra lift as the upper level low
lifts through Idaho and drags a cold front and a negatively
tilted trough across the state. This could lead to some locally
heavy rain. As a result, we have a marginal risk of excessive
rainfall across western Wyoming for this time period. However,
flow is fairly fast at the mid levels, as high as 50 knots at
500 millibars, so any showers or storms would be of the moving
and not stationary variety.

Monday at this point will be a cooler day behind the front. A few
showers will linger across western and possibly northern Wyoming
with as some moisture wraps around the upper level low. The main
story Monday will be a gusty to strong wind. There will be a
tight pressure gradient between the upper level low and the
ridge to the south and east. Wind at 700 millibars could be as
strong as 40 knots in portions of the area. Most of the area has at
least 3 in 5 chance of wind gusts past 30 mph. Some of the more wind
prone areas, like Casper have a 2 out of 5 chance of wind gusts to
50 mph on this day. Relative humidity looks to remain above critical
levels, so fire weather highlights are not likely at this point.
However, elevated fire weather looks to be a near certainty.

Drier air then returns to the area for Tuesday into Wednesday as
flow turns more zonal across Wyoming. Temperatures should run
somewhat above normal but not hot. There could also be a gusty
breeze in some locations and with humidity lowering we may need Fire
Weather Highlights in some locations, especially on Tuesday.
Monsoonal moisture may return to the area for the end of the week.
At this time, most guidance is showing the core of it remaining to
the south, so this could be more of a glancing blow. However, with
how poorly the models have handled the monsoon this summer,
confidence is low with this solution, especially several days out.
So, the sum up this weekend, all areas will have at least one day
with most locations having two. The first part of the weekend also
looks better than the latter half.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 350 PM MDT Sat Sep 5 2026

VFR conditions prevail through the period. Winds decrease overnight,
with a mostly clear sky. Moisture begins to increase from the
southwest after 18z Sunday. Increasing clouds ahead and with this
moisture is expected. Rain chances increase around KBPI, KPNA, and
KRKS starting about 18z, with KJAC soon to follow. Chances spread to
other sites near the very end of the period and soon after. Sunday
afternoon is also expected to be windy, with widespread gusts 15
to 25 knots, nearing 30 knots at KRKS.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued at 143 AM MDT Sat Sep 5 2026

Low humidity and locally breezy conditions will bring elevated
to near critical fire weather to most of the area this afternoon
and early this evening. At this time, any critical fire weather
conditions look to be isolated and of fairly brief duration.

&&

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

&&

$$

UPDATE...Myers
DISCUSSION...Hattings
AVIATION...Wittmann
FIRE WEATHER...Hattings