Grand Targhee

000
FXUS65 KRIW 060349
AFDRIW

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

.KEY MESSAGES...

- Skies will be mostly clear overnight with temperatures
  dropping to the 40s and 50s across central and western WY,
  with a couple of upper 30s in Yellowstone. Dry conditions will
  persist into Sunday morning as winds decrease as well.

- Weather systems from the Pacific Northwest and Great Basin
  will approach WY and 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"). Southwest winds will develop as 
  will with gusts from 20 to 30 mph. Elevated fire weather 
  conditions can be expected Sunday afternoon in the lower 
  elevations east of the continental divide.

- Cooler and windy conditions are expected on Monday with
  widespread gusts of 30 to 50 mph during the afternoon.
  Scattered showers will continue over northwest WY through the
  day.

- Dry conditions with near normal temperatures are expected
  Tuesday through Friday, with breezy afternoon winds. There is
  a low chance of a storm developing over WY next Saturday and
  Sunday.

&&

.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 /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 915 PM MDT Sat Sep 5 2026

VFR conditions will continue tonight and through the morning
Sunday, with mostly clear skies. Most areas have seen winds
decrease, but a few gusts are still going in the vicinity of
KCOD, which will need a few more hours to steady. 

A weather system will sweep across the state late afternoon and
through the night Sunday night, with increasing clouds through
the later afternoon. Areas of rain will increase at all sites
late afternoon. Minimal to no change in timing of the start of 
precipitation, with areas west of the Divide between 22Z and 
00Z/07, while areas to the east will pick up between 00Z and 
03Z/07. Right now, cigs will hold above 6000 feet, except for 
some lower cigs approaching MVFR at KJAC. Once precip starts, 
shower activity will continue through most of Sunday night for 
all sites.

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...Straub
FIRE WEATHER...Hattings