660
FXUS65 KRIW 010740
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
140 AM MDT Tue Sep 1 2026

.KEY MESSAGES...

- A weather system and a plume of monsoonal moisture will bring
  scattered showers and storms today, most numerous in the
  morning and in western Wyoming.

- Mainly dry and warm from Wednesday through Friday. Low
  humidity and a gusty wind will bring elevated to potentially
  critical fire weather each afternoon.

- The next better chance of showers moves in this weekend, most
  likely on Sunday with the next plume of monsoonal moisture.

&&

.DISCUSSION...
Issued at 134 AM MDT Tue Sep 1 2026

This morning we say goodbye to meteorological summer for another
year. And we enter what is the favorite month for many Wyoming
residents, September. The number of tourists drops substantially,
especially after Labor Day, so we have the state more to ourselves.
It is usually a fairly dry month (with some exceptions). And
temperatures are usually pretty nice, with comfortably warm days and
cool nights. However, I have witnessed a few snowstorms in
September in my almost 15 years at the office. But, none of
those on the horizon, at least for the next week.

We are seeing some needed moisture this morning, in the form of a
plume of monsoonal moisture and a shortwave lifting northward across
the area. We do have some showers across the area this morning but
as of right now, the heaviest showers are across Idaho. Most
guidance does shift this eastward toward dawn and into the morning,
with decent model agreement showing northwestern Wyoming seeing the
steadiest and heaviest showers. Ensemble guidance is giving at least
a 1 in 2 chance of a quarter of an inch of rain from through
midnight tonight across the Tetons, Wind River Range and northern
Salt and Wyoming Range, with the adjacent valleys having a 4 in 5
chance of at least a tenth of an inch of rain. The chance of an inch
or rain is small though, generally less than 1 out 3 and this is
only at the highest elevations. As for flooding potential, it looks
small at this point given that there is steering flow with these
showers, so they should not be stationary. Precipitable water is
also coming in about 15 to 20 percent less than yesterday, generally
running 125 to 150 percent of normal versus 150 to 175 percent
yesterday. The chance is not zero though. The best chance this
morning would be in areas prone to southerly upslope, like the
western Wind River Range. The chance is small though, generally less
than 1 out of 5. There look to be two periods of showers. The
initial one this morning with the wave lifting northward. The second
will be this afternoon, as a trailing shortwave moves into western
Wyoming. Most of this wave would remain West of the Divide, as
downsloping west to southwesterly flow develops behind the wave.
(Around a 1 in 4 chance west of the Divide, less than 1 out of 5
East of the Divide except near the mountains). A few of these
showers will linger into the evening and possibly after midnight
before ending before dawn on Wednesday as drier air punches in from
the south and west.

And now for something completely different, an 180 degree flip as it
would be. Drier air will move across most of the area, with
precipitable water values dropping back to close to normal values.
We can`t rule out a stray high based shower or thunderstorm but
these would be few and far between (less than a 1 in 6 chance). For
a few days, we will be shutting off the monsoon machine as it goes
in for maintenance. However, we are in the process of changing the
fluids in the famous Wyoming Wind Machine, which will be fired up
for the first time in a while starting Wednesday. It will only be
running at half power though, not red lining it Wyoming will be
close to the middle point between and upper level low over the
Pacific Northwest and a strong ridge over the southwest. The result
will be a tight height gradient. We will also have around an 80 knot
jet nearby. The result will be a breezy to windy afternoon across
much of the area. With 700 millibar winds only expected to be around
35 knots, high wind is not expected. Much of the area has at least a
3 out of 5 chance of wind gusts over 30 mph. There is at least a 1
in 2 chance of wind gusts past 40 mph in the favored southwest flow
locations from South Pass through southern Johnson County. And
humidity will be lower. However, it does not look quite low enough
for critical fire weather at this time. The most likely spot would
be Wind River Basin to the eastern Bighorn Basin where there is
around a 1 in 2 chance of humidity falling to under 15 percent. It
may not be for three hours though. So, for now we will hold off on
any Fire Weather Watches. We will let the day crew take another look
as they can issue watches and we could upgrade tomorrow night if
need be. And with the downsloping flow, it will get quite warm. The
warmer spots like the Bighorn Basin have at least a 1 in 2 chance of
high temperatures reaching 90.

Thursday at this point looks like a dry day across the area, With
the jet still nearby, it will still be breezy to windy. However, dew
points will be around 5 to 10 degrees lower. This will lead to a
more widespread area of relative humidity under 15 percent. Wind may
not be quite as strong, but there is still a greater than 1 out of 2
chance of gusts over 25 much across much of the area. As a result,
this looks like the best chance (greater than 3 out of 5) of
critical fire weather across portions of the area. We will likely
need some fire weather watches but it is a bit too far out at this
point. Temperatures should remain above normal, but a few degrees
cooler than Thursday.

Fire conditions should begin to improve somewhat on Friday as
humidity begins to rise. There could still be some elevated fire to
possibly critical fire weather though as the wind will remain rather
gusty and relative humidity could approach 15 percent again.
These concerns should end Saturday as the wind decreases and
humidity rises further in advance of another plume of moisture
from the southwest. The main players on this will be an upper
level low off of the Pacific coast and the ridge of high
pressure which will gradually retrograde from the southeastern
United States to the southern High Plains. The wild card will be
potential hurricane off of the Mexican Coast and it`s eventual
track. If the tropical system can move closer to the coast,
there is chance that some of it`s moisture could get caught in a
moist conveyor belt and moved into the Great Basin and
potentially Wyoming. There is a still a lot of uncertainty as to
the track of the possible hurricane though. There is decent
agreement in the delayed arrival of the moisture though.
Yesterday at this time, it was late Saturday into Saturday
night. It is now trending more toward Sunday now. And, with the
ridge shifting back to the west, temperatures will remain above
normal. It does not look like record heat, but temperatures will
be above normal through at least Saturday, with some location
having at least a 1 in 2 chance of a high temperature each day
from Thursday through Saturday before more potential cloud cover
on Sunday brings cooler conditions. So, we will have a wide
variety of weather over the next several days, from potential
heavy rain to critical fire weather to more summer like
temperatures. It is what keeps the weather in Wyoming
interesting.
&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1030 PM MDT Mon Aug 31 2026

Increasing chances of showers moving in from the south to start
the TAF period, with showers being widespread west of the Divide
by 09Z. These showers will continue to spread northward through
the morning, crossing the Divide between 09Z and 12Z. Little to
impact is expected, with conditions remaining VFR. There is a
30% chance for MVFR ceilings at KJAC between 14Z and 18Z.
Showers will primarily end over northern portions of the state,
with isolated showers lingering through the rest of the
afternoon. Winds will increase after 16Z as clouds clear from
south to north, with gusts of 20 to 25kt through the rest of the
day at KRKS, KRIW, KLND and KCPR. These winds will subside by
00Z.

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 134 AM MDT Tue Sep 1 2026

No fire weather concerns today as relative humidity should
remain above 20 percent. From Wednesday through Friday, a
combination of humidity falling into the teens, above normal
temperatures and wind gusting over 30 mph will bring the
likelihood of elevated to critical fire weather each afternoon,
with Thursday afternoon looking like the period of most concern
with the lowest humidity.

&&

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

&&

$$

DISCUSSION...Hattings
AVIATION...LaVoie/Lowe
FIRE WEATHER...Hattings