537
FXUS65 KRIW 010002
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
602 PM MDT Mon Aug 31 2026

.KEY MESSAGES...

- Clouds will continue to increase this evening with scattered
  showers and thunderstorms starting to develop across southern
  WY.

- Showers spread over portions of central and northern WY
  overnight into the morning hours on Tuesday. Locally moderate
  to heavy rain is possible with any thunderstorm.

- Mainly dry, warm, and breezy to windy Wednesday through
  Friday. Low humidity and breezy winds will lead to elevated to
  near critical fire weather each afternoon, especially east of
  the Divide.

- Moisture returns again for the weekend, with an increased
  chance of showers and thunderstorms.

&&

.UPDATE...
Issued at 1128 AM MDT Mon Aug 31 2026

Few clouds are seen across the area as of now. The better moisture
that is projected to move in is currently to our south. As the
shortwave moves through overnight, so do shower and thunderstorm
chances. These will move through overnight and into Tuesday morning,
with all locations having a shot of rain as the main moisture
moves through. The latest hi-res guidance suggests the main
"line" of showers and storms push north starting 5pm in southern
Wyoming and exiting northern Wyoming between 10am and noon
(some uncertainty with timing).

&&

.DISCUSSION...
Issued at 1244 AM MDT Mon Aug 31 2026

I have arrived at work for another shift and for the first time
in a while the radar is clear, no showers in the county warning
area as drier air has spread across almost all of the area.
However, the monsoon machine has been unrelenting this year,
going almost nonstop for the last six to seven weeks with only
a few stops for maintenance. We are in one of the those now, but
it won`t last. The lock out / tag out on it will be removed
later today and will again be fired up across the area tonight.

But today looks like a fairly nice start to the week for most,
now the last day of August and of meteorological summer. After a
rather chilly start, with some frost possible in the deeper
valleys in the west with temperatures dropping close to
freezing, it should be comfortably warm this afternoon.
Temperatures look to be fairly close to normal with light to
moderate wind for most locations. The monsoonal moisture will
begin to creep northward late today, with a few showers and
thunderstorms south of roughly an Afton to Wamsutter line before
sunset. Moisture then spreads northward tonight, across much of
the area by sunrise. As for the potential of heavy rain and
possible flooding, we once again have some factors in favor of
it and some against it. Well above normal precipitable water
values will again move across the area, as much as 175 percent
of normal. The moisture will also be riding a shortwave lifting
from the southwest, enhancing lift and mid level forcing. There
are some factors against it as well. Most of the precipitation
will fall at night, with limited surface instability and less of
a chance for thunderstorms. There will also be some mid level
flow, so any showers or storms will not be stationary. At this
point, the marginal risk of excessive rainfall looks reasonable
for now, mainly in the west and south where the greatest lift
will be. Chances of showers will continue into Tuesday as well,
although chances of showers and heavy rain should decrease
through the day as the deeper moisture moves away to the east.
Most showers should end by midnight Tuesday night. As for
amounts, most of the ensembles are giving at least a 2 in 3
chance of a quarter of an inch or rain across most of the
western mountains and valleys. The best chance of over a half an
inch or rain will be in the Wind River Range and Tetons with a
greater than 1 out of 2 chance. Some locations will probably end
up with an inch of rain but with the nature of convection this
is impossible to pinpoint.

Much drier air will the move across the area for the middle of the
week as the moisture gets pushed to the east. However, we have
another concern for these days. On these days, we will be between an
upper level low over the Pacific Northwest and ridging over the
southeast. The jet stream will also be nearby, as much as 90 knots,
a decent speed for this time of year. The result will be a gusty
wind developing across much of the area. Most areas will have at
least a 3 out of 5 chance of wind gusts over 30 mph on Wednesday,
Thursday and even into Friday. Some of the more wind prone areas
could even see gusts to 50 mph, especially on Wednesday when the
wind is expected to peak. The limiting factor will be humidity. The
main area of concern looks to be East of the Divide where most
locations have at least a 1 in 2 chance of relative humidity below
15 percent. We will likely (around a 3 out of 4 chance) need
some fire weather highlights for some if not much of the period.
It is still a bit too early right now though. And, we can`t
completely rule out a stray shower over the mountains but the
chance is less than 1 out of 8 each day. Temperatures should be
well above normal on these days, with the warmer spots, like the
eastern Bighorn Basin, having at least a 1 in 2 chance of 90
degree highs each day.

The monsoon looks to make a return for next weekend as the ridge
over the southeast retrogrades and moves back toward the southern
plains. At the same time, an upper level low will be moving into the
central Pacific coast. This may create a moist conveyor belt of
moisture into the area for Saturday or Sunday with some possible
adding of moisture from a possible tropical storm or hurricane
off of the western Mexico coast. The GFS is very moist with
precipitable water, showing around 200 percent of normal
Saturday night into Sunday. This model does tend to run a bit
hot on this, so reality is probably 20 to 25 percent less.
However, the chance of showers and storms really increases on
then. With the ridge remaining fairly stationary for a few days,
this pattern looks to continue into early next week. And with
this, temperatures will remain above normal, generally around 7
to 12 degrees above normal. So, the monsoon continues to stick
around like an house guest that overstayed its welcome. However,
it will be leaving for a few days at least.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 345 PM MDT Mon Aug 31 2026

VFR conditions expected throughout the entirety of the period.
Winds 10kts or less to start the period with the next round of
rain showers pushing in from the southwest first at RKS after
02Z, then spreading to BPI/PNA/JAC after 05Z, and LND/RIW
towards 08Z. Winds increase to slight breezes up to 18kts with
lower but VFR ceilings with light rain before lifting out after
07-14Z respectively. This pushes to CPR after 12Z and COD/WRL
after 14Z through 16-17Z. There after, expect improving skies
and light winds 10kts or less with a few scattered passing mid
to upper level clouds. Expect continued improved skies and
flying conditions into Tuesday night through Wednesday as well.

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

&&

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

&&

$$

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...Lowe