093
FXUS65 KRIW 220421
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1021 PM MDT Fri Aug 21 2026

.KEY MESSAGES...

- Scattered showers and isolated storms continue across the
  southwestern half of the area this evening. Showers will
  linger across southwestern Wyoming overnight, spreading east
  of the Divide Saturday morning.

- Monsoonal moisture increases on Saturday with more numerous
  showers and thunderstorms, some may produce heavy rain and
  local flooding. Marginally severe weather looking more
  possible in the afternoon across Natrona/Johnson Counties,
  with damaging winds and small hail the main threats at this
  time.

- Lingering moisture will bring a daily chance of showers and
  thunderstorms into early next week, with temperatures more
  seasonable for this time of year pushing towards the end of
  August.

&&

.UPDATE...
Issued at 1245 PM MDT Fri Aug 21 2026

Hottest of the afternoon still in progress, bumped up a few
spots a couple degrees east of the Divide as it has already
warmed past the previous noon forecast. Otherwise, much of the
forecast still intact and on schedule. Monsoonal moisture
visible on IR pumping up from the south, but more limited and a
bit west than it was yesterday currently. This should extend
into western parts of the state by mid-afternoon that will
spark thunderstorm activity. HiRes models haven`t been very
reliable the past couple of days missing the morning showers
across Johnson County and a bit late on development thus far.
PoPs will be adjusted as needed once things kick off, but main
concerns (if you want to call it that) is gusty outflows over 40
mph. A rogue storm or two could push down the Wind River Basin
between 4-6PM with some of the models picking up on this still,
although chances low around 15-20%. Even some isolated storms
developing currently near South Pass with slow eastward movement
due to weaker steering flow aloft. Not much should come from it
but a mere breeze.

Otherwise, weekend precipitation looks on track for now but
maybe a bit stronger severe weather wise across eastern counties.
HiRes models do indicate better instability and stronger
multicellular formations across Natrona to Johnson County during
the afternoon and early evening hours. Even though SPC pulled
the Marginal further east, this should be back in place east of
the Bighorns by Saturday morning if not having a slight closer
to the eastern parts of the CWA. Stay tuned for updates
overnight into early Saturday morning.

&&

.DISCUSSION...
Issued at 241 AM MDT Fri Aug 21 2026

We are now returning to my favorite (note sarcasm, this is by far
the thing I hate the most about the weather around here) thing to
forecast, the monsoon. And, the models are off to a bad start.
We have some showers across northern Wyoming that none of the
models had. So, they are 0 for 1 right now. And this isn`t
baseball where batting .300 makes you an all star.

Other than these light showers, things are quiet right now, and it
should stay this way through the morning. However, we will see
another round of isolated showers and thunderstorms today, probably
in similar positions to yesterday. Coverage should remain less than
1 out of 3 for all areas though, as some warm air aloft should
prevent widespread convection. This is mainly across the western
half of the state. Dewpoint depressions remain fairly high and
we still have inverted-V soundings, so this will likely be
another day of strong wind gusts from any shower or thunderstorm
with only light rain. There could be a stray shower further
east, but the chance is less than 15 percent so we left the
forecast dry. But, let`s just say my confidence is not high with
how poorly the models handle the monsoon with weak forcing.
Some showers will linger through the night across the west as
well, but the core of the moisture likely holds off until
Saturday.

The other story today will be the heat, as temperatures will peak
today as 700 millibar temperatures climb to around 20 Celsius.
All areas below 6500 feet have at least a 4 out of 5 chance of
high temperatures over 90 this afternoon. Areas below 5500 feet
have at least a 1 in 2 chance of highs over 95. As for triple
digits, it is smaller. However, temperatures have been running
higher than guidance so I give it around a 1 in 3 chance in
places like Greybull and Worland. The warmest temperatures would
be east of the Divide with the lower elevations and more
sunshine.

The monsoon returns with a vengeance on Saturday. Precipitable water
values climb to around 150 to 200 percent of normal across most of
the area. We will also have a shortwave moving across the area to
bring some additional forcing. The result will be a chance for all
areas to see a shower or thunderstorm, but most guidance favors
western Wyoming. There is a Marginal risk of Excessive Rainfall
across areas west of the Divide as I write this around 1:15 am.
This could be expanded east, we will know around 2:30 am. There
is some steering flow, especially at 500 millibars, so this
should mitigate the flooding threat somewhat but it will be
there. Most convection should end by midnight, but some showers
will continue after midnight. Confidence on placement of showers
Saturday night is very low, as this is where the models have
struggled the most with the monsoon. The best chance will be in
western Wyoming, but exactly where is still somewhat uncertain.
As for temperatures, they will remain very warm east of the
Divide where there will be some morning sunshine, but about 5
degrees cooler than Friday. Areas west of the Divide should see
a greater drop, with more cloud cover and a greater coverage of
showers. And, some storms could become strong. We do have a
Marginal risk of severe weather east of the Bighorn Range. It
may be a bit overdone, since there is only weak shear. The main
threat would be strong wind gusts, like most showers or storms
in Wyoming.

Monsoonal moisture then looks to stick around into the first part of
next week. This will lead to daily chances of mainly diurnal showers
and thunderstorms each day. Guidance has poor consistency with the
details though. So, it is still difficult to nail down details on
where the most numerous convection will be each day and timing of
the storms. So, we kept things broad brushed for the most part.
Temperatures will decrease somewhat with more cloud cover. However,
with the mean ridge remaining in place, above normal temperatures
should continue into next week. There are some indications that
drier air may try to move in toward the end of next week,
bringing a return to drier conditions for a few days through the
weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1015 PM MDT Fri Aug 21 2026

Light rain showers will continue over western portions through
12Z. PROB30 groups remain in the forecast at KJAC, KPNA and
KBPI. Mountaintop obscurations will be possible through this
time. These showers will cross the Divide into the central
basins after sunrise Saturday morning, moving toward the
northeast. The greatest chance for thunderstorms will occur as
this wave crosses the Bighorns and moves over Johnson County by
21Z. Additional showers will be possible across the rest of the
area in the afternoon, but confidence has decreased in the
threat level. Confidence has shifted to thunderstorms moving
over far western portions by 23Z/00Z, quickly spreading
eastward through the rest of the period. Have adjusted the
timing of PROB30 groups for all TAF sites, accordingly. Wind
gusts of 30 to 40kt and marginal VFR conditions continue to be
the main threats with the storms. The storms in Johnson and
Natrona counties could be strong to severe.

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

&&

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

&&

$$

UPDATE...Lowe
DISCUSSION...Hattings
AVIATION...LaVoie