170
FXUS65 KRIW 211845
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1245 PM MDT Fri Aug 21 2026

.KEY MESSAGES...

- Today will be the hottest day of the week with isolated
  thunderstorms mainly in southern and western Wyoming, with a
  15-20% chance of a couple isolated storms pushing down the
  Wind River Basin after 4-5PM.

- 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 in tact 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 across eastern counties severe weather
wise. 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) 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. Dew point 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, bring a
return to drier conditions for a few days.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1105 AM MDT Fri Aug 21 2026

After 20Z, scattered virga, showers, and thunderstorms are forecast
to develop across the higher terrain of western Wyoming and move
west to east. Virga, showers, and thunderstorms are forecast in the
area of or at KJAC, KRIW, KRKS, KLND, KPNA, and KBPI through 03Z, at
the latest. PROB30 groups have been maintained at the aforementioned
terminals given the scattered nature of the convection and
uncertainty in direct impacts to the terminals. However, gusty
outflows up to 45 kts are possible in and around any virga, showers,
or thunderstorms. KCOD, KCPR, and KWRL have less than a 15 percent
chance during this timeframe. Between 08Z and 17Z, model guidance is
showing a cluster of showers moving across western and central
Wyoming (KJAC, KRIW, KRKS, KLND, KPNA, and KBPI). However,
confidence in exact timing and impacts is rather low so went with
PROB30 groups at this time. Otherwise, VFR flight conditions are
anticipated through 18Z Saturday with winds remaining 12kts or less.
KCPR could se occasional gusts around 20kts Friday afternoon.

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...Gerhardt