275
FXUS65 KRIW 211512
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
912 AM 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.

&&

.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 /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 429 AM MDT Fri Aug 21 2026

Mainly VFR conditions are expected through 12Z Saturday. After a
mainly dry start there will be another day of isolated showers
and thunderstorms. We kept the PROB30 groups at all TAF sites
that had them before (KRIW, KLND, KJAC, KBPI and KPNA). With
recent model trends we also added a group to KRKS starting at
20Z this afternoon. Most showers and storms will have little
rain. However, wind gusts of 40 knots will be possible in and
around any shower or thunderstorm. Most showers should end by
04Z. A few showers may linger in vicinity of KJAC, KBPI and KPNA
overnight, but with chances generally 20 percent or less, we
left them out of the TAFs for now. Most locations should see
winds generally less than 11 knots except near any showers or
thunderstorms. An exception will be in vicinity of KCPR where
gusts to 20 knots will be possible.

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

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 430 AM MDT Fri Aug 21 2026

Mainly VFR conditions are expected through 12Z Saturday. After a
mainly dry start there will be another day of isolated showers
and thunderstorms. We kept the PROB30 groups at all TAF sites
that had them before (KRIW, KLND, KJAC, KBPI and KPNA). With
recent model trends we also added a group to KRKS starting at
20Z this afternoon. Most showers and storms will have little
rain. However, wind gusts of 40 knots will be possible in and
around any shower or thunderstorm. Most showers should end by
04Z. A few showers may linger in vicinity of KJAC, KBPI and KPNA
overnight, but with chances generally 20 percent or less, we
left them out of the TAFs for now. Most locations should see
winds generally less than 11 knots except near any showers or
thunderstorms. An exception will be in vicinity of KCPR where
gusts to 20 knots will be possible.

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.

&&

$$

DISCUSSION...Hattings
AVIATION...Hattings