946
FXUS65 KRIW 210845
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
245 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.

- Monsoonal moisture increases on Saturday with more numerous
  showers and thunderstorms, some may produce heavy rain and
  local flooding.

- Lingering moisture will bring a daily chance of showers and
  thunderstorms into early next week, with temperatures
  remaining near to above normal.

&&

.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 /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 946 PM MDT Thu Aug 20 2026

Any lingering showers end by the start of the TAF period. Monsoonal
moisture will begin to move into southwestern portions of the area
on Friday. Most of this moisture will be above the surface, so it
will be enough for scattered rain showers and thunderstorms to
develop, but not enough for much rain to reach the ground. Showers
and storms will first develop near KBPI and KPNA around 20Z, with
KJAC, KRIW, and KLND following around 21Z. There may be showers and
storms near KRKS as well, but the chance was too low (15-20%) to
include a PROB30 group at this time. There is about a 15% chance of
showers at KBPI, KPNA, and KJAC late Friday evening once PROB30
groups end at 03-04Z.

Wind will remain below 10 knots tonight into Friday morning. Then,
wind will increase late Friday morning and Friday afternoon to
around 10 knots for most terminals. KCPR will be a bit gusty with
gusts around 20 knots through the afternoon hours. The main threat
with showers and storms will be strong downdrafts and outflows.
Convective wind gusts may reach as high as 50 mph Friday afternoon.

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...Rowe