079
FXUS65 KRIW 221021
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
421 AM MDT Sat Aug 22 2026

.KEY MESSAGES...

- Increased moisture brings an increased chance of showers and
  storms across the area today. Some storms could have heavy
  rain, strong wind or hail.

- The chance of showers and storms lingers through much of the
  week, although today looks the most active.

- Temperatures will gradually decrease to near normal levels for
  much of the week.

&&

.DISCUSSION...
Issued at 141 AM MDT Sat Aug 22 2026

The monsoon has once again arrived across the area, I believe this
is the third or fourth appearance so far. So far, it has behaved. We
do have some light showers across mainly the western half of the
area but nothing heavy so far. The main concern in the forecast will
be the potential for heavy rain and possible strong thunderstorms
today. Right now, in this meteorologists opinion, potential is mixed
for if we will see widespread of either.

In regards to heavy rain, the main factor will be the amount of
moisture in the atmosphere. Dew points have been slow to rise so
far, but they should this afternoon, rising into the 50s in portions
of the Big Horn Basin as well as Johnson and Natrona Counties.
Precipitable water values should also be rather high. The GFS, for
example, has precipitable water values about 175 to 200 percent of
normal. This had been overdone by around 15 to 20 percent the past
few events though. Even with this, precipitable water values over
150 percent of normal look fairly likely. There will also be some
forcing in the form of a shortwave moving in from the west. This
should enhance chances for all areas, so everyone will have a
chance. The best chance still looks to be in northwestern Wyoming,
generally a 1 in 2 chance, and there is decent model agreement in
this. As for severe thunderstorms, there looks to be a decent amount
of instability. This is especially so in the favored portions of our
area, mainly East of the Bighorn Range in Johnson and Natrona
Counties. Lifted indices fall as low as minus 4 to minus 5 with
close to 1500 J/Kg of surface CAPE here. There is also a decent
amount of directional shear in this area. This, plus the ample
moisture, may increase the hail threat in this area.  We do have a
marginal risk of severe weather in portions of Johnson and Natrona
Counties, and this could be expanded westward with the 13Z update.
All areas could have a thunderstorm but with less instability and
shear most storms would be sub severe.

However, there are some factors against possible hazards. In regards
to flooding and thunderstorms, it is cloud cover. Much of the area,
especially western Wyoming, is covered by clouds as I write this
around midnight. If it stays mostly cloudy for much of the day, this
could limit instability somewhat and decrease the chance of stronger
thunderstorms. There will likely be some areas that will be sunnier
though, especially further east. And there is one more mitigating
factor for heavy rain and possible flooding. With the shortwave
moving through, there will be some steering flow today. It won`t be
strong, but most areas will have 15 to 25 knots at 500 millibars
which should keep storms moving somewhat. We continue to have a
marginal risk of excessive rainfall, and persistent storms could
still cause flooding. But it may decrease the chance somewhat. The
shortwave should clear the area by midnight, but some showers will
likely linger through the night. As for temperatures, more cloud
cover should lower them around 5 to 8 degrees, but it will still be
above normal for almost all areas.

The forecast from Sunday through much of next week looks fairly
similar. We will remain on the edge of the ridge. Some monsoonal
moisture will stick around, with precipitable water values remaining
above average through much of the period. However, they do look
lower than today, so the chance of convection should be somewhat
lower.  Most guidance favors western Wyoming at this time. However,
given the poor performance of the models in regards to timing and
placement of the showers and storms, confidence in the details
remains low. Temperatures will also cool off somewhat, back down to
near to just slightly above normal levels for late August. There
continue to be some indications of drier air moving in for the end
of next week or next weekend, possibly shutting off the monsoon and
bring a period of warm, but not really hot weather for the end of
August.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 414 AM MDT Sat Aug 22 2026

Rain showers will spread into western Wyoming during the morning
hours, with PROB30 groups at all terminals West of the
Continental Divide (KJAC, KPNA, KBPI AND KRKS). These showers
then spread East of the Divide after 13 to 14Z and move
eastward. Additional showers are possible through the day with
thunderstorms possible mainly after 19Z although exact timing of
the periods of showers and thunderstorms is uncertain. Most
storms should end after 04Z tonight although some showers will
linger through at least 08Z tonight and some even until sunrise
on Sunday. The main threat from most storms would be heavy rain
that could bring brief MVFR conditions with the heavier periods
of rain and wind gusts over 35 knots. Stronger storms are
possible in Johnson and Natrona Counties, including KCPR where
storms may become strong to severe. Locally stronger storms are
also possible in northwestern Wyoming, mainly in vicinity of
KJAC.
&&

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

&&

$$

DISCUSSION...Hattings
AVIATION...Hattings