923
FXUS65 KRIW 291152
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
552 AM MDT Sat Aug 29 2026

.KEY MESSAGES...

- An active monsoonal pattern continues today with more showers
  and thunderstorms, especially west of the Divide. Stronger
  storms with heavy rain, strong wind and hail are possible.

- Turning somewhat drier and cooler Sunday and Monday. Patchy
  frost is possible in the western valleys Monday morning as
  temperatures drop into the 30s.

- Monsoonal moisture returns for much of next week, with a
  continuing chance of mainly afternoon showers and
  thunderstorms.

&&

.DISCUSSION...
Issued at 257 AM MDT Sat Aug 29 2026

I have returned after a break and this pattern reminds me of one of
my favorite types of candy from my youth and the movie it was from.
The movie was "Willie Wonka and the Chocolate Factory", the original
with Gene Wilder. And the candy was the "Everlasting Gobstopper."
And this reminds me of the pattern we have been in since the middle
of July, expect I would title this "The Everlasting Monsoon."

And this is no exception this morning, as we continue to have a
plume of monsoonal moisture streaming northward over the area,
mainly the northwest half of the region. Dew points are also rather
high, most locations in the 50s. I know people from the East Coast
or Gulf Coasts are laughing at this statement, but that is rather
high in this part of the country. Precipitable water values also
remain rather high, topping out at over an inch in some locations,
which is close to 200 percent of normal. Most guidance continues to
put the main impact West of the Continental Divide. The HREF
ensemble gives a greater than 1 in 2 chance of a half an inch or
rain or more across much of western Wyoming through 7 am Sunday.
And, with the rain being enhanced by a shortwave and approaching
cold front, the forcing gives me greater confidence in the solution.
Exact positioning of the storms is still not certain though. There
are a couple of factors against excessive rainfall though. One is
cloud cover, it may be fairly substantial today, this would limit
instability and possible thunderstorm development. The other is that
we do have some decent flow at 500 millibars, as much as 40 knots on
some models. This should keep the storms moving. There could still
be some training though. We continue to have a marginal risk of
excessive rainfall across this area as well. So, we will continue
the flood watch as is for now. Like tonight, showers and possible
thunderstorms will continue through the nighttime hours as well.

We also have to talk about the possibility of severe thunderstorms.
The entire state is under a marginal risk with western Sweetwater
and southern Lincoln County under a slight risk of severe
thunderstorms. Hodographs are generally of the unidirectional
variety which would lead to wind and hail being the main threat. We
did have some wet microbursts on Friday and this looks to once again
be the main threat today. The Storm Prediction Center has roughly
the southern half of Wyoming in a Conditional 1 area for convective
wind (wind gusts to 75 mph), and this looks reasonable given
the largely straight hodographs and decent mid level forcing and
wind. Again, the main concern against the severe weather may be
cloud cover which could limit instability. So, severe storms
are not definite but the possibility is definitely there.

The cold front will move across the area on Sunday. Much drier air
will move in behind it through the area, bringing below normal
precipitable waters from west to east across much of the area by
sunset on Sunday evening. This should also help shut off or limit
the amount of convection, The main area of concern on this day would
be southern and eastern portions of the area. We still have a
marginal risk of excessive rainfall on this day, roughly south of an
Evanston to Casper line. As for stronger thunderstorms, it would be
more limited and largely across the usual suspects of Johnson and
Natrona Counties where more instability would build before the cold
front arrives. And then, to quote Monty Python, "Now for Something
Completely Different".

Much cooler air will move in behind the front, and with a clearing
sky, light wind and the longer nights of late August, it will get
quite cool. And this brings the possibility of potential below
freezing temperatures and possible frost. The best chance would be
across the western valleys. The NBM ensemble gives around a 1 in 2
chance of portions of the Jackson Valley falling to 32 degrees
Monday morning. Most other locations are less than 1 out of 3
though. The chance of temperatures below 28 remains very low though,
less than 1 out of 10. At this point, if there is any frost it would
be patchy. There is still time to evaluate though so we will let the
day shift take another look.

On Monday, monsoonal moisture will make another push northward
although it looks to be a later arrival at this point except in far
southern Wyoming. With the night time arrival could limit excessive
rainfall though with little instability and most precipitation just
being showers despite precipitable waters again rising to over 150
percent of climatological normals. Tuesday looks like more
widespread activity. Wednesday may be rather active as well although
another approaching shortwave and weak cold front and drier air
behind it may dry things out somewhat. Timing is still uncertain
this far out though. Thursday may also be drier but given how
poor the models have been with the monsoonal moisture in the
medium to long range, confidence is low with the details. As for
temperatures, the general trend will be for above normal but
not hot temperatures with a possible cooler day on Wednesday or
Thursday depending on the timing of the cold front. So, it looks
like the year of the monsoon will roll on through much of next
week, although with a few breaks possible.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 551 AM MDT Sat Aug 29 2026

Numerous showers and thunderstorms are expected through the period
as a moist southwesterly flow continues. All terminals are expected
to see impacts, most persistently at KBPI, KJAC, KPNA, and KRKS.
Brief heavy rain and low clouds will lead to occasional MVFR
conditions. The more widespread impact will likely be strong outflow
wind gusting 30 to 40 knots. There is also a more isolated threat of
gusts to around 50 knots at KCPR, KLND, KRIW, and KRKS between 18Z
and 00Z as stronger thunderstorms move through the area. After a
relative lull around 00Z Sunday, the next wave of showers will
follow quickly, first across KRKS with chances increasing to the
north through overnight into Sunday morning. Low clouds will lead to
frequent mountain obscuration through the period, especially across
western Wyoming.

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...
Flood Watch through Sunday morning for WYZ001-002-012>014-023-
024.

&&

$$

DISCUSSION...Hattings
AVIATION...Myers