675
FXUS65 KRIW 301824
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1224 PM MDT Sun Aug 30 2026

.KEY MESSAGES...

- Showers and thunderstorms diminish through the afternoon as
  drier air makes its way into the state. Best chances (40-60%)
  remain along and south of Interstate-80 through this evening.

- A cool night over western Wyoming, with lows in the low-to-mid 30s.
  Patchy frost is possible.

- Most of Monday is dry, with increasing shower and thunderstorm
  chances late in the day from the southwest.

&&

.UPDATE...
Issued at 1224 PM MDT Sun Aug 30 2026

Isolated showers and thunderstorms are moving through the area early
this afternoon. Current water vapor imagery depicts the drier air
starting to make its way into western Wyoming, which will greatly
limit additional shower and thunderstorm chances today. The best
remaining chances (40-60%) occur over southern Wyoming, around I-80
and south, where the best moisture remains today. Some moderate to
heavy rain remains possible in this area. After the last couple
showers diminish this evening, dry conditions are expected overnight
and into Monday morning. With a clearing sky overnight, cooler
temperatures occur over western locations, with lows in the low-to-
mid 30s. Some patchy frost is likely in cold prone locations, low-
lying areas, and near rivers.

&&

.DISCUSSION...
Issued at 236 AM MDT Sun Aug 30 2026

Another night is upon us and, once again, it is more monsoonal
activity with some showers and isolated storms scattered through the
area, although as I write this around 1 am most are west and south
of Wyoming. These will continue through the night as well as this
morning. And the main story for the next seven days will be, once
again, the ebbs and flows of the monsoon, although we also have
something else to talk about.

For the next 24 hours we have two concerns. The first concerns the
convection. Overall, things looks somewhat less active today. Most
guidance does show drier air spreading across much of the area from
west to east starting this morning, but especially so this
afternoon and evening as a cold front and trough move through the
area. It could end up being below normal for the
first time in a while across most of the area. There will still be
one area of concern though, and that is largely south of a Casper to
Evanston line. Moisture will linger here and so will the chance of
heavy rain. Precipitable water remains around 150 percent of normal
here. especially in southern Sweetwater County which has a slight
risk of heavy rainfall, with a marginal chance further north. The
main mitigating factor again is that there are decent low to mid
level winds so the storms will be moving, so any threat of local
flooding would be from training convection. Further north, the main
threat from storms would shift to strong wind gusts, as we have
larger dew point depressions, higher cloud bases, steep lapse rates
as well as inverted V soundings. This should also be an earlier
show, as most guidance shows the dry air making all the way to the
southern border, and this should bring a dry period from late
tonight to Monday morning. And, with the threat of heavier rain
ending further north, we will likely cancel the flood watch sometime
this morning.

The other concern is the coolest temperatures so far this season.
There will be cooler air moving in behind the front, with lows
falling into the 30s for many. The question is the concern for a
frost or freeze. Temperatures have trended a bit warmer this
morning. The only lower elevation locations with a greater than 2
out of 5 chance of lows below 32 degrees is the northern Jackson
valley and around Bondurant. As for 28 degrees of below, the chance
is less than 1 out of 4 everywhere. So, we will not issue a freeze
watch at this time. If we still issued frost advisories, I would
have considered one though. Just about all locations will have the
coolest night they have seen in a while.

Monday should start dry. However, the monsoonal moisture will begin
pushing northward again, and once again increase the chance of
showers and thunderstorms, especially in southern and western
Wyoming. It may take a while to arrive though, with the deepest
arriving after sunset. This may limit heavy rain potential somewhat
with the lack of instability, despite precipitable water values once
again rising to over 150 percent at times, especially in southern
Wyoming. And with southwest flow returning, warmer temperatures will
returning to somewhat above normal levels.

The rest of the forecast period is again dependent on the movement
of the monsoonal moisture. At this point, Tuesday looks to be more
active with above normal precipitable waters and a shortwave moving
through the area. Wednesday or Thursday could then be less active
as drier air moves in from the west with another uptick in
convection for Friday and into the weekend. However, the models
have been terrible in predicting these ups and downs in the
monsoonal moisture so may confidence in timing the surges of
moisture is low at best. With the southwest flow continuing,
temperatures will remain above normal. With the center of the
ridge possibly retrograding back to the west toward next
weekend, it could get quite warm again with highs in the 90s
possible for the upcoming holiday weekend. So, it continues to
more of the same with the monsoon continuing to dominate the
weather conversation.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1125 AM MDT Sun Aug 30 2026

Primarily VFR flight conditions are expected through the period.
Isolated to widely scattered showers and thunderstorms are again
expected today, though less widespread than on Saturday. Brief MVFR
conditions at terminals remains possible with any heavy rain. The
more widespread impact potential continues to be strong outflow wind
gusts of 30 to 40 knots through the day. Rain will come to an end
everywhere by sunset this evening, with lighter wind and decreasing
clouds through Monday morning.

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...Wittmann
DISCUSSION...Hattings
AVIATION...Myers