320
FXUS65 KRIW 210806
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
206 AM MDT Thu Aug 21 2025

.KEY MESSAGES...

- Cooler today as a cold front moves through. A cooling trend
  occurs through early next week.

- Mostly dry showers and thunderstorms possible (15% to 50%
  chances) today, mainly for southern and central Wyoming.
  Lightning and gusty outflow winds 30 to 50 mph are the main
  hazards.

- Daily rain chances occur through early next week as monsoonal
  moisture makes it to the area.

&&

.DISCUSSION...
Issued at 205 AM MDT Thu Aug 21 2025

A shift in the weather pattern starts today as a cold front moves
through. This drops high temperatures 8 to 15 east of the Divide and
3 to 9 west of the Divide compared to the record-breaking high
temperatures yesterday. Even with these large drops, temperatures
are seasonal to just above seasonal today. A combination of
northwest flow and the continuation of a trough to the northeast
will trend the temperatures down into early next week. Early
indications have possible highs in the upper 70s Monday or Tuesday,
with warmer basins in the lower 80s.

The other influential feature is high pressure centered around the
Four Corners area. This will aid in monsoonal moisture flowing
around its western extent and into southwestern Wyoming through at
least early next week. For today, the NAM indicates precipitable
water (PWAT) values up to 200 percent of normal (about 0.6 inches to
just over one inch). As the trough moves through, there will be
chances of showers and thunderstorms across much of the area today.
Hi-res models are in fairly good agreement of the overall coverage
of showers/storms, with the best chances occurring from an Afton to
Greybull line and east (though an isolated shower is still possible
over northwestern Wyoming). The main hazards with any storms will be
similar to previous days, lightning and gusty outflow winds 30 to 40
mph, though there is potential for gusts to around 50 mph. With the
best instability, Johnson and Natrona Counties will have the best
chances of storms today. Showers will likely linger into this
evening, with the last showers over Johnson/Natrona Counties ending
shorty after midnight.

Looking ahead, the pattern remains comparable through the weekend,
with daily shower and thunderstorm chances and cooler temperatures.
Depending on where subtle upper-level features occur, the daily rain
chances will shift locations. For example, the aforementioned high
may shift westwards a bit Saturday, cutting off moisture supply and
lowering rain chances that day. EFI/SOT has trended down the QPF
percentiles across southern Wyoming on Monday compared to yesterday,
which may indicated less monsoonal moisture than previously
forecasted for early next week; there is still plenty of time for
this to reverse, however. Will have to watch how this trends.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1025 PM MDT Wed Aug 20 2025

Very light/virga showers will continue along a roughly KAFO-to-
KSHR line to start the forecast. This line will gradually shift
southward to a KEMM-to-KBYG line by 12Z. These showers will
continue through the rest of the morning, becoming more
widespread and develop into thunderstorms across areas east of
the Divide through the afternoon as a trough moves over the
area. This may eventually necessitate PROB30 groups for
KRIW/KLND/KWRL/KCPR, but low confidence precludes mentioning
this for now. These showers and storms will exit the forecast
area by the end of the TAF period. A cold front will move
through northern portions after 09Z, becoming stationary across
portions of Natrona and Fremont counties in the afternoon. A
northerly or easterly wind direction will be more predominant
for terminals east of the Divide after 00Z, as the front pushes
further south. Winds will generally stay light through the
period, with gusts 20 kt or less in the afternoon (outside of an
potential outflow boundaries).

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...
Air Quality Alert until 1 PM MDT this afternoon for WYZ005>007-
017.

&&

$$

DISCUSSION...Wittmann
AVIATION...LaVoie