810
FXUS65 KRIW 211101
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
501 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 /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 456 AM MDT Thu Aug 21 2025

An area of showers and thunderstorms will be developing
somewhere across central Wyoming today, but coverage will be
isolated as they move through. Felt confident enough to add
PROB30 to the KCPR TAF, but the threat will also be present for
the remaining sites except KJAC. Main threat will be gusty winds
and lightning threat. A front is still expected around sunset
this evening, bringing north to northeast winds at northern and
central TAF sites. Since this occurs in the evening, winds will
remain light.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Air Quality Alert until 1 PM MDT this afternoon for WYZ005>007-
017.

&&

$$

DISCUSSION...Wittmann
AVIATION...Straub