235
FXUS65 KRIW 220353
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
953 PM MDT Thu Aug 21 2025

.KEY MESSAGES...

- Isolated showers will continue to wane and end in the next few
  hours tonight. A cold front will continue to progress across
  central and southern portions tonight, with easterly winds
  occurring in its wake.

- Rain chances will be very isolated until Sunday, with dry lightning
  a distinct possibility each afternoon.

- A monsoonal type pattern will develop late this weekend into
  early next week, which will provide chances for rain showers
  and thunderstorms each day beginning Sunday, mainly across
  southwest Wyoming.

- The increased moisture will also lead to increased cloud cover
  and overall much cooler temperatures next week.

&&

.UPDATE...
Issued at 113 PM MDT Thu Aug 21 2025

Cloud cover will continue to increase across the southern two thirds
of the area this afternoon. Isolated to widely scattered rain
showers and thunderstorms will continue to develop and push through
the area this afternoon into this evening. Most of the showers and
storms will be dry, but if a couple of storms develop and move over
the same area there could be a good wetting rain in that area. There
is a very low chance (5%) of a severe wind gust or large hail in
northeastern Johnson County later this afternoon into this evening
as storms move through. Gusty and erratic outflow wind may cause
problems for active wildfires this afternoon and evening, even
though RH is slightly higher than yesterday. Dry lightning may cause
new fire starts today as well. With monsoonal moisture expected next
week there will remain the threat of dry lightning and gusty
outflows around the periphery of the moisture in central and
northeastern portions of the area. How high of a threat will depend
on the northeastern extent of the low-level moisture values
advecting into Wyoming.

&&

.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 degrees east of
the Divide and 3 to 9 degrees 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 SATURDAY/...
Issued at 950 PM MDT Thu Aug 21 2025

Most leftover showers and isolated thunderstorms should end by
midnight. Another surge of cooler air will move south into the
area through Friday morning, bringing an increased northerly
wind to KCOD. Showers and thunderstorms will be less widespread
on Friday, but still possible in some locations. We have added
PROB30 groups to most of central Wyoming, mainly in vicinity of
KWRL, KCPR, KRIW and KLND. This could turn out to be a later
show as well, with most activity possibly holding off until
after 00Z Saturday.

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 Friday for WYZ005.

&&

$$

UPDATE...LaVoie/Rowe
DISCUSSION...Wittmann
AVIATION...Hattings