871
FXUS65 KRIW 161758
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1158 AM MDT Wed Sep 16 2026

.KEY MESSAGES...

- A warmer day for most today. Isolated to scattered (15-40%)
  thunderstorms this afternoon and evening, mainly in southern
  and western Wyoming. The strongest storms could have very
  small hail and gusty outflow winds.

- Greater chance of showers and storms Thursday and Friday, with
  Friday having the most widespread chances. Brief moderate to
  heavy rain is possible.

- Temperatures remain near to above normal through Friday, with
  cooler temperatures for the weekend. Drier weather returns by
  Sunday.

&&

.UPDATE...
Issued at 1154 AM MDT Wed Sep 16 2026

The light showers from this morning, and the associated cloud cover,
has mostly exited to the east. With a mostly sunny sky, isolated to
scattered (15-40%) showers and thunderstorms are expected to form
during this afternoon, mainly for western and southern Wyoming. Most
of this activity dissipates this evening, with some lingering
overnight across the far south as moisture continues to advect
northward.

&&

.DISCUSSION...
Issued at 216 AM MDT Wed Sep 16 2026

Have you ever had someone that overstayed their welcome, be it at a
party or staying at your house? Well, the weather is doing that
again. Our friend the monsoon (which those of you who read my
discussions know I love so much, note sarcasm) has returned. And
will be around for a few more days.

We do have some shower and isolated thunderstorms around as I write
this, but nothing strong and the storms are moving. Monsoonal
moisture will continue to push northward today and increase the
coverage of showers and storms across the area. Coverage still looks
scattered at best though (at most a 1 out of 3 chance). There is
decent model consensus in keeping a vast majority if the showers and
storms along and west of the Continental Divide and also near Casper.
We can`t rule a shower or storm out anywhere, but the chance is only
around 1 in 10 or less. As for potential flooding, probably not
through Wednesday night. The core of the deepest moisture should
remain south of Wyoming today. There is also some steering flow,
about 15 to 20 knots worth, which should keep the showers and storms
of the moving and not of the stationary variety. As for temperatures,
this looks to be the warmest day of the week with flow turning
to south and still some sunshine. Some of the warmer spots will
likely see highs around or over 80 degrees.

Coverage should increase on Thursday. An upper level low will slowly
approach from the Pacific Northwest. At the same time, a strong
ridge over the southeast will begin to retrograde to the west and
direct deeper moisture into the area, as much as 175 percent of
normal. At the same time, a weak shortwave will be pivoting around
the upper level low giving a bit more lift. However, there is a
limiting factor. It is that with the moisture in the atmosphere,
much of the day may end up being fairly cloudy. This may limit
instability somewhat, especially since it is now mid September and
not July, limiting daytime instability. Nevertheless, there will be
an increased chance of showers, especially across western Wyoming.
With the increased cloud cover, temperatures will also be cooler,
generally by about 5 to 10 degrees compared to Wednesday.

The upper level low will begin to make a closer approach by Friday.
Of all the days, this looks like the most active day across the area
with almost all areas having at least a 1 in 3 chance of convection
during Friday and Friday evening. I have one more concern about this
day as well. Most guidance is showing a 20 percent drop in
precipitable water. However, this could lead to a fairly sunny
morning that could bring more instability and a greater chance of
convection, especially East of the Divide. If there could be local
flooding, this could the most likely day. The one mitigating factor
continued to be steering flow though, as much as 40 knots at times.
So the storms will be moving. And with more sun, temperatures will
be warmer, but likely not as warm as today.

The low will pass close to the area on Saturday, turning flow
more to the northwest. This should result in a noticable cooler
day across the area. There will still be enough wrap around
moisture for some showers and isolated thunderstorms to
continue. However, most guidance shows precipitable water values
much lower on this day, so the threat of heavy rain looks less.
Uncertainty then increases from Sunday into early next week. as
a lot depends on the movement of the upper low. Some guidance
moves it far enough east for drier weather to move in for
Sunday, other guidance keeps showers around through Sunday
night. Confidence is not high in specifics as the models have
had different solutions in regards to speed and track of the
upper level low for the past few days. However, we do have high
confidence of unsettled weather through at least Saturday.
Somewhat drier and warmer weather may then return for early next
week.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1130 AM MDT Wed Sep 16 2026

Breezy winds are anticipated at KCPR and KRKS the first part of the
TAF period. Scattered showers and thunderstorms are forecast across
Wyoming this TAF period, with impacts possible (30 percent chance)
at KRKS, KRIW, KLND, KPNA, and KBPI Wednesday afternoon and evening.
Chances at KJAC this afternoon are less than 20 percent due to the
forecasted shower activity likely remaining over the nearby
mountains. Overall dry conditions are expected overnight at all
terminals with relatively light winds. Showers and thunderstorms
are forecast as early as 14Z Thursday at KRKS and after 16Z
Thursday around KCPR and KPNA, with chances at other terminals
increasing after 18Z Thursday. Localized MVFR flight conditions
and gusty winds cannot be ruled out in any shower or
thunderstorm. Occasional mountain obscurations are possible due
to increasing cloud cover.

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...Gerhardt