821
FXUS65 KRIW 162237
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
437 PM 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 slightly 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 of 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 the 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. 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 in the forecast, 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 continues to be the
steering flow, 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 /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 437 PM MDT Wed Sep 16 2026

Scattered showers and a few isolated thunderstorms will be present
across western and southern Wyoming through sunset Wednesday, with
KPNA and KBPI having the highest chances (30 percent) of impacts.
All other terminals have a 10 to 20 percent chance of seeing a
shower or thunderstorm through sunset. Anticipate relatively light
winds and increasing cloud cover overnight. Shower and thunderstorm
potentially exists at most terminals Thursday, with chances ranging
from 20 percent to 70 percent. 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