768
FXUS65 KRIW 171335
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
735 AM MDT Thu Sep 17 2026

.KEY MESSAGES...

- Deeper moisture moves over the area today with an increased
  chance of showers and thunderstorms. The most numerous showers
  will be in western and southern Wyoming.

- Showers will remain likely through at least Saturday and
  possibly into Sunday, although the focus of the most numerous
  and heaviest showers will shift to northern Wyoming starting
  on Saturday. The most numerous storms should be on Friday.

- Temperatures will be cooler today with more cloud cover.
  Friday will be a warmer day before cooler temperatures move in
  Saturday and especially on Sunday.

&&

.DISCUSSION...
Issued at 206 AM MDT Thu Sep 17 2026

The story today and for the next few days, as it has been for
seemingly forever (actually the past two and a half months) is
monsoonal moisture. Satellite imagery shows an area of moisture
advancing northward with showers and embedded thunder moving toward
Sweetwater County as I write this around 1 am. This area of
showers will spread northward tonight. Meanwhile, most of
northern Wyoming should remain dry tonight.

This deeper moisture will overspread the entire area today, with
precipitable water levels rising to 150 to 175 percent of normal.
Just about all areas will have at least a 1 in 6 chance of a shower
or thunderstorm today. The best chances though will be across
western Wyoming where there will be more forcing, and also across
areas East of the Big Horns, with the least chance in the center,
especially the central basins. There continues to be one factor that
makes me think coverage of thunderstorms may be less though.
There will be a decent amount of cloud cover around the area,
and this could limit surface instability somewhat. And it is we
are now almost to the autumnal equinox, not the middle of
summer. So, the sun is weaker and daytime heating will be less.
This may also limit the chance of heavy rain and any flooding
potential. Steering flow is less than yesterday, but we still
have 10 to 20 knots and this should be enough to keep storms
moving somewhat. However, with the high precipitable waters
there could still be isolated minor flooding, especially west of
the Continental Divide. Most showers should end after sunset,
but a few showers will likely linger through the night. And it
will probably be more than expected, the models have been
horrible this season with nocturnal monsoonal convection.

An upper level low that has been helping to funnel the moisture into
the area will finally begin to move eastward today, drifting
toward Idaho by tomorrow. This may be the most active day for
convection of the period, especially thunderstorms. Most
guidance is showing a clearer sky in the morning, allowing for
more daytime heating and surface instability. Instability
parameters are decent for this time of year, with CAPE
approaching 700 J/Kg and lifted indices as low as minus 4. With
the approaching upper level flow, there will also be some decent
directional shear, especially in western Wyoming. The result
will be a chance for some stronger thunderstorms, again
especially in western Wyoming where there is a marginal risk of
severe weather. The main threat would be hail given the low wet
bulb zero levels. With more sun and southwest flow continuing,
this will also be a warmer day, especially East of the Divide
where warmer spots may get close to 80 for a high. Showers will
continue into the night as well. With the added instability and
a shortwave rotating around the low, if there are any flooding
problems, this would be the time for it.

The latest guidance continues to slow the progression of the low,
only slowly moving it over Wyoming by Saturday. On this day,
the best chance of showers and storms would be across northern
Wyoming. Most guidance is in decent agreement on this. The
deepest moisture would be over Montana, but most areas will
have a chance. There may also be a dry slot that moves in
portions of the area, but exactly where is still in question.
The low will slowly cross the state then on Sunday, but will
still be close enough for some wrap around moisture to bring
more showers, mainly to northern and eastern portions of the
area. This also looks like a cooler day as flow turns to the
northwest, with temperatures anywhere from 5 to 10 degrees
cooler than on Saturday. Following this, there is more
divergence in the models with some bringing another system
across the area early next week with another chance of showers
and others suppressing it to the south, keeping the area largely
rain free. All guidance shows south to southwesterly flow
returning though, so temperatures look to be back above normal
as we head into the start of autumn, which arrives on Tuesday,
September 22nd at 6:04 pm.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 452 AM MDT Thu Sep 17 2026

Moisture will spread across the area today with all TAF sites
having a chance of a shower or thunderstorm. The chance is the
lowest in vicinity of KWRL (20 percent), so we left an mention
of showers out of the forecast for now. An area of showers will
move out of Colorado and move northward through the morning,
impacting mainly KRKS, KCPR, KRIW and KLND through around 18Z.
Other showers will impact KJAC until around 16Z this morning.
Additional showers will be possible later this afternoon and
evening and possibly again late tonight although there is still
high uncertainty with timing and position of the afternoon and
nighttime showers. Wind should generally remain less than 11
knots. The exception will be in and around any thunderstorms or
even showers where a gust to 35 to 40 knots can`t be ruled out.
Conditions should be largely VFR, although occasional MVFR
conditions are possible around showers and thunderstorms.
Mountains will be obscured at times around showers at times,
especially above 9000 feet.

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.

&&

$$

DISCUSSION...Hattings
AVIATION...Hattings