955
FXUS65 KRIW 121845
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1245 PM MDT Sat Sep 12 2026

.KEY MESSAGES...

- Dry and near seasonable temperatures for the weekend.

- The next weather system approaches late Sunday and Sunday
  night bringing a small chance of showers, mainly East of the
  Divide.

- A gusty wind and low humidity will bring elevated to near
  critical fire weather for Monday.

- A wetter pattern possible for later in the week, although
  details on timing and placement of heaviest precipitation
  remain uncertain.

&&

.UPDATE...
Issued at 1245 PM MDT Sat Sep 12 2026

No change in the quiet and benign short term forecast.
Seasonable temperatures and partly cloudy skies with a bit of a
breeze for the wind corridor over the weekend. Any storm/shower
activity will remain east and south of the CWA. The next system
still on track for later Sunday and overnight into Monday, but
track continues its northward progression with the CWA remaining
relatively dry. Otherwise, the cold front will bring in cooler
than average temperatures heading into the new work week being
more fall- like.

&&

.DISCUSSION...
Issued at 125 AM MDT Sat Sep 12 2026

It definitely felt more like autumn when I came into work this
evening. This was one of the first times I broke out my windbreaker
since last spring. An we do have a gusty breeze this evening
following the passage of a cold front. We still have a few light
showers out there this evening, but nothing heavy.

Today looks like a rain free day for a vast majority of the area as
much drier air passes in from the west, dropping precipitable water
values to under a half an inch for most locations. The one exception
will be across far southern Wyoming, mainly along and south of
Interstate 80, where a bit more moisture will linger. We could see
an isolated shower or thunderstorm here, but the chance is less than
1 out of 7 and therefore the POPs will be largely silent. Most
locations should see a light to moderate wind today. Again, the
exception will be Sweetwater County, where a gusty breeze will
continue. And as for fire weather concerns, elevated will be likely
but humidity should remain above critical levels. Temperatures will
be cooler than yesterday, but still remain above normal for most.
Northern Wyoming will be a bit cooler though, with places like
Buffalo and Cody struggling to get above 70.

Moisture begins to increase tonight. Enough mid level moisture may
return later tonight for a few isolated showers in the western
mountains late tonight. The bulk of the moisture should hold off
until late in the day and especially after sunset. The culprits for
this will be a plume of monsoonal moisture moving in from the south,
impacting areas largely East of the Divide. At this same time and
upper level low will be moving northward from Idaho and into western
Montana, dragging a negatively tilted trough and cold front through
the area. And there could be some decent rain in eastern
portions of the area. For factors for, we will have deep
moisture, with precipitable water values approaching 175 percent
of normal. The trough and cold front will also add some extra
lift. The main factor against will be steering flow, which looks
rather strong. There is generally 30 knots at 700 millibars
with up to 50 knots at 500 millibars. So, any showers and storms
will be moving at a decent pace. The main concern would be
possibly training storms, or in southwesterly upslope areas,
like the southern end of the Bighorn Range. The greatest chance
would be largely along and east of a Kaycee to Rock Springs
line, with lessening chances further north and west. And with
the southwest flow ahead of the system, it will be a warm end to
the weekend with many locations below 6000 feet well into the
80s for high temperatures.

Most showers should clear the area by sunrise on Monday, replaced by
another cooler and drier airmass, with temperatures falling back
toward normal levels. Concerns on this day shift to fire weather
again. There will still be a decent height and pressure gradient
between ridging over the southern United States and the upper level
low moving into the Prairie Provinces. The result will be a gusty
wind across much of the area. And with very dry air moving into the
area, we will likely see at least elevated fire weather. The
southern two thirds of the area; especially East of Rock Springs and
Riverton, has at least a 1 out of 2 chance of relative humidity
falling under 15 percent. We may need some fire weather highlights
for this day, it is a bit too early to issue right now though.

Tuesday looks mainly dry at this point, although signs are pointing
to another shot of monsoonal moisture starting to move into southern
Wyoming for late Tuesday as another low moves toward the area. Most
of the moisture looks to hold off until Wednesday though. A
preponderance of the guidance takes this low more to the east
and closer to the Cowboy State, bringing a better chance of
showers for Wednesday and into Thursday and possibly into Friday
as well. Details of precipitation amounts and timing of the
heaviest precipitation remain in flux though as there are
different solutions in regards to the exact timing and placement
of the low. Nevertheless, we are looking at an increased chance
of showers for later in the week.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1101 AM MDT Sat Sep 12 2026

VFR conditions through the period. West of Divide locations will
have gusts 15 to 30 knots through this afternoon, decreasing this
evening. Besides some high cirrus clouds and a few afternoon
cumulus, it remains mostly clear today.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Lowe
DISCUSSION...Hattings
AVIATION...Wittmann