492
FXUS65 KRIW 272145
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
345 PM MDT Sun Sep 27 2026

.KEY MESSAGES...

- A mainly dry day today. Showers will increase from the
  southwest late this afternoon into the evening, becoming
  widespread through the overnight hours.

- Cooler and unsettled Sunday night through Monday. Showers
  will be ongoing over western and southern portions by sunrise
  Monday morning, spreading eastward through the rest of the
  morning.

- Mainly dry weather returns Tuesday and lasts through at least
  Friday with seasonal temperatures.

&&

.UPDATE...
Issued at 1110 AM MDT Sun Sep 27 2026

Light rain showers earlier this morning have ended with dry
conditions expected through the afternoon. Winds will not be as
strong today, with gusts up to 25 mph mainly over areas west of the
Divide. Temperatures this afternoon will be slightly cooler east of
the Divide, with most areas across the CWA rising into the 70s.
Showers will increase across the area after 23Z (5pm), starting over
far northwestern Sweetwater County to South Pass. These showers will
spread northward through the evening, becoming widespread between
06Z and 08Z. Isolated thunderstorms will be possible during this
time as well. Showers will be ongoing over western and southern
portions by sunrise Monday morning, spreading eastward through the
rest of the morning and dissipating by early afternoon. Temperatures
will be cooler Monday, with readings west of the Divide about 10
degrees cooler.

&&

.DISCUSSION...
Issued at 144 AM MDT Sun Sep 27 2026

Things are relatively quiet here at the humble Riverton weather
abode early this Sunday morning. We will have some active weather in
the forecast, but also a decently long period of fairly nice early
autumn weather, so there is a little bit for everyone, depending on
your taste.

The active weather will be front loaded. Most of the day today
should be fairly quiet, although a few showers will be possible,
With the greatest number in the afternoon and in and near the
mountains. However, a vast majority of the area should remain
rain-free through the daylight hours. The main cause of the
active weather, a shortwave that will be rotating around an
upper level low over California, will begin making a closer
approach tonight, increasing the coverage of showers and
thunderstorms. With the southwest flow ahead of the wave, the
main area of showers and isolated thunderstorms will be across
southern and western Wyoming, with drier conditions east of the
Divide where downsloping flow may keep a lot of the showers
away. As for temperatures, they will remain above normal, but
more cloud cover will keep them a few degrees cooler than on
Saturday.

The most widespread showers and thunderstorms still look to be
on Monday. The shortwave will be passing through Wyoming at that
point. There will also be some upper level support from the
left front quadrant of a 90 knot jet streak that will be passing
across southeastern portions of the area. As for threats,
locally heavy rain is possible. Precipitable water values will
be from 125 to 150 percent of normal with the additional upper
level forcing from the jet streak. However, instability may be
limited as there could be a decent amount of sky cover around
the area. In addition, there will be steering flow so storms
will be of the moving variety. In addition, most locations have
been mainly dry since last Sunday, so there is room in the soil.
So, although the threat of flooding is not zero, it is low. As
for overall amounts, most of southern and western Wyoming has at
least a 1 in 2 chance of a tenth of an inch of QPF through
Tuesday morning. Most of western Wyoming, especially the favored
south and southwesterly upslope flow locations, have at least a
1 in 2 chance of a quarter of an inch of QPF. The highest
amounts would be in the highest elevations of the southwestern
slopes, like the western Wind River Range and the southern
Absarokas where locally over a half an inch or rain could fall.
And, with low wet bulb zero levels, small hail may also be
possible with the stronger storms. Most showers decrease after
sunset with only isolated to scattered leftover showers after
midnight.

And we have to talk about the possibility of high elevation
snow. The 700 millibar temperatures do fall as low as 2 Celsius
later Sunday night and Monday morning, bringing snow levels
down to around 9500 to 10000 feet. It would not surprise me to
see some snowflakes on the higher passes like Beartooth or
Togwotee. However, any accumulation would be above 11000 feet
and even here it looks light. The only location with a greater
than 1 in 2 chance of over an inch or snow is the highest
elevations of the Wind River Range where impacts would be slim
to none.

We then enter the mainly dry period of the forecast. At this point,
we are around 90 percent sure that we will have a decent period of
nice autumn weather. The one wild card is that will happen with the
combination of the upper level low moving eastward from California
when it interacts with the moisture from the remnants of Hurricane
Polo after it makes landfall in Mexico. Almost all the ensemble
members keep the moisture south and east of Wyoming, but a few
try to bring the edge into southern Wyoming. We will watch this
but odds are against it at this point. Temperatures should
average above normal for the most part as ridging builds into
the area. A brief is likely Wednesday night and Thursday as a
weak cold front moves through the area. With the lack of
moisture, it will likely only bring some clouds and some
breezes. Temperatures then warm again for Friday and into
Saturday. The next chance of showers would be later Saturday
into Sunday with another trough and cold front approaching.
There has been poor model consistency with this feature though,
so confidence in the details this far out is very low.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 345 PM MDT Sun Sep 27 2026

Overall, VFR flight conditions persist much of the TAF period at
most terminals. Gusty winds at West of the Divide terminals (KJAC,
KRKS, KPNA, and KBPI) will begin to diminish around or shortly after
sunset. Elevated to breezy winds are forecast to reach KRIW and KLND
between 01Z and 03Z and be present through 08Z Monday. There exists
varying shower chances throughout the TAF period at most terminals.
Scattered light rain showers are forecast to develop Sunday evening
and overnight across southern and central Wyoming. Given the
scattered nature of these showers, the chances are reflected in
PROB30 groups at KRKS, KRIW, KLND, KPNA, KBPI, and KWRL.

A broader area of showers begins to move into western Wyoming around
08Z and push east on Monday. All terminals have some chance of a
rain shower. An isolated thunderstorm is also possible, but with the
chance 20 percent or less there is not enough confidence to include
in the TAFs. There is a 10 to 30 percent chance of MVFR ceilings and
visibilities with any heavier rain shower, with confidence being
greatest at KRKS and KPNA Monday morning.

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...LaVoie
DISCUSSION...Hattings
AVIATION...Gerhardt/Zaprzalka