140
FXUS65 KRIW 281751
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1151 AM MDT Mon Sep 28 2026

.KEY MESSAGES...

- A weather system moving across the area will bring areas of
  showers and isolated thunderstorms today into this evening.

- Locally heavy rain will be possible with any shower or
  thunderstorm, but confidence is low for any adverse impacts.

- Weather conditions stay quiet the remainder of the week, with
  only a couple of chances of light showers. Temperatures will
  stay seasonal, except on Friday. Highs will be about 5 degrees
  above normal.

&&

.UPDATE...
Issued at 915 AM MDT Mon Sep 28 2026

Showers will be ongoing over central and southern portions of
the CWA by midday, gradually pushing into Johnson County and
over the Bighorns. Showers will become more scattered in
coverage late in the afternoon, as additional showers and
thunderstorms move over far western portions. Although locally
heavy rain will be the main threat this afternoon, confidence
remains low due to a lack of instability and strong dynamics.
Precipitation will then wane across the area through the evening
and end before midnight. Temperatures will be slightly cooler
tonight, with near-freezing temperatures possible over in the
western valleys.

&&

.DISCUSSION...
Issued at 129 AM MDT Mon Sep 28 2026

We are currently getting some much needed moisture across the area
with some areas of showers and isolated thunderstorms. This is
courtesy of a shortwave moving toward the area. It is getting an a
assist from a 95 knot jet streak that is approaching southwestern
Wyoming and will cross southern Wyoming this afternoon and evening,
putting much of the area in the left front quadrant of it to enhance
lift somewhat. As a result, almost all areas will have a chance of
showers today. There will also be some embedded thunderstorms.
However, with cloud cover around the area, surface instability may
be limited. The main threat from any storm would be locally heavy
rain, with precipitable water values generally running anywhere
from 125 to 150 percent of normal. A mitigating factor will be
that there will be steering flow, so any storms will have decent
motion and not be able to anchor. With more cloud cover,
temperatures will also run around 5 to 10 degrees cooler than
Sunday. Coverage of showers and thunderstorms should decrease
after sunset, ending generally from northwest to southeast,
although with the jet still in the area overnight, isolated
showers and thunderstorms may linger south of a Casper to Rock
Springs line through the night.

And, we could see some higher elevation snow from this. Any
accumulation would likely be restricted to above 11000 feet
though. The highest elevations of the Wind River Range, like the
Gannett Peak, have a 1 in 2 chance of seeing 3 inches of new
snow. There should be little or no impact to travel though.

We should then enter a quieter period of weather as ridging builds
into the area from the west, pushing drier air into western and
central Wyoming. There may be just enough leftover moisture for some
isolated showers in southern Wyoming, but this looks like the
exception rather than the rule. Following that, things look mainly
dry through the week and even into the weekend. A large majority of
the guidance keeps the moisture associated with an upper level
low and the remnants of Hurricane Polo to the south of the area
through the forecast period. A weak cold front will move into
northern and central Wyoming Wednesday night. This front is
moisture starved though, so it will likely only bring some
clouds and somewhat cooler temperatures and a gusty breeze,
especially to northern Wyoming. Another front may move through
Friday night or Saturday, but again it has little moisture to
work with so any showers would be isolated with the only impacts
being somewhat cooler temperatures and some gusty breezes.
Temperatures will generally average above normal, with Thursday
and Sunday looking like the coolest days behind the front. But
even here, temperatures will only fall back to near normal
levels. So, once we get past the next 24 to 36 hours, things
look fairly uneventful as we head into October.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1150 AM MDT Mon Sep 28 2026

Showers and isolated thunderstorms will be present across Wyoming
through at least sunset today. All terminals have a chance of seeing
a shower or thunderstorm directly impact the sites. Conditions will
largely remain VFR, but instances of MVFR to IFR ceilings and
visibilities are possible (up to 30% chance). Occasional mountain
obscurations are expected through at least sunset. Gusty 30 to 35kt
winds are also possible in and around convection. Showers and
thunderstorms will generally end from northwest to southeast, with
isolated showers across southern Wyoming, including KRKS, until
sunrise on Tuesday. Outside of thunderstorms, winds largely remain
12 kts or less at all terminals.

There is a low chance (10% to 20%) of fog and low cloud development
at or in the areas of KJAC, KRIW, KCPR, KRKS, KBPI, and KPNA early
Tuesday. This development is dependent on how quickly overnight
clouds clear. Given the low confidence, there is no mention in TAFs
at this time.

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