995
FXUS65 KRIW 151825
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1225 PM MDT Mon Sep 15 2025

.KEY MESSAGES...

- Chances for showers and thunderstorms through today. Hazards
  include small hail and gusty outflow winds. Strong wind over
  50 mph is possible across central and northern areas through
  this evening.

- Cooler air brings the potential for frost in western valleys
  and basins tonight. Freeze Warnings are in effect. These
  temperatures are again likely the next few nights.

- Daily shower and thunderstorm chances through Wednesday.
  Drier and warmer air moves in late this week.

&&

.UPDATE...
Issued at 1221 PM MDT Mon Sep 15 2025

The main consideration for this update is freezing temperature (32F
or less) potential tonight across western valleys and basins. Models
are in good agreement with a dry slot moving into the area as the
upper low moves over Wyoming tonight. If that happens, the drier air
should limit clouds across central areas, including Star Valley,
Jackson Valley, and the Green River Basin. Therefore, confidence is
fairly high (70%) that widespread temperatures of 32 degrees or
lower will occur. This is very dependent on cloud cover decreasing,
however. Freeze Warnings have been issued to cover the likelihood of
frost formation. Cold prone locations, like along rivers and near
Bondurant will see the coldest lows, potentially in the mid-20s.

As the aforementioned low moves over Wyoming tonight, the drier air
should limit precipitation chances across central areas, with
chances continuing across portions of northern and southern Wyoming
through Tuesday. Central areas could see better storm chances pick
up Tuesday afternoon as moisture fills in. Snow levels could be as
low as about 9000ft at times through Tuesday.


&&

.DISCUSSION...
Issued at 200 AM MDT Mon Sep 15 2025

The next trough is moving into western WY as I type this, with
showers and thunderstorms on the increase. These showers and
thunderstorms will continue to spread to the northeast
generally across the northern half of the state through this
afternoon. Midlevel support will peak during the mid afternoon,
which, along with dry air near the sfc, will lead to some gusty
winds from stronger storms this afternoon, mainly across central
WY. The midlevel trough stalls around northern WY tonight,
leading to continued showers and thunderstorms for northern WY.
This will also bring a band of showers across southern WY
increasing after midnight tonight. The trough continues to spin
across the state on Tuesday, leading to another round of showers
and thunderstorms for most of western and central WY. As the
trough slowly drifts to the east Wednesday morning, rain
chances will finally start to decrease, with central WY being
the final area to see an end to precip. While most of western WY
is settling into total accumulations of 0.10" to 0.25", Johnson
and Natrona Counties are seeing an increasing probability of
0.50" (currently near 40% chance).

The cold pool associated with this wave will also bring some
cooler temperatures overnight tonight and Tuesday night, mainly
for lower elevations west of the Divide. Probabilities of
temperatures below freezing remain modest (20-30%), the most
likely locations would be the upper Green River Basin (Hoback
to Bondurant), and across most of Yellowstone NP.

The latter half of the week will be a return to more stable
conditions. Models are differing on the final wave rotating
around the main trough moving east. If this wave stays far
enough west, then Thursday may see one more day of showers for
the I-25 corridor. Once this is through, a more stable regime
sets up, with warmer temps and minimal rain chances.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1134 AM MDT Mon Sep 15 2025

A low pressure system will remain overhead through the period. This
will result in periods of showers and thunderstorms. A wave of
showers that moved through this morning will continue to impact KCPR
and KCOD terminals at the start of the period, but will quickly exit
to the northeast by 19Z. Then, showers and thunderstorms are
expected to develop along and east of the Divide this afternoon.
Gusty winds and possibly small hail are the main threats with any
storms. PROB30 groups cover the most likely timing of these storms
through the afternoon.

Skies then gradually clear overnight. With recent moisture, there is
a 20 to 30% chance that fog could develop along the Jackson Hole
Valley. Confidence in not high enough to include vsby/cig reductions
KJAC TAF but have hinted at the potential with a FEW002 cloud group
beginning at 10Z Tuesday morning. If fog develops, there is a 30%
chance it could linger through the end of the period.

Another wave of showers looks to develop overnight, pushing into
southern WY and eventually into KCPR area Tuesday morning. There is
50 to 60% confidence in prevailing rain at KRKS for several hours
through the morning with this wave, and slightly lesser confidence
(40 to 50%) at KCPR. VFR conditions are expected to prevail through
the period despite these showers, however, scattered low clouds
around FL010 to FL020 are possible and a cig could develop (20% to
30% chance) at KCPR with the band of showers after 14Z Tuesday.

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...
Freeze Warning from 1 AM to 7 AM MDT Tuesday for WYZ013-023-
025-026.

&&

$$

UPDATE...Wittmann
DISCUSSION...Straub
AVIATION...Hensley