Granite Lake, Wyoming Weather Forecast Discussion

018
FXUS65 KRIW 211812
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
1212 PM MDT Sun Sep 21 2025
.KEY MESSAGES...
- Shower and thunderstorm chances increase through the afternoon as
  a system approaches, especially across western Wyoming.
- Cooler and wetter conditions will set in on Monday behind a
  cold front. The highest precipitation amounts are expected
  east of the Divide.
- Freezing temperatures are expected across western valleys
  Tuesday morning, including Jackson Hole and Star Valley.
- Drier and warmer weather returns midweek.
&&
.UPDATE...
Issued at 1210 PM MDT Sun Sep 21 2025
The forecast remains on track today. Satellite imagery shows the
leading wave from the upcoming system beginning to enter western
Wyoming. This will kick off additional showers and thunderstorms
this afternoon, most widespread across northwestern Wyoming. Hi-res
models continue to show convection developing across central Wyoming
as well, likely in response to a surge of moisture and a vorticity
max passing overhead. The cold front is expected to push in late
this evening through Monday morning bringing much cooler and wetter
conditions - especially east of the Divide.
&&
.DISCUSSION...
Issued at 206 AM MDT Sun Sep 21 2025
We have one more day of astronomical summer, before autumn
arrives with weather conditions that will feel like a slap in
the face given the warm weather we have experienced over the
past several days. Fall arrives at 12:19 pm on Monday, when the
sun drops southward across the equator, otherwise known as the
autumnal equinox.
Most areas will have one more warm day today, especially east
of the Divide. Most model guidance has slowed the arrival of the
Pacific cold front somewhat, with most keeping most areas east
of the Divide largely dry through today and even much of the
night. There could be a stray shower or thunderstorm this
afternoon, but this would be the exception (less than a 1 out of
6 chance). With the delay in the cold front, it should get
warmer so we raised temperatures a few degrees today. And with
a southwesterly breeze increasing, there could be some elevated
fire weather this afternoon, especially across the Wind
Corridor from east of Rock Springs to Casper which is favored
area ahead of a frontal passage. West of the Divide, things
will turn wetter as the front moves toward the area, bringing
some showers and thunderstorms. There is still some spread in
the models with placement of the most numerous storms, so we
kept things fairly generic right now. This will be the best
chance of a shower or thunderstorm we have had in several days
though.
On Monday, the main weather maker post frontal will be an upper
level low dropping southward out of Canada. We have a bit more model
consensus this morning, with most guidance showing the low dropping
across western Wyoming. There continues to be some uncertainty with
details of the timing and placement of the heaviest
precipitation. The GFS continues to be slower than the ECMWF.
In addition, the GFS has a jet couplet and a weak 700 millibar
circulation that may enhance precipitation rates Monday night
into Tuesday from around Riverton toward Casper. The ECMWF is
faster and kicks the heaviest precipitation south of the area by
midnight Monday night. As for ensemble guidance, the highest
probability of greater than a half an inch of precipitation is
found across areas from around Jackson and north, with an
average of a 1 in 2 chance of half an inch of precipitation.
East of the Divide, it runs from east of Riverton to Casper,
with as much as a 3 in 4 chance through Tuesday afternoon. There
is even a small area of a 1 in 3 chance of greater than an inch
of rain in eastern Fremont and western Natrona County. This
would likely be more stratiform rain and given the recent dry
conditions, any threat of minor flooding would be minimal, but
not zero.
Now for the colder portion of the storm. The latest model run came
in a bit colder, with 700 millibar temperatures falling as low as
minus 1 Celsius on Monday night across much of central Wyoming.
This could drop snow levels down to 8500 or even 8000 feet at
times. There could be a period of northerly upslope flow developing
as well. And this could lead to a few inches of snow,
especially in the Wind River Range. Ensemble guidance gives a
greater than 2 in 3 chance of greater than 3 inches over the
higher peaks with a 1 in 4 chance of over 6 inches. This
shouldn`t cause any travel problems given the warm ground
temperatures, but places like South Pass and Powder River Pass
could see some snowflakes in the air late Monday into Monday
night. There could even be a coating of snow at the top of
Casper Mountain. Temperatures will also be much colder, with
highs on Monday generally 20 to 25 degrees less than today. This
will also be accompanied by a gusty north to northwest wind,
with gusts over 25 mph common. Many areas east of the Divide
will see gusts past 30 mph. And, in the favored northwest flow /
cold advection locations, like the northern Bighorn Basin and
Johnson County, there is a 1 in 3 chance of wind gusts over 45
mph from later tonight into Monday, making it feel even cooler.
And on Monday night, there is a chance for a freeze in the
western valleys, especially if skies can clear.
By Tuesday night, the low should be moving away from the area.
Following that, we look to have a return to more late summer
like weather as strong ridging builds across the Northern
Rockies and brings a prolonged period of warm and dry weather
from Wednesday possibly into next weekend or even longer.
&&
.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1057 AM MDT Sun Sep 21 2025
A complicated TAF period. A cold front moving through tonight will
be the main influence through the period. Today, shower and
thunderstorm chances occur, mainly along and west of the Divide.
Best chances occur at KJAC through mid-afternoon (about 20z), though
lingering showers and storms continue through this evening.
The upper-level low will then increase cloud cover tonight as it
moves towards southwest Wyoming. Low to mid-level clouds increase
through the night for all sites. VFR to MVFR ceilings are possible
outside any rain chances, and could be briefly IFR with rain
showers. As the cold front moves in from the north, winds shift to
be from the north, with upsloping increasing overcast clouds for
near mountain sites, like KCOD, KCPR, and KLND, with lesser impacts
to KRIW.
Rain shower chances occur for all sites starting tonight. Although
much of it will likely be stratiform, there should be enough support
for convective elements. Overnight, this should mostly be in the
form of moderate to heavy showers, though would not be surprised to
see a thunderstorm pop-up. Thunderstorms should then become more
likely by late Monday morning, with continued chances through Monday
afternoon. Temperatures Monday will also be much cooler (10 to 25
degrees) behind the cold front.
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...Myers
DISCUSSION...Hattings
AVIATION...Wittmann

NWS RIW Office Area Forecast Discussion