Rock Springs, Wyoming Weather Forecast Discussion

473
FXUS65 KRIW 092131
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
331 PM MDT Tue Sep 9 2025
.KEY MESSAGES...
- Showers and thunderstorms through the afternoon will mainly be
  focused across western WY (25 to 50% chances) with 15 to 30%
  chances of showers and storms across the rest of the region
  through the evening. Gusty winds 25 to 35 mph are the main
  threat with these showers and storms.
- Widespread elevated fire weather conditions are expected along
  the Rock Springs to Casper Wind Corridor Wednesday and
  Thursday, with focus on stronger winds Wednesday and Thursday.
- Afternoon and evening showers and thunderstorms are expected
  again Wednesday through Friday. Isolated stronger storms are
  possible on Thursday (10 to 15% chance).
- A weather system looks to bring cooler and wetter weather to
  the region Friday and Saturday.
&&
.UPDATE...
Issued at 1238 PM MDT Tue Sep 9 2025
A wave of weak showers continues to move across the Divide as of
1200L. The associated cloud cover has helped stabilize things thus
far, and CAMs have backed off on shower and thunderstorm chances for
the afternoon. There has been enough clearing west of the Divide,
that with help of the terrain, expect to see convective development
towards the middle part of the afternoon. CAMs have been fairly
consistent with a cluster of showers and storms developing along the
western WY terrain by around 1400L, and spreading across much of
western WY by 1500-1600L. Gusty winds 25 to 35 mph would be the
primary concern with these showers and storms, but an isolated
stronger storm producing small hail (peas to dimes) cannot be ruled
out.
As this activity pushes across the Divide later in the afternoon/
evening, a few weak showers and storms are possible to sustain east
of the Divide, and will be primarily outflow driven. Lingering
shower and thunderstorm activity should wane quickly towards sunset,
once diurnal heating is lost. A few outflow driven showers and weak
storms could continue across portions of northern WY and western WY
overnight, but chances are generally less than 10%.
Wednesday generally looks quieter in terms of shower and
thunderstorm chances, but elevated fire weather conditions continue
to look likely (70% confidence) across the Wind Corridor from Rock
Springs through Casper. RHs for Thursday have notably come in wetter
with the latest model runs, so despite the wind continuing on
Thursday, the fire weather concern looks to primarily be focused to
Wednesday.
&&
.DISCUSSION...
Issued at 120 AM MDT Tue Sep 9 2025
An upper level low has moved onshore near the CA/OR border earlier
Monday evening and is expected to dig southward over northern CA
through the day today. This will cause a ridge axis over the Cowboy
State to amplify and gradually shift eastward through the day. This
eastward shift will allow the winds aloft to turn more
southwesterly. Showers and thunderstorms over portions of UT will
continue to move toward the northeast and look to reach southern
Lincoln County and the Flaming Gorge area around 09Z. These showers
will continue to trek northeast across the Upper Green River Basin
and portions of Sweetwater County through the rest of the morning.
Little to no impacts are expected, other than isolated lightning
strikes. Additional showers and thunderstorms will be widely
scattered (20-30% chance) across the forecast area this afternoon,
as additional shortwaves move over the area. The Absaroka and
northern end of the Wind River Mountains, as well as the Cody
Foothills, will have the better chances. The showers and storms from
these areas will move over the Bighorn Basin this evening through
06Z Wednesday. Wind gusts of 35 to 45 mph, small hail and lightning
will be the main threats. Any smoke that made its way into the
forecast area Monday will once again get pushed northward out of the
area through the day.
This overall pattern will continue Wednesday and Thursday, as the
upper level low slowly rotates over portions of OR, CA, NV and ID.
The southwest flow aloft will continue over the Cowboy State, as a
high develops over the TX Panhandle and strengthens the ridge over
the Plains. These winds will usher in more drier air once again and
set up a pseudo-dryline over eastern portions of Johnson/Natrona
counties Wednesday afternoon. Any convection looks to be confined to
western portions of the state along and west of a Kemmerer-to-Cody
line and be widely scattered (20% chance) through the afternoon and
overnight hours. Elevated fire weather conditions will be in place
Wednesday and Thursday, with gusts up to 30 mph each afternoon. RH
values are expected to stay above critical thresholds (15%), but
will still be low in the 18 to 25% range. Showers and thunderstorms
will once again develop across areas west of the Divide Thursday
afternoon, as a leeside low develops over southern MT and the
associated cold front leads to the convection over western portions.
Wind gusts of 40 to 55 mph, small hail and lightning will be the
main threats through the afternoon and evening.
Isolated showers will remain possible over western portions Thursday
night into Friday morning, as the eastern fringes of the upper low
approach. Temperatures will be 5 to 10 degrees cooler Friday, as a
result of this low. Cloud cover will certainly aid in these cooler
temperatures. Precipitation chances will quickly increase over much
of the CWA Friday afternoon, as a portion of the low rounds the base
of the trough and moves northward. Precipitation will continue
through the overnight hours, as the low moves over southern and
eastern portions of MT by 12Z Saturday. Precipitation chances will
be varied Saturday, as the low stays in MT and a trough rotates
around the low and moves eastward over the forecast area.
Temperatures will remain on the cool side with highs in the 60s west
of the Divide and lower to middle 70s east of the Divide. Freezing
temperatures could occur for portions west of the Divide Friday and
Saturday nights in the more cold prone locations, like Cokeville and
Bondurant. Sunday looks to remain mainly dry and seasonal, with
isolated showers and storms possible over far southwestern portions
late in the afternoon.
&&
.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 330 PM MDT Tue Sep 9 2025
VFR conditions through the period, with lessening winds through this
evening. Lingering showers and storms occur through the first few
hours of the period, with PROB30 groups to indicate timings. Any
shower or storm could produce a gust up to about 40kts.
Wednesday morning, a generally southerly wind increases across the
area. Wind gusts will start to pick up late morning. All sites are
expected to have gusty winds 20 to 30 knots through Wednesday
afternoon. Convection will be more limited compared to Tuesday. Only
KCOD has a PROB30 group included at this time.
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...Hensley
DISCUSSION...LaVoie
AVIATION...Wittmann

NWS RIW Office Area Forecast Discussion