Alpine, Wyoming Weather Forecast Discussion

251
FXUS65 KRIW 250722
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
122 AM MDT Thu Sep 25 2025
.KEY MESSAGES...
- Temperatures 5 to 15 degrees above normal and dry weather can
  be expected through Saturday.
- An unsettled weather pattern returns on Sunday across the west
  with multiple shower and thunderstorm chances (20% to 50%)
  through the middle of next week.
&&
.DISCUSSION...
Issued at 118 AM MDT Thu Sep 25 2025
Today will be another warm day with above normal temperatures, sunny
skies, and dry weather conditions as an upper-level ridge shifts
east over the Intermountain West. The above normal temperatures can
be attributed, in part, to 500mb temperatures being above the 99th
percentile of climatology. However, the center of the upper ridge
will not sit over the region for long so temperatures will remain
well below any record values. Relative humidity values around 15%
are forecast across the eastern Bighorn Basin and Johnson County.
However, wind gusts are forecast to remain below 20 mph so fire
weather concerns remain low at this time.
There is good agreement that the upper-level ridge will be situated
over the central CONUS by late tonight or early Friday morning. At
the same time an upper-low will be developing over southern
California. A slight moisture uptick associated with the upper-low
and a separate approaching trough and will trek across the state
Friday afternoon, and a weak cold front. The trough is being pushed
across the Northern Rockies by an upper-level jet that is centered
over southwestern Canada and Montana. This puts Wyoming in the right
exit region of the jet - an unfavorable location for convection. So
the only noticeable sign that a cold front is passing through is an
increase in cloud cover, breezy winds, and highs being a degree or
two cooler than today. That being said, a very isolated terrain
driven shower or thunderstorm initiating off the Unitas and moving
east is possible (less than 10% chance). Chances in anything
sustaining for long is even lower given the location in proximity to
the upper-level jet.
High pressure briefly returns on Saturday with continued dry weather
and above normal temperatures. There remains consensus among
ensemble guidance that the California low and it`s moisture will be
absorbed into a trough and push northeast into the Intermountain
West on Sunday. This will kick off the first day of unsettled
weather for portions of the state. The greatest moisture and support
aloft is forecast to remain west of the Divide so shower and
thunderstorm chances (15% to 30%) will be confined to these areas on
Sunday. Early next week, a separate longwave trough will push into
the region from the west with Pacific moisture. This is forecast to
bring shower and thunderstorm chances (30% to 50%) and cooler
temperatures to Western Wyoming through the middle of next week.
Current forecasted snow levels are around or above 10,000 feet AGL,
so any snow would be across the higher elevations of the Western
Mountains.
&&
.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 946 PM MDT Wed Sep 24 2025
VFR conditions and SKC will prevail through the TAF period. Wind
will remain light, less than 10kts through the TAF period for
most terminals. The exception is KCPR, where a southwest wind
will increase towards 17Z Thursday, with gusts around 15 to
20kts through the afternoon.
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.
&&
$$
DISCUSSION...Gerhardt
AVIATION...Rowe

NWS RIW Office Area Forecast Discussion