Mammoth Hot Springs, Wyoming Weather Forecast Discussion

322
FXUS65 KRIW 060347
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
947 PM MDT Fri Sep 5 2025
.KEY MESSAGES...
- Cool overnight temperatures into Saturday morning.
- Warming trend through the weekend and into early next week.
- Moisture begins to spread slowly northeast into the state
  Saturday, leading to isolated showers and thunderstorms across
  southwest Wyoming. These showers and storms will be more
  widespread Sunday, especially along and west of the
  Continental Divide.
&&
.UPDATE...
Issued at 1255 PM MDT Fri Sep 5 2025
Cyclonic flow aloft and moist 700mb upslope flow allowed for a deck
of clouds 040-060 AGL to linger through much of Friday morning
across the Wind River and southern Bighorn Basins. Pacific Northwest
and British Columbia wildfire smoke continues to stream into central
and southern portions of the state, while improvement has been seen
across the northern-third. Most areas remain dry and mostly clear
through Friday night with the exception of the far southwest. Weak
instability and just enough mid-level moisture should trigger
isolated showers and perhaps a thunderstorm or two between 2 PM and
8 PM Friday. The best chances are across southern Lincoln and
western Sweetwater Counties during this time period. The pressure
gradient is still looking favorable for a period of 15 to 25 mph
easterly wind across portions of southwest Wyoming Friday evening.
Saturday finds moisture slowly returning to the state from the
southwest. Isolated showers and thunderstorms develop over southern
Lincoln County between 1 and 2 PM before slowly spreading north and
east by 5-6 PM Saturday. It looks as though enough mid-level
moisture reaches the Teton and Gros Ventre Ranges late in the day to
at least generate isolated showers and storms in those mountains.
Convection may struggle to make its way east off the higher
terrain and into the basins given lower dew points in the 30s.
Temperatures do rebound to seasonal readings Saturday as the
push of moisture and warmer air arrives from the southwest. HRRR
smoke indicates this slight northward push in the large-scale
pattern should lead to smoke returning to the northwest and
Bighorn Basin late Saturday.
&&
.DISCUSSION...
Issued at 150 AM MDT Fri Sep 5 2025
Dry conditions will be in place today, as the Cowboy State remains
under a cyclonic flow pattern in the wake of Thursdays cold front.
Highs today, mainly in the 70s, will be 5 to 10 degrees cooler as a
result of the front. Unfortunately, the smoke will continue for most
of the CWA. Far northern portions look to be the least impacted from
the smoke and should have mostly clear skies. Winds will generally
be light as well, with gustier northwest winds over far western
portions by mid-afternoon. There also looks to be another strong
push of easterly winds, with gusts up to 30 mph, moving across
portions of the Upper Green River Basin after 00Z and subsiding by
06Z. Rain showers will be possible over portions of southern
Lincoln, western Sweetwater and southern Sublette counties this
afternoon. A few lightning strikes will be the main concern and
little to no precipitation is expected.
Similar conditions return for Saturday, as the ridge that has been
to the west begins to shift eastward over the region. Elevated fire
weather conditions will be more widespread, with critical RH values
returning across eastern portions of the Bighorn Basin. However
winds will remain light across much of the forecast area, with gusty
southwest winds returning over southwestern portions of the state
through the afternoon. Wind gusts of 25 to 35 mph will be common in
this area. Additionally, some of these gusts could occur as outflow
boundaries as showers and a few thunderstorms will again be
possible.
The ridge axis will be over the Cowboy State by Sunday, with a broad
ridge remaining in place on Monday. A shortwave trough, undercutting
the ridge, will allow for more widely scattered showers to occur
across areas west of the Divide Sunday and northwestern portions
look to have the better chances as the trough moves over Monida Pass
and southern MT. Conditions look to remain dry Monday, as a more
diffluent flow pattern aloft looks to take shape. This will be due
to an upper level low over the EPAC moving onshore over the PACNW.
Temperatures will be much warmer as a result, with highs in the 80s
returning to areas east of the Divide. Tuesday looks to be the most
active day with chances for widespread precipitation returning. This
will be due to a shortwave trough rounding the base of the upper low
and moving over WY through the afternoon and evening. PWATs look to
increase to 0.6 to 0.8, which is in the 90th percentile for this
time in September. So, chances could be good for a wetting rain.
Southwest winds aloft will continue Wednesday as the upper low
remains over the PACNW and a upper high sits over the Southern
Plains. Precipitation chances do not look as good, but are still
possible from additional subtle embedded shortwaves. However, it
looks like this could be a dry slot as the trough on Tuesday could
push all the moisture to the north as it moves through. Hopefully
all the smoke and haze will mix out with the change in wind
direction and chances for precipitation next week.
&&
.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 947 PM MDT Fri Sep 5 2025
The main weather element through the period will be smoke. To start
the period, smoke is over a majority of the area, though is notably
less at KCOD and KWRL. Visibilities should generally range from 6 to
10 miles, though KJAC may prevail lower (4 to 8 miles). A shortwave
begins to push in from the southwest Saturday evening. This feature
brings shower and thunderstorm chances to western sites (KBPI, KJAC,
KPNA, KRKS) starting around 23z. There are indications of
showers/storms developing over the mountains west of KBPI and around
KJAC starting around 20z, but have put PROB30 groups in the TAFs for
the more likely onset times. This shortwave also pushes the smoke
towards the northeast, with lessening smoke for southwestern Wyoming
sites (KBPI, KPNA, KRKS) and increasing smoke across northern and
eastern sites (KCPR, KCOD, KWRL).
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...
Air Quality Alert until 1 PM MDT Sunday for WYZ001-002-012>019-
023>030.
&&
$$
UPDATE...CNJ
DISCUSSION...LaVoie
AVIATION...Wittmann

NWS RIW Office Area Forecast Discussion