Clark, Wyoming Weather Forecast Discussion

699
FXUS65 KRIW 061009
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
409 AM MDT Sat Sep 6 2025
.KEY MESSAGES...
- Moisture begins to spread slowly northeast into the state
  today, leading to scattered showers and thunderstorms across
  southwest Wyoming. These showers and storms will be more
  widespread across the forecast area Sunday.
- Warming trend through the weekend and into next week.
- Elevated fire weather conditions across areas east of the
  Divide today.
&&
.DISCUSSION...
Issued at 140 AM MDT Fri Sep 5 2025
Mostly dry and smoky conditions will be in place again today, as the
ridge that has been to the west begins to shift eastward over the
region. As a result of this eastward shift, a shortwave trough will
move over areas west of the Divide this afternoon and increase
moisture levels across this area. This will result in showers and
thunderstorms being possible along and west of a Jackson-to-
Wamsutter line. Any convection is expected to come from Uinta County
and move into southern Lincoln and western Sweetwater counties
around 20Z. Showers and thunderstorms will then become more
widespread through the rest of the afternoon as the trough
approaches the state. Thunderstorms are not expected to be strong or
severe, as SBCAPE values will range between 500-1000 J/kg and the
greater levels of instability will be confined to southern Lincoln
and western Sweetwater counties. Outflow wind gusts up to 40 mph,
brief heavy rain and lightning will be the main threats. This
activity will become isolated after 06Z and is not expected to
impact areas east of the Divide. This will be due to the continued
cyclonic flow that will be in place due to the upper low over the
Upper Midwest. This is the same upper low that is associated with
the cold front that moved through on Thursday. Speaking of,
conditions will be dry east of the Divide today with elevated fire
weather conditions. Critical RH values will return across the
Bighorn Basin this afternoon, with values of 14 to 18 percent
occurring in the Wind River Basin and Johnson and Natrona counties.
Highs will be about 10 degrees warmer, with readings in the lower
80s returning in the central basins. The smoky/hazy skies will
remain in place today.
The ridge axis will be over the Cowboy State by Sunday, with a broad
ridge remaining in place on Monday. The timing for the next
shortwave trough has slowed somewhat and now looks to reach
southwestern MT/the Monida Pass area around 03Z. Additionally, the
remnants from Saturdays shortwave will continue to move over the
state and exit over eastern portions by Sunday afternoon. These
features will result in precipitation chances being more widely
scattered across the forecast area. Once again, severe thunderstorms
are not expected. Even though there will be modest instability,
there is no good upper level support. Any upper level diffluence
associated with the PFJ will be over OR. Otherwise, the warming
trend over areas east of the Divide will continue, along with
elevated fire weather conditions. Winds are expected to remain
light, outside of any outflow boundaries. The smoky/hazy conditions
could begin to improve Sunday as well due to this activity and the
upper level flow pattern shifting.
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 middle to upper
80s returning to areas east of the Divide. Chances for widespread
precipitation look to be in place again Tuesday. This will be due to
a shortwave trough rounding the base of the upper low and moving
over WY through the afternoon and evening. These chances look to
return again Wednesday and Thursday, as a second upper low develops
and digs a longwave trough further south over CA/NV. This will lead
to embedded shortwaves rounding the base of the trough and
increasing moisture from the south. The CA/NV low looks to become
the more dominant feature with widespread precipitation chances
continuing into Thursday.
&&
.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 405 AM MDT Sat Sep 6 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 22Z. 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.
&&
$$
DISCUSSION...LaVoie
AVIATION...LaVoie/Wittmann

NWS RIW Office Area Forecast Discussion