687
FXUS65 KRIW 061709
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1109 AM MDT Sat Sep 6 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms for southwestern Wyoming
  after noon through sunset. Atmospheric moisture is above
  normal, and storms will be slow moving, leading to a small
  chance (5 to 10%) of brief flooding of dry washes.

- Near seasonal temperatures today, warming to about 5 degrees
  above normal for Sunday through Wednesday. Cooldown expected
  later in the week.

- Isolated showers and thunderstorms expected Sunday, with
  basins having a 10 to 20% chance of seeing measurable
  precipitation in the vicinity.

&&

.UPDATE...
Issued at 1055 AM MDT Sat Sep 6 2025

Looking at 1200Z high-resolution model guidance, the previous
forecast remains largely on track. MLCAPE values west of the
Continental Divide, where showers and thunderstorms are
forecast, are notably lower than surface based values, showing
150 to 250 J/kg, meaning storms are unlikely to be strong. The
additional atmospheric moisture over southwestern Wyoming will
keep inverted V soundings from being quite as wide, with smaller
T-Td spreads at the surface, leading to outflow winds being
more mellow, likely gusting 35 to 45 mph in general. Since
storms will be slow moving, there is a small chance (5 to 10%)
for isolated flooding in normally dry washes and locations with
poor drainage. Thunderstorms should quickly diminish after
sunset, but a few showers could persist until around midnight.

Despite low afternoon relative humidities (12 to 18 percent) east
of the Divide, surface winds this afternoon should be minimal,
keeping the fire weather conditions there more benign. In the
southwestern part of the state where gusty outflow winds are
more likely, the air mass will be more moist, keeping relative
humidities higher.

&&

.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 /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1109 AM MDT Sat Sep 6 2025

Smoke continues across much of the area, with visibilities from 4
to 10 miles. To start the period, smoke is most concentrated across
western Wyoming (KBPI, KJAC, KPNA). A shortwave moves through late
today through tonight from the southwest. This will push the smoke
towards the northeast, with lessening concentrations over the west
and southwest, and increasing over the north and east sites (KCPR,
KCOD, KLND, KRIW, KWRL). Though some smoke will still remain, the
thickest should be exiting to the northeast late Sunday morning into
Sunday afternoon.

The aforementioned shortwave also brings shower and thunderstorm
chances for western sites, (KBPI, KJAC, KPNA, KRKS) mainly between
22z and 06z. A few earlier showers and storms are possible over the
mountains west of KBPI and around KJAC, starting about 20z. PROB30
groups were included in mentioned TAFs to account for these 20% to
40% chances. Gusty outflow wind up to 30 knots is the main hazard.
Notably, as this wave moves to the northeast. there could be some
gusty winds coming off the mountains west of KCOD, so have included
wind shear there starting about 03z.

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...VandenBoogart
DISCUSSION...LaVoie
AVIATION...Wittmann