638
FXUS65 KRIW 161820
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
Issued by National Weather Service Billings MT
1220 PM MDT Wed Jul 16 2025

.KEY MESSAGES...

- Wednesday daytime highs east of the Continental Divide will be
  15 to as much as 30 degrees below normal. Highs west of the
  Continental Divide will be seasonal.

- Showers and thunderstorms develop over the Uinta Mountains of
  Utah between 12 and 2 PM Wednesdayand track east-northeast
  across southern Wyoming into the early evening hours. Isolated
  to scattered virga showers/dry thunderstorms will be capable
  of producing outflow gusts 35 to 45 mph.

- Elevated fire weather conditions exist until 8 PM Wednesday across southwest Wyoming.

- Warmer temperatures return Thursday through at least early
  next week. The best chance for afternoon/evening showers and
  thunderstorms exist Friday and Saturday, especially along and
  east of the Continental Divide.

&&

.UPDATE...
Issued at 1216 PM MDT Wed Jul 16 2025

Moist boundary layer conditions in north to northeast upslope
flow has enabled lower cloud decks to persist east of the
Continental Divide Wednesday morning. Low clouds and fog in the
Jackson Valley cleared between 9 and 10 AM as drier air allowed
for abundant sunshine and surface mixing. It will be early
afternoon, probably 2 to 3 PM, before low clouds give way to
sunshine across much of the Wind River and Bighorn Basins.
Clouds will hang on through the afternoon along the Cody
Foothills of the western Bighorn Basin, while Natrona and
Johnson Counties see the cloud cover persist through most of the
evening. Daytime temperatures will be below normal across all
areas east of the Divide Wednesday.

A shortwave in eastern Montana and the presence of the right
entrance region of a departing jet allowed for weak convection
to persist overnight and most of Wednesday morning across the
far north. Showers have gradually been fading as the jet shifts
eastward. Expect there will be enough moisture to generate
afternoon isolated convection over the Bighorn Range Wednesday
afternoon. Attention then shifts to the southern-third of the
forecast area Wednesday afternoon as mid-level moisture remains
elongated along an west-east axis trailing the aforementioned
shortwave. Isolated showers have already formed late Wednesday
morning within this plume. Deeper convection fires over the
Uinta Range between 12 and 2 PM Wednesday, and then moves east-
northeast through the afternoon and evening. A ribbon of
isolated/scattered showers and storms runs to southern Natrona
County through 8-9 PM Wednesday, before fading with the setting
sun. Other locations across the forecast area should be dry as
drier mid-level air has been filtering into the state in west-
northwest flow aloft.

Thursday is looking dry and warmer across the forecast area as
the drier west-northwest flow prevails. There could be some
areas of low clouds that linger along the I-25 corridor
overnight and early Thursday morning, but latest guidance has
these clouds hanging out just to our east. Either way, any cloud
cover in Natrona and Johnson Counties should give way to
sunshine by mid-morning Thursday as surface heating increases.
High temperatures rebound to seasonal or slightly above seasonal
normals Thursday with the exception of cooler temperatures in
the 80s lingering across the far north. Convection again fires
over the Uinta Range early Thursday afternoon, but moisture
appears to be confined to only the far southwest.

.DISCUSSION...
Issued at 309 AM MDT Wed Jul 16 2025

The upper-level trough that drove Tuesdays thunderstorms will
be over the Upper Great Plains by early afternoon, leaving
Wyoming in zonal flow aloft. At the surface, a frontal boundary
will remain stationary along the Divide. East of the Divide,
saturated northerly flow will keep temperatures much cooler than
what we have been experiencing the past week or so.
Temperatures here will be about 5 to 12 degrees cooler today (in
the 60s and 70s), which will be 10-20 degrees below normal for
mid-July.

West of the Divide, unsaturated westerly flow with mean 700-mb
temperatures will keep temperatures largely near normal today.
Deep mixing in conjunction with the already unsaturated westerly
flow, will help drop relative humidity values to around 15-20%
west of the Divide with the lowest values across the Green River
Basin. The surface pressure gradient will remain somewhat tight
west of the Divide, leading to west-northwest winds gusting 20
to 30 mph. Critical fire weather conditions will be marginal and
not widespread, as a result at this time, thus no highlights
are being issued. However, with the westerly winds being
funneled down the Green River Basin, relative humidity values
below 15% is not completely out of the question this afternoon.

An embedded disturbance aloft is forecast to trek across
southern Wyoming late this afternoon and evening and provide
support for isolated to scattered showers and thunderstorms
across Sweetwater County. With mean SBCAPE forecasted around 500
J/kg at its highest and 0-6km shear around 20kts, storms are
not expected to become severe. However, given the dry low-levels
with dewpoint depressions between 30-40 degrees a virga shower
or dry thunderstorm will be capable of producing gusty outflows
around 40 mph.

Ensemble model guidance is in consensus in zonal flow aloft and
weak unsaturated low-level flow on Thursday. This will begin
the first day of a warm up to near to above normal temperatures
across the entire area. By Friday afternoon, an upper-level low
develops over southern California and a trough begins developing
over the Pacific Northwest and this set-up ushers in increased
moisture into the Intermountain West. This increased moisture
aloft and a weak disturbance aloft will support afternoon
isolated to scattered showers and thunderstorms across southern
Wyoming and east of the Divide Friday afternoon. These scattered
shower and thunderstorm chances are expected again on Saturday
across southern Wyoming as another disturbance treks across the
area and a surface boundary situates itself near the southern
Wyoming border. Drier weather and near normal temperatures are
expected to return Sunday through early next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1135 AM MDT Wed Jul 16 2025

East of the DivideKCOD/KCPR/KLND/KRIW/KWRL Terminals.

Terminals east of the Divide continue to be under the influence
of low clouds at the start of the period. For KLND and KRIW,
models indicate that these should scatter out between 19Z and
20Z this afternoon. KWRL should also begin to scatter out around
that time, as satellite is beginning to indicate some breaking
up of lower cloud decks. KCPR and KCOD will hold onto the low
clouds much longer, with KCPR possibly holding onto MVFR decks
through 06Z. Hi-res guidance does indicate a potential brief
clearing at KCPR during the afternoon, but given continued moist
northeasterly flow, would expect this to be only brief, so have
covered the potential with a TEMPO for now. All terminals are
expected to be SKC by 09Z Thursday.

West of the DivideKBPI/KJAC/KPNA/KRKS Terminals.

SKC and quiet conditions will prevail for much of the TAF
period. Convection is expected to initiate off the Uinta
mountains this afternoon and will have the potential to impact
KRKS with most likely timing covered by the PROB30 group.
Otherwise, west-southwest winds gusting to around 20 to 25kts
will be common through the afternoon, diminishing towards
sunset. Light winds and SKC prevail overnight and through the
end of the TAF period.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC
for the latest information on smoke, icing, and turbulence
forecasts.

&&

.FIRE WEATHER...
Issued at 1216 PM MDT Wed Jul 16 2025

Persistent dry, westerly flow will continue to generate
elevated to near critical fire weather conditions across
southern Wyoming between 2 PM and 8 PM through at least
Thursday. Daily minimum relative humidity drops into the teens,
while wind gusts blow at 20 to 30 mph. Fire zones in the BLM
High Desert District will be most susceptible to these
conditions. With borderline relative humidity values, have opted
to not issue highlights Wednesday afternoon. Isolated/scattered
showers and thunderstorms form over the Uinta Mountains each
afternoon, leading to the potential of erratic outflow wind
gusts 35 to 45 mph.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE... CNJ
DISCUSSION...Gerhardt
AVIATION...Hensley
FIRE WEATHER... Gerhardt/CNJ