240
FXUS65 KRIW 280116
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
716 PM MDT Thu Aug 27 2026

.KEY MESSAGES...

- Showers and thunderstorms continue to move across the state
  this evening. Frequent lightning and gusty winds will be the
  main hazards in any thunderstorms. Showers may linger into the
  morning hours Friday, especially across western WY.

- Confidence is increasing in widespread showers and
  thunderstorms Friday afternoon through Sunday morning. The
  greatest chances exist across west and northwest Wyoming late
  Friday through midday Saturday.

- Near to above normal temperatures through Saturday. Daily
  shower and thunderstorm chances (20% to 40%) exist the first
  half of next week with widespread cooler temperatures,
  including lows in the 30s for western Wyoming.

&&

.UPDATE...
Issued at 1249 PM MDT Thu Aug 27 2026

Showers and thunderstorms have begun to increase across western
WY as expected. While stronger thunderstorms are unlikely, there
may still be some locally gusty winds and frequent lightning
through this evening. With showers moving east quickly, overall
accumulations in any one place will remain low.

Main moisture surge continues to approach, arriving across
western WY tomorrow morning. Overall QPF values only adjusted
slightly, with the main focus still on the NW Parks Friday
through Sunday. Percentage chances over a half inch are still
over 90% for most of NW WY, with 50% chance for an inch of rain.
Precip chances outside of higher elevations remain much lower,
with most areas east of the Divide holding around 0.10". Some
slight increases were noted across Sweetwater County, where
potential for a half inch has increased to around 50%.

&&

.DISCUSSION...
Issued at 247 AM MDT Thu Aug 27 2026

An upper level high over has been rooted over the southwest CONUS
over the past few days. The associated ridge axis is currently (230
AM) positioned over far western Wyoming, where it has been for the
past day or so. The ridge axis will begin to slowly shift east
across Wyoming today and bring another day of widespread warm
temperatures. With subtle height rises as the ridge progresses east,
temperatures east of the Continental Divide are forecast to be in the
high 80s to mid 90s. Western and southern Wyoming will likely see
widespread highs in the 80s. However, today`s forecasted highs
across western Wyoming could be a few degrees too warm due to
increased cloud cover this afternoon.

The increased cloud cover is a result of an influx of moisture with
a shortwave disturbance on the upstream side of the ridge this
afternoon and evening. Much like the past few days, showers and
thunderstorms are expected to initiate across the higher terrain of
western and southern Wyoming this afternoon and move east. Model
forecast soundings are indicating DCAPE values ranging from 700 J/kg
across western Wyoming to 1500 J/kg in central and southern Wyoming.
Furthermore, dew point depressions across central and southern
Wyoming are in the 45 to 55 degree range. Thus, another day of gusty
outflows will be possible in any shower or thunderstorm across
central and southern Wyoming late this afternoon. A couple gusts
could be closer to that 55 to 60 mph mark, but most gusts will be in
the 35 to 45 mph range. Unlike the past few days, shower and
thunderstorm coverage is expected to be greatest across west and
northwest Wyoming. This is due to those areas being in better
proximity to the highest moisture content. Showers and thunderstorms
are largely forecast from 1PM through at least 10PM, with light rain
possible across the northern half of Wyoming until 2AM Friday.

By Friday, the upper level ridge will be positioned over the central
CONUS, allowing for flow to be predominately southwesterly. This
deep southwest flow will advect deep Pacific moisture into the
region as embedded shortwaves traverse the state. Shower and a few
embedded thunderstorms will be possible (up to 30% chance) as early
as sunrise Friday across northwest Wyoming as the nose of the
Pacific moisture arrives. These shower and thunderstorm chances will
only increase through the day on Friday as a continuous stream of
Pacific moisture advects into the state. Showers and thunderstorms
are expected to initiate and be most numerous over the higher
terrain in the afternoon and move east/northeast into the lower
elevation areas. PWATs are expected to increase across the entire
state through the afternoon and evening as southwest flow
strengthens. Ensemble guidance indicates that PWATs will increase
from the the 90th percentile of climatology in the morning to above
the 99th percentile of climatology by Friday afternoon, especially
across west and northwest Wyoming. This provides increased
confidence in widespread showers and thunderstorms bringing at least
a few hundredths of rain for many locations. The increased cloud
cover across the state will also limit how warm we get on Friday,
with forecasted highs in the 70s and 80s.

The active weather will continue Friday night through Saturday
afternoon as a cold front traverses across the area. There will be
plenty of available moisture, lift, and support aloft for widespread
showers and thunderstorms. The highest chances (90%+) are still
across the western half of Wyoming due to remaining in better
proximity to the greatest moisture content. The current forecast has
anywhere from 0.5" to 1.0" of rain between 6AM Friday through
Midnight Sunday morning (42 hours) across Yellowstone, the Teton and
Gros Ventre Mountains, the northern Wind River Mountains, and
Jackson Valley. Amounts during this same timeframe elsewhere across
the state range from 0.01" to 0.40". Showers and thunderstorms are
not forecast to be slow moving. However, if multiple showers or
thunderstorms with moderate to heavy rainfall rates go over the same
area in a short amount of time, localized flash flooding is possible
across west and northwest Wyoming.

Showers and thunderstorms are expected to persist Saturday night
through Sunday afternoon, although coverage will gradually decrease.
This is decreasing trend will largely be a result of westerly flow
cutting off the deeper moisture. Flow becomes more westerly and
strengthens ahead of a shortwave trough rounding a Pacific Northwest
low. This shortwave and an associated cold front is forecast to trek
across the Intermountain West Sunday afternoon. This cold front is
forecast to come with a "punch" of cold air, especially west of the
Continental Divide. There is a 60% to 90% chance of temperatures on
Monday morning falling into the 30s across the Green River Basin and
the western valleys. For the higher elevations, there is a greater
than 90% chance of temperatures falling below to or below freezing
on Monday morning.

Ensemble guidance is in consensus that multiple shortwaves will
traverse across the state Monday through Wednesday. Moisture does
not look to be anything of note at this time so the current forecast
holds a 20% to 40% chance of showers and thunderstorms each
afternoon Monday through Wednesday. Temperatures also look to be
near to below normal for the end of August and the start of
September.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 429 PM MDT Thu Aug 27 2026

Showers and thunderstorms will again be the primary aviation concern
this period. This activity will continue to move northeastward
through the evening. Brief heavy rain may bring occasional MVFR
visibilities, though the main concern continues to be strong outflow
wind gusts of 30 to 40 knots. While most of this will decrease after
sunset, lingering showers will be more widespread tonight,
especially across western Wyoming. A broken VFR cloud deck is
expected for most terminals overnight. The next wave will follow
quickly, with more persistent showers expected to begin at KJAC by
16Z Friday morning. Showers and thunderstorms will become more
widespread after 18Z, with a PROB30 group now added for all
terminals for Friday afternoon.

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

&&

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

&&

$$

UPDATE...Straub
DISCUSSION...Gerhardt
AVIATION...Myers