661
FXUS65 KRIW 281110
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
510 AM MDT Thu Aug 28 2025

.KEY MESSAGES...

- Another cooler day with shower and isolated thunderstorm
  chances (50-90%). The greatest coverage of showers and
  thunderstorm is forecast across northwestern Wyoming and east
  of the Divide.

- More scattered shower and thunderstorm chances (30-70%) Friday
  and Saturday east of the Divide.

- Temperatures increase to near to slightly above normal Sunday
  into early next week with limited precipitation chances.

&&

.DISCUSSION...
Issued at 200 AM MDT Thu Aug 28 2025

08Z water vapor imagery shows a negatively tilted upper-level ridge
over the Intermountain West and an upper-level low center over the
Pacific Northwest extending into Idaho and Montana. As today
progresses, a building upper ridge over the Great Basin will push
the low north and Intermountain West ridge east. With enhanced
vorticity aloft and deep moisture, another day of rainy weather is
forecast. However, differing from Wednesday, the greatest coverage
of showers and isolated thunderstorms will be east of the Divide,
where support aloft is maximized and mean PWATs generally range from
0.75 inch to 1.2 inch. Showers and isolated thunderstorms will also
be common across western Wyoming, including Yellowstone National
Park, given favorable upsloping flow. Additionally, an upper-level
wind maximum rounding a Four Corners upper-high will be present over
southern Wyoming this afternoon along with a tightening pressure
gradient. This translates to frequent 20 to 25 mph gusts across
southern Wyoming.

On Friday, the upper-level Great Basin ridge continues to build
northward and the upper-low is forecast to roughly be centered over
the Montana/Canada border. This leaves Wyoming in deep northwest
flow. The low`s surface reflection is expected to trek southeast
from Montana into the South Dakota/Nebraska area, focusing shower
and thunderstorm chances east of the Divide. The greatest rainfall
amounts, generally around 0.25 inches, will be across the northern
Bighorn Mountains due to upsloping. By Saturday, a shortwave will
provide support aloft for scattered showers and potentially
thunderstorms east of the Divide during the afternoon and early
evening hours.

There is consensus among ensemble guidance members that the upper-
level ridge will continue to build across the western CONUS Sunday
and remain through much of next week, keeping the Intermountain West
in northwest flow aloft. Associated height rises at the surface and
a lack of moisture will limit precipitation chances Sunday through at
least the middle of next week. Despite northwest flow aloft, a warm
up to near to slightly above normals looks favorable during this
timeframe.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 510 AM MDT Thu Aug 28 2025

Mainly VFR conditions to start the period with tempo conditions
at JAC for lower ceilings into the IFR status with the lower
stratus advecting in from the southwest. This should linger
around through 15Z before lifting to VFR with all other TAF
sites for the remainder of the period. Otherwise, some breezy
winds in the afternoon and early evening west of the Divide as
well as CPR gusting up to 18kts, likely highest at RKS over
20kts possible.

Mid level ceilings through much of the day with increasing rain
shower/storm activity for all but RKS affecting the northern
half of the CWA today. Lower confidence for BPI/PNA carrying
VCSH with all other locations PROB30 groups. Earliest timing at
JAC after 20Z spreading to COD/WRL after 21Z, RIW/LND after 22Z,
and off to CPR after 00Z. These should linger for at least a
couple hours but confidence wanes (20-30%) on timing length
dependent how fast steering flow becomes in the afternoon.
Regardless, lifting ceilings scattering out into the upper
portions of the mid levels (12-15kft) with lower ceilings
around 5kft lingering for JAC overnight into Friday morning.

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.

&&

$$

DISCUSSION...Gerhardt
AVIATION...Lowe