415
FXUS65 KRIW 292306
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
506 PM MDT Fri Aug 29 2025

.KEY MESSAGES...

- Isolated to scattered shower and thunderstorm chances (up to
  70%) along and east of the Divide and across southern Wyoming
  today. Isolated heavy rain is possible across Johnson County.

- Isolated to widely scattered shower and thunderstorms chances
  (up to 70%) again on Saturday. The greatest chances (50%-70%)
  are across Johnson and Natrona Counties.

- A warmer and drier trend Sunday through Tuesday of next week.

&&

.UPDATE...
Issued at 1216 PM MDT Fri Aug 29 2025

No major changes to the forecast with this update. Have
adjusted precipitation chances with the latest hi-res guidance.
Overall, shower/storm coverage has decreased a little. Models
are in high consensus for three main features. The first is
storms across Johnson County between noon and 8pm MDT today.
There is also good consensus for showers across southern
Sweetwater County through late this evening, which could bring
outflow gusts to around 40 mph. The third feature is a cluster
of showers and storms forming around the Fremont/Hot Springs
County line this evening (roughly 6pm, though there are model
differences up to a couple hours in either direction); this
feature then tracks eastwards through Fremont and Natrona
Counties overnight, exiting the Casper area around 8am tomorrow.
The HRRR in particular highlights this feature, with lesser
agreement in the other models.

&&

.DISCUSSION...
Issued at 224 AM MDT Fri Aug 29 2025

An upper-level ridge extending from New Mexico to the Pacific
Northwest and an upper-level low over eastern Montana, both evident
on 08Z water vapor imagery, will be the main players for our weather
today and Saturday. There is consensus that the upper-level ridge
will continue to build west of Wyoming today as the low expands into
into the Dakotas. This leaves Wyoming in deep west to northwest
flow. Two separate areas of increased support aloft, vorticity
maxima, will be present today. One area will be across southern
Wyoming and the other is associated with the upper-low for areas
along and east of the Divide. Thus, these will be the areas of
interest today for scattered showers and isolated thunderstorms.

The greater deep moisture will be associated with the upper-level
low so areas along and east of the Divide have a greater chance (40%-
70%) at seeing more rain shower activity through the day. Brief
periods of heavy downpours are possible across the aforementioned
area, however, the chances of widespread rain amounts over 0.10" are
overall less than 30%. The exception is east of the northern Bighorn
Mountains (Johnson County), where moisture is greatest (mean PWATs
around 1.0") and northerly low-level flow favors upsloping. There is
a 60%-80% chance of at least 0.25" today and tonight for the
aforementioned area. Across southern Wyoming, shower and thunderstorm
chances (15%-30%) are much lower compared to the northern half of
the state and will likely be more isolated in nature and diurnally
driven. Dewpoint depressions across southern Wyoming peak around 40
degrees F, so isolated 40 mph gusts cannot be ruled out during the
afternoon. Temperatures across the area are forecast to be slightly
below to near normal (70s).

A somewhat similar pattern is forecast again on Saturday, with the
only difference being that the upper-level ridge builds and shifts
east into Wyoming. This ultimately will keep shower and thunderstorm
chances widely scattered along and east of the Divide. Temperatures
on Saturday are forecast to be similar to today with more sunshine.

Ensemble guidance remains in consensus that the upper-ridge will be
over Wyoming with high pressure at the surface Sunday through much
of next week. With limited moisture, dry weather and near to
slightly above normal temperatures look to return, despite weak
disturbances aloft each day. The day that could be the exception is
Wednesday. There are indications that on Wednesday there could be a
slight uptick in available moisture along a frontal boundary for
areas along and east of the Divide. However, it is unlikely that we
will see any substantial precipitation with this system at this time.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 504 PM MDT Fri Aug 29 2025

Scattered showers and a few isolated thunderstorms continue
across portions of southern and northern WY this evening. As
the shortwave moves through, additional convection still looks
possible (20-30%) across the Wind River Basin, developing near
KRIW, which will slide eastward overnight, impacting KCPR with
showers after 09Z with showers potentially lingering into mid-
morning. A little better chance(30-40%) for scattered showers
and storms exists at KWRL overnight. Winds remain light (less
than 12kts) for most terminals through the period. Similar weak
convection across similar locations is expected on Saturday
afternoon and evening, but confidence is too low to include for
most terminals at this time.

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...Wittmann
DISCUSSION...Gerhardt
AVIATION...Hensley/Rowe