268
FXUS65 KRIW 132158
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
358 PM MDT Thu Aug 13 2026

.KEY MESSAGES...

- Showers and thunderstorms become widespread across the state
  this afternoon and evening, with most locations having a 60%
  to 90% chance of rain. Convection begins to diminish from
  south to north after sunset.

- Moderate to heavy rain and localized instances of flash
  flooding are the primary threats with showers and
  thunderstorms today, especially in steep terrain and along
  I-25 and east.

- Scattered shower and thunderstorm potential exists Friday
  through Sunday with overall cooler temperatures.

&&

.UPDATE...
Issued at 135 PM MDT Thu Aug 13 2026

The forecast for the afternoon and evening remains on track
with widespread showers and thunderstorms across the state.
Around 11AM this morning, a slow moving thunderstorm tracked
over Casper and dropped 0.5" to 1.5" of rain in less than an
hour which prompted a Flood Advisory to be issued. This is
expected to be the slowest moving storm/storms of the day due to
that convection being focused along a weak stationary boundary
and overall slow storm motion (model soundings signaled 14 to
15kts of cloud layer wind/motion). However, if the Casper area
sees another thunderstorm that produces moderate to heavy rain
rates today, the ground may not be able to handle too much
additional rain. This will need to be monitored throughout the
afternoon and evening.

As of 1PM local time, showers and a few embedded thunderstorms have
started to push into southwestern and western Wyoming. These
showers and thunderstorms are rooted to a vorticity
maximum/shortwave axis and a surface frontal boundary. Shower
and thunderstorm coverage is expected to increase as the system
progresses to the northeast this afternoon, with any given
location having a 60% to 90% chance of rain by 9PM. The 18Z
balloon sounding in Riverton shows 16kts of cloud layer
wind/motion, a deep warm cloud layer, and precipitable water
amounts of 1.01" (well above the 90th percentile). This suggests
that the atmosphere is primed for shower and thunderstorm
activity. Storms are expected to move somewhat faster compared
to what Casper saw this morning due to the system being more
progressive. It is important to note that model soundings show
an increase in the cloud layer wind/motion this afternoon as
well. That being said, instances of moderate to heavy rainfall
rates are possible as this system traverses across the state, so
isolated flash flooding is possible. The areas of most concern
would be urban areas, poor drainage areas, and any steep terrain.
It is important to stay vigilant and not pass through any roads
or drainage areas that may have minor flooding. A bulk of the
shower and thunderstorm activity is forecast to be out of the
forecast area/Johnson County by 2AM Friday. Lingering light rain
showers remain possible (up to 30% chance) through the
overnight hours across northwestern Wyoming.

&&

.DISCUSSION...
Issued at 152 AM MDT Thu Aug 13 2026

Chances for precipitation will be widespread across the CWA today.
The threat for heavy rain and localized flash flooding is still in
the forecast, with areas east of the Divide having the better
chances. Precipitable water values will be higher here, ranging
between 1 and 1.3". WPC Excessive Rainfall Outlook continues to have
a Slight risk in the forecast for Johnson County and portions of
Natrona County (mainly along and east of I-25). Showers will move
northward over Lincoln County and the Green River Basin after
sunrise and spread over western portions through the rest of the
morning. Scattered showers and thunderstorms will develop across the
rest of the CWA between 18Z and 20Z, moving toward the northeast
through the afternoon. Convection will begin to decrease by 00Z,
ending from southwest to northeast through the evening. Light
showers will still be over northern portions by 06Z and linger
through the rest of the night. Thunderstorms will be possible across
the area this afternoon, with portions of Lincoln and Sublette
counties having the best chances as a shortwave from NV/UT
moves over this region.

Convection will not be as extensive Friday, being more scattered
through the day along and north of a Afton to Midwest line. Heavy
rain and localized flash flooding will continue to be the main
threat from these storms as PWAT values remain abnormally high,
ranging between 0.7" to 1.1". The hazy/smoky skies looks to no
longer be an issue by Friday, as the southwest flow aloft cuts off
the source region and the atmosphere becomes more mixed.

The cooler and wetter pattern will continue through the
weekend, with showers and thunderstorms possible each day. The
threat for heavy rain will not be as strong by Sunday, as a bit
more drier air filters into the region. More drier conditions
look to return by Monday, but temperatures are trending to
remain seasonal for the early part of next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 351 PM MDT Thu Aug 13 2026

A system will continue to bring showers and thunderstorms to the
area through much of the period. Rain will be heavy at times, and
occasional MVFR to IFR conditions are expected at terminals.
Thunderstorms will carry an outflow wind gust concern, though wind
will otherwise remain light. The best chance for showers and storms
will be before 02Z as this activity moves northeast. Coverage
decreases overnight, though isolated showers will linger especially
across western Wyoming through Friday morning. MVFR ceilings are
expected at KCOD and KWRL overnight as the rain comes to an end
there. Frequent mountain obscurations are expected through the
period. Additional convection is expected to develop after 18Z
across western Wyoming, with the best chances for terminal impacts
at KBPI, KCOD, KJAC, and KPNA.

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...

&&

$$

UPDATE...Gerhardt
DISCUSSION...LaVoie
AVIATION...Myers