060
FXUS65 KRIW 090347
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
947 PM MDT Fri Aug 8 2025

.KEY MESSAGES...

- Clear skies in the wake of a cold front will lead to freezing
  temperatures tonight for areas in the western basins and
  valleys, especially for the Upper Green River Basin.

- The threat of critical fire weather decreases Saturday, as
  winds decrease, and should come to an end on Sunday.
  Temperatures increase next week.

&&

.UPDATE...
Issued at 1236 PM MDT Fri Aug 8 2025

The two main considerations with this update are fire weather for
tomorrow (Saturday) and freeze potential for tonight. Will start
with fire weather. The focus is on southern Wyoming, mainly southern
Lincoln and Sweetwater Counties. Relative humidities (RH) see
little change compared to today, but winds decrease. Current models
have winds in those mentioned locations right around 25 mph for a
few hours in the afternoon. Favoring hi-res guidance, (like the
HRRR) have decided to hold off on extending the current Red Flag
Warnings into tomorrow for fire zones 277 and 279. The HRRR has a
few hours of localized gusts 25, to maybe 30 mph, as a subtle trough
pushes south. This would be the period where critical fire weather
would be most probable, but the HRRR keeps it as localized gusts and
only for a few hours. The overnight shift can take one last look to
see if models increase either the widespread higher winds or
increases the duration of gusty winds.

Now to freeze potential. The areas of interest are the lower
elevations west of the Divide, including the Green River Basin, Star
Valley, Jackson Valley, and southern Lincoln County. Factors in
favor of cold temperatures are the dry air and forecasted clear sky.
Currently, the Green River Basin looks most probable to have
temperatures reach 32 degrees, with about 60 to 70 percent chance at
Big Piney and Pinedale. Better chances occur elsewhere in the basin,
especially along any rivers. Therefore, have issued a Freeze Warning
for temperatures of 32 degrees or less. The chance for widespread 28
degree temperatures is 10 percent. For Jackson and Star Valleys, NBM
v4.3 mean temperatures are 33 to 35 degrees, with NBM v5.0 a
little colder at 32 to 34 degrees. The coldest temperatures are
located in northern Jackson Valley and around Afton. Confidence
is just not high enough in widespread 32 degrees, so have held
off on freeze highlights for these valleys. Would currently give
Jackson a 40 percent chance of hitting 32 degrees. It should be
noted that temperatures right near the surface can be up to a
few degrees colder than the air temperature, so some frost is
possible in colder areas. Be prepared if you have sensitive
outdoor plants. Another location is in and around Cokeville.
This tends to be a colder spot, so temperatures around and a bit
below 32 degrees is likely (80%). The rest of the lower
elevations of southern Lincoln County should remain above 32
degrees.

&&

.DISCUSSION...
Issued at 233 AM MDT Fri Aug 8 2025

The headline of the National Weather Service Daily Weather edition
today continues to be fire weather concerns. We have a cold front
moving through the state right now, and it has produced some strong
wind gusts, including a few over 50 mph in Park County. At the
humble Riverton weather abode, we have had some gusts over 40 mph.
These should decrease somewhat toward morning.

However, it won`t last as the atmosphere mixes through the morning
and cold advection kicks into the area. This will be aided by a 95
knot jet streak that will be dropping southward across the area. The
result will be another day of gusty wind, but a bit more widespread
than yesterday with the jet support. Most of the area has at least a
1 in 2 chance of wind gusts past 25 mph today, with at least a 1 in
2 chance of gusts past 35 mph across the Rock Springs to Casper
Wind Corridor today according to the NBM ensemble. The limiting
factor for critical fire weather will be the cooler
temperatures and somewhat higher humidity. This is especially
the case across northern Wyoming. Elevated fire weather is
likely with the gusty wind, but humidity has more than a 1 in 5
chance of being under 15 percent for three hours. So, no
highlights for this area. This will not be the case in central
and southern Wyoming. So, we left the the Red Flag Warning as is
for now.

The other concern is convection. We do have some showers on the
radar now, but little is reaching the ground. The main threat of any
convection will be from the Bighorns and eastward, where there will
be greater upper level divergence from the left front quadrant of
the jet streak and somewhat deeper moisture. One or two are also
possible near the Absarokas. All convection should end sunset.

The next concern is the chilly low temperatures that will accompany
the dry airmass as it moves into the western valleys. Chances have
backed off somewhat today. The area with the best chance in the
lower elevations looks to be the western Green River Basin with
around a 1 in 2 chance anywhere from Daniel to Marbleton as the cold
air drains with the clear sky and calm wind after the sun sets and
the wind decouples. Another area of concern is the Bear River Valley
around Cokeville, another location with a 1 in 2 chance. In
other areas the chance is only at most 1 out of 3. The chance of
a hard freeze, ending the growing season, is at most, a 1 out
of 4 though. We will hold off on any highlights given the Red
Flag Warning, but the day shift may want to think about some if
trends continue.

Things are somewhat quieter on Saturday. We still have some fire
weather concerns to think about though. They would be more
restricted though, mainly to Sweetwater and southern Lincoln
Counties. Single digit relative humidity will be possible once
again. However, wind looks borderline at this time. There is a
greater than 1 in 2 chance of wind gusts past 25 mph. However,
at this time it looks like a brief time and may not reach 3
hours. We will not extend at this time, but the day crew or
tomorrow`s midnight shift can take another look. And, there
could be just enough mid level moisture for an isolated
thunderstorm in far northwestern Wyoming. The chance is only
around 1 in 10 though, so we left things dry for now.

Fire weather concerns should end Sunday, as the wind finally
decreases. Mainly dry weather with near to slightly above normal
temperatures are expected for Sunday and Monday. Fire weather
concerns may return Tuesday or Wednesday as another front and low
approach, but there has been poor consistency regarding this
system so confidence and timing this far out remains low.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 945 PM MDT Fri Aug 8 2025

SKC skies will be in place through 12Z, before clouds increase
across areas west of the Divide. SCT-BKN FL150-250 will continue
over this area through the rest of the day Saturday, spilling east
of the Divide after 18Z. BKN-OVC skies will be in place across
central and western portions between 21Z and 02Z. Virga showers will
be possible over these areas as well during this time, with KLND
having the best chance (20%). Rain showers will be possible (30-40%)
over far northern portions, but are not expected to impact KCOD
between 01Z and 04Z.

Winds will not be as strong Saturday, with gusts of 18 to 22 kt
occurring for the terminals west of the Divide, including KCPR.
These winds will subside in the evening by sunset. Otherwise, winds
will be light (13kt or less) through 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 233 AM MDT Fri Aug 8 2025

Gusty winds with a cold front and continued low humidity will
bring another day of critical fire weather across the southern
half of the area. Elevated fire weather is likely most other
locations, but either wind will not be strong enough or humidity
should remain largely above 15 percent. Elevated to possibly
critical fire weather will be restricted to southern Wyoming on
Saturday and then end on Sunday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Warning from 1 AM to 8 AM MDT Saturday for WYZ025-026.

&&

$$

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...LaVoie
FIRE WEATHER...Hattings