253
FXUS65 KRIW 060154
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
754 PM MDT Tue Aug 5 2025

.KEY MESSAGES...

- Critical fire weather remains probable from Wednesday through
  Saturday, with Thursday possibly the most concerning day due
  to the passage of a mostly dry cold front.

- Temperatures will remain above normal through Thursday,
  cooling for Friday and Saturday.

&&

.UPDATE...
Issued at 1222 PM MDT Tue Aug 5 2025

No major changes from the previous forecast (see full discussion
below). Focus remains on the elevated to critical fire weather this
week, with Thursday/Friday being the most impactful days as a cold
front moves through. Current water vapor depicts the shortwave
moving eastwards over Montana. No precipitation is expected today,
though an isolated shower/storm (15%) could pop-up over eastern
Johnson County this afternoon.

With the cold front passing late this week, highs Friday drop 10 to
15 degrees compared to Thursday. Nighttime lows will also drop for
Friday night/Saturday morning. The Green River Basin (places like
Big Piney and Pinedale) currently have forecasted lows around 34
degrees, so sensitive plants may be impacted. Lows are in the mid-
30s for Star and Jackson Valleys.

&&

.DISCUSSION...
Issued at 222 AM MDT Tue Aug 5 2025

The main forecast concern today and for the next few days reminds me
of one of my favorite bands from the 70s, Earth, Wind and Fire. In
that, the combination of low humidity and gusty Wind could help
any Fire to spread and scorch the Earth. Anyway, lets dive into
the details.

We have some showers lingering until around midnight tonight,
but these have moved into Montana. Much drier air and ridging
will control the weather across the area today, with
precipitable water values generally running anywhere from 60 to
80 percent of normal. This means a dry day for a vast majority
of the area. We can`t completely rule out a stray afternoon
shower or thunderstorm in far eastern Johnson County. However,
with the chance generally less than 1 out of 10, we left it out
of the forecast. Now for fire weather concerns. Humidity is a
slam dunk here, with many areas seeing relative humidity below
15 percent with some locations under 10 percent. The question is
wind. This looks very likely across the previously warned
southern zones (greater than a 4 out of 5 chance). The question
was across Fremont and Natrona Counties for this afternoon.
Using the NBM ensemble, we see that much of this area has at
least a 3 out of 5 chance of a 3 hour period of wind gusts above
25 mph. With this, we have upgraded the Watch to a Warning.

Dry weather will continue on Wednesday. Somewhat of a ridge axis
will be over the area on this day, cutting wind somewhat, mainly in
central Wyoming. This is why we chose not to continue the Red Flag
Warning into Wednesday for central areas, as the chance of reaching
the wind criteria is less than 1 out 3. Critical fire weather
conditions will likely continue across southern Wyoming though.

The day I am really concerned about is Thursday. A cold front will
begin to approach the area from the north and west along with an
associated upper level low. This will tighten the pressure gradient
across the area, especially in the favored southwest flow areas,
like Rock Springs to Casper and even the Wind River Basin. This will
also raise temperatures, making this the hottest day of the week.
Highs will be well into the 90s east of the Divide, with areas
like Casper possibly making a run to near 100. This will also
lead to widespread relative humidity below 10 percent. Many
locations across southern Wyoming also have at least a 1 in 2
chance of wind gusts past 35 mph. So, of all the days this week,
at this point Thursday looks like the worst fire day of the
period.

The cold front will move across the area on Friday. This will lower
temperatures somewhat and also raise humidity a bit. However, it
still looks critical across the southern half of the area, with the
NBM ensemble mean progging humidity values at 15 percent or
lower. Wind looks to be of similar speeds on Friday as well.
So, although it may be a bit better with the higher humidity (a
relative term for sure), things still look critical for many
areas. And some guidance, especially the ECMWF, is showing a
few showers and thunderstorms. The surface looks very dry
though, with some locations having less than one half of
climatological normals of precipitable water. That means any
storms would be high based with little or no rain. They could
be capable of very strong wind gusts though, as some dewpoint
depressions could approach 70 degrees.

Conditions should begin to improve on Saturday. Humidity will begin
to creep higher again, especially in central Wyoming. Wind will also
begin to decrease. It could still be breezy enough for some
additional Fire Weather highlights though, but this would be mainly
across southern Wyoming. By Sunday, wind should be light enough in
all areas to have no critical fire weather. The first part of
next week looks mainly dry with near normal temperatures.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 412 PM MDT Tue Aug 5 2025

Areas of CU will continue across the forecast area to start the
forecast period, dissipating through the evening and leaving
SKC skies by 06Z. Any gusty winds will also decrease by 02Z.
Similar conditions return Wednesday afternoon, with some of the
gustier winds (over 20 kt) occurring by 22Z. Conditions will
remain dry through the period, although there is a 10% or less
chance of a virga shower near KWRL Wednesday afternoon.

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 222 AM MDT Tue Aug 5 2025

A combination of a gusty breeze, humidity falling under 15
percent and hot temperatures will bring elevated to critical
fire weather to central and southern Wyoming this afternoon and
evening. Critical fire weather looks likely through at least
Friday and possibly into Saturday as well. The most concerning
day looks to be Thursday, when a dry cold frontal passage may
bring wind gusts to around 40 mph.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT Friday for WYZ277>279-288-289-
414-416.

&&

$$

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