338
FXUS65 KRIW 021745
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1145 AM MDT Wed Sep 2 2026

.KEY MESSAGES...

- Trending drier today with widespread near critical fire
  weather across much of the area this afternoon with lower
  humidity as low as 15 to 25 percent and gusty southwest winds
  up to 30 to 45 mph.

- Thursday still looks to be the day of most concern with the
  lowest humidity as low as 10 to 15 percent. The fire weather
  watch has been upgraded as a result to red flag warnings.
  Further near critical fire weather is possible on Friday into
  Saturday.

- Monsoonal moisture returns later this weekend into early next
  week with an increased chance of showers and thunderstorms,
  but details on placement of the most numerous showers remains
  uncertain.

&&

.UPDATE...
Issued at 1145 AM MDT Wed Sep 2 2026

Mid-day convection has kicked off across the Winds and northern
Sweetwater County, and HiRes models did not do well on
initiationfor both timing and location. The NAMNest has come
around to it finally, with HRRR shortly behind as per usual.
Some activity to the west as well and will continue through the
afternoon and early evening hours. Not much in the way of
instability for anything stronger to severe, but gusty outflows
over 40 mph and frequent lightning the main hazards. These will
wane towards sunset with nothing of note overnight into
tomorrow.

Otherwise, fire weather watches for Thursday afternoon and early
evening have been upgraded to red flag warnings with good
confidence on low humidity values and gusty winds. East of the
Divide will be the best areas of concern in regards to fire
weather. Friday and Saturday afternoons will not be quite as
windy but still elevated to near critical conditions to be re-
evaluated after Thursday if any products need to be issued. The
system for late Sunday and into next week still uncertain at
this time, but timing continues to be pushed back a little each
model run. Stay tuned for updates in the coming days.

&&

.DISCUSSION...
Issued at 222 AM MDT Wed Sep 2 2026

My favorite thing in the whole world (note my sarcasm) is
continuing this morning, nocturnal monsoonal convection, with
the models doing their usual great job with it (more sarcasm).
They are gradually weakening though. A couple of showers may
linger into the morning but this would be isolated. The trend
today will be for convection chances to trend downward through
the day as much drier air punches in from the north and west.
There may be enough moisture for a stray shower or thunderstorm
across southern Wyoming this afternoon, but the chance is small,
generally less than 1 out of 5. We have some slight POPs in
this area to cover for this. However, our attention will turn to
something else.

And that is fire weather. The pattern will be the same for the next
three days or so. The set up is the same as was discussed yesterday
Wyoming will be roughly the midpoint between an upper level low off
the Pacific Northwest coast and riding over the southeastern United
States. As a result we will have a fast flow over the area. The 700
millibar winds are fairly strong for this time of year as well,
generally peaking at 35 to 40 knots. Much of the eastern half
of the area has at least a 1 in 2 chance of wind gusts past 30
mph as a result. The limiting factor for fire weather today is
humidity. Mean relative humidity among the ensemble members is
generally above 15 percent, although it does get close in
Johnson County where it drops to around 17 percent. So, for now,
we will not issue any fire weather highlights. However,
elevated to near critical fire weather is likely today for most
locations East of the Divide. Temperatures will be rather warm
as well, generally about 5 degrees above normal with the warmer
spots like the eastern Bighorn Basin making a run at 90 degrees.

Thursday looks like most concerning day for fire weather. This will
be the driest day across the area, with humidity generally running
around 5 percent lower than on Wednesday. Wind will be a bit lighter
though, generally be around 5 mph. However, there is still a greater
than 1 in 2 chance of wind gusts past 25 mph across much of the
area, With this, we will issue a fire weather watch across most of
the area East of the Divide as well as most of southern Wyoming,
although humidity will be a bit more borderline in this vicinity.
The chance of showers looks to be close to zero as well, with the
only place with a chance the far northwest where the chance is less
than 1 in 5 and largely in the morning. On Friday, we will still
have the wind. However, humidity looks somewhat higher,
especially across southern Wyoming. We may need additional fire
weather highlights on this day but will hold off for now since
things look more borderline.

As we head into the weekend, humidity will continue to creep up
and wind should decrease somewhat on Saturday, and this should
be the end of the critical fire weather. The next question is
about the next surge of monsoonal moisture and any possible
interaction with a tropical cyclone in the western Pacific.
Models trends are still slower, with most holding off until
Sunday now, and continuing into Monday. Most guidance also takes
the tropical cyclone further off of the coast, limiting the
moisture somewhat. However, some guidance still gives over an
inch of precipitable water for Sunday and Monday. Details the
placement of the most numerous showers are still uncertain given
it is still several days out though. Precipitable waters are
still expected to climb to over an inch though, so some locally
heavy rain is still possible. As for temperatures, with the
ridge still expected to retrograde closer to the area, expect
above somewhat above normal temperatures to continue into the
weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1137 AM MDT Wed Sep 2 2026

Isolated to scattered showers and thunderstorms are expected
initiate off the higher terrain and move northeast this afternoon.
There is higher confidence (30 percent) that KCOD, KCPR, and KRKS
will see a shower or thunderstorm Wednesday afternoon. All other
terminals have a 10 to 20 percent chance of seeing convection at or
around the terminal. The main impact in any shower or thunderstorm
will be gusty 35 to 50kt winds and brief instances of lower
visibilities. Convection chances diminish quickly after sunset,
except at KCPR where light rain is possible overnight. Outside of
any showers or thunderstorms, gusty 25 to 35 kt winds are forecast
through this evening before diminishing between 01Z and 05Z
Thursday. Anticipate relatively light winds overnight before
increasing and becoming gusty late Thursday morning.

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 Wed Sep 2 2026

Widespread elevated to near critical fire weather is expected
today. There is a greater than 60 percent chance of wind gusts
past 30 mph across much of the area. Relative humidity should
remain above critical levels for the most should remain above 15
percent, although brief periods of critical fire weather are
possible in some locations this afternoon. The day of greatest
concern continues to be Thursday, when humidity will be the
lowest. Further elevated to possible critical fire weather is
possible Friday before conditions improve this weekend.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon to 9 PM MDT Thursday for WYZ275>283-
285>289-300.

&&

$$

UPDATE...Lowe
DISCUSSION...Hattings
AVIATION...Gerhardt/Zaprzalka
FIRE WEATHER...Hattings