869
FXUS65 KRIW 062153
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
353 PM MDT Sun Sep 6 2026

.KEY MESSAGES...

- Low humidity (10 to 20 percent) and breezy winds (gusts of 20
  to 30 mph) will contribute to near-critical fire weather
  conditions through sunset.

- An approaching system will return showers and thunderstorms to
  the area today, starting mid-afternoon across southwestern
  Wyoming. This activity will move northward through tonight.
  Western Wyoming will have the best rain chances (50-70% chance
  of at least 0.25").

- Cooler and windy on Monday with widespread gusts of 30 to 50 mph
  during the afternoon.

- Wind and low humidity will lead to continued fire weather
  concerns through the week. A late week system may bring cooler
  and wetter conditions though uncertainty remains high at this
  time.

&&

.UPDATE...
Issued at 1244 PM MDT Sun Sep 6 2026

Fire weather concerns continue this afternoon, with near-critical
conditions for much of the area given a gusty southwest wind and
humidity near or below 20 percent. Showers and thunderstorms with
the incoming system are expected to start across southwestern
Wyoming between 3 PM and 6 PM, which is a little later than forecast
yesterday. By sunset this activity will become more widespread
across western and southern Wyoming. Chances then increase through
tonight farther north, though especially across western Wyoming. One
thing to note, even after sunset, is the potential for outflow wind
gusts of 30 to 40 mph. Convection-allowing models are showing this
most widespread east of the Divide this evening as showers move in
there, also aided by a deepening surface low across northern
Wyoming.

&&

.DISCUSSION...
Issued at 105 AM MDT Sun Sep 6 2026

I was thinking back to where I was 18 years ago tonight. I had just
finished my last shift at my previous job for a private company in
Pennsylvania, and begin a 5000 mile drive to start my career in the
National Weather Service, in Alaska. At this time I was sound asleep
in a motel room in Sparta, Wisconsin. I was quite tired after
driving 800 miles after working a midnight shift and having to
endure the Chicago rush hour traffic. And this ties into the weather
pattern coming up the next week, mainly with transitions and
changes. As temperatures changed from the warm late summer
temperatures in the lower 48 states then cooling as I drove
northward, especially when I reached the Yukon, Northwest
Territories and Alaska. This reminds me of the pattern over the next
week, when we will see a little bit of everything. This includes
some continued warm late summer temperatures, gusty to strong wind,
elevated to potentially critical fire weather and maybe, just maybe,
the first real taste of fall for next weekend.

Today will be another very warm day across the area as ridging holds
for another day, with the warmest spots likely seeing highs around
90. We have a couple of concerns over the next 24 hours. The first
is, once again, fire weather. The set up looks similar to yesterday.
Some areas will be see relative humidity under 15 percent,
especially East of the Divide. Others will see a gusty afternoon
wind, especially in southern Wyoming and West of the Divide. Most of
these areas are not together though. With this, along with
increasing cloud cover and dew points ticking higher later in the
day, we will not issue any Red Flag Warnings. Elevated fire weather
still looks likely though. The other concern, especially late today
and especially tonight, will be the chance of showers and possible
thunderstorms. Deeper moisture will moving into southwestern Wyoming
later this afternoon, spreading across the area tonight. At this
same time, an upper level low will be lifting northward in Idaho.
This will drag a Pacific Cold Front and negatively tilted trough
across the area. This will be a typical monsoon set up though, in
that all areas will have a chance of showers, but not everyone will
see one. Models have come into decent agreement in keeping the most
numerous showers in western Wyoming, with lessening chances further
east. We still have a marginal risk of excessive rainfall across
this area as well. Precipitable water values will rise close to an
inch in some locations, which is over 175 percent of normal.
However, there is plenty of steering flow, with some 500 millibar
winds approaching 50 knots. So, the main threat of from training
storms. With the strong wind level wind as possible mixing from the
frontal passage, there could be some strong wind gusts as well. The
deepest moisture will be through the area by Monday morning, but a
few showers will be possible into Monday with wrap-around moisture,
mainly in the northern and western Wyoming.

Monday will be a cooler day behind the front. The main concern here
shifts to wind. We will have a fairly pressure gradient between the
low to the northwest and the ridge to the south and east. That means
a gusty to potentially strong wind across much of the area. Much of
the area, especially East of the Divide, has at least a 1 in 2
chance of wind gusts past 35 mph. With 700 millibar winds
approaching 40 knots at times, there could be some gusts to around
50 mph, especially in the higher elevations and in some wind prone
areas like the south side of Casper. As for fire weather, the wind
will be there but there should be enough residual humidity to keep
conditions above critical. It will be another day of elevated fire
weather though. The real day of fire weather concern will be
Tuesday, when dew points really crater and a gusty wind continues.
As of now, there is at least a 2 out of 3 chance of seeing some Fire
Weather Watches issued for this time period. Dry zonal flow will
dominate through Friday with dry conditions, warm temperatures and a
continued gusty breeze. Further fire weather highlights may be
needed on these days bit wind may be borderline in some locations.

And now to the potential change I was talking about. The main word
of this sentence is POTENTIAL, since this is a week out a lot can
change between now and then. Both the European model and the
GFS show a decent system coming out of Canada for the weekend.
There are differences in timing and details, as would be
expected this far out. In addition, the GFS has more of a front
/ open wave while the European has a closed low. I won`t even
try to hash out the details at this point. However, the
potential for the coolest temperatures we have seen since last
spring are possible. Both models show below zero temperatures at
700 millibars at some point next weekend, with some showing as
low as minus 5. And with some moisture, this could also mean
some snow for the mountains. Again, this is still a long way out
so uncertainty is still very high. However, this could be the
first taste of fall and some relief for people who are sick of
the long and hot summer we have had.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 353 PM MDT Sun Sep 6 2026

Clouds have been increasing from the southwest this afternoon,
associated with the moisture ahead of an incoming cold front.
Precipitation chances will increase from the southwest within the
first few hours of the period. Chances spread northeastwards through
the night, with most ending by Monday morning. Mountain rain shadow
look to lessen chances at KLND, KRIW, and KWRL compared to other
locations. In any case, clouds will be over the area, but look to
clear fairly quickly Monday morning into early afternoon. Monday is
also windy, with gusts starting between 15z and 18z. Widespread
gusts 25 to 40 knots are expected through Monday afternoon.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued at 105 AM MDT Sun Sep 6 2026

Today will be similar to yesterday. Humidity will fall into the
teen at times this afternoon, especially East of the Divide with
gusty breeze developing, especially in southern Wyoming.
Although critical fire weather is not expected, elevated to near
critical fire weather conditions will be fairly widespread this
afternoon. Elevated to critical fire weather will be possible
much of next week, with low humidity and breezy to windy
conditions continuing. At this point, Tuesday looks like the day
of greatest concern.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

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