589
FXUS65 KRIW 040345
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
945 PM MDT Wed Sep 3 2025

.KEY MESSAGES...

- A strong cold front pushes south through Wyoming Thursday.
  North wind gusting 25 to 40 mph follows the front and blows
  until sunset across areas along and east of the Continental
  Divide, before reaching Sweetwater County Thursday evening.

- Isolated showers and thunderstorms return to the far west
  Friday before becoming more widespread this weekend into
  early next week.

- The middle of next week (September 10-11) could turn warm,
  dry, and breezy/windy.

&&

.UPDATE...
Issued at 120 PM MDT Wed Sep 3 2025

The broad trough over the northern Great Lakes will keep northwest
flow aloft across the region through the next 36 hours. Mid-level
atmospheric moisture is limited Wednesday afternoon, so there is
only the slimmest of chance for a mountain shower over the eastern
Bighorn, southern Absaroka, or southern Wind River Range late in the
day. The northerly surface flow has ushered smoke into northern
Wyoming, and the Wyoming DEQ has issued a Air Quality Alert until
late morning Thursday.

A secondary shortwave rounds the backside of the broad trough
Thursday. This shortwave sweeps a vigorous cold front south through
areas east of the Continental Divide during the day. The front
pushes through the Bighorn Basin and Johnson County around mid-
morning, reaches the Continental Divide mid-afternoon Thursday,
and progresses westward across Sweetwater County early Thursday
evening. Post-frontal north to northeast wind produces
widespread 25 to 40 mph gusts. The trend with this forecast
update was to increase wind speeds and gusts across the Bighorn
Basin and Johnson County, and even those changes may not
adequately capture the potential for gusts 40 to 50 mph at KBYG
(70 percent chance of 40 mph, 20 percent chance of 50 mph)
Thursday afternoon. Minimum relative humidity remains above fire
weather critical thresholds across northern and central
Wyoming. However, minimum relative humidity may drop as low as
10 percent across southwest Wyoming in the pre-frontal
environment. Northwest wind 10 to 20 mph appears to stay just
below critical thresholds in this region, so fire highlights
should not be needed. Sweetwater County sees an abrupt wind
shift to the east-northeast between 7 and 9 PM Thursday as the
cold front pushes west, eventually reaching southern portions of
Lincoln and Sublette Counties. Eastern Sweetwater County sees
the strongest gusts Thursday evening, but these will diminish
farther to the west.

While smoke dispersal will improve with the increased wind, the
volume of smoke moving south across Alberta from British
Columbia wildfires is impressive. Therefore, smoke may continue
to be a concern east of the Divide. Isolated showers were added
to the southwest Wyoming forecast Thursday afternoon given added
lift and at least marginal mid-level moisture associated with
the tail end of the shortwave. Several CAMs also showed this
potential, which added confidence to these changes. Taking a
peek at Friday, temperatures look to be 7-10 degrees below
seasonal averages.

&&

.DISCUSSION...
Issued at 232 AM MDT Wed Sep 3 2025

It is another quiet night across western and central Wyoming. A weak
cold front will move through the area today. It has just passed
through Buffalo around 2 am with a wind shift to the northwest.
Effects look rather minimal though. The only impact will be
some local breezes as temperatures averaging about 5 degrees
cooler across far northern Wyoming. Otherwise, it will be
another warm and mainly dry late summer day across the region.
We will again have a few, emphasis on few, afternoon showers and
thunderstorms over the northern mountains, Mid levels are even
drier than yesterday, so chances are at most 1 out of 6 and will
be confined to the mountains with little chance to move far
from them. Temperatures elsewhere should be fairly similar to
yesterday. As for fire conditions, there could be locally
elevated fire weather is the central and southern Wyoming, but
wind should not reach critical levels.

Another cold front will drop southward into the area on Thursday,
and this one looks more interesting. Not in the precipitation
department though. There will be a few showers and thunderstorms
with it but most areas will see nothing. The main impact will be the
wind, and likely strong in some locations. Most areas East of the
Divide have at least a 1 in 2 chance of seeing wind gusts past 30
mph in the afternoon, when wind is expected to peak. The most
impacted areas will be usual suspects with northwest flow and cold
advection, mainly the northern Big Horn Basin. Both these area have
at last 1 in 2 chance of wind gusts past 40 mph. The question this
morning is about potential high wind though. And this would mainly
be in Johnson County. For now, we are leaning against it. The latest
NBM ensemble guidance is showing less than a 1 in 2 chance of wind
gusts past 50 mph in Johnson County. Deterministic guidance is
split, with the GFS MOS showing a period of sustained 40 mph wind
Thursday afternoon while the NAM and European keep sustained wind
under 35 mph. There are a couple of other factors though. One, 700
millibar wind is only 40 knots and I would prefer to see 50 knots.
The main jet energy is also further east in the Dakotas. So, we will
hold off on any highlights. If the wind trends up we can issue this
afternoon or tomorrow night. Wind should drop from north to south
Thursday night as the pressure gradient relaxes. As for fire weather
concerns, humidity looks to remain high enough to prevent critical
conditions. Elevated fire weather is certainly possible.

Friday looks like the first taste of early autumn across the area as
highs in the 70s look fairly widespread with places like Buffalo
remaining in the 60s. Our focus then shifts with an increasing
chance of thunderstorms as monsoonal moisture moves in from the
west. Most guidance shows the moisture being delayed somewhat,
so on Friday and storms would be largely confined to west of the
Divide and even here the chance is only 1 out of 5. Chances
then increase on Saturday as deeper moisture moves in and
spreads East of the Divide, especially on Sunday. The chance of
showers and storms looks to linger into early next week as well.
And with flow turning southwest, temperatures will likely climb
back to above normal levels by Sunday and stay there for much
of next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 945 PM MDT Wed Sep 3 2025

VFR conditions expected throughout the TAF period at all
locations. Mainly clear skies and light winds overnight into the
morning hours. A cold front will start to sag southward passing
through COD/WRL after 15Z, making its way south through RKS by
23Z as timing still looks on track. Winds were bumped up a bit
for some of the western sites at BPI/PNA as well as increasing
to 40kts at CPR between 00-02Z. The strongest winds behind the
front look to occur an hour of two after FROPA when the "cooler"
air advects through. Otherwise, winds will diminish at all but
CPR/RIW/RKS after 01-02Z towards sunset. The other locations
look to weaken into the next TAF cycle after 06Z into Friday
morning. Dry conditions expected with no indications for any
showers near TAF sites remaining over the Winds/Absarokas.

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...
Air Quality Alert until 11 AM MDT Thursday for WYZ009>011.

&&

$$

UPDATE...CNJ
DISCUSSION...Hattings
AVIATION...Lowe