636
FXUS65 KRIW 140355
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
955 PM MDT Sun Sep 13 2026

.KEY MESSAGES...

- Scattered showers and thunderstorms persist into early Monday
  morning east of a line from Rock Springs to Buffalo. Gusty
  wind to 40 mph and small hail are possible.

- A weather system over the Pacific Northwest tracks east across
  Montana by early afternoon Monday. An associated cold front
  pushes across western and central Wyoming between midnight and
  6 AM MDT Monday leading to gusty west to northwest wind
  through the rest of the day.

- The combination of gusty west to northwest wind and low
  relative humidity of 10 to 15 percent creates critical fire
  weather conditions across portions of Fremont, Natrona and
  Sweetwater Counties between 12 PM and 8 PM MDT Monday.

- Dry and seasonal conditions Tuesday give way to a steadily
  increasing chance of showers Wednesday through Friday.
  Temperatures are expected to remain seasonal through the week.

&&

.UPDATE...
Issued at 1200 PM MDT Sun Sep 13 2026

Some passing clouds on IR as winds have started to become a
little breezy for some across the CWA this afternoon.
Temperatures still on track to be a bit warmer than Saturday
afternoon ahead of the next storm system later tonight into
Monday morning. Winds will increase ahead of FROPA from the WNW
this evening and into the overnight hours, pushing through to
the east by sunrise Monday morning. HiRes models still
indicating high winds for the Absarokas to Cody Foothills (aided
by funneling and downslope), as well as the upper Wind River
Basin near and around Dubois. High wind warnings have been set
in place for these areas after midnight tonight through early
Monday afternoon before winds start to diminish but remain
elevated through the rest of Monday.

Otherwise, the fire weather watch has been upgraded to red flag
warnings with high confidence in gusty winds and low humidity
values for portions of Sweetwater through Fremont to Natrona
Counties of the wind corridor Monday afternoon and evening.
Behind the cold front, expect more seasonable temperatures to
start the week with freezing temps for the lower valleys west of
the Divide to trigger the last zone for the freeze warning in
terms of growing season. Chilly morning continue Tuesday before
a slight and steady warmup the rest of the week and
precipitation chances for the second half of the week into the
weekend.

&&

.DISCUSSION...
Issued at 218 AM MDT Sun Sep 13 2026

Some people who read my discussions know that I like the reference
old song lyrics to compare to the weather. And today`s reference is
another 1980s song, Madonna`s second hit, "Borderline". In the song,
she talks about pushing her love over the borderline. In the
weather, the question is whether or not the weather can cross the
borderline into needing highlights. And we have two of these
dilemmas this morning.

The borderline conditions are not today for the most part though.
Today will be a warm late summer / early fall day across the
area. An approaching cold front and a trough axis will tighten
the pressure gradient across the area, and bring a breezy to
windy afternoon across the area. Temperatures East of the Divide
will rise well into the 80s, with a few of the warmer spots
possibly getting close to 90. As for fire weather, critical fire
weather does not look likely as dew points will be rising ahead
of the front, although some locations will see isolated fire
weather at times this afternoon. As for the chance of
precipitation, the morning looks generally dry. The one fly in
the ointment is the European model does have some light showers
over the far northern and eastern areas before noon. However,
this model is the outlier and even the this model is further
east than the 12Z run. So, we kept the forecast dry for now, but
a 10 percent chance of a shower is still possible. A few
showers and thunderstorms will break out this afternoon, but the
bulk of the activity should hold off until this evening.

All guidance shows a chance of showers tonight, with the best chance
in southeastern portions of the area. Many areas will have a chance
though. There is still disagreement in rainfall amounts though,
especially with the convective nature of the precipitation. As for
flooding potential, there could be some locally heavy rain but with
strong steering flow, showers and storms will be moving so the
threat looks minimal. This is where we reach the first borderline
situation. And this is for the potential for high wind. We do have
one of the indications for high wind across portions of the area, we
do have some 700 millibar winds of 50 knots. There will also be a
decent jet streak moving in, around 100 knots this evening. There
are some indications against it though. For one, it looks like a
brief period of higher wind aloft, generally less than 6 hours.
Also, ensemble guidance, both the HREF and NBM ensembles, gives less
than 1 in 3 chance of wind gusts over 55 mph except in the
mountains. The one location I am most concerned about is the Cody
Foothills, where there are some indications of a possible mountain
wave or gap flow, especially between midnight and 5 am. The best
chance of this would be near Clark, where criteria is higher. We
will hold off on any wind highlights for now and let the day shift
take another look. Any showers should end by shortly after sunrise
on Monday.

Then we move to Monday, where we have our other situation of
borderline highlights. The 700 millibar wind will remain strong, but
will gradually decrease through the day as the jet streak moves
away. The borderline situation is potential Fire Weather. Areas like
Natrona and Sweetwater County will have the wind, with a greater
than 4 out of 5 chance of wind gusts over 30 mph through the day.
Humidity looks borderline though. Further west in Fremont County,
the humidity will be plenty low. However, wind looks more borderline
with only a 1 out of 2 chance of wind gusts over 30 mph. Also, if
there is a decent amount of rain in Natrona County, it could wet the
fuels. However, it has been very dry. And, with thinking that fire
weather is the greater threat than high wind, we have issued a Fire
Weather watch for the above mentioned areas. It will also be a
cooler day behind the front, with high temperatures 10 to 15 degrees
less than on Sunday.

Tuesday looks like a dry and seasonable day across most of the
area with less wind. A bit of moisture may come far enough
north for another chance of a shower or thunderstorm across
Sweetwater County. And, much like yesterday. Wednesday into next
weekend remains in flux with a lot of uncertainty. The main
driver this far out will be an upper level low that will be
located over the Pacific Northwest on Wednesday. The models
continue to struggle with its movement, especially the European
which has trended slower and further north. At this point, it
looks like there will be a chance of showers and thunderstorms
each day from Wednesday through Friday. However, confidence in
picking a more active day and especially pinpointing the most
numerous showers remains very low. So the forecast is subject to
change in the extended portion of the forecast. And with the
southwest flow ahead of the low, temperatures should rebound to
above normal levels once again for later in the week.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 955 PM MDT Sun Sep 13 2026

An upper low over the Idaho Panhandle late Sunday evening will spin
eastward across most of Montana by midday Monday. A 300mb 100kt+ jet
streak ahead of this feature has initiated scattered light showers
across northwest Wyoming late Sunday evening. These showers
spread east across mainly the north half of Wyoming before
ending between 10Z-13Z/Monday. Impacts are most likely at KJAC
and KCOD where 3-hour PROB30 groups exist overnight. To the
southeast, late Sunday evening scattered showers and isolated
thunderstorms in a swath from around KRKS to KCPR gradually
slide east and decrease in coverage between 06Z-08Z/Monday.
Attention then turns to a strong cold front associated with the
upper low. This cold front pushes through all but KCPR by
10Z/Monday with gusty west to northwest wind in its wake given
8mb/3-hour pressure rises. The strongest winds of 30-45kts
likely occur at KRIW where the surface wind will be from a
favored west-northwest direction. Other terminals to see
routine 20-30kt winds for a 3 to 4-hour period before
diminishing as the front pushes eastbound. These diminished wind
speeds linger most of the morning before the west-northwest
wind steadily increases through mid-afternoon Monday. Speeds
again come down between 01Z-04Z/Tuesday as the pressure gradient
relaxes and the atmosphere decouples. Mountain tops across
northwest Wyoming occasionally obscured through 12Z/Monday.

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

&&

.FIRE WEATHER...
Issued at 218 AM MDT Sun Sep 13 2026

A gusty afternoon breeze may bring some elevated fire weather
this afternoon. The main concern will be on Monday, when a
combination of a very day airmass with humidity under 15 percent
and a gusty to strong southwest wind may bring critical fire
weather Monday afternoon, mainly in Natrona, Sweetwater and
Fremont Counties. Elsewhere, elevated fire weather is possible
but critical fire weather chances are less than 40 percent.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 2 PM MDT Monday for WYZ002-003-016.

Red Flag Warning from noon to 8 PM MDT Monday for WYZ279-280-
283-289-300.

Freeze Warning from 3 AM to 9 AM MDT Monday for WYZ023.

&&

$$

UPDATE...Lowe
DISCUSSION...Hattings
AVIATION...CNJ
FIRE WEATHER...Hattings