569
FXUS65 KRIW 151000
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
400 AM MDT Tue Sep 15 2026

.KEY MESSAGES...

- A mainly dry and seasonable day today. A few showers are
  possible in southern Wyoming this evening and tonight.

- Increasing monsoonal moisture increases the chance of showers
  starting on Wednesday, with Thursday and Friday looking like
  the most active days.

- Temperatures will remain near to above normal through most of
  the week, with a cool down likely for the weekend.

&&

.DISCUSSION...
Issued at 149 AM MDT Tue Sep 15 2026

The last two nights I used music to describe the weather of the day.
So, why not make it three days in a row. Lets step back another
decade today, to the 60s. And let us also change music genres again,
this time to country. And this has to with a pattern that you
would expect more during the middle of summer, not a week from
the start of autumn. The song is by the late, great Johnny Cash,
Ring Of Fire. Basically, a strong ridge sets up in the southern
United States and moisture rides along the northern edge of it.

There continues to be an strong ridge over the southeastern and
south central United States, bring more record high temperatures.
For us in Wyoming, we will be close to the top of the ridge, with an
increasing chance of showers. For today and most of tomorrow, the
bulk of the moisture looks to remain to the south of Wyoming. Just
enough moisture will creep northward late today and especially
tonight to bring some showers to the southern third of the state.
But these still look fairly isolated and even here most locations
should not see one (less than a 1 out of 4 chance). Moisture will
try to creep further north on Wednesday but even on this day the
bulk will remain south of the border with only isolated activity. As
for temperatures, today looks fairly close to normal. With southerly
flow increasing Wednesday, temperatures look warmer with highs
rising to around 80 in some of the warmer spots.

Chances of showers should increase beginning Wednesday night and
especially into Thursday and Friday. The ridge over the southeastern
United States with retrograde toward the southern plains. At the
same time, low pressure over the Pacific Northwest will begin to get
pushed eastward. moving toward Idaho by Thursday night. This will
push the plume of monsoonal moisture further north and west and into
the Cowboy State. Precipitable water values could be rather high.
The GFS has values approaching an inch, which is approaching 200
percent of normal Wednesday night and Thursday. This model does tend
to overcook this somewhat, so reality may be closer to 150 percent
of normal. Nevertheless, the chance of showers and storms will
increase. Model guidance continues to have disagreements in the
placement of the most numerous showers though. So, we have higher
confidence in Thursday and Friday being the two more active days,
but placement of the most numerous showers and precipitation amounts
remains uncertain. Temperatures will remain somewhat above normal,
although with more cloud cover a bit cooler than on Wednesday. And
like most monsoonal events, all areas will have a chance of a shower
or storm, but not everyone will see one.

There is more uncertainty for the weekend. There is still a decent
model spread on how fast the low will exit the area. Most guidance
now takes the low north of the area across Montana. With flow
turning northwestward, Saturday looks like a cooler day across the
area. How fast the low moves will determine the extent of any
precipitation though since it will be of the wrap around variety.
Most guidance favors northern Wyoming given the position of the low,
with lesser chances further south. Sunday looks somewhat drier
although a few areas may still see a few showers around. Ridging
should build back over the area to start next week, bring a return
to dry and seasonable weather.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 356 AM MDT Tue Sep 15 2026

Largely VFR conditions should continue through 12Z Wednesday.
The day will start of clear but increasing moisture from the
south will bring high clouds from south to north through the
day. Clouds may build up more by 22Z with some showers and even
a thunderstorm possible. Deeper moisture slowly spreads
northward through the night, bringing some showers to
southwestern Wyoming. We have added PROB30 groups for showers in
vicinity of KRKS, KBPI and KPNA between roughly 03Z to 09Z, A
stray shower may get close to KJAC, but with the chance less
than 15 percent we decided not to add to the TAFs. Wind should
remain under 11 knots for the most part, except near KCPR where
some gusts to 15 to 20 knots will be possible this afternoon.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Warning until 9 AM MDT this morning for WYZ023.

&&

$$

DISCUSSION...Hattings
AVIATION...Hattings