322
FXUS65 KRIW 180405
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1005 PM MDT Fri Jul 17 2026

.KEY MESSAGES...

- Rain showers will continue into the early overnight hours.

- The threat for thunderstorms will be more widespread across
  the forecast area Saturday, with central and southern portions
  having better chances than have occurred through the week so
  far.

- The chance of showers and thunderstorms continues each day
  through the next seven days, with the chance of localized
  flooding each afternoon and evening.

&&

.UPDATE...
Issued at 238 PM MDT Fri Jul 17 2026

Thunderstorm activity was delayed slightly this morning due to
midlevel cloud cover holding back daytime heating, but has now
begun to bubble along the western mountains and the Bighorns.
Storm motion remains low, and storms have been tapping deeper
monsoon moisture, so thunderstorms have been producing ample
rainfall. PW values from 18Z sounding noted 0.94" still, so the
main threat with any storms remains localized downpours. The
Flash Flood Watch continues through 10PM tonight, and was
expanded west to include Greater Yellowstone National Park,
where models have noted showers and thunderstorms may last well
into the evening. Remaining forecast looks on track for today.
Remember: if you see flooded roads, Turn Around, Don`t Drown!

&&

.DISCUSSION...
Issued at 157 AM MDT Fri Jul 17 2026

Have you ever been stopped at a train crossing watching a really
long freight train go by? One of those really long ones that tend to
always show up when you are running late for something. Well, I am
comparing that long freight train to the monsoonal pattern that we
look to be locked in for at least another week. There will be day to
day fluctuations, but there will be at least some chance of showers
and thunderstorms each day, and possibly the chance of flooding.

We will have one of those fluctuations today, as the ridge builds
back to the west somewhat. This will lead to somewhat higher
heights and warmer temperatures. The biggest impact will be
that precipitable waters will drop about 15 to 25 percent
roughly west of a Buffalo to Riverton to Rock Springs line. This
means that the chance of storms and the chance of flooding
should be somewhat reduced today in this area. However, this
will not be the case for the Bighorn Basin as well as the
western mountains. So, the main area of showers and storms will
be here, with tapering chances further south and east. And with
the continued chance of flooding, we will issue another Flood
Watch for the Absarokas and much of the Bighorn Basin given the
saturated soils. We thought about one for the western mountains
but rainfall hasn`t been as heavy here and we will hold off for
now. However, much like yesterday, any shower or storm could
have heavy rain given the abnormally moist atmosphere. Like
yesterday, most storms will be over shortly after sunset, but a
few may linger until after midnight.

The moisture axis will shift a bit further to the east on Saturday,
and as a result so will the chance of showers and thunderstorms.
Precipitable waters climb back to levels similar to Thursday, so the
chance of storms should be similar as well. Again, the Bighorn
Basin looks to be the area under the gun as well as the
Absarokas as the low level easterly upslope flow continues. Far
eastern areas, like Kaycee and Casper, who have been wondering
where their rain is, will have another dry day on this day. This
will also be a warmer day, and some locations in the eastern
Bighorn Basin might make a run at 100 degrees if they can get
enough sunshine.

It still looks like the transition day will be on Sunday. On this
day, some drier air will begin to move into western Wyoming, drying
things out somewhat. The main focus then moves to east of the
Divide, including locations like Casper that have remained hot
and dry through much of the week. Precipitable water values will
again average around 175 percent of normal, and again, sounding
like a broken record, the threat of heavy rain and localized
flooding. With more clouds shifting eastward, temperatures
should begin to cool down as well on this day, although they
will remain well above normal.

There is a wrinkle for Monday, as there is some split in the
guidance. Some ensemble members, as well as the deterministic
ECMWF model run, now builds the ridge over the state, shutting
off most of the convection. Others keep things quite active
though. For now, the compromise was to dry out northern Wyoming
and have the highest PoPs across southern Wyoming. There is
still plenty of moisture in the atmosphere so it would not take
much for a few storms to fire.

Following Monday, there is greater model agreement in bringing the
deep monsoonal moisture back over the area, especially east of
the Divide. Precipitable water values look exceptionally high on
some model runs. This is still 5 or more days out, so take this
with a grain of salt. For example, at Riverton, our mean
precipitable water value is 0.63 inches. On Wednesday, some
ensemble members and the deterministic GFS is showing
precipitable waters of over 1.3 inches. This is over 200 percent
of normal. It is also close to the record high precipitable
water value of 1.46 inches that was set back in 2003. Again, it
is a long way out but there is the potential for some very heavy
rain from any thunderstorm that could develop for the middle of
the week. The abnormally high precipitable water values look to
stick around through at least Thursday. So, the monsoonal
freight train will continue to transport moisture into the
area, along with the chance of drenching thunderstorms, for the
next several days.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1000 PM MDT Fri Jul 17 2026

Scattered rain showers will continue through about 18/0900Z,
with variable breezy winds based on where showers are tracking
in the vicinity of area terminals.

Skies partially clear during the early morning hours. By
18/1800Z, scattered showers and storms will again begin forming
over the high terrain, and will slowly drift northeastward into
basins, possibly (30% chance) affecting terminals across
western and central Wyoming. Heavy rain reducing visibility and
lightning will be the main threats with storm Saturday. Expect
variable gusts up to 35 kt with any storms that do form near
airports. Storm and shower activity taper off after sunset.

&&

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

&&

$$

UPDATE...Straub
DISCUSSION...Hattings
AVIATION...VandenBoogart