310
FXUS65 KRIW 160352
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
952 PM MDT Wed Jul 15 2026

.KEY MESSAGES...

- Showers and thunderstorms will continue through the next
  several hours, ending late tonight. Locally heavy
  rain/localized flash flooding will remain possible near
  Thermopolis for the next couple of hours.

- More shower and thunderstorm chances Thursday through Saturday
  for western and northern Wyoming. This potential becomes more
  widespread across the state Sunday through the middle of next
  week. Locally heavy rain and localized flash flooding will be
  the main threats with these storms through the rest of the
  week into the weekend.

- Hot temperatures are expected through Sunday with highs in the
  upper 80s to upper 90s.

&&

.UPDATE...
Issued at 1200 PM MDT Wed Jul 15 2026

The forecast for today is on track with showers and thunderstorms
over western and northern Wyoming as a shortwave traverses overhead.
As of 18Z/12PM, the shortwave is located over eastern Idaho and will
continue northeastwards within the southwesterly flow aloft. Across
the western valleys, morning cloud cover is expected to limit the
overall coverage of thunderstorms early this afternoon. Shower and
thunderstorm coverage should be greatest from the Gros Ventres and
Wind River Mountains to the Absarokas and Yellowstone due to
favorable orographics and sufficient daytime heating. Thunderstorms
then move into the Bighorn Basin and upper Wind River Basin after
3PM. A few thunderstorms could become severe across the northern
Bighorn Basin due to proximity to the shortwave and over 1,000 J/kg
of BLCAPE. With DCAPE values around 1,200 J/kg, gusty winds up to 60
mph will be the primary severe threat. There is an additional threat
for localized heavy rainfall, but as mentioned in the morning
discussion, the flash flooding threat is on the lower end due to the
storms moving. Thunderstorms will begin to diminish in coverage
after sunset, with only lingering showers and thunderstorms through
1AM along the shortwave axis from the northern Bighorn Mountains to
southern Lincoln County. Otherwise, the forecasted shower and
thunderstorm chances continue Thursday through the middle of next
week.

&&

.DISCUSSION...
Issued at 133 AM MDT Wed Jul 15 2026

I am coming off of a short break. When I left Sunday morning, I was
talking about all time record high temperatures. And we had several
across the area. What a difference 72 hours makes. Now, we are
talking about a much more moist atmosphere and the potential for
heavy rain and possible flash flooding over the next several days.
The reason is that the monsoon is now here.

We had a few showers out there yesterday and a couple of light ones
right now, but for the most part most of the action is in
Montana right now. Most of the models, both the synoptic and
convective ones, have the main area of action across the
northwestern half of the area, largely west and north of a
Buffalo to Riverton to Evanston line. And, all models show the
most numerous storms over the northwestern Bighorn Basin and the
Absarokas, where the combination of a passing shortwave and
some low level easterly upslope flow will bring more numerous
storms. The big difference today is the amount of moisture in
the atmosphere. Climatologically, precipitable water values are
the highest around this time, averaging around 0.60 inches in
Riverton. Today, some of these will exceed this by 150 to 175
percent, especially in the aforementioned Bighorn Basin where
values may climb as high as 1.30 inches, close to 200 percent of
normal. Instability parameters are also decent here, with
lifted indices of minus 4 and CAPE over 1000 J/kg, so we could
see a stronger storm as well. The main threat will still be
heavy rain though. If there is one thing against flash flooding,
it is that there is some decent flow at 500 millibars so the
storms will be moving somewhat. This looks like a diurnal event,
with the vast majority of storms ending by sunset and almost
all done by midnight. And, it will be another hot day. Probably
not record breaking, but some of the hotter locations could
approach 100 degrees again this afternoon.

At this time, Thursday looks fairly similar to today, with mainly
diurnal showers and thunderstorms and in roughly the same
locations. The deeper moisture may press a little further to the
east, but areas like Casper and Rock Springs continue to look mainly
dry. Precipitable water values also look fairly similar, generally
150 to 175 percent of normal across the northwestern half of the
area, along with easterly upslope, and the maximum chance of
convection in the easterly flow upslope locations. I think the
chance of local flooding may be greater on this day though, since
flow in the 700 to 500 millibar layer looks lighter, so storms would
be slower moving. Also, temperatures should continue to be on
the downward trend, although it will still be above normal with
widespread 80s and 90s for highs.

The latest guidance now shows ridging building back over Wyoming
Friday and Saturday, with 500 millibar heights rising anywhere
from 20 to 40 meters. Precipitable water values also drop about
10 to 20 percent, but this still averages around 150 percent
above normal. The result will be somewhat less coverage of
storms and the main areas of storms being pushed a bit further
to the west, although all locations have at least some chance.
The emphasis will again be on easterly upslope locations
(noticing a pattern here). The mean 700 to 500 millibar flow is
also stronger, so the threat of flooding may decrease a bit with
better steering flow and a slightly, emphasis on slightly, less
moist atmosphere on these days. Temperatures will also increase
somewhat on these days, with some 100s again possible in the
eastern Bighorn Basin.

The main chance of storms may finally begin to shift east of
the Divide on Sunday as the main area of moisture moves eastward
as the core of the ridge moves southeastward and moisture can
be pushed further east. There is still a bit of a spread on
exact timing of this happening and it is impossible to pinpoint
a more active day this far out. However, the eastern portions of
our area may finally begin to get some needed moisture next
week. As for temperatures, they look to remain above normal but
more cloud cover and the core of the ridge moving away should
keep record high temperatures away for now.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 945 PM MDT Wed Jul 15 2026

Showers and thunderstorms will continue at the beginning of the
TAF period. The exception will be over the Upper Green River
Basin, which will last through 09Z. Have added new PROB30 groups
to KBPI/KPNA to account for this activity. Otherwise, clouds
will continue to scatter out through the rest of the night, with
SKC skies expected across the area by 16Z.

Showers and storms will redevelop over the mountains by 19Z/20Z
and move to the northeast through the afternoon. KJAC, KCOD,
KBPI and KPNA will be the first terminals to be impacted, with
KWRL, KRIW/KLND and KRKS being impact after 00Z. Strong wind
gusts of 40 to 45kt and heavy rain will be the main impacts with
these storms. Showers and storms may be ongoing over areas east
of the Divide by the end of the forecast period.

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...
None.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...LaVoie