435
FXUS65 KRIW 151536
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
936 AM MDT Wed Jul 15 2026

.KEY MESSAGES...

- Scattered showers and thunderstorms are forecast this
  afternoon and evening across western and northern Wyoming.
  Localized heavy rainfall and gusty winds are possible in any
  thunderstorms.

- 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.

&&

.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
this 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 area of main 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, avenging 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
some decent flow at 500 millibars so the storms will be moving
somewhat. This looks like a diurnal event, with the vast
majority over 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 slow
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 heights 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 normals. 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 /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 355 AM MDT Wed Jul 15 2026

Largely VFR conditions are expected through 12Z Thursday. There
is a less than 15 percent chance of a shower in vicinity of
KJAC until 14Z. Increasing moisture will bring the chance of
showers and thunderstorms this afternoon and evening (roughly
between 20Z and 04Z) at all TAF sites with the exception of KCPR
where the chance of a shower or storm in less than 10 percent.
The PROB30 groups were continued at all sites due to uncertainty
in pinpointing the storms. The one site we had a thought of
prevailing thunder was in vicinity of KCOD. However, with the
chance around 40 percent, we decided to keep the PROB30 for now.
All convection should end by midnight with VFR conditions after
that. Wind should remain at 14 knots or less except in and
around any shower or thunderstorm where wind gusts to 30 to 40
knots are possible.

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.

&&

$$

DISCUSSION...Hattings
AVIATION...Hattings