056
FXUS65 KRIW 211739
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1139 AM MDT Tue Jul 21 2026

.KEY MESSAGES...

- Increasing monsoonal moisture brings an increase in coverage
  of showers and thunderstorms today, especially in western
  Wyoming. Locally heavy rain and local flooding is possible
  with any shower or storm.

- The most active day looks to be Wednesday, with a possibly
  record moist atmosphere along the best chance of showers and
  thunderstorms, as well as the highest chance of flooding.

- A good chance of thunderstorms lingers into Thursday, although
  likely fewer than on Wednesday. Things finally dry out
  somewhat Friday with a return of above normal temperatures.

&&

.UPDATE...
Issued at 1003 AM MDT Tue Jul 21 2026

Some light showers are moving across the area this morning as the
monsoonal moisture begins to push in. Current water vapor imagery
shows the plume of higher moisture wrapping around the upper-level
high situated over eastern Colorado and a low around the Arizona/New
Mexico border. Showers and thunderstorms will be present this
afternoon, mainly across western Wyoming, with chances increasing
for locations east of the Divide mainly late this afternoon and
evening. Rain chances diminish overnight, though some showers will
be around. Flood Watches continue through Wednesday night.

Smoke has also moved in from Oregon/Washington and is sitting mainly
east of the Divide and over far northwest Wyoming. With the
monsoonal push making its way in, the smoke should get pushed out
through the next 24 hours, with really good conditions by the end of
Wednesday. Any falling rain will also aid in taking smoke out of the
air.

&&

.DISCUSSION...
Issued at 158 AM MDT Tue Jul 21 2026

After a lull for the past couple of days, the monsoon is coming back
with a vengeance. Showers and embedded thunder have broken out across
much of western Wyoming as I write this shortly after midnight. Most
of these should continue until around 3 am before tapering to just a
few stragglers for the rest of the overnight like last night.

Things ramp up considerably today, as much deeper monsoonal moisture
moves over the area. Precipitable waters, which right now are
running around 150 percent of normal, are now expected to climb as
high as 1.3 inches at out office and as high as 1.5 inches in other
locations. This is more than 200 percent of normal for many
areas. The Weather Prediction Center has also increased the
slight risk of excessive rainfall over southwestern Wyoming,
with a marginal risk across all but the northeastern portion of
our forecast area. As a result, with some rainfall rates that
could be over 2 inches an hour, we will hoist a flash flood
watch starting at noon today for all areas across our county
warning area. The best chance today would be West of the Divide,
but the models, both the synoptic and high resolution, have
been doing a terrible job in showing the placement of the
convection. Case and point, showers popped up in Washakie and
Johnson County and no models had it there. It does look like the
progression will start in western Wyoming and spread east
through the day. And like the past few nights, most convection
should end by midnight, but some showers and a few storms should
linger through into Wednesday morning. As for temperatures,
with more cloud cover, temperatures will still be cooler than
yesterday. Still quite warm, but generally 3 to 8 degrees cooler
than Monday.

The main event still looks to be on Wednesday. Precipitable water
values will reach a zenith on this day, topping out at around 1.5
inches at our office (around 250 percent of normal). If this comes
to fruition, this would be a new record high for the office (the old
record is 1.42 inches). The main difference will be a shortwave that
will tracking across the area during peak heating of the day, adding
some additional lift. With the combination of the possibly
record moist atmosphere and the passing shortwave, this looks to
be most likely day with flooding problems. The Weather
Prediction Center also agrees and has expanded the slight risk
of excessive rainfall to cover most of the County Warning Area.
As a result, we will continue the flood watch through this day
as well. Another difference here is that a lot of the convection
will linger through the evening and possibly even into the
night time hours, much for than the previous couple of nights.
And with this, the flood watch was run though Wednesday night.

We should begin a downward trend starting on Thursday. Most
guidance shows the main plume starting to moving to the east
and south with precipitable water values dropping, especially
West of the Divide. All in all, values look to drop between 20
and 30 percent. However, this is still close to 200 percent of
normal, especially across areas East of the Divide. However,
coverage should be less with somewhat less moisture and the lack
of a trigger like Wednesday`s shortwave. There is still the
risk of flooding on this day given the remaining moisture and
possible saturated ground from the previous days convection,
although it should be somewhat less than Tuesday and especially
Wednesday. We will let future shifts make the decision whether
or not to extend it through Thursday.

Friday is when it finally looks like the main area of moisture will
retreat out of the area and chances of convection finally go back to
normal, giving the people of Wyoming and meteorologists a break
(including me, I HATE the monsoon!). There could be another
secondary surge over the weekend, but it should not be anything like
it has been during the middle of this week. In addition, the ridge
will still be present over the area through the period. So, after a
few days of cooler weather with more cloud cover. Well above normal
temperatures look likely to return with triple digit high
temperatures possible in eastern Big Horn Basin possibly on both
Saturday and Sunday.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1140 AM MDT Tue Jul 21 2026

With upper-level high pressure centered over northeast Colorado
today, a southwest US monsoon pattern has set up to impact the
Great Basin into Wyoming. Light morning showers continue over
parts of WY, but are dissipating late in the morning. Afternoon
heating and increasing moisture will bring widespread showers
and thunderstorms from mid afternoon through about sunset. -TSRA
is included in every TAF today with varying times, but generally
21/22Z to 22/04Z. Moderate to heavy rain is likely today, with
a good chance of MVFR conditions with reduced vsbys and lowered
ceilings as showers and storms impacts terminals. Gusty outflow
winds of 25-35 kts are also possible around the storms,
especially across southwest WY. With such a moist atmosphere,
localized flooding is a possibility both today and tomorrow.
Showers are expected after sunset through about 22/09-12Z for
most of the area as well. MVFR conditions are possible (40-60%)
overnight in areas where moderate precipitation leads to a very
moist boundary layer.

Early morning showers are likely again Wednesday morning, with
the best chances north of KCPR and across NW WY. More numerous
showers and thunderstorms are expected from Wednesday afternoon
into the evening, with a higher chance of localized flooding
given such a moist atmosphere and another disturbance moving
from the northern Great Basin into northern WY. Winds Wednesday
afternoon will generally be north (east of the divide) or WNW
(west of the divide). Moderate rain and isolated thunderstorms
are expected to continue after midnight into early Thursday
morning.

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late Wednesday night for WYZ001>020-
022>030.

&&

$$

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...McDonald