054
FXUS65 KRIW 201842
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1242 PM MDT Mon Jul 20 2026

.KEY MESSAGES...

- One more hot day with highs around 100 in the warmer spots. A
  few showers and thunderstorms will be around, mainly in and
  near the mountains and in southern Wyoming.

- Deeper monsoonal moisture returns Tuesday and brings a better
  chance of showers and thunderstorms, especially in western
  Wyoming.

- Coverage of showers and thunderstorms peaks on Wednesday and
  Thursday, with very heavy rain possible from any storm, along
  with potential for flooding.

&&

.UPDATE...
Issued at 1242 PM MDT Mon Jul 20 2026

The forecast and key messages remain largely unchanged. Today,
isolated showers and storms will be possible around the mountains
and across southern Wyoming (generally around and south of I-80). A
boundary also pushes south through the area during the day, bringing
a gusty north to northeast wind shift to locations east of the
Continental Divide. One addition to the forecast was for the
introduction of smoke the next couple days, as smoke from
Oregon/Washington sinks southward into the state with this boundary
moving through. Attention then remains focused on the monsoon
moisture for Wednesday as models continue to support high-end
moisture values across the area.

&&

.DISCUSSION...
Issued at 139 AM MDT Mon Jul 20 2026

It was finally fairly quiet on a night shift, with only very
isolated showers over Johnson County and the Wind River as I
write this. These should be gone over the next couple of hours.
And for the most part, the morning should be dry.

We have two issues to deal with this afternoon. The first is
the one we have been talking about ad nauseam for the past five
days, the monsoonal moisture and the potential for convection.
For the most part, the deepest moisture will remain south of the
area over the next 24 hours. This, plus some warmer air at the
mid levels should help cap the atmosphere somewhat. This should
keep shower and thunderstorm coverage fairly sparse.
Precipitable water values remain rather high though, generally
about 150 percent of normal, approaching 180 percent in
Sweetwater County. This is where the best chance of convection
will be, but even here it is only around a 1 in 4 chance for the
most part. As for heavy rain and potential flooding, the
Excessive Rainfall Outlook is still out of the area. I could see
a few heavier storms and the chance of flooding is not zero.
However, since the storms would be isolated and this area has
seen little rain since the monsoon started, the chance is low.
The moisture deepens later at night, but with the instability
limited this would likely only be showers.

The second issue is the heat. There are a few locations that have
had the warmest string of high temperatures in station history over
the 9 days (since last weekends heat wave began). And, we will have
one more hot day today. High temperatures look similar to
yesterday, with the warmer spots in the Bighorn Basin, like
Worland and Greybull, having a 1 in 2 chance of highs of 100 or
higher. Elsewhere, like yesterday, highs in the 90s will be
almost certain in locations below 6200 feet, with 80s in the
rest of the populated areas.

The monsoon really makes itself felt starting on Tuesday.
Precipitable water levels return to levels similar to much of
late last week, generally about 175 to 200 percent of normal.
The one thing that is missing is some kind of a forcing
mechanism, like a jet streak or a shortwave. So, these storms
look like more of the random pop up variety. There will be a
better chance of flooding with the higher precipitable water
content and expected slow movement. If there is one good thing,
it that much of the area has been largely dry the past 48 hours
or so, so the soil won`t be as saturated. Nevertheless, the
marginal risk of Excessive Rainfall looks justified at this
time. The most numerous storms will be in western Wyoming on
this day, with less coverage north and east. And with more cloud
cover, temperatures will finally cool off somewhat, an average
of about 5 degrees when compared to Monday. Showers and storms
should decrease after sunset, although some will linger through
the night.

The main event still looks to be on Wednesday. The deepest moisture
moves over the area during the afternoon and evening. The models
continue to show a possible record breaking precipitable water
value, over 1.50 inches, breaking the old record of 1.42 inches. In
addition, a shortwave will be passing through the area, bringing
additional lift and an increased coverage of thunderstorms. If there
is one thing working against flooding, it is that with the
shortwave, there should be more steering flow and this may mean more
movement with the thunderstorms. However, the threat of flooding is
there. The Excessive Rainfall Outlook continues to show a slight
risk across much of the area, which has basically not moved
since yesterday. And, with the shortwave, showers and storms
may continue well into the evening and even the overnight hours.
We will likely need a flood watch, possibly as early as Tuesday
afternoon but we will let future shifts make this decision.

Thursday also looks like a rather active day as high precipitable
waters remain, although about 10 to 20 percent less than Wednesday.
Nevertheless, this is still 200 percent of average for many
locations. So, expect another of fairly widespread showers and
thunderstorms across the area. Following that, somewhat drier air
will begin to push into the area from the north and west,
bringing an end to the widespread showers and thunderstorms.
There will be some around each day into next weekend, but more
scattered and more limited to the favored locations like the
mountains. However, above normal precipitable waters will linger
into the weekend, although more on the 125 to 150 percent of
normal. And with less cloud cover, temperatures should return to
above normal levels Friday and continue into next weekend, with
some triple digit highs possible once again in the warmer
spots.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1125 AM MDT Mon Jul 20 2026

Upper-level high pressure centered over NW CO continues to
dominate the weather pattern over western and central WY today
into Tuesday. Moisture will be limited today with fewer showers
and thunderstorms expected. Southern WY will have the best
chances for showers and thunderstorms late this afternoon into
the evening hours, though mid level clouds will increase from
mid-afternoon into the evening. Thus, VFR conditions are
generally expected at all TAF sites, though KRKS could see MVFR
conditions with thunderstorms from 20/23-21/04Z. KLND has an
outside chance of a thunderstorm or two, but the probability is
less than 10%, so have not mentioned it in the TAF. Winds are
relatively light today, through showers and thunderstorms could
produce gusts 25-40 kts. In addition, a boundary will be moving
from MT into northern and central WY this afternoon and evening,
bringing gusty NNE winds to sites east of the divide starting at
20Z at KCOD, pushing into KLND/KRIW/KCPR between 00-01Z. The
gusty NE winds will push into KRKS around 04Z and influence wind
dir at KBPI/KPNA as well.

Mid-level clouds are expected to remain overnight and a few
isolated mountain showers are possible. Moisture increases
Tuesday, with showers and thunderstorms initiating between
18-20Z across western and southwest WY. Scattered showers and
thunderstorms will increase Tuesday afternoon and evening,
impacting all TAF sites except KWRL and KCOD. Gusty winds of
30-40 kts and brief heavy rain could bring MVFR conditions.

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...
None.

&&

$$

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