367
FXUS65 KRIW 210347
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
947 PM MDT Thu Aug 20 2026

.KEY MESSAGES...

- Scattered rain showers, mainly virga, will continue to move
  east across southern portions of the area until midnight. Wind
  gusts up to 50 mph will be the main threat.

- Another degree or two warmer Friday. Isolated storms/rain
  showers (~15-20%) Friday afternoon and evening for western
  portions of the area with stronger outflows the main threat.

- Deeper monsoonal moisture returns to the area on Saturday with
  an increased chance of showers and thunderstorms, with heavy
  rain possible with any storm.

- The daily chance of mainly afternoon showers and thunderstorms
  continues Sunday into early next week.

&&

.UPDATE...
Issued at 1200 PM MDT Thu Aug 20 2026

Not much in way of significant changes and/or edits in the short
and long term forecast. PoPs were decreased a bit for the
southern half of the CWA west of the Divide as HiRes models have
declined in activity for this afternoon and evening. Similar
conditions likely Friday as well with more of a cap to prevent
more active storms. Saturday still on track in terms of
precipitation into the weekend and early next week. Increased
PoPs a bit across Yellowstone Saturday morning with some early
shower activity that seems to be more likely many models are
missing coming out of Montana, as this aligns with neighbors as
well. Otherwise, status quo, onward and upward.

&&

.DISCUSSION...
Issued at 110 AM MDT Thu Aug 20 2026

Yesterday was one of those late summer days that make me happy to
live in Wyoming. Warm temperatures, low humidity, no precipitation
and lots of sunshine. However, that will be the last one of these
days we will see for a while. Portions of the area may see one from
time to time, but not all at once.

All is quiet right now across Wyoming and the humble Riverton
weather abode. However, things will begin to turn more active again
starting today. However, it will be a gradually ramp up rather than
flipping a switch. And, we will start the day off dry today.
However, monsoonal moisture, which was suppressed to the south
yesterday, will begin to come back into the area today. However, as
is the usual trend, it will start at the mid levels and mainly in
western Wyoming. The result will be a scattering of late day and
evening showers and storms across this region. However, most
locations will not see one. The chance at any one spot is no more
than 1 out of 4, with the highest chance in places like southern
Lincoln and western Sweetwater County having the highest chance.
Mainly dry conditions should continue East of the Divide. I can`t
completely rule out a stray shower to cross the mountains, but the
chance is small, less than 1 in 10. And, these storms would likely
produce more wind than rain. Many locations will have a greater than
50 degree dew point depression with the dry lower levels of the
atmosphere. This, plus the tell tale inverted V soundings, show a
good possibility of strong wind gusts with any shower or storm. This
does have a good chance to be one those "little green blob" days.
Most activity should be diurnal and end shortly after sunset. And,
with the ridge expanding northward and 700 millibar temperatures
rising, it will be hot day across much of the area. Most locations
below around 5500 feet will see highs over 90.

Friday at this point looks fairly similar, with most convection
confined to the western mountains, and chances remaining small at
any particular location, generally less than 1 out of 4 once again.
The bigger story on this day will be the heat. The 700 millibar
temperatures peak on this day, rising as high as 20 celsius. Most
locations below 6000 feet have at least a 1 in 2 chance of high
temperatures over 95. There is even an outside chance of a few of
the warmer spots, like Greybull and Worland, touching 100.

Chances of showers and storms begin ramping up Saturday. However,
the latest guidance does show a somewhat slower arrival of the
deeper moisture. As a result, POPS were cut back late Friday night
with most locations likely remaining dry through sunrise on
Saturday. A shortwave should be tracking across the area in the
afternoon and will help to enhance shower or thunderstorm activity
across the area. Precipitable water values do rise considerably, to
as much as an inch in portions of the area East of the Divide. As
for area of greatest coverage, most guidance favors area West of the
Divide. However, not only there. This will be a situation where all
areas will have a chance of a shower or storm, but not everyone will
see one. Much of the area is also under a marginal risk of
excessive rainfall, so heavy rain and possibly local flooding is
not out of the question. And this far out, it is impossible to
pinpoint. Most guidance shows most activity weakening and ending
after midnight. The models lately, both the synoptic and
convective, have been absolutely terrible in handing nighttime
convection with the monsoon. So may confidence on a dry Saturday
night is rock bottom at this point. Temperatures will cool
somewhat with more cloud cover, but remain well above normal,
with highs in the 90s East of the Divide.

Near to above normal precipitable water is expected to hang around
through Sunday and into next week. It will not be as deep as on
Saturday. The result will be a daily chance of mainly diurnal
showers and thunderstorms, although it is difficult to nail down a
more active area or day this far out. With ridging remaining over
the area, above normal temperatures are likely to continue although
record high temperatures are not expected.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 946 PM MDT Thu Aug 20 2026

Any lingering showers end by the start of the TAF period. Monsoonal
moisture will begin to move into southwestern portions of the area
on Friday. Most of this moisture will be above the surface, so it
will be enough for scattered rain showers and thunderstorms to
develop, but not enough for much rain to reach the ground. Showers
and storms will first develop near KBPI and KPNA around 20Z, with
KJAC, KRIW, and KLND following around 21Z. There may be showers and
storms near KRKS as well, but the chance was too low (15-20%) to
include a PROB30 group at this time. There is about a 15% chance of
showers at KBPI, KPNA, and KJAC late Friday evening once PROB30
groups end at 03-04Z.

Wind will remain below 10 knots tonight into Friday morning. Then,
wind will increase late Friday morning and Friday afternoon to
around 10 knots for most terminals. KCPR will be a bit gusty with
gusts around 20 knots through the afternoon hours. The main threat
with showers and storms will be strong downdrafts and outflows.
Convective wind gusts may reach as high as 50 mph Friday afternoon.

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...Lowe
DISCUSSION...Hattings
AVIATION...Rowe