404
FXUS65 KRIW 201512
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
912 AM MDT Sun Sep 20 2026

.KEY MESSAGES...

- Showers and thunderstorms are forecast through sunset across
  Johnson and Natrona Counties, with heavy rain and isolated
  flooding possible. A few showers and thunderstorms are
  possible (30% chance) across the Absarokas, Wind River
  Mountains, and Wind River Basin between 2PM and 8PM.

- Elsewhere across the state, drier conditions under partly
  sunny skies are expected throughout the day today. Expect
  widespread highs in the 60s.

- Apart from shower and thunderstorm chances (20% to 60%) on
  Tuesday afternoon and evening, this work week is forecast to
  be dry with above normal temperatures.

&&

.DISCUSSION...
Issued at 237 AM MDT Sun Sep 20 2026

Rain continues to be ongoing across portions of the area, mainly
central Wyoming as low pressure continues to move across the area,
It is now fairly close to our office just after midnight with
moisture wrapping back into the Wind River Basin. Some area had a
lot of rain yesterday afternoon and evening, with some street
flooding in Lander. Most of the convective rain is done now, it now
mainly of the stratiform variety. The main area of rain will
continue move slowly east through the night and into tomorrow with
gradually drying as the low tracks to the east. We will still have
the chance of some flooding today as well, especially later in
the day if there is some destabilization of the atmosphere to
get some thunderstorms to fire. However, the impacted area will
be more restricted. Precipitable water values remain over 150
percent of normal along and East of the Bighorns. So, we will
maintain the flood watch here for the rest of the day. Across
the western zones, with the deeper moisture moving away, we will
cancel the watch with issuance this morning. There will be
enough moisture for isolated showers and storms to fire back
into the Basins East of the Divide and the adjacent mountains,
but coverage looks rather sparse (generally less than a 1 out of
3 chance). Showers should gradually end this evening across the
eastern zones this evening with all areas dry by midnight. Some
locations in Johnson and Natrona Counties could pick up 0.75 to
1.50 inches of additional rain today, especially where storms
persist. However, not all areas will see heavy rain.

Once we get past this evening, it looks like we will enter a
quieter pattern. Monday looks like a nice last day of
astronomical summer for 95 percent of the area with near normal
temperatures and a sunny to partly cloudy sky. There may be just
enough moisture leftover for an isolated, and this means
isolated with a capital I, shower or thunderstorm in and near
the mountains East of the Divide. The chance is at most 1 out of
6 though, and nothing strong is expected. The only fly in the
ointment is an upper level low moving northeastward from the
Desert Southwest that will open into a trough and pass through
Tuesday afternoon through Wednesday morning. The GFS has trended
toward the European, so the chance of showers and maybe a few
thunderstorms looks better. This system is a fast mover so no
heavy rain or flooding is expected out of it at this time. Any
precipitation from this system should clear the area by sunset
on Wednesday. Following that, ridging should build across the
area and bring dry weather for the rest of the workweek and
possibly into the start of next weekend. And with generally
southwest flow across the area, temperatures should average
above normal, in some cases well above normal. Some of the
warmer spots may see highs in the mid or even the upper 80s from
Thursday through Saturday.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 454 AM MDT Sun Sep 20 2026

Rain will continue at issuance roughly east of a KGEY to KWRL to
KRIW line this morning. Rain will slowly move to the east, with
gradually clearing skies from west to east. Showers and possible
a thunderstorm remain possible in vicinity of KCPR this
afternoon. Later this afternoon, there is a 20 to 25 percent
chance of a shower or thunderstorm along the eastern slopes of
the mountains, especially in vicinity of KLND and KCOD until
around 01Z Monday. Largely VFR conditions should dominate in
areas West of the Continental Divide. There will be mountain
obscurations at times especially in the morning and around the
Wind River Range, Bighorn Range, Absarokas and Casper Mountain.
Patchy fog may impact a few terminals but not enough confidence
to include in the TAFs. Wind will average 5-10 knots through
the TAF period, with gusts up to 20 knots at KCPR Sunday
afternoon, with locally higher gusts in and around any
thunderstorm.

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 until 6 PM MDT this evening for WYZ007>011.

&&

$$

DISCUSSION...Hattings
AVIATION...Hattings