685
FXUS65 KRIW 202344
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
544 PM MDT Sun Sep 20 2026

.KEY MESSAGES...

- Showers across Johnson and Natrona Counties, the Bighorn
  Mountains and Wind River Basin continue to wane and will end
  by sunset this evening. Temperatures will be a bit chilly
  tonight, with lows in the 30s and middle 40s.

- Areas of fog are possible Monday morning across much of central
  and western Wyoming, especially in low-lying areas.

- 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 returning by Thursday.

&&

.UPDATE...
Issued at 1258 PM MDT Sun Sep 20 2026

The rainy and stormy weather that we have been experiencing the past
few days is coming to an end for most as the upper level low drifts
into the High Plains. As shared in the morning discussion, abundant
moisture will continue to wrap around the upper low into the far
eastern Bighorn Basin, the Bighorn Mountains, and Johnson County
this afternoon. These areas can expect showers and thunderstorms
through sunset, with instances of heavy rainfall and localized flash
flooding remaining possible. Thus, the Flash Flood Watch will remain
in effect until 6PM this evening. Portions of Natrona County could
also see instances of heavy rainfall and localized flooding, but the
chances are much less due to drier air shifting into the area.
Additionally, a few diurnal and terrain-driven showers and
thunderstorms are expected across and around the Absarokas and
Wind River Mountains between 1PM and 8PM.

The only addition to the forecast this afternoon is the potential
for fog on Monday morning. Skies are forecast to be mostly clear
tonight, so sufficient radiational cooling is expected. These clear
skies, combined with the recent widespread moisture, creates
favorable conditions for fog on Monday morning. Low lying areas
such as basins and valleys will be the most susceptible
locations. However, if we dry out enough this afternoon from the
sunshine, less areas would see fog. Regardless, anticipate
potential impacts for the Monday morning commute so be sure to
stay vigilant tomorrow morning and take it slow.

&&

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

Rain continues across portions of the area, mainly central
Wyoming, as low pressure continues to move through the area. It
is now fairly close to our office just after midnight with
moisture wrapping back into the Wind River Basin. Some areas had
a lot of rain yesterday afternoon and evening, with some street
flooding in Lander. Most of the convective rain is done now,
and is now mainly of the stratiform variety. The main area of
rain will continue to move slowly east through the night and
into tomorrow with gradual drying as the low tracks to the east.
We will still have the chance for 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 Continental
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 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 ECMWF, 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 work week 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 middle or even the upper 80s
from Thursday through Saturday.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 405 PM MDT Sun Sep 20 2026

VFR conditions continue through 06Z/Monday as a slow-moving low
pressure system finally moves into South Dakota and Nebraska.
Ongoing convection along and east of the Continental Divide will
generally wind down between 02Z-04Z/Monday. A few PROB30 groups
linger into Sunday evening as a result. As the low slowly pinwheels
east overnight, persistent mid-cloud ceilings rise and cloud cover
dissipates. The clearing sky may introduce areas of fog east of the
Continental Divide and at KJAC. Given what occurred at KJAC Sunday
morning, have included a TEMPO group for at least MVFR fog Monday
morning given continued moist boundary layer. The heavy rain at KRIW
and KLND should lead to late night fog in the Wind River Basin, but
this may stay closer to the river valleys. Therefore, confidence is
not as high at these two terminals and have not included late night
impacts at either terminal. KWRL could also be in the same boat. All
terminals to be VFR after 18Z/Monday. The gusty surface wind 10-
20kts at KCPR fades early in the period. All terminals to see wind
speeds less than 10kts through the remainder of the forecast period.
Occasional mountain top obscurations through 06Z/Monday, with
occasional foothill obscurations in the western valleys and central
basins after 10Z/Monday.

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...CNJ/LaVoie