645
FXUS65 KRIW 220730
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
130 AM MDT Tue Sep 22 2026

.KEY MESSAGES...

- Chilly temperatures return again Tuesday morning with lows in
  the 30s and middle 40s. Clouds will increase from the south
  through the overnight hours. There will be a 10 to 20% chance
  for low clouds/fog near Casper Tuesday morning through about
  9 AM.

- Shower and thunderstorm chances (40% to 70%) on Tuesday
  afternoon and evening, with gusty outflow winds up to 50 to 55
  mph the main threat through sunset.

- Dry conditions and a warming trend through the end of the week
  into the weekend with the next system pushing in late Sunday
  into early next week.

&&

.DISCUSSION...
Issued at 130 AM MDT Tue Sep 22 2026

IR currently shows an upper level low over Nevada with cooling
cloud tops pushing into northern Utah and far southwestern
reaches of Wyoming. This will be the ignition for thunderstorm
development come later Tuesday afternoon and evening as it
pushes northeast through the morning hours. WV shows the main
finger of the PFJ extending around a low over the Gulf of Alaska
extending into Canada, with a weak finger of 60-70kts around the
aforementioned low over Nevada. With not much support, it is
forecasted to become more of an open wave as it moves over the
CWA in the afternoon and evening. With mostly clear skies for
much of the CWA, coupled with recent rainfall, fog development
is possible in eastern Natrona County around Casper by sunrise
Tuesday morning, but confidence low with small hint of upslope
easterly flow by the FV3 sticking with 10-20% chance.
Regardless, this will burn off quickly with a slightly warmer
afternoon in store than it was Monday by 2 to 5 degrees.

With the approaching system from the southwest, expect scattered
rain shower activity with a few isolated thunderstorms to push
in from northeastern Utah west of the Divide by 11AM to noon
with northward progression through the evening hours. This will
extend across the Absarokas and Bighorn Basin by late afternoon
exiting into Montana by 7-8PM weakening with radiational cooling
towards sunset. The main concern will be with a stronger push of
PVA anomaly into western Sweetwater County by 4-5PM extending
over the southern Winds into the Wind River Basin by 6-7PM
before exiting across Natrona/Johnson Counties to the northeast
by 9-10PM. The main threats will be gusty outflow winds up to 50
to 55mph. MLCAPE values are forecasted to increase to 6-700 J/kg
by late afternoon with forecast sounding output of shear values
over 30kts that will help aid in the stronger winds. Not quite
severe is expected, with much of that activity into northern
Utah to Uinta County due to stronger aid of peak heating of the
afternoon earlier than when it pushes into the RIW CWA. Heavy
downpours will not be as high of concern being a bit more
progressive than over the weekend with the low becoming more
open with its northeastward progression into the upper Great
Plains by midnight Tuesday night. It fills with time as well
with decreased divergence aloft and ridging builds in from the
south.

With that said, a warming trend and dry conditions expected for
the rest of the work week into much of the weekend. Above
average temperatures in the afternoon by Thursday extending
through Saturday ahead of the next system that could affect the
upper Rockies by late Sunday from the previously mentioned GOA
low. Longer term model solutions aren`t in total agreement on
its progression to the Pacific northwest and later through the
Rockies but something to watch in the coming days for late
Sunday into early next week. For now, enjoy the rain chances for
Tuesday, as it will be a few days at least until the next decent
chance.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1045 PM MDT Mon Sep 21 2026

VFR conditions will prevail across all terminals throughout much of
the TAF period. The one caveat to this may be some patchy fog
possible nearby KCPR around 10Z to 16Z Tuesday morning. Otherwise
winds remain light at 5 to 10 knots through the overnight into
Tuesday morning. A disturbance is expected to begin to near the
region Tuesday bringing with it chances for showers and
thunderstorms across nearly all terminals. Showers and storms look
to start developing shortly after 18Z with PROB30 groups starting at
KJAC, KPNA, KBPI, and KRKS by 21Z through 03Z Wednesday. Terminals
east of the Divide look to see precipitation arrive closer to 00Z
Wednesday with many terminals seeing PROB30 groups start around 23Z
Tuesday through 04Z Wednesday. KCPR and KCOD may see lingering
showers and storm into the end of the TAF period. The stronger
storms if moving directly overhead a terminal may create brief MVFR
to IFR conditions. The main concerns with any convection will be
brief periods of moderate to heavy rain and strong gusty winds of 35
to 45 knots. Strong wind gusts nearing 50 knots cannot be ruled out
especially east of the Divide. Mountains will likely be obscured
through the afternoon and evening Tuesday with some improvement
possible to end the period as precipitation dissipates.

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

&&

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

&&

$$

DISCUSSION...Lowe
AVIATION...Dziewaltowski