Arminto, Wyoming Weather Forecast Discussion

073
FXUS65 KRIW 290747
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
147 AM MDT Mon Sep 29 2025
.KEY MESSAGES...
- Widely scattered (10-20%) showers and thunderstorms across
  western portions this afternoon and evening.
- Chances are low (10%) for locally heavy rain with the
  stronger storms this afternoon.
- Shower and thunderstorms will be possible over western
  portions again on Tuesday, spreading east of the Divide late
  in the afternoon into the evening.
- Chances for widespread precipitation and high elevation snow
  increase Friday and Saturday.
&&
.DISCUSSION...
Issued at 130 AM MDT Mon Sep 29 2025
Showers and isolated thunderstorms are ongoing across areas east of
the Divide, but are beginning to wane as of 05Z. These are expected
to end through 09Z, with only very isolated weak showers remaining
possible through 12Z. The convective activity today is not expected
to be as energetic or widespread today, as the mechanism that led to
the widespread showers and storms Sunday will not be in place again
today (namely the diffluence that occurred on the north side of the
remnant Desert Southwest upper low. Also, the low has been weakening
and filling as it moves northward Sunday night/Monday morning).
Showers and thunderstorms will redevelop over western portions of
the CWA, including areas near Dubois, Cody and Meeteetse, as well as
the Bighorns, after 19Z as weak, subtle shortwaves move northward.
Wind gusts up to 40 mph will be the main threat, with a small chance
of locally heavy rain once again. PWATs of 0.6" to 0.8" will be in
place again, with the higher amounts occurring over areas east of
the Divide. Precipitation will end across the area through the
evening hours. Otherwise, there will be little change in
temperatures today, remaining about 5 to 10 degrees above normal.
A longwave trough will begin to move onshore over the West Coast
through the day today. It will fill and weaken as it moves over the
PACNW through Tuesday. The southern end of the trough will initiate
isolated showers over western portions Tuesday morning, with more
widespread showers and thunderstorms occurring by 21Z as the trough
approaches the Cowboy State. Most of the precipitation east of the
Divide will occur with the main line of showers and storms as the
trough progresses eastward. The main threats with the stronger
storms will again be wind gusts up to 40 mph and locally heavy rain,
as the main mechanism for the storms will be the trough. Instability
will be weak, with CAPE values only ranging between 200-500 J/kg.
Additionally, any jet support will be well to north over Canada
(indicative of the weakening nature of the trough). Precipitation
will end across the area through the evening hours as the trough
exits to the east. However, isolated showers will remain possible
over far western portions through the overnight hours.
A southwest flow pattern will be in place Wednesday through Friday.
A closed low associated with the longwave trough will make little
progression eastward due to a strong high center over the mid-
Mississippi River Valley. Any precipitation Wednesday and Thursday
will be confined to far northwestern portions. Temperatures be
relatively unchanged through Thursday, staying in the aforementioned
5 to 10 degree above normal range. By Friday, the upper low will dig
southward over the Great Basin. Models remain split on timing of
when this storm moves over the Cowboy State. The ECMWF remains the
faster solution, bringing widespread precipitation across the area
through the day Friday and ending Friday night as the storm moves
over the Plains. The GFS continues to be the slower solution,
setting up a possible overrunning scenario over central portions of
the CWA. The associated cold front will move across areas east of
the Divide Friday morning, becoming stationary along the Divide and
increasing precipitation chances from the south through the
afternoon. Snow levels will range between 8000 and 9000 ft, with
showers continuing across much of the area Friday night into
Saturday as the low slowly tracks across northern CO. The system
will exit over the Northern Plains and Upper Midwest, as a second
storm system drops southward from western Canada Sunday.
&&
.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1050 PM MDT Sun Sep 28 2025
Scattered showers currently ongoing across the area will continue to
decrease over the next few hours, generally coming to an end by 09Z.
Clouds will similarly be on the decrease through the morning
hours. One caveat to this trend will be the possibility for
lingering showers in the KCPR vicinity overnight through mid
Monday morning, though confidence is not high enough to include
any impacts in the TAF. Another shortwave will arrive Monday
afternoon, returning generally low shower and thunderstorm
chances to western Wyoming. The best chances for impacts will be
at KCOD and KJAC after 21Z. Otherwise, clouds will increase but
terminals are expected to remain VFR. Any precipitation should
come to an end between 03Z and 06Z Tuesday.
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...LaVoie
AVIATION...Myers

NWS RIW Office Area Forecast Discussion