Midwest, Wyoming Weather Forecast Discussion

813
FXUS65 KRIW 300756
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
156 AM MDT Tue Sep 30 2025
.KEY MESSAGES...
- There is a 10% chance for showers over western portions this
  morning, increasing to 40-60% of showers and thunderstorms
  after 1200. There is a 30-50% for showers and thunderstorms
  over the central basins and southern portions between 3 PM and
  9 PM.
- Wind gusts up to 40 mph and locally heavy rain will be the
  main threats with the stronger storms today.
- There will be a 10% chance for showers over the western
  mountains Wednesday.
- Chances for widespread precipitation and high elevation snow
  increase Friday and Saturday, with cold temperatures possible
  Saturday night and Sunday night.
&&
.DISCUSSION...
Issued at 137 AM MDT Tue Sep 30 2025
A longwave trough continues to move across the PACNW early this
morning. The overall forecast for today remains largely unchanged.
The trough will weaken as embedded shortwaves move northward through
the flow aloft. The southern end of the trough will initiate
isolated showers over western portions between 10Z and 12Z, with
more widespread showers and thunderstorms developing by 19Z 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 after 21Z. 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 become isolated across the CWA by 03Z Wednesday,
becoming more focused over western portions by 08Z as another
shortwave moves over the area. These showers will move over the
central basins between 10Z and 12Z Wednesday morning as the
shortwave continues its eastward track and finally end by late
morning.
A southwest flow pattern will be in place Wednesday in the wake of
this trough. A closed low will slowly make its way toward the
Puget Sound area, filling as it does so, and moving onshore by
00Z Friday. Isolated showers will be possible over the western
mountains through the afternoon Wednesday, as a result of the
flow pattern. Conditions will be mainly dry Thursday, with any
precipitation staying over far northwestern portions. The trough
will begin to dig over northern CA and the Great Basin Thursday
and Thursday night, with a new closed low developing over the
Great Basin during the day Friday. Friday will become more
active, as a leeside low develops over eastern WY and showers
and thunderstorms occur over western portions of the CWA through
the afternoon. Chances for elevated fire weather conditions are
looking better across the Wind Corridor (Rock Springs to
Casper), as 700mb winds up to 35 kt and temperatures of 10C to
12C will be in place. Showers and isolated thunderstorms will
continue across the west Friday night, with snow levels around
9500 ft. These showers will spread across the rest of the
forecast area Saturday as the upper low moves over the area with
snow levels dropping through the day, ranging between 7500 and
8500 ft. The system will exit over the Northern Plains Saturday
night, with precipitation ending across the area as it does so.
A second trough is still progged to drop southward from western
Canada on Sunday, bringing another chance for precipitation and
colder temperatures. Model trends are still leaning toward a
positively-tilted trough developing by Monday, with a low center
over the Great Basin once again. This would keep much of the colder
air further north. Overall, confidence remains medium of a storm
system impacting the region this weekend. This is partly due to a
previously mentioned, relatively strong high center that will be in
place over the Ohio River Valley through the rest of the week.
Models are split with the Friday/Saturday storm as a result, as well
as showing signs of a strong low pressure system in Canada next week
which could bring the first push of cold air Sunday/Monday.
&&
.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1030 PM MDT Mon Sep 29 2025
VFR conditions expected for the entirety of the period for all
TAF sites. Light winds and some scattered mid to upper level
clouds overnight. Winds will increase west of the Divide after
15Z up to 18kts for KJAC/KBPI/KPNA as well as KCOD, and up to
25kts at KRKS/up to 30kts at KCPR. KLND/KRIW will not see the
increased winds until after 18Z-19Z. These winds will subside
after 00Z-01Z towards sunset with radiational cooling.
Rain shower activity increases after 15Z for KJAC spreading
east to KBPI/KPNA/KRKS after 18Z-21Z. Isolated thunder is
possible during this peak heating of the afternoon. These will
weaken towards 00Z-01Z as TAF sites east of the Divide will
remain dry.
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...Lowe

NWS RIW Office Area Forecast Discussion