Granger, Wyoming Weather Forecast Discussion

826
FXUS65 KRIW 302343
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
543 PM MDT Tue Sep 30 2025
.KEY MESSAGES...
- Rain showers remain possible (30 to 50% chances) through the
  rest of the evening across the west and portions of central
  WY.
- Isolated showers and storms linger across western WY for
  Wednesday with mostly dry conditions expected elsewhere.
- Chances for widespread low elevation precipitation and high
  elevation accumulating snowfall increase for late Friday and
  Saturday.
- Cold temperatures following the weekend storm system could
  bring the first freezing temperatures to lower elevations east
  of the Divide Sunday/Monday (20 to 30% confidence)
&&
.UPDATE...
Issued at 1239 PM MDT Tue Sep 30 2025
The forecast remains on track for the next few days with a longwave
trough gradually making its way over the PACNW today. Weak embedded
shortwaves will move through the region over the next few days as
the long wave trough inches farther east. This will bring chances
for showers and thunderstorms over the next few days with the best
chances (30-60%) remaining across western WY. Isolated areas of
heavy rainfall along with gusty outflow winds will be the main
concern with any stronger storms. Warm above normal temperatures
persist across the eastern half of the state. Highs today will be in
the mid 70s to low 80s east of the Divide. Highs in the west will be
closer to seasonal values ranging in the low to mid 60s. These
temperature ranges remain unchanged through most of the remainder of
the week.
A cold front moves through for the end of the week as a result of
the longwave trough nearing the area. An eye will need to be kept on
this as there is the potential for a rather potent system developing
for the weekend. Forecast models have very little agreement among
themselves at this time. The only real constant among them all,
being some kind of low developing over the weekend. The time frame
for this currently looks to be late Friday into Saturday and
depending on the model possibly Sunday. Impacts currently vary
greatly with the GFS showing a low developing and moving farther
north while other models such as the Euro slow the low down and keep
it farther south. Due to the high amount of uncertainty going into
further detail is not beneficial at this time. However, one
thing to keep in mind is that the potential for a system
continues to increase but the impacts and timing still remain
unknown. Behind this system looks to be some much colder air and
may bring the first real freeze to locations east of the Divide
for the start of next week. Overall, confidence still remains
low with high uncertainty regarding the upcoming weekend
forecast but a better idea of what to expect should come to
fruition over the next few days.
&&
.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 /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 537 PM MDT Tue Sep 30 2025
VFR conditions prevail through the period at all terminals. A few
showers continue across the region to start the period. Gusty winds
may occur with these showers, most notably near KCOD, KRIW, and
KRKS. KJAC will see showers linger the longest with a second round
of showers also expected later tonight. MVFR conditions are possible
at KJAC with these showers. There are hints that fog could develop
at KBPI/KPNA/KJAC early Wednesday morning, but mid- and high clouds
should be thick enough through the night that fog development is
unlikely (less than 10% chance). Breezy winds 10 to 15kts will also
remain a concern through much of the period, most notably at KCPR
and KRIW. Gusts should generally decrease at the beginning of the
period, however, with gusts returning at KCPR by mid morning.
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.
&&
$$
UPDATE...Dziewaltowski
DISCUSSION...LaVoie
AVIATION...Hensley

NWS RIW Office Area Forecast Discussion