Mantua, Wyoming Weather Forecast Discussion

308
FXUS65 KRIW 191001
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
401 AM MDT Fri Sep 19 2025
.KEY MESSAGES...
- A warm and dry day is expected today.
- Chances for showers and thunderstorms increase this weekend,
  especially Sunday as a cold front approaches.
- Early next week looks cooler and wetter, with snow possible in
  the higher elevations.
&&
.DISCUSSION...
Issued at 220 AM MDT Fri Sep 19 2025
We are nearing the end of astronomical summer, as autumn arrives
shortly after noon on Monday, September 22nd. And those of us that
have lived in Wyoming for a while, know that the days of warm
weather are numbered. However, we still have some warm days coming
up, although they will be interrupted by a taste of cool fall
weather. And, possibly a few inches of the white stuff, a.k.a. snow,
in some locations. As always, details will be below.
Today will be one of those nice September days, much like yesterday.
Satellite imagery does show some mid and high level clouds
approaching from the south though, so it will not be as sunny as
yesterday. Nevertheless, a nice day for the vast majority of the
area. The only small exception is there may be just enough moisture
for a stray shower or thunderstorm this afternoon in the southwestern
mountains. This chance is very small though, at most 1 out of
10. We have left it out of the official forecast but if you are
in these locations, don`t be completely surprised if there are
one or two around.
Saturday looks like a nice start to the weekend for a majority of
the area as well, with warm temperatures and light to moderate wind.
The aforementioned moisture will deepen somewhat and increase
the chance for some afternoon and evening convection across the
western mountains. The bulk of it remains west and south of
Wyoming, so the chance of a shower or storm tops out at around 1
out of 4 and even these places should be largely dry for most
of the day. Also, with southwest wind increasing, there could be
some elevated fire weather in the afternoon with the low
humidity in place.
The taste of fall will move start to move in on Sunday as a cold
front approaches from the west, increasing the chance of more
widespread showers and thunderstorms. There are still some model
differences in the timing of the front though. The American
models are a bit slower with it and the ECMWF a bit more
progressive. For now, we compromised and went with the best
chances across western Wyoming, with much of central Wyoming
likely being mainly dry most of the day. Areas east of the
Divide should have one more warm day, with cooler conditions
moving into the west.
A second part of the system, in the form of a trough that will
likely close off into an upper level low as it crosses Wyoming then
moves in for Monday. This is where temperatures will fall to
cooler levels, below normal for many locations. And this is
where the white stuff conversation comes in. In the lower
elevations, with low wet bulb zero levels, the white stuff could
come in the form of small hail or possibly even graupel at
times. And the other white stuff, snow, would be in the
mountains. It should remain fairly high up though. The 700
millibar temperatures fall to around 0C, which would put snow
levels around 8500 feet in the northern mountains. At this
point, travel problems should be minimal given the warm ground
temperatures. However, those venturing into the higher
elevations, especially the Bighorns, should be prepared for wet
and cool conditions. The higher peaks in the Bighorns could
pick up as much as 6 inches of new snow with the favorable
upslope flow, although the 50th percentile on most ensemble
guidance is around 2 to 3 inches. The air mass is of Pacific
origin, so at this time the chance of snow in the lower
elevations is very low. Monday night looks like a cool night as
well, although cloud cover should keep any freeze conditions in
the lower elevations at bay.
Things are more uncertain for Tuesday, as there are differences
in how fast the models move the low out of Wyoming. Some, like
the ECMWF, are more progressive and move the low out by Tuesday
morning. The GFS keeps things cool and unsettled. We didn`t
change much this far out, since model guidance tends to not
handle upper level low movement very well. Wednesday and
Thursday look dry and warmer though as ridging builds back
across the Rockies.
&&
.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 400 AM MDT Fri Sep 19 2025
High clouds will continue to increase from the west through the
morning, with SCT-BKN FL180-250 skies in place over areas west
of the Divide after 18Z. BKN-OVC skies will then be in place
across the forecast area after 00Z. Isolated showers will be
possible over southern portions between 00Z and 06Z, but chances
and confidence remain too low to include in the forecasts for
KBPI, KPNA and KRKS. Winds will continue to be light (10kt or
less) through the TAF period.
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...Hattings
AVIATION...LaVoie

NWS RIW Office Area Forecast Discussion