Midwest, Wyoming Weather Forecast Discussion

033
FXUS65 KRIW 200440
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
1040 PM MDT Fri Sep 19 2025
.KEY MESSAGES...
- Warm and mostly dry weather continues this afternoon and
  evening.
- Shower and thunderstorm chances increase this weekend, with
  the greatest chances ahead and along a cold front Sunday into
  Monday.
- Cooler and wetter conditions early next week with snow
  potential across higher elevations. Dry and warm conditions
  return the middle next week.
&&
.UPDATE...
Issued at 1208 PM MDT Fri Sep 19 2025
The forecast remains unchanged this afternoon and evening with warm
temperatures, increasing cloud cover, and little to no
precipitation chances. 18Z satellite imagery shows mid to high
level clouds slowly moving into Wyoming from the south and west.
As clouds shift into the state and limit the amount of
radiative heating from the sun, high temperatures are forecast
to remain in the low to mid 70s across west and southwest
Wyoming despite mean 700mb temperatures around 10C. Elsewhere,
where the sun is able to shine for longer, high temperatures are
forecast to be in the high 70s and low 80s. Temperatures could
be a few degrees warmer this afternoon, should cloud cover be
less than forecast. Tonight`s lows are forecast to be anywhere
from 5 to 10 degrees warmer than last night due to increased
cloud cover limiting the amount of radiational cooling.
Rain chances are highest (10-14%) across far southwest Wyoming this
afternoon and evening. These very low precipitation chances
will be focused along a very weak area of PVA. Chances remain
very low due to a lack of both substantial support aloft and
available moisture. However, by Saturday, a shortwave, along
with a slight uptick in moisture and instability, is forecast to
move in from the southwest and provide support for isolated to
widely-scattered shower and thunderstorm chances across
southwest Wyoming beginning in the early afternoon. Saturday`s
forecasted mean PWATs (0.80") are around 0.30" greater than this
afternoons PWATs (0.50") and this is why instability (SBCAPE
around 500J/kg) and shower and thunderstorm chances (15-30%) are
greater Saturday compared to today.
There remains some uncertainty in the exact timing of the
Sunday into Monday cold front. However, confidence remains high
(70-80%) in shower and thunderstorm chances across western
Wyoming Sunday ahead of the front. These chances push east of
the Divide, along with the cold front and an upper low, Monday
morning. Temperatures on Monday are currently forecast to be 10
to 15 degrees cooler than what we feel this afternoon. These
colder temperatures will also be favorable for snow across high
elevations of northern Wyoming on Monday.
&&
.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 /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1035 PM MDT Fri Sep 19 2025
VFR conditions continue through the next 24 to 36 hours. Cigs
will remain restricted to high cirrus overnight tonight into
Saturday morning. Shower activity may approach KRKS late
Saturday afternoon, but chances still remain very low (under
20%), so no notation in TAFs yet. Even if showers do not
approach, midlevel cloud cover will thicken here and at
KBPI/KPNA late afternoon, lowering to near 8000ft. Increased CU
will also build along the higher elevations either side of KJAC
during the afternoon, but should stay out of the main approach
corridors.
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...Gerhardt
DISCUSSION...Hattings
AVIATION...Straub

NWS RIW Office Area Forecast Discussion