309
FXUS65 KRIW 041107
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
507 AM MDT Sat Oct 4 2025

.KEY MESSAGES...

- A wetter and cooler day today with numerous showers and
  isolated thunderstorms.

- Turning much cooler tonight and Sunday with continued periods
  of rain and mountain snow.

- A frost or freeze is possible (around a 3 out of 5 chance) Sunday
  night or Monday across many of the central basins.

&&

.DISCUSSION...
Issued at 314 AM MDT Sat Oct 4 2025

It is rather common to experience large swings in the weather in
Wyoming. For example, from Friday through Sunday, over three days we
will experience weather in three different months. Friday was
September, with warm temperatures, elevated fire weather and
thunderstorms. Today will be October with cooling temperatures and
more showers and isolated thunderstorms, and snow at the highest
elevations. And Sunday will be November, with a cold rain in the
lower elevations and snow in the mountains, possibly with some
snowflakes in some of the lower elevations. So, let us get started.

Radar is showing some showers and isolated thunderstorms moving
north across the area, even with a couple of distant lightning
strikes seen from the humble Riverton weather abode. Most of the
heavy precipitation is still to the west of the area across Idaho
and Utah. This will gradually shift eastward through the day as a
low pressure system lifts across the area this afternoon. And this
is when we expect the most widespread steadiest precipitation, from
this afternoon into this evening, with some decrease as the low
moves into Montana. We do have a small area of marginal risk for
excessive rainfall where showers may train a bit across western
Wyoming, but the risk of flooding remains low, mainly since it has
been fairly dry and there is room in the soil. Temperatures will be
cooler today as the first cold front moves through, but not really
cool as the air behind the first on is of Pacific origin. As a
result, 700 millibar temperatures should remain above 0 celsius for
most of the day, keeping snow levels fairly high and travel problems
somewhat minimal. Highs will be about 15 to 20 degrees cooler, but
these highs are not far away from seasonal normals.

Temperatures will begin to noticeably fall as a second system, this
one from Canada, starts to rotate into the area. On Saturday night,
700 millibar temperatures will fall to anywhere from minus 2 in
southern Wyoming to as cold as minus 5 degrees northern Wyoming.
This puts snow levels anywhere from 7500 to as low as 6000 feet. And
this is where the accumulating snow will really begin, as well as
the travel problems. The best moisture and dynamics will have moved
away at this point, but there will still be enough lift to periods
of light to moderate rain and snow through Sunday and evening Sunday
evening. As for snowfall amounts, continuity looks fairly
reasonable. Ensemble guidance gives at least a 4 out of 5 chance of
6 inches or more of snow across the areas with the advisories. As
for the chance of over a foot of snow, chances have increased
a bit. However, the areas with a greater than 1 out of 2 chance
are largely above 9000 feet and are not in the areas with
roads. The one exception may be Granite Pass, there is around a
1 in 2 chance of over a foot. However, the snow may have trouble
sticking to roads initially and during the day on Sunday, given
the warm ground and still decent sun angle. We will continue
the advisories for now but the day crew may want to take a
second look, mainly for Bighorn Range, if we need a warning. As
for the lower elevations, the chance for any accumulation would
be above 6000 feet. For example, Dubois has about a 2 in 5
chance of an inch of snow or more, and Meeteetse about a 1 in 5
chance. Elsewhere, there will likely be nothing more than
flurries or a light coating.

As for precipitation amounts, all areas except for southern
Sweetwater County have at least a 3 out of 5 chance of at least a
quarter of an inch of QPF. The northern half of the state has at
least a 3 out of 5 chance of at least a half an inch of rain.  As
for over an inch of rain, the best chance would be in the northern
third of the state, as well as some eastern facing slopes since
there could be some easterly upslope flow on Sunday, squeezing out
more moisture.

Wind may also be an issue at times, especially tonight following the
passage of the second cold front in the northwest flow / cold
advection locations. There is at least a 3 out of 4 chance of wind
gusts past 40 mph in the Bighorn Basin and northern Johnson County.
The 700 millibar wind only maxes out at around 35 knots though, so
high wind is not expected. It will make things feel much colder
though. With highs on Sunday only in the 40s and lower 50s, it will
feel like it is only in the 30s at times. And, this day will be the
November day.

And that brings us to one more problem. the likelihood of a freeze
on Sunday night. Chances of a freeze remain about the same as
yesterday at this time, anywhere from 1 out of 2 to 3 out of 4.
Again, the mitigating factor may be lingering cloud cover which
could keep temperatures up a bit. We won`t issue any freeze watches
yet but there is a possibly we may be issuing some at this time
tomorrow night.

As for next week, we didn`t have time to take a long look. However,
most guidance favors mainly dry conditions with flat ridging
building across the area. this should, after a cool Monday, allow
temperatures to return to near to somewhat above normal levels for
much of the week. A majority of the ensemble members are showing a
weak shortwave moving through Wednesday, and this could bring a
windy day. It has little moisture though, so any showers would be
minimal.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 503 AM MDT Sat Oct 4 2025

Showers continue to develop across the state this morning, with
main impacts currently approaching KJAC. Showers will continue
to impact all airports by 18Z, and will continue on and off
through the rest of the day. After sunset, wraparound rain will
build, with much lower cigs below 3000 feet for most sites
through the rest of the night. Cigs and vis will stay MVFR to
IFR. Only sites that might see any breaks would be KBPI/KPNA
after 12Z Sunday.

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...
Winter Weather Advisory from noon today to 6 PM MDT Sunday for
WYZ002-012-015.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Sunday for WYZ008-009.

&&

$$

DISCUSSION...Hattings
AVIATION...Straub