560
FXUS65 KRIW 130829
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
229 AM MDT Mon Oct 13 2025

.KEY MESSAGES...

- A mainly dry and seasonably day today, with any isolated
  showers confined to southern Wyoming.

- The next batch of showers spreads northward tonight, with the
  most numerous showers across western Wyoming.

- An unsettled and wetter than normal pattern is expected
  starting Tuesday night and continuing through at least
  Thursday, but details on timing and amounts of precipitation
  are uncertain.

&&

.DISCUSSION...
Issued at 223 AM MDT Mon Oct 13 2025

As I have mentioned in some of my past discussions, I am a native of
the northeast, New England to be exact. And back there, it is now
fair season. And one of the most popular rides at a fair is the
carousel, also called a merry-go-rounds. And what happens in this
ride? Wooden horses or other animals rotate around it. And this is
similar to the weather over the next four days of so. With the
central part of the carousel being an upper level low now spinning
over the northwest, with the horses being a few shortwaves that will
passing through the day before the main low moves through the area.

Most areas should start of dry today. There could be a few showers
in eastern Sweetwater County, but nothing of consequence. The first
horse, AKA the first surge of moisture and shortwave, will move
in from the south late today but especially tonight and bring
the first chance of showers to the area, with an emphasis on
southern and western Wyoming. Instability is somewhat limited,
but lifted indices do drop down to around 0 late in the day so
we have allowed for some isolated (1 out of 6 chance) thunder
across western and southern Wyoming. Rainfall amounts look
relatively minor at this point. There is at most a 1 out 5
chance of a quarter of an inch of QPF through 6 am Tuesday, with
most areas less than 1 out 10. Snow levels generally look to
remain at 8000 feet through the period, so any impacts from snow
look limited.

Tuesday generally looks like a lull in the activity as we will be in
between waves. There will still be a few showers across the west,
but this looks to be the least active period. Areas East of the
Divide look largely dry though. And, with flow turning southward as
the low starts moving eastward in California and toward the
Great Basin, temperatures will begin to rise again, back to
near to somewhat above normal temperatures.

The second horse, or shortwave will move toward the area on Tuesday
night, increasing chances of showers across mainly western Wyoming
at first with spreading eastward through the night. Confidence in
the forecast details begins to drop past this point though.
This is where the models begin to diverge on the exact track of
the main upper level low and the speed it moves through, with as
much as a 24 hour spread in timing. We do have a fairly high
confidence (about a 4 out of 5 chance) of at least a 1 in 3
chance of showers across most of the area, with the best chance
of higher QPF in the west. However, placement and timing of the
heaviest precipitation remains in flux. Probabilistic ensemble
guidance gives around a 2 out of 3 chance of a half an inch of
QPF from Tuesday morning through Wednesday night across the
west, with lesser chances further east. The general gradient is
highest in the north and west, with lesser amounts further south
and east.

Now for precipitation type. With southerly flow ahead of the low,
850 temperatures generally remain above 0 through Wednesday morning,
keeping snow levels above 8500 feet. Levels will drop later as the
low moves through the flow turns west to northwesterly, possibly
down to 6500 feet if things set up right. Again, exact timing
depends on the speed of the low. As for amounts, ensemble guidance
gives greater than a 1 in 2 chance of 6 inches or more across the
western mountains for the 72 hours from Tuesday morning through
Thursday night. But again, amounts and timing of the changeover
depend on the exact speed and placement of the low.

One more factor to consider is wind, possibly strong wind. The best
chance for this would be Wednesday, as an 120 knot jet core will be
moving over Wyoming. The most likely spot for this looks to be
southern Wyoming. This will be especially so in eastern Sweetwater
County, where the right front quadrant of the jet will enhance
downward momentum. The NAM MOS is giving sustained wind of 34
knots Wednesday afternoon. However, other guidance is not as
impressed, with ensemble guidance giving only a 1 out of 5
chance of wind gusts past 50 mph. It also looks like 700
millibar winds only top out at around 40 knots, when I would
like the see 50 knots for high wind. So, it appears unlikely at
this time. It will be rather breezy across many areas, especially
the eastern half of the area.

Conditions should begin to improve by Friday, although there could
still be some wrap around showers across northern Wyoming. Things
are uncertain for Saturday as well, as another wave may bring some
showers to northern Wyoming. As for temperatures through the period,
given the Pacific origin of the air masses, should average fairly
close to seasonal normals for the latter portion of the week.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1044 PM MDT Sun Oct 12 2025

A calmer period of weather is expected tonight through Monday
afternoon, with VFR conditions and mostly clear skies. Wind will
remain light until Monday afternoon increasing into the evening as
the next weather system moves into the area. Cloud cover will
increase from south to north late Monday afternoon as wind turns
more southerly. Scattered rain showers and isolated thunderstorms
will develop and move northward into the area Monday evening, with
PROB30 groups at KRKS, KLND, and KRIW for -SHRA and MVFR conditions.
Just after the TAF period into Monday night rain showers will
overspread much of the area, but chances will remain moderate around
50%, so some terminals may be spared.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Warning until 9 AM MDT this morning for WYZ004-006-010.

&&

$$

DISCUSSION...Hattings
AVIATION...Rowe