403
FXUS65 KRIW 092314
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
414 PM MST Sat Nov 9 2024

.KEY MESSAGES...

- Seasonably cool and mostly dry today. Light rain
  showers/drizzle remain possible (30%) in southeast Natrona
  County through this afternoon before shifting east this
  evening.

- A warmup will occur Sunday and Monday, with much of the area
  rising into the 50s!

- There is a moderate chance (30-40% chance) for snow in
  northwestern WY Sunday afternoon.

- Next consequential weather system brings impacts to WY late
  Monday through Tuesday night. Snow showers linger across
  northwest WY through next week.

&&

.DISCUSSION...
Issued at 1207 PM MST Sat Nov 9 2024

The warming trend will continue through this weekend as high
pressure keeps it clear/dry for most of the area after this
afternoon. High temps look to rise into the low 50s east of the
Continental Divide/middle 40s west of the Continental Divide by
Sunday, and Monday temps look to be similar. Model guidance has a
weak, quick-hitting shortwave on Sunday/Sunday night. This shortwave
has a chance (The NBM has probs of 20-40% for 0.1" of QPF) of
bringing snow to northwestern WY mountains and YNP. The timing of
this shortwave has come into focus, with the time period of light
snow being mainly Sunday afternoon/evening. A ridge will build back
in after this shortwave passes through, so temps won`t change much
until early next week with the next impactful weather system.

There is now high confidence in the timing of a large trough pushing
into the area from the west late Monday into Tuesday. Wind will
increase ahead of this weather system on Monday into Tuesday with
southwesterly flow developing and tightening of the pressure
gradient. This is not looking like a high wind scenario/event even
where the strongest wind occurs (NBM probs have the chance for
greater than 50 mph wind gusts at around 5-10% for southern Natrona
County), so while there is a low chance of wind gusts above 50 mph
it is not the favored outcome. There will be widespread gusts of 15
to 25 mph and gusts of 25 to 35 mph along the Wind corridor from
Eastern Sweetwater County up into southern Johnson County Monday
afternoon. Wind will increase Monday night along the Wind Corridor,
with gusts of 30-40 mph (higher gusts to 50 mph at the higher
elevations) as the trough approaches the area. As the trough pushes
through on Tuesday wind will increase across the area, with gusts of
20-40 mph through Tuesday afternoon.

Model guidance is in good agreement that this will be an open wave
trough that progresses quickly from west to east across the region.
With this type of weather system western WY predominantly takes the
brunt of the precip, with lee side mountain shadowing preventing
much precip from occurring east of the Continental Divide. NBM probs
for 48 hour precip (QPF) greater than 0.1" for lower elevations east
of the Continental Divide are 30% and less from Monday night through
Tuesday night. Overall the trend is for lower precip (QPF) amounts
across the area with this weather system. Now for western mountains
the NBM probs for 48 hour precip (QPF) greater than 0.5" are in the
20-40% range for most mountain locations. There is a higher chance
(70-80%) on the Pitchstone Plateau where upslope is favored. As
previously mentioned, this weather system appears to be progressive
in nature and while it will very likely (90%) bring colder temps
back to WY for a few days it will take the cold air with it as it
leaves as a ridge develops behind it for the latter half of the
week. Another trough looks to come in from the west next weekend and
right now it looks like it will provide a better chance for
widespread precip chances.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 413 PM MST Sat Nov 9 2024

With the large storm system now over the plains, weak high pressure
is moving over western and central Wyoming tonight into Sunday.
Mostly clear skies this evening will become partly cloudy early
Sunday morning as the flow becomes more westerly, and moisture from
the west coast starts to move into the region.  Northwest WY will
see the most clouds Sunday morning, then thicker clouds will develop
in the afternoon as a weak system moves into western and central
Montana. While VFR conditions are expected at all TAF sites through
Sunday evening, KJAC has a low chance of MVFR due to showers late
Sunday afternoon.  Breezy and gusty west-southwest winds at KRKS and
other sites will decrease after sunset this evening.  As southwest
mid-level flow increases overnight, southwest winds will increase at
KCPR after 10/06Z and increase to 15g25kts during the day Sunday.
KJAC, KRKS, and KCOD should also see WSW winds of 10-12 kts at times
during the day.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Rowe
AVIATION...McDonald