178
FXUS65 KRIW 081917
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1217 PM MST Wed Jan 8 2025

.KEY MESSAGES...

- Minor snow chances this afternoon and evening, mainly east of
  the Divide.

- Dry and warming trend Thursday and Friday.

- More widespread weather system Friday night and into the
  weekend west of the Divide and higher terrain to the east.

&&

.DISCUSSION...
Issued at 1154 AM MST Wed Jan 8 2025

IR currently depicts increasing clouds across northwestern
portions of the state as the next system moves in out of
Montana. High pressure has weakened and started to push out to
the east with increasing divergence aloft ahead of the
shortwave. The bulk of the stronger PVA anomalies continue to
trend northeast and east of the CWA for the heavier snowfall,
but regardless, some will fall, mainly east of the Divide and
for higher terrain. Radar shows a bit of light snow pushing in
for Yellowstone that will spread east and southeast out of
Montana into the Bighorn Basin mid to later this afternoon. It
could clip eastern parts of the Wind River Basin around sunset
but best chances later towards midnight on the backside of this
progressive wave. The only concern will be for Johnson County
north of Buffalo along the interstate on how much snow comes
down from Sheridan. As the front passes, stronger northwest
winds behind it for a couple of hours, so there could be a
limited window of impacts around the midnight time frame across
I-90 and I-25 corridors of Johnson County. Low confidence at
this point as it is mainly the NAM Nest indicating even this
little hazard. Otherwise, a colder afternoon for much of the
area, especially east of the Divide for the basins where fog
lingered towards the noon time frame limiting sunshine ahead of
the increasing cloud cover with the aforementioned system.
Highest probabilities for over an inch will be the upslopes of
Casper Mountain (60-70%) to the northern Bighorns (>90%), and
even a smaller chance for the Lander foothills (~10-15%). Most
other areas east of the Divide have decent chances over 40-50%
for at least a dusting or measurable snowfall. As is such, not
near as cold overnight with the cloud cover even with the
pseudo- front being more of a surface trough than any distinct
temperature change behind it. Behind FROPA, improving skies and
diminishing winds towards sunrise Thursday morning as any snow
exits to the east and southeast.

Upper level ridging builds in from a strong high off the
California coast aiding in the wildfire and Santa Anna winds
there, but dry and a slight warming trend for Thursday and
Friday. Outside of a slight breeze each afternoon, no
significant weather concerns to end the work week with ample
sunshine ahead of the next system.

Increasing clouds Friday afternoon as the aforementioned system
nears by sunset pumping in ample Pacific northwest moisture as
the the jet sags southward. Divergence ahead of the shortwave
will allow for snow chances to increase for western mountains
overnight into Saturday morning before spreading to the Divide
by lunch time. As the main base of the trough continues its
southward trend, as it usually does, northern flow aloft will
continue to bring in the reinforced cooler air and sporadic
snow chances for the weekend and into early next week. The main
upper level low across the Hudson Bay will deepen but have the
main energy and colder air for the northern plains and Great
Lakes with mainly mountain snow for the CWA to include some
western valleys. Best chance for the lower elevations east of
the Divide look to be late Saturday afternoon and evening,
albeit low confidence of much snowfall if any at all at this
point in time.

Ridging then builds in by Tuesday next week bringing in a
weaker upper level flow with a warming/dry trend through much of
the remainder of the week. Longer term models indicate it
blocking much of the systems northwest and north as the PFJ
migrates back north along the Canadian border dipping down to
the Great Lakes and impacting the east coast. The next system
of note beyond that will be by Friday to see where that goes as
it pushes down from the GOA once again. Stay tuned for updates
but high impact events are not expected at this time.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1030 AM MST Wed Jan 8 2025

Weak high pressure overnight is giving way to a quick moving
weather system today approaching the forecast area from the
northwest. Some low clouds and fog are slow to burn off in the
Wind River Basin, and may keep KRIW in VLIFR until heating and
wind dissipate the fog before 08/20Z. Otherwise, VFR conditions
are expected this afternoon, transitioning to MVFR this evening
from north to south as snow showers move from NNW to SSE,
especially for sites east of the divide. These sites should see
a few hours of MVFR/marginal IFR with the snow before improving
before sunrise. TAF sites west of the divide could see a stray
snow shower during the late evening hours, but chances are
better east of the divide. Lingering showers are likely through
mid-morning Thursday as well. Winds are quite breezy and gusty
at KCPR this afternoon from the southwest, then decrease after
sunset before snow showers move in during the evening. At this
time, not expecting any areas of fog early Thursday morning due
to the cloud cover an slightly increased north winds.

As mentioned, a few snow showers will linger Thursday morning,
but skies should clear from NW to SE through the day. NW-N-NE winds
will increase around mid-day, but decrease by sunset. High
clouds will return to NW WY Thursday night, with snow showers
returning to the western mountains late Friday afternoon in
advance of the next storm system.

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...Lowe
AVIATION...McDonald