799
FXUS65 KRIW 171815
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1115 AM MST Fri Jan 17 2025

.KEY MESSAGES...

- A potent arctic front sweeps across the Cowboy State,
  bringing periods of gusty winds to parts of the state today.

- Light to moderate snow develops behind the front bringing accumulating
  snow to much of the state especially locations east of the
  Divide.

- Some of the coldest temperatures we have seen in nearly a year
  are forecast for Sunday morning through Tuesday morning.

- Morning temperatures will be dangerously low ranging from the
  -10s to -20s with even colder values possible in some
  locations.

&&

.DISCUSSION...
Issued at 208 AM MST Fri Jan 17 2025

Change is on its way for the Cowboy state, to be more specific,
a potent arctic front is gradually moving across southern
Montana. There looks to be three parts to this incoming system,
the first being wind, the second being snow, and the last being
frigid cold temperatures. The front is expected to make its way
into the state by the mid morning hours Friday. The front will
move north to south over the state bringing periods of gusty
winds, especially across the northern CWA such as the Bighorn
Basin, Johnson County and the northern extent of the Wind River
Basin. Temperatures will drop behind the front with many
locations expected to see their high temperatures sometime
Friday morning.

The second aspect of this event will be quick moving but potent snow
showers that will develop behind the front. Nearly all of the
CWA will have a chance of seeing some snow but the prime
locations are going to be east of the Divide. Central Wyoming
especially across the Wind River Basin and parts of Natrona
County look to see the brunt of the impacts. Probabilistic
guidance shows these locations having a 50 to 70% chance of
seeing 3 or more inches of snow. Hi-res models have continued to
show a brief period of northerly/northeasterly 700mb winds
develop during the afternoon/evening Friday. This looks to
create a period of heavier snowfall in locations that are prone
to upsloping with these winds. Those locations would be the
Lander Foothills, Casper Mountain, northern faces of the
Rattlesnake/Green Mountains and portions of the Owl Creek and
Bighorn Mountains. In regards to lower elevation locations, the
Lander Foothills are the area to monitor as guidance shows a 30
to 50% chance of seeing 6 or more inches of snow. Locations that
do not see the favorable upsloping will need to keep an eye on
the possibility for some heavy snow banding/squall development
throughout the afternoon and evening. These squalls will likely
create periods of moderate to heavy snowfall, which would
quickly reduce visibility and make travel difficult. The best
locations to see this banding develop look to be along the US
20/26 Corridor and the I-25 Corridor. Winter Weather Advisories
remain unchanged and begin late Friday morning through late
Friday evening. Overall, the bulk of the snowfall comes to an
end by the late evening Friday. Snow showers are expected to
linger throughout the overnight hours into Saturday morning and
mostly end by Saturday afternoon.

The last act for this three part system is the frigid cold arctic
air that moves into the region behind the initial push of the cold
front. However, it will not manifest itself until most of the
snowfall has come to an end, which looks to be Saturday.
Forecasting the extent of the cold is going to be tricky as
there are multiple small scale factors that can greatly
influence temperatures. These factors are cloud cover, wind,
snow depth, and elevation. These factors will first be seen
with high temperatures on Saturdays, which are currently
forecast to range from the single digits to teens. Cloud cover
will greatly limit the degree of daytime heating that many
locations see, along with the freshly fallen snowfall. Clouds
are expected to gradually clear out overnight Saturday into
Sunday morning. This looks to be the real start of the frigid
dangerous cold, as forecasted Sunday morning lows are all sub-
zero with values in the negative teens for much of the CWA. As
mentioned earlier, there are a lot of small scale factors at
play here and there is a good chance that lows will be even
colder than what is currently forecasted. Having these factors
in mind, models are highlighting Monday morning and Tuesday
morning as having the best chances of seeing the coldest
temperatures. Current forecasted lows range from the negative
twenties to negative upper teens for both days. Probabilistic
guidance is pinpointing portions of the central Wind River
Basin, Bighorn Basin, Upper Green River Basin, and parts of
Jackson Hole/Star Valley as having the best chance of seeing
lows near or below negative 20 on these two mornings. Depending
on these smaller scale factors at these locations, temperatures
may end up being colder if the perfect conditions are met. As we
are still a little over two days out, no cold temperature
related highlights will be issued but it will likely need to be
revisited during the Friday night shift.

Tuesday onward will see much of the arctic air move out as a brief
period of ridging tries to build back in over the region.
Temperatures are looking to warm back up across much of the CWA,
but there may be some exceptions to this. Lower elevation basins
with fresh snowpack will need to be monitored as stubborn
inversions may limit how much mixing makes it down to the
surface. One area of note is the central Wind River Basin, which
based off previous events, has had difficulty mixing out after
inversions develop. So while many will see warmer temperatures
for the second half of next week, some locations may be socked
in, with cold temperatures prevailing until those inversions can
be broken.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1114 AM MST Fri Jan 17 2025

A winter storm and its cold front has made its way into Wyoming
bringing snow or snow chances and windy conditions to each terminal.
The cold front is currently (18Z) extended across the southern half
of Wyoming and has already caused winds to become northerly at KCOD,
KWRL, KCPR, and KRIW. At all other terminals winds will become
northerly over the next few hours with KRKS being the last to shift
by 00/01Z.

Snow should be diminishing at KCOD over the next few hours and done
by 00/01Z as the main forcing for winter system treks south. All
other terminals should expect snow to continue or begin as the day
progresses into evening with KRKS being the last terminal to see the
onset of snow. KCPR and KLND will have snow linger through much of
the TAF period with upsloping due to the northerly winds. KPNA and
KBPI will likely only see snow showers in the area with little
confidence (less than 30% chance) in direct impacts at KBPI and
persistent impacts at KPNA. At KJAC, expect snow in the mountains
and in the area through 23Z with reduced visibilities for light
snow. IFR to low MVFR visibilities and ceilings will accompany the
heaviest of snow showers. The tempo groups are the anticipated
timing of heaviest snowfall but it is important to note that it
could linger an hour or two later. Mountains will be obscured with
the snow and lower clouds.

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...
Winter Weather Advisory until 11 PM MST this evening for
WYZ005-007>009-017>020-022.

&&

$$

DISCUSSION...Dziewaltowski
AVIATION...Gerhardt