932
FXUS65 KRIW 101128
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
428 AM MST Fri Jan 10 2025

.KEY MESSAGES...

- An approaching weather system bring moderate amounts of snow
  for the western mountains tonight and Saturday.

- Strong wind is likely late today and tonight from Muddy Gap
  through Casper and in the Lee of the Absarokas.

- A prolonged period of snow is possible for the Bighorns from
  tonight through the weekend.

&&

.DISCUSSION...
Issued at 251 AM MST Fri Jan 10 2025

One of the most popular flavors in a coffee is a half and half. And
this is what this morning`s forecast reminds me of. One half of the
forecast is rather active, and the other half is rather tranquil. We
will, of course, focus on the active half for the most part.

Today will have transitory ridging over the area, bringing a dry
although chilly day. The main issue today will be wind, as the
pressure gradient tightens ahead of an advancing Pacific weather
system. The main areas with the impacts look to be the usual
suspects, mainly from Rock Springs to Casper as well as in the lee
of the Cody foothills. The next question is that of potential wind
highlights. We will start with around Cody. Probabilistic guidance
does give a 1 out of 2 chance of wind gusts past 50 mph around Cody
starting this evening. However, the chance of wind gusts past 60 mph
is less than 1 out of 4 in most locations. In addition, 700 millibar
winds only top out around 35 knots, despite the right front downward
forcing from a jet streak. The exception is near Clark, but criteria
is higher here. So, we will go with a Special Weather Statement
mentioning wind gusts of 50 to 55 mph starting this afternoon and
continuing until around midnight when the cold advection will end
the threat. It is the same situation from Muddy Gap through Casper,
although 700 millibar winds do approach 50 knots here. The chance of
wind gusts past 55 mph according to the probabilistic guidance only
exceeds 2 out of 5 in the more wind prone areas on the south side of
Casper. So, we will again go with a Special Weather Statement for
wind gusts past 50 mph from later this afternoon into tonight for
the wind prone areas.

Now on the snowy side of the system. Snow will begin around sunset
in the western mountains and continue into Saturday, with the
heaviest snow expected tonight. We did issue some winter weather
advisories here though. The emphasis will be in the Tetons and Salt
and Wyoming Range, where there is at least a 3 out of 5 chance of 8
inches or more of snow for the 24 hour period from tonight through
Saturday, although most of the snow will fall tonight. Much of the
Star Valley had at least a 3 out of 4 chance of 4 inches or more, so
we went with an advisory here as well. Elsewhere, with less than 1
out of 2 chance of advisory criteria, we did not issue. The next
question is what to do further east. The Bighorn Range looks to be
the most impacted here. We will likely need advisories here but in
conjunction with surrounding offices we will let the day shift issue
here. Areas around Buffalo look rather interesting as well, but a
bit of a dilemma as well. Most probabilistic guidance only gives a 1
in 5 chance of 2 inches of snow or more. However, there will be
usual post frontal northwest flow / cold advection pattern where
strong winds are a good possibility later tonight through Saturday
and even into Saturday night. There at least a 1 in 2 chance of wind
gusts past 40 mph through this period and this is usually underdone.
An advisory may be needed here for impacts of blowing snow and
reduced visibility even though amounts should remain sub advisory.

Most of the snow from the system should end by Saturday night.
However, with unstable northwest flow continuing, snow showers will
continue through much of the area. The Bighorns will continue to see
upslope snow with this. The only area I could see justifying a
warning here would be far northern areas of the range north of
Granite Pass but impacts would be few since most roads are closed
for the season.

Cyclonic curvature will keep some snow showers going through Monday
as well, mainly across northern Wyoming. Following that, high
pressure will build in and bring the other half of the forecast. And
that is the tranquil portion. Temperatures will average near to
somewhat below normal, but the chances of any precipitation are
rather small if any, with the next chance not arriving until next
weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 422 AM MST Fri Jan 10 2025

Fog has not developed into any local aerodromes so far this
morning, having remained well to the west through the night.
There is still a small possibility of some fog in the vicinity
of KJAC thru 16Z, but any fog that does form would be brief and
patchy. Otherwise VFR conditions through this evening with
increasing high clouds. Gusty winds return today for
KRKS/KCPR/KCOD, with some gusts above 35kts this afternoon.

Next round of snow moves into the state around 06Z, with low
cigs and vis west of the divide. Some snow may shift east of the
divide, but would be closer to end of forecast period around
12Z/11.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Saturday for WYZ012-024.

Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Sunday for WYZ023.

&&

$$

DISCUSSION...Hattings
AVIATION...Straub