Winter Storm Warning 3, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
1013 AM MST Thu Jan 30 2025

.KEY MESSAGES...

- One more dry and quiet day today.

- A prolonged snow event across western Wyoming begins Friday
  night and lasts at least into early next week.

- Strong to possibly high wind possible in portions of central
  Wyoming Friday through Saturday.

&&

.DISCUSSION...
Issued at 212 AM MST Thu Jan 30 2025

Things are looking quite active for most of the forecast period, due
to an atmospheric river, which was previously known as the Pineapple
Express, which was previously known as a fire hose. I swear its
changed names more than Elizabeth Taylor changed husbands.

We will have one more quiet day today with ridging in control of the
weather. The only concern looks to be temperatures in the basins
where inversions continue to be around, although they are weakening
and thinning. For now, we will keep the temperatures cooler than
guidance. If the inversions break and it does warm up more, the
vast majority of people will be happy and impacts would be
minimal. Otherwise, most areas will have above normal
temperatures.

With the large contrast in weather, we will split the rest of the
discussion into west and east of the Divide.

West of the Divide...It looks like snow lovers will really like this
forecast. Friday will start off largely dry. However, the first
snowfall will move into the area later Friday afternoon as
increasing moisture and isentropic lift moves into the area. This
looks to be a long duration snow event for the area. Our current
Watches go out through Sunday evening. For now, we will
continue the highlights as is. However, when the Watches are
upgraded (and there is almost a 100 percent chance of this),
these will likely have to be extended through at least Sunday
night and possibly into Monday as well. The other feature of
this system will be a powerful jet that will be over Wyoming
from Friday night and into next week. If there is uncertainty
with this system, it will be with the timing of the individual
waves of snow, as the models have different ideas. At this
point, the best guess is for an initial batch on Friday night.
Snow will continue on and off through Sunday, but here there is
more uncertainty for timing of the batches of the snow. The
main threat for Saturday could turn out to be blowing snow, with
a greater than 1 in 2 chance of wind gusts to or over 40 mph,
including over portions of the western valleys. Agreement does
increase somewhat for Sunday night, as this may be the heaviest
period of snow. The main culprits for this will be the passage
of a trough axis as well as a left front / right rear 140 knot
jet couplet to enhance upper level divergence. Snowfall rates
could approach 2 inches an hour during this time across northern
Wyoming and this includes Teton and Togwotee Passes. The jet
will also bring strong wind, with gusts over 30 mph possible
with the possibly of near-blizzard conditions. Travel around
northwestern Wyoming will not be very fun Sunday night into
Monday. As for snowfall amounts, the valleys have a nearly 100
percent chance of 6 inches or more from Friday through Monday
with a greater than 1 in 2 chance of 12 inches or more. However,
the most impressive totals will be found in the mountains.
Almost all of the western mountains have at least 1 in 2 chance
of 30 inches or more through Monday night and some areas could
receive over 40 inches by Monday evening.

The heaviest snow should end by Monday night. However, moist
westerly to southwesterly flow will continue to bring light to
occasionally moderate snow to the western mountains through the
middle portion of next week, with an emphasis on northwestern
Wyoming.

East of the Divide...The main concern here shifts to strong to
possibly high wind. The usual suspects will be the primarily
effected locations, mainly from Muddy Gap through Casper with
increasing southwest flow. It looks like a good setup for it as 700
millibar wind increases to 50 to 60 knots later Friday into Friday
night and good downward forcing from the right front quadrant of an
120 knot jet streak. Probabilistic guidance shows a greater than 1 in
2 chance of wind gusts past 60 mph on the south side of Casper from
Friday afternoon through Friday night. High Wind Watches may be
needed, but we will let the day shift make the decision and we
could upgrade to a warning tomorrow night if we need to. Gusty
wind will continue into Saturday, but the chance of high wind
gusts decreases to around 1 in 4 as 700 millibar winds decrease
as the jet shifts slowly northward.

Temperatures will remain above normal through Sunday. There is still
some question if the Wind River Basin can break out of the
inversion, especially on Friday. There is a better chance on
Saturday and Sunday, with a greater than 3 out of 5 chance of
reaching 40 degrees each day. Cooler temperatures then return
on Monday after the cold front passage.

The next question is potential snow. Only light showers are expected
through Sunday as westerly to southwesterly flow will bring
downsloping flow. There is more uncertainty though as we head
toward Sunday night and Monday. Some model runs are showing some
snow associated with some jet forcing following the cold front
passage. Most guidance is showing the best chance across
northern Wyoming, especially north of a line from Cody to
Greybull where there is around a 2 in 5 chance of 2 inches or
more from Sunday night through Tuesday afternoon. Other model
runs however show this across central Wyoming, from the Wind
River Basin to Natrona County. This will likely be banded snow,
and this is tough to pinpoint 24 hours out, let alone 4 days
out. We kept the PoPs low for now, but we will have to watch
this closely. Mainly dry weather then looks to return for the
middle of the week.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1012 AM MST Thu Jan 30 2025

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals

Mostly clear skies and VFR conditions prevail through this afternoon
into Friday morning, but a prolonged period of unsettled weather is
on the horizon. An Atmospheric River of moisture will begin to
impact western Wyoming just beyond the TAF period (Friday
afternoon). Snow and low clouds will bring MVFR/IFR conditions to
KJAC Friday afternoon. This weather system will also bring strong
wind to the area, beginning Friday afternoon.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

Mostly clear skies and VFR conditions prevail through the TAF
period. Wind will remain light at most terminals through tonight. A
prolonged period of strong westerly flow will bring strong wind to
the area beginning Friday morning, mainly at KCPR initially.

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 Storm Watch from Friday afternoon through Sunday
afternoon for WYZ001-002-012>015-023>025.

&&

DISCUSSION...Hattings
AVIATION...Rowe

Hazardous Weather Outlook
National Weather Service Riverton WY
218 AM MST Thu Jan 30 2025

WYZ001>020-022>030-311400-
Yellowstone National Park-Absaroka Mountains-Cody Foothills-
North Bighorn Basin-Southwest Bighorn Basin-Southeast Bighorn Basin-
Owl Creek and Bridger Mountains-Bighorn Mountains West-
Bighorn Mountains Southeast-Northeast Johnson County-
Southeast Johnson County-Teton and Gros Ventre Mountains-
Jackson Hole-Wind River Mountains West-Wind River Mountains East-
Upper Wind River Basin-Wind River Basin-Lander Foothills-
Green Mountains and Rattlesnake Range-
Natrona County Lower Elevations-Casper Mountain-Star Valley-
Salt River and Wyoming Ranges-Upper Green River Basin Foothills-
Upper Green River Basin-South Lincoln County-
Rock Springs and Green River-Flaming Gorge-East Sweetwater County-
218 AM MST Thu Jan 30 2025

...WINTER STORM WATCH IN EFFECT FROM FRIDAY AFTERNOON THROUGH SUNDAY
AFTERNOON FOR WESTERN WYOMING...

This Hazardous Weather Outlook is for Western and Central Wyoming.

.DAY ONE...Today and Tonight.

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday through Monday...The next chance for snow moves in from
the west late Friday, with snow chances through the weekend
through Monday, heavy at times.

Very strong westerly winds are also expected, with high winds
possible from South Pass through Casper, especially on Friday and
Saturday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today.