619
FXUS65 KRIW 310958
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
258 AM MST Fri Jan 31 2025

.KEY MESSAGES...

- A prolonged snow event begins late today and continues through
  early next week in western Wyoming.

- Strong to high wind is likely today and tomorrow from Muddy
  Gap through Casper and the lee of the Absarokas.

- Some snow is possible East of the Divide starting later
  Sunday, but details are still uncertain.

&&

.DISCUSSION...
Issued at 256 AM MST Fri Jan 31 2025

The period looks quite active tonight and we have a lot of talk
about. Like last night, given the contrast we will split the
discussion into West and East of the Divide.

West of the Divide...Things are still quiet this morning. However,
this won`t last long as we will have multiple periods of snow moving
though over the next several days. The current warnings may be a
bit fast, snow shouldn`t break out until around 2 pm with the
steadier snow likely holding off until around sunset. The set up
looks the same, as an atmospheric river (I always liked the
term fire hose better, just more descriptive and not as geeky)
will bring the first round of steady snow tonight with good
isentropic lift moving into the area. This will be combined with
a potent 150 knot jet that will remain over the state through
the weekend. As a result, periods of snow will continue through
the weekend. There is still some uncertainty in the models in
the timing of the heavier periods of snow. There could even be a
few breaks in the snow. Nevertheless, snow could be heavy at
just about anytime with snowfall rates over 1 inch an hour. If
there is a bit of a break, there is a decent amount of agreement
that it would be during the day on Sunday. Then another period
of heavy snow, possibly the heaviest, could be Sunday night with
a left front / right rear jet couplet brings additional lift,
with snowfall rates possibly approaching 2 inches an hour at
times. This will be combined with gusty wind with a greater than
1 in 2 chance of wind gusts past 30 mph in most areas except
for the Star Valley, bringing areas of blowing and drifting
snow. As for the thing more people are interested in, snowfall
amounts, the western valleys have at least a 2 in 3 chance of 10
inches or more, easily warning amounts. The higher elevations
of the mountains will obviously see the most, with all the
higher elevations of the Tetons, Salt and Wyoming Range and
western Wind Rivers having at least a 1 in 2 chance of 30 inches
of snow. Some areas will likely end up with over 40 inches by
Monday morning.

The warning may have be extended as well. Probabilistic guidance
shows at least a 1 in 2 chance of 6 or more inches Monday into
Monday night, especially across northwestern Wyoming. Chances are
less further south though, as the jet will shift further north at
this point. Additional snow may continue through Wednesday as well,
but guidance diverges more here so confidence is low.

As for Sweetwater County, things should remain dry through Friday
night. Showers will then be possible the weekend. There will be
less jet forcing here. In addition, temperatures will be warmer,
with 700 millibar temperatures rising from minus 4 to minus 2. This
would raise snow levels from 6500 to 7500 feet and result in some
mixing with or changing to rain, limiting accumulations. As a
result, accumulations would be light, with the chance of 2 inches
or more mainly north of Interstate 80 and even here it is less than
1 out of 2. There could be some slick roads at night though as
temperatures drop below freezing at night.

East of the Divide...The main concern here shifts to wind. Much of
the area, will be in the right front quadrant of the jet, enhancing
downward forcing. The main area of concern is the usual suspect with
southwesterly flow, from Muddy Gap to Casper. The set up looks good
for the wind with 700 millibar winds of 50 to 60 knots and the
downward forcing from the jet. Probabilistic guidance shows at least
a 1 in 2 chance of wind gusts past 60 mph across most of the Casper
area through tonight. As a result, we will continue to the warning.
The mid level winds do decrease somewhat on Saturday though so it
may need to be cancelled early. As for the Cody Foothills and
Absarokas, things are more borderline there, 700 millibar winds do
not look as strong, maxing out at around 50 knots. We will continue
the watch for now and let the day crew decide on any upgrades. Don`t
know if we need it in the Absarokas, as the winter storm warning
should cover it. We will continue it for now though.

As for precipitation, downsloping flow should keep most areas dry
through Sunday. There could be some blowover showers in a place like
Lander or Casper. However, temperatures will be warmer and amounts
light so any impacts should be minimal through the period. The time
of concern here will be from later Sunday through Monday as the cold
front moves through. Models have come into somewhat better agreement
with the focus across northern Wyoming. I do have some concerns as
this is a good set up for a system to overperform in regards to
snow. There will be a frontal boundary in the area with the jet in
the area and the Pacific moisture overrunning the cold dome.
Probabilistic guidance, for example, shows a 2 out of 3 chance of 3
inches of snow in Cody, which tends to be hardest hit in this
pattern. The greatest area of concern is generally north of a
Meeteetse to Greybull to Buffalo line though. It is too early for
highlights but some advisories may be needed if this trend
continues. Amounts elsewhere look light, mainly less than an inch.

As for temperatures, they should remain above normal through Sunday
before the cold front. The question remains if the Wind River Basin
can break out of the inversion. We did go cold for one more day
today but there it should break on Saturday, with temperatures near
40 for Saturday and Sunday before the cold front. Things are a bit
more uncertain early in the week though, as there is uncertainty
as to how far the cold air can get. This is a situation where Cody
could be 10 degrees and Lander could be 40.

That is the discussion. Now, excuse me as I have to ice down my
wrists after typing this novel.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 942 PM MST Thu Jan 30 2025

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

VFR conditions will continue overnight through Friday morning.
However, clouds will begin to increase by late morning as a major
winter storm begins pushing into western Wyoming. This will first
bring impacts to KJAC around 00Z, with chances spreading to KBPI and
KPNA through Friday evening. MVFR to IFR conditions will continue
through the rest of the period, with KJAC the most likely to see
persistent IFR conditions. KRKS will be the exception and will not
see snow impacts for this period.

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

VFR conditions are expected through the period with snow impacts
remaining across western Wyoming through this period. The main
impact east of the Divide will be strong wind, most notably at KCPR
where gusts to 40 mph are expected by late Friday morning. KCOD will
also see wind with gusts to 30 mph after 00Z. Otherwise, clouds will
generally increase for all terminals Friday afternoon.

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 Warning from 11 AM this morning to 11 PM MST
Sunday for WYZ001-002-012-014-015-024-025.

High Wind Watch from late tonight through late Saturday night
for WYZ002-003-016.

Winter Storm Warning from 11 AM this morning to 11 PM MST
Saturday for WYZ013-023.

High Wind Warning from 5 PM this afternoon to 5 AM MST Sunday
for WYZ019-020.

&&

$$

DISCUSSION...Hattings
AVIATION...Myers