Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 121009
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
409 AM MDT Tue Mar 12 2024

.KEY MESSAGES...

- Light to moderate snow returns to western Wyoming today.

- Snow spreads East of the Divide late tonight and especially
  Wednesday and Wednesday night, with the maximum impacts on
  Wednesday night.

- Uncertainty remains high with details on snowfall amounts East
  of the Divide Wednesday and Wednesday night.

&&

.DISCUSSION...
Issued at 251 AM MDT Tue Mar 12 2024

As I write this around 2 am, we are in a lull in the precipitation
across the area. Radar does show some echos across eastern Idaho,
and these will move eastward, moving into western Wyoming before
sunrise. Snowfall amounts have changed little, so the highlights
across the west will be kept as is for now. The steadiest snow will
fall further the day today though. And with temperatures expected to
be close to freezing and the strong mid March sun angle,
accumulations should be small on roads, especially in the valleys.
Meanwhile, mainly dry conditions should continue East of the Divide.
Gusty wind will continue from Muddy Gap through Casper. However,
with higher humidity, fire weather concerns should be less. Some
snow showers could also move into Sweetwater County this evening as
the trough moves eastward.

Attention then shifts to East of the Divide tonight and especially
Wednesday into Wednesday night. There was a lot of uncertainty with
details last night. An unfortunately, this remains the case this
morning. In the deterministic model realm, the GFS and European have
decided to pull a Freaky Friday and basically reverse positions with
what they were last night, with the GFS now fast and the European
slower. The ensembles also have a similar spread. We are still
fairly confident in a couple of main ideas though. First, the main
impacts would be Wednesday night. Why, you may ask? Well, the
combination of temperatures near freezing, warm ground from the
recent mild temperatures and strong March sun angle (the sun is now
as strong as late September) will make it difficult for much
accumulation on roads until after sunset. We are also fairly
confident in the highest accumulation being in areas favored by
northeasterly upslope flow, as that flow becomes dominant as lee
cyclogenesis occurs over Colorado and the low moves into the central
plains. So, the most impacted areas would be areas like Casper and
Lander as well as the adjacent mountain ranges. The problem is,
there will be some banding as well, and we still have little
confidence of the bands this far out. Some of the ranges on the
probabilistic guidance, are to put in bluntly, rather large.
Such as, in contrast between the 10th and 90th percentiles,
Trace to 7 inches at Buffalo, 1 to 10 inches at Casper and my
favorite, 2 to 16 inches at Lander. Add to this that a lot of
the snow that falls during the day on Wednesday probably won`t
stick to roads and even some non paved surfaces. All this is
making me pull out more of my thinning hair. As for highlights
here, we will issue some Winter Storm Watches based on the 50th
percentile of warning amounts, mainly the Lander Foothills and
the eastern slopes of the Wind River Range, where there is some,
emphasis on some, agreement on placement. In other areas, we
are leaning toward advisories but since the main impacts would
be Wednesday night with the setting sun and passage of the cold
front, we will hold off for now.

As for Thursday, even more uncertainty creeps in as some models move
the moisture out and others show some snow showers continuing with a
baggy trough hanging around. We made few changes to continuity for
now but confidence in any solution is low, less than 1 in 4 for any
possibility. Friday at this point looks dry and chilly. The next
system move in for Saturday in the form a shortwave from Canada.
With the continental origin of this system, any showers would be
limited though. The main impact would probably end up being
strong winds in the favored northwest flow areas like Buffalo
and Greybull. Drier and warmer weather then moves in for Sunday
into next week as ridging builds across the west.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 402 AM MDT Tue Mar 12 2024

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

VFR conditions to start the TAF period for all locations as snow
pushes in from Idaho near JAC. MVFR ceilings and visibility
expected at JAC through 17Z with light snow on station. Snow
will spread eastward to BPI and PNA after 14Z through 18Z but no
indications below VFR at this time. RKS will not see snow on
station until the end of the period with a more widespread snow
for southern areas of the CWA. Southwest winds less than
10-12kts throughout the period with RKS seeing 25kt gusts after
22Z ahead of an approaching cold front for Wednesday.

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

VFR conditions expected for all TAF sites throughout the
entirety of the period. Mid level clouds will increase in
coverage ahead of the next storm system affecting western
zones. Southwest to westerly winds 10-15kts for all sites with
CPR seeing 30kt gusts after 15Z with daytime heating and mixing
to the surface. Winds will subside after 23Z with radiational
cooling becoming less than 10kts after 06Z. Dry conditions
remain until later on Wednesday in the next TAF cycle or two.
No other weather elements are expected at this time.


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 MDT this evening for
WYZ001-012.

Winter Weather Advisory from midnight tonight to 6 AM MDT
Thursday for WYZ008-009.

Winter Storm Watch from Wednesday morning through Thursday
morning for WYZ015-018.

&&

$$

DISCUSSION...Hattings
AVIATION...Lowe


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