Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 120857
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
257 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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1005 PM MDT Mon Mar 11 2024

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

A weak trough axis traversing the state Monday evening has
initiated snow showers across portions of southwest Wyoming,
particularly around KRKS. Satellite trends indicate these are on
the wane. However, the residual moisture and a clearing sky
early Tuesday could lead to fog and low clouds at KRKS. Short-
term ensemble members are not high on this possibility, but with
a moist boundary layer there is concern of IFR or less
conditions at times. Final adjustments are likely to be made
closer to TAF issuance time. To the north, KJAC will see snow
increase right around 12Z/Tuesday as the next shortwave reaches
western Wyoming. IFR looks likely for much of the morning with
the potential for heavier snow and 1 to 1.5 mile visibility from
14Z-17Z/Tuesday. Similarly, this time period presents the best
chance for MVFR snow showers at KBPI and KPNA. The moisture
shifts southeast through the afternoon with improvement noted at
these terminals. Late day MVFR snow showers are possible at
KRKS. Mountain tops frequently obscured.

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

All terminals to be VFR through 06Z/Wednesday. Gusty southwest wind
persists at KCPR with an uptick to 20G30kts from 16Z-23Z/Tuesday.
KCOD, KRIW, and KLND could see some late afternoon westerly wind
around 12kts. Mid-cloud cover increases and gradually lowers toward
the end of the forecast period.

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 from 6 AM this morning to 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...Jones


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