520
FXUS65 KCYS 270432
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1032 PM MDT Fri Apr 26 2024

.KEY MESSAGES...

- Significant mountain snow is expected in the Snowy and Sierra
  Madre ranges, with 1-2 feet likely in the Snowys from this
  evening through Sunday morning. Winter Weather Advisories and
  Winter Storm Warnings have been issued.

- Confidence is increasing in snow levels lowering sufficiently
  for heavy snow above 7500 feet over the South Laramie Range
  and I-80 Summit. However, uncertainty remains with the overall
  travel impacts due to the heaviest snow occurring during the
  daytime hours. A Winter Weather Advisory has been issued.

- Widespread rain and high-elevation snow will impact a large
  portion of southeast Wyoming and the western Nebraska
  Panhandle through Sunday morning. Rain may change to snow as
  far east as the I-25 corridor into Cheyenne Saturday night,
  with minor accumulations possible on grassy surfaces.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 314 PM MDT Fri Apr 26 2024

Minimal changes made to the short term forecast through Sunday
afternoon. We are still looking at rain showers and thunderstorms
this afternoon into evening across southeast Wyoming and western
Nebraska with snow in the mountains. Late tonight, rain will begin
to transition over to snow at elevations as low as 7500 feet. Opted
to upgrade the Winter Storm Watch to a Winter Weather Advisory in
the South Laramie Range with potential snow totals being 5 to 11
inches, but the heaviest snow is most likely to fall during the day
Saturday. Therefore, with the high sun angle due to it being late
April, any snow that does fall will likely begin melting relatively
soon after. In addition, if this area does not start receiving more
substantial snowfall overnight, then this area will also have to
overcome warmer temperatures before they`ll fully transition to snow
Saturday afternoon. The 12Z HREF ensemble shows the heaviest snow
ending near the Summit around 6 PM, but light snow may still
continue over the next few hours. It is still uncertain whether the
snow will get into the Cheyenne Ridge area. HiRes guidance has rain
potentially transitioning to snow along the Cheyenne Ridge between 2
PM Saturday and 7 PM Saturday. While there will be decently strong
easterly, upslope winds with colder air, since this timeframe is
during the day, the likelihood that there will be any accumulation
outside of elevated and grassy surfaces is low. Looking at the Snowy
and Sierra Madre Mountains, the winter weather headlines issued
remain in effect still expecting 6 to 12 inches in the Sierra Madre
Mountains, and 1 to 2 feet in the Snowy Mountains, which is
consistent with easterly winds and the Sierra Madre Mountains being
shadowed by the Snowy Mountains. Saturday evening and overnight as
the low pressure system ejects eastward, wraparound moisture sticks
around along the eastern Nebraska panhandle, resulting in the
potential for an additional 0.3-0.6 inches of rainfall. Across the
High Plains, chances for precipitation taper off quickly Sunday
afternoon and evening, but areas west of the Laramie Range are
expected to receive an additional round of light precipitation that
will bring snow mainly above 8500 feet, and rain below.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 314 PM MDT Fri Apr 26 2024

Quasi-zonal flow aloft early next week will lead to daily chances
for precipitation along with warmer temperatures compared to this
weekend. A surface low passage to the north across northeast WY
could lead to a brief bora-wind event Tuesday morning. Local in-
house guidance suggests near 50% chance of wind gusts exceeding 58
mph along the I-25 corridor from Cheyenne to Douglas strengthening
mslp gradients across the Laramie Range and favorable downward omega
fields spreading across the adjacent foothills with the passage of
the trough axis. While likely to be short-lived, this event may need
to be monitored for potential high wind headlines.

Headed into the middle of next week, forecast uncertainty grows
quite a bit with the progression of a shortwave trough dropping out
of the PacNW behind a larger scale upper level low near the Northern
Great Plains. NBM interquartile range for high temperatures shows a
20F degree spread from Wednesday onward. While the deterministic GFS
and Euro appear to be in line with the progression of the shortwave
trough passage mid-to-late next week, cluster analysis shows
multiple camps of ensembles with varying timing in the trough`s
progression drastically effecting environmental conditions and focus
points for areas of precipitation. With the high degree of
uncertainty, current forecast highs from Wednesday onward could
swing a few degrees in either direction from the current forecast of
near normal for early May. Right now, decided to lean a little
towards the cooler side of guidance with a trend towards a more
amplified wave pattern and slower progression of the north-central
CONUS trough. Additionally, slightly increased PoPs from the latest
NBM though noteworthy QPF totals are not expected at this time.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1028 PM MDT Fri Apr 26 2024

Starting to see the effects of the next in the series of upper
level lows moving through tonight into Saturday. Rain has begun
out near KRWL and KLAR. So far only MVFR conditions being
observed. But rain is expected to turn over to snow the next few
hours and IFR conditions expected to be common west of the
Laramie Range. Short range guidance showing lowering conditions
this overnight at KRWL...KLAR and eventually KCYS as snow
develops and continues through the day Saturday. For the
Nebraska Panhandle...look for lowering conditions later Saturday
morning...more after 12Z that once it develops...will last
through the day.


&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 6 PM MDT Sunday for WYZ112.
     Winter Storm Warning until 6 PM MDT Sunday for WYZ114.
     Winter Weather Advisory until midnight MDT Saturday night for
     WYZ116.
NE...None.

&&

$$

SHORT TERM...LEG
LONG TERM...MB
AVIATION...GCC