216
FXUS65 KCYS 280355
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
955 PM MDT Sat Apr 27 2024

.KEY MESSAGES...

- Significant mountain snow is expected in the Snowy and Sierra
  Madre ranges, with total snowfall accumulations of 1-2 feet
  likely in the Snowys 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.

&&

.UPDATE...
Issued at 953 PM MDT Sat Apr 27 2024

Getting dense fog on WYDOT webcams just west of Cheyenne this
evening. Reports as low as a tenth of a mile on WYDOT sensors.
Decided to issue a Dense Fog Advisory through 8 AM Saturday.
Updates sent.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 217 PM MDT Sat Apr 27 2024

Winter Weather Advisories and Winter Storm Warnings remain in effect
for the South Laramie Range until midnight tonight and the Sierra
Madre and Snowy Ranges through tomorrow afternoon for periods of
moderate to heavy snowfall. So far the Sierra Madre Range has
received approximately 4 to 10 inches, with 10 to 13 inches in the
Snowy Range and almost 6 inches of snow at Crow Creek in the South
Laramie Range. All that being said, the storm is going nearly
exactly as expected. As the low has shifted further east in southern
Colorado, the moisture tap for the Sierra Madre Range has mostly
ended, so they may only see an addition 1 to 3 inches of snow. This
shift has heavily favored the Snowy Range and South Laramie Range as
the winds turned east, providing direct upslope flow and increased
surface moisture right into the eastern slopes. An additional 2 to 6
inches of snowfall is possible with the closer to 2 to 3 inches
along the Interstate 80 corridor between Cheyenne and Laramie. Here
in Cheyenne, as a heavier band moved up from Colorado, we briefly
changed over to snow, but will likely continue to see a rain-snow
mix through the afternoon as these bands move through. By this
evening, the low is expected to eject northeast, continuing to
wraparound moisture into the Nebraska panhandle and keeping rain
showers/thunderstorms around, but overall cut the moisture off along
and west of Interstate-25. Sunday as the low continues to eject
northeastward, an additional, messy, weak shortwave embedded in the
flow follows right behind it. This shortwave may kick off a few
showers and maybe a rumble of thunder west of the Laramie Range.
These showers and thunderstorms will likely remain weak and not
produce much substantial precipitation as the main forcing and
moisture will have moved east. As skies begin to clear Sunday
afternoon, high temperatures may reach well into the 50s outside of
the mountains. As this messy, quasi-zonal flow begins to take back
over winds will likely be increasing due to the mainly downslope
west winds, drying the air out quickly.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 217 PM MDT Sat Apr 27 2024

WPC clusters are showing a weak upper level ridge building over
the area on Monday before a strong upper level trof dives
southeast into the northern Rockies on Tuesday. The models are
showing quite a bit of variability on temperatures on Monday
ahead of the trof, but this could be fairly dependent on the
cloud cover. The latest GFS tends appears to show very limited
cloud cover moving into the area on that day. Therefore, this
limited cloud cover combined with downsloping winds, we are
leaning towards the 75th percentile of the NBM. These gusty
downsloping winds will most provide milder conditions Monday
night as well with the higher gusts taking place in our wind
prone areas of Arlington and Bordeaux Monday night. This strong
upper level trof is also expected to drive a fairly strong front
through the area from the west to east late Monday night into
Tuesday morning. The front appears to be mostly dry with a few
showers possible, but the main threat will be the strong bora
type winds. We did bump up wind speeds on Tuesday for most
locations to reflect this potential. In fact, I would not be
surprised to see a few more places than the wind prone areas
reach high wind criteria for a short while after the frontal
passage.

Wednesday-Friday looks very cool and unsettled as more
shortwaves ride through the upper level trof in the northern
Rockies. Quite a bit of uncertainty on the strength of these
shortwaves moving through the trof, especially with the ECMWF
looking much more amplified than the GEM/GFS on Thursday into
Friday. Meanwhile, the general consensus of the WPC clusters
are not really latching on to that more amplified signal yet.
Ahead of the trof, it looks more favorable for more showers and
thunderstorms on Thursday. However, depending on how amplified
this trof becomes we may have to contend with the potential for
another rain/snow mix on Friday. Stay Tuned!

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 555 PM MDT Sat Apr 27 2024

A soggy overnight will lead to widespread IFR conditions for
terminals east of the Laramie Range. IFR conditions will mainly be
caused by stratus, leading to low CIGs for the majority of the
overnight and into Sunday morning. Widespread light to moderate rain
can also be expected tonight into tomorrow morning, especially
in the Nebraska panhandle. Some snow may mix in. Visibility
drops possible in moderate rain. Conditions improve Sunday
afternoon, but scattered showers will be possible.

&&

.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 tonight for WYZ116.
     Dense Fog Advisory until 8 AM MDT Sunday for WYZ116>118.
NE...None.

&&

$$

UPDATE...GCC
SHORT TERM...LEG
LONG TERM...REC
AVIATION...SF