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000
FXUS65 KCYS 190215
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
815 PM MDT Thu Apr 18 2024

.KEY MESSAGES...

- Light snow showers will continue in the southern Laramie Range
  this afternoon. Visibility will remain less than 1 mile with
  low clouds and fog over the I-80 summit.

- A weak wave will move through on Friday into Saturday
  enhancing upslope flow. A few inches of snow are possible in
  far southeast Wyoming through Saturday morning.

- Much warmer temperatures are expected for Sunday through
  Thursday.

&&

.UPDATE...
Issued at 811 PM MDT Thu Apr 18 2024

No major changes made as the forecast remains largely on track
as of 02Z this evening. Dense Fog Advisory remains in effect for
the South Laramie Range, including the I-80 Summit, Buford,
Pumpkin Vine, and Vedauwoo. Visibility as low as a quarter of a
mile likely in the most dense areas. Expecting fog to mix out
starting around midnight tonight as winds begin to increase over
the Summit.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 1241 PM MDT Thu Apr 18 2024

Afternoon satellite imagery reveals a broad area of low stratus
cloud cover over much of southeast Wyoming and far southwest
Nebraska. Moist easterly upslope flow has remained persistent
throughout the day with limited mixing taking place. Farther
north over east-central Wyoming and northwest Nebraska, surface
flow has turned to more north to northwesterly, ushering in a
drier boundary layer airmass. High-res short-term guidance
remains consistent showing a deep moist boundary layer with upslope
flow at the surface continuing through the night and into
Friday. Aloft, a few very weak pieces of energy will traverse
the base of a broad trough over the northern Rockies. Once such
piece of energy will work through overnight into Friday. As it
does, very weak ascent aloft will result in a few light snow
showers. A seeder/feeder type scenario is possible with this
type of setup, given some of the lighter snow from the energy
aloft making it into deeper boundary layer moisture. With that
being said, a light coating of snow, possibly up to a half inch
or so, is possible in southeast Wyoming and far southwest
Nebraska tonight through Friday morning. With warm ground
temperatures and surface air temperatures only dropping into
the mid to upper 20s, it will be hard to accumulate any of this
snow on paved surfaces. However would not be surprised to see up
to an inch or so in the grass from the I-80 summit through
Cheyenne and Pine Bluff/Kimball by Friday afternoon.

After a brief lull in activity after the departure of this
system, another weak wave will move through the Great Basin and
pass through the central Rockies/high plains on Friday night
into Saturday. This system will bring slightly more Pacific
moisture from the west in aloft. At the surface, a reinforcing
shot of cold air will firmly entrench the cold front up against
the Front Range of the Rockies from Wyoming southward through
Colorado. A period of favorable lift moves through from around
Midnight on Saturday through Saturday late morning, coinciding
with the deepest surface moisture soundings in the high plains.
Currently, forecast guidance keeps the greatest QPF values just
south of the state line, in the northern Front Range of
Colorado. However notably the NAM is a bit farther north,
pushing advisory-level snow totals into southeast Wyoming. A
slight trend southward with the push of moisture aloft would
likely result in much lower totals than currently forecast in
this package. However, given the subtle differences between
mesoscale and global models at this time, have opted to keep
totals on the threshold of advisory levels for now. It is likely
(over 60% of high res guidance) that at least 3 inches of snow
will fall in the south Laramie Range including the I-80 summit,
with less confidence (40% or less) in 2-3 inches of snow from
Cheyenne on eastward. It is important to note that with warm
ground temperatures, any snow that does fall will have a hard
time sticking on paved and especially treated surfaces.
Therefore, given the potential low-impacts of this snow event
even if we do get near the 3" mark, we will opt to hold off on
any Advisory products for now.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 1241 PM MDT Thu Apr 18 2024

A shortwave ejecting eastward followed by upper-level ridging will
increase temperatures back to near-normal Sunday afternoon and keep
conditions mostly dry. Monday, another shortwave drops south out of
Canada dropping temperatures a few degrees and increasing
precipitation chances and wind gusts. Long range models are showing
the shortwave skimming the northern and eastern portions of our
forecast area, followed by more upper-level ridging. Due to the
quick movement eastward, minimal precipitation amounts are expected,
if any, across southeast Wyoming and western Nebraska. Mostly dry
and warm, above-normal conditions then expected until Wednesday
night into Thursday when the ridge begins to break down and a
shortwave moves northeast across southeast Wyoming and western
Nebraska. In turn, more gusty winds and increased precipitation
chances expected headed into the end of the week.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 458 PM MDT Thu Apr 18 2024

Another weather disturbance will slide north from CO overnight.
We will see MVFR and IFR CIGs overnight for KLAR and KCYS
through Friday morning, with low end VFR for the remaining
terminals through the forecast period. Included a brief timeline
of MVFR for KSNY on Friday morning, but confidence is low to
medium on this happening. Wind gusts for KCDR and KAIA of 20-25
knots this evening should dissipate by 2z. KCDR will see
elevated winds pick back up Friday afternoon to 20-25 knots. All
other remaining terminals will see wind of 12 knots or less
through the forecast period.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Dense Fog Advisory until 1 AM MDT Friday for WYZ116.
NE...None.

&&

$$

UPDATE...AM
SHORT TERM...MAC
LONG TERM...LEG
AVIATION...BW