665
FXUS65 KCYS 271000
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
400 AM 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.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 400 AM MDT Sat Apr 27 2024

Light to moderate rain showers are slowly propagating northward
early this morning. Areas across the higher elevations above
8000 feet are reporting snow showers. Expect this trend of light
to moderate rain showers to persist through the remaining
morning hours as chilly temperatures are also forecast to
continue. Unfortunately, the cloud cover and upslope flow from
the surface to upper levels of the atmosphere will keep it very
cool outdoors today. Not expecting a large recovery in daytime
highs as we remain largely in the 30s and 40s for the lower
terrain, and below freezing in the mountain zones where winter
headlines remain in effect through today. The forecast remains
on track with a prolonged period of accumulating snowfall in the
Snowy/Sierra Madre Mountains, so have not changed the
headlines. The warmest areas that will see partly cloudy skies
remain in our northern forecast zones, where temperatures may
eclipse the 50 degree mark temporarily, before mostly cloudy
skies become prevalent for the majority of the cwa. Several
areas should see a wetting rainfall of 0.10 inches while the
rain showers continue.

Tonight and into early Sunday, this slow-moving weather system
will become occluded for all intents and purposes. This will
keep the window for rain and snow showers present across our
eastern half of the cwa, with the orographic enhancement keeping
accumulating snowfall in the mountain zones for another good
while. Some of the model guidance members have the eastern tier
of the south Laramie Range foothills and Central Laramie Range
wet bulbing to near or below freezing, which could assist with
light snow accumulations on the grass by daybreak Sunday
morning. Have gone against this as the Kuchera ratio method for
both the HRRR/NamNest have less than a half inch accumulating.
As the day presses onward, expect the rain showers to change
over to light snow showers for the NE Panhandle, primarily after
sunset. Would expect up to perhaps a dusting of snow to
accumulate on grassy surface total. Daytime highs will rebound
slightly, with most locations see the lower to upper 50s, except
the mountain zones where 30s will be present.

We will have another weak shortwave bring the potential for
light snow showers on Monday for the higher terrain.
Additionally, upper level winds at 700mb are trending upwards by
the late afternoon period. This coupled with subsidence/mountain
wave signatures from model guidance are increasing confidence of
gusty winds for southeast WY, especially the higher terrain.
Have kept the breezy to gusty winds in the forecast. At this
time of inspection, high wind headlines do not appear to be
necessary, but 40-50mph wind gusts in the wind prone corridors
by late Monday afternoon are highly favored. Partly sunny skies
will become present by the afternoon for most of the cwa,
outside of the higher terrain. This combined with the
downsloping should assist with westerly winds warming the lee
side of the Laramie Range and the western NE Panhandle nicely.
Expect daytime highs in the 60s to lower 70s for the high
plains.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Issued at 400 AM MDT Sat Apr 27 2024

Long Range models start off in good agreement early to midweek,
and then start to diverge by the end of the week as another
Pacific storm system moves eastward into the Front Range and
Intermountain West. Quasi-zonal flow aloft is expected to
dominate the weather pattern late Monday through Wednesday with
progressive upper level disturbances rapidly moving eastward
every 18 to 24 hours near or colocated with the 100 to 120 knot
jet across Wyoming and the Dakotas. This pattern typically
results in near average temperatures for this time of the year
with highs in the mid 50s to mid 60s and lows in the 20s to mid
30s. Some drier air will move into the area by Tuesday, limiting
coverage of precipitation. However, can rule out isolated to
widely scattered showers and a few thunder showers late Monday
and Tuesday. Kept POP between 10 to 20 percent for now. Strong
Pacific storm system will begin to move eastward and start
impacting the region Wednesday with a higher coverage of showers
and thunderstorms in the afternoon and evening hours.

A brief period of strong gusty winds are possible late Monday
night into Tuesday morning as a fast moving Pacific cold front
brings some precipitation to southern Wyoming Monday evening. A
Bora-like wind event is possible across southeast Wyoming with
strong subsidence behind the cold front and 700mb winds of 55 to
60 knots early Tuesday morning. In-house wind models do show the
potential for locally strong wind gusts, mainly along and west
of the I-25 corridor. Increased winds near High Wind criteria
and will need to continue to monitor this trend through the
weekend for the possibility of High Wind Watches, especially in
and near the wind prone areas.

For later next week (Thursday and Friday), model solutions
diverge quite a bit as the Pacific storm system moves over
Wyoming. Ensemble guidance shows a large spread of max and min
temperatures as a strong cold front will be located near the
Wyoming and Colorado border at this time and models are having a
hard time resolving timing and location of the front.
Temperature spreads are as high as 15 to 25 degrees with closer
to 40 degrees comparing the bottom 5th-10th percentile to the
top 90th percentile. Most guidance, including the NBM, is
showing this. This forecast period will likely become the
primary forecast concern since all models show a cold enough
airmass for widespread snow down to 5000 feet Thursday and
Friday. Did not go that aggressive in the official forecast due
to low confidence and uncertainty. Lowered max and min
temperatures below average for the end of the week, but kept
mainly rain and some thunderstorm activity below 7000 feet.

&&

.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 tonight for WYZ116.
NE...None.

&&

$$

SHORT TERM...BW
LONG TERM...TJT
AVIATION...GCC