062
FXUS65 KCYS 061145
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
445 AM MST Sat Dec 6 2025

.KEY MESSAGES...

- High Wind Warnings remain in effect for much of southeast
  Wyoming through Saturday.

- Areas of blowing snow will impact travel conditions in the
  typical wind prone locations.

- Prolonged mountain snow will continue through early Sunday.
  The heaviest snowfall rates are expected this morning.

- There is potential for additional periods of high winds in the
  wind prone areas, with short lulls in between, on Sunday and
  again on Monday.

- A long duration, high-end, wind event is increasingly likely
  in the Tuesday through Thursday time period. Areas of snowfall
  will be possible too, but the details remain uncertain.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 400 AM MST Sat Dec 6 2025

An active northwest flow pattern will dominate the weather over our
area for the forseeable future. Current satellite imagery shows
increased cloud cover over most of the area as a subtle shortwave
traverses overhead. This feature is supporting surface pressure
falls over east central Wyoming this morning, helping to tighten the
low-level pressure gradient across our mountain barriers. High winds
have been observed for several hours now in the typical wind prone
areas. The pressure gradient is likely near its maximum now through
about sunrise as the surface low begins to quickly move east. This
process should help to spread winds into the zones adjacent to the
wind prone areas, including more of the I-25 corridor and into the
Laramie Valley. While most of the day today should see gradually
decreasing gradients, parameters remain supportive of high
winds through about the early afternoon hours. Today`s soundings
are actually somewhat unstable, which should help mix down the
stronger winds during the daytime, even as winds aloft decrease
through the day.

Aside from the wind, we also have a complex near-term forecast for
snow potential across the area today. Current radar shows a region
of light precipitation over far eastern Wyoming and into the
Nebraska panhandle associated with a region of weak isentropic lift
ahead of the primary shortwave trough axis aloft. This is
inconsistently reaching the ground, but is grounds to keep PoPs over
the area for the next several hours. Further west, moist
northwest flow continues to push into the higher peaks of
southeast Wyoming. SNOTEL observations suggest that about 4 to
12" of additional snow has fallen since Friday evening, with
moderate to heavy snowfall expected to continue through about
midday before some modestly drier air is introduced into the
middle atmosphere and cuts down on snowfall rates. Light to
moderate snow (possibly showery type and thus locally or briefly
heavy) will continue into Sunday morning. Snow has spilled out
of the higher mountains thanks to the vorticity advection aloft
and modest frontogenesis pushing up against the higher terrain.
The best forcing to spread snow to surrounding areas will also
be present from now through about midday. While strong downslope
winds will cut into accumulation potential along and east of
I-25, we could see a brief moderate snow shower drop a quick
dusting. Northwest flow can be favorable for the Laramie area as
well as the I-80 summit, so we will need to monitor
observations here. There is not enough confidence to expand the
Winter Weather Advisory, but these areas could see up to a few
inches of snow accumulation on the higher end of today`s
possible outcomes.

We should see snow shower coverage drop off this afternoon as winds
come down as well. However, we will remain under moist northwest
flow and near to a decent baroclinic zone through Sunday, while
continued vort-maxes move overhead. As such, it`s hard to completely
rule out snow shower activity at anytime before about midday Sunday.
Therefore, low-end PoPs are retained for much of the period. We are
also expecting a break in the high winds this evening and overnight,
but it might be rather short lived. Pressure gradients begin to
creep up once again as early as Sunday morning. To avoid confusion
with the current high wind event, no headlines will be issued at
this time. The wind potential Sunday also looks considerably more
marginal and lower confidence than today`s event, with high wind
probabilities around 30 to 50% for the wind prone areas at this
time. This should be mainly confined to the typical gap areas as
well, and is not expected to spill into adjacent zones. Another
brief lull in the wind is possible Sunday evening.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 400 AM MST Sat Dec 6 2025

The main weather concern in the week ahead is the potential for a
long-duration, high-end wind event. There is high confidence that
the overall synoptic weather pattern will remain northwest flow, and
that a powerful jet stream aloft will park itself over the northern
Rockies for much of the week. There is good to excellent agreement
through early Wednesday. Monday looks to remain windy with high
winds likely returning to the wind prone areas in the early morning
hours. This portion looks like generally a fairly typical high wind
event for the area. However, things get much more interesting late
on Monday as a powerful upper level low races in from the northwest
and tightens the pressure gradient. Ensemble median 700-mb winds
exceed 65 knots over much of southeast Wyoming Monday night into
early Tuesday, providing unusually high confidence in a high-tier
wind event, even at this lead time. NAEFS mean 700-mb winds exceed
the 99th percentile of climatology over most of southern Wyoming,
and are fairly close to climatological maximums. Wind speeds were
increased in the official forecast to better capture the potential.
700-mb winds of 65 knots would easily support gusts of 75+MPH, so
this was introduced to the forecast. Overall, this may be a
very difficult week for travel for light-weight and high-profile
vehicles across the state of Wyoming.

Looking ahead past Wednesday, nearly all ensemble members agree on
the general set-up, but there is considerable variance in the exact
position of the jet core, which results in wildly divergent
solutions for the surface weather, although this might be a pick-
your-poison type choice. A more amplified western CONUS ridge would
shift the jet core just slightly to our north, promoting warmer
temperatures and drier conditions, but would likely mean the strong
to extreme wind event continues into Thursday or even early Friday.
This scenario is supported moreso by the ECMWF ensemble. A weaker
ridge would place the jet core slightly further south, and allow
arctic cold fronts associated with shortwave troughs riding overhead
in the northwest flow to push through our area. About 25% of
ensemble members show fairly widespread snowfall with a potent
arctic cold front in the Wednesday/Thursday time period. The bright
side of this scenario is that it would likely bring a break in the
strong winds. This pattern was a little tricky to capture in the
official gridded forecast. This is because ensemble mean
temperatures are actually rather warm for Wednesday and Thursday, in
the 40s to 50s. The NBM thus puts rain in the forecast due to the
possible precipitation against the ensemble mean temperatures.
However, every scenario that has precipitation has the arctic front
slipping to our south, and thus precipitation would be all snow.
Without spending too much time wrestling with this, decided to
simply lower the minimum snow level to at least force the grids to
contain rain/snow rather than just rain. It should be noted that
rainfall is rather unlikely though, and this is more an artifact of
the way our system describes a highly uncertain forecast
scenario.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 439 AM MST Sat Dec 6 2025

The initial round of light snow is departing the Nebraska
panhandle over the next hour or two, which will be followed by a
wind shift to the northwest and an increase in gusts. Snow
showers will continue in the vicinity of Wyoming terminals
through much of the day today. Confidence is low in the timing
of any VIS/CIG reductions, but we will need to watch for a quick
drop in flight category for much of the day as these move
through. Otherwise, look for gusty winds to overspread the area
today, with frequent gusts of 30 to 40 knots in Wyoming and 20
to 30 knots in Nebraska. An occasional gust up to 50 knots is
possible at WY terminals. Winds will weaken this evening, and
die off in Nebraska. A period of low CIGs may impact CDR and AIA
overnight tonight.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 5 PM MST this afternoon for WYZ106-109-
     110-115>117.
     High Wind Warning until 2 PM MST this afternoon for WYZ107-118.
     Winter Weather Advisory until 5 PM MST this afternoon for
     WYZ110.
     Winter Storm Warning until 5 AM MST Sunday for WYZ112-114.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...MN
AVIATION...MN