771
FXUS65 KCYS 072251
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
351 PM MST Sun Dec 7 2025

.KEY MESSAGES...

- A High Wind Warning is in effect from 11 PM tonight through 11
  AM Monday for Arlington, Bordeaux, and the Laramie Summit wind
  prone locations.

- Generally light mountain snow expected for the Sierra-Madre
  and Snowy ranges tonight through tomorrow morning.

- A High Wind Watch has been issued for most of the CWA
  beginning late Monday, with portions lasting through late
  Tuesday, while much of the high terrain and adjacent zones of
  Wyoming will stay in a watch through early Thursday.

- Uncertainty remains in the forecast between late Wednesday
  through Friday due to an arctic cold front that will stall
  very near or over our area. A small shift in the front
  location will mean wildly different outcomes.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
Issued at 259 PM MST Sun Dec 7 2025

This morning`s high wind warning was a bust as winds struggled
to reach criteria across the gaps of our windy prone locations.
That being said, decided to err on the side of caution and
issued another high wind warning, once again for our wind prone
locations of Arlington and Bordeaux but also included the
Laramie Summit from 11 PM tonight and continuing through 11 AM
Monday. A quick moving shortwave should promote 700mb pressure
gradients from Craig to Casper in excess of 60 meters, and near
surface gradients of 65-75 meters, aided by a 700mb jet of
around 50-60 knots and favorable downward Omega values.
Otherwise tonight into tomorrow morning we should see some light
snow showers moving across the mountains of the Sierra-Madre
and Snowy Ranges from this quick passing system, with amounts
generally sub-advisory outside of the highest peaks, but just
low enough that an advisory didn`t feel necessary, particularly
after the dumping of snow these mountains just got.

Moving into Monday during the morning and beginning of the
afternoon, we`ll see highs jump back into the 40`s to 50`s east
of the Laramie Range as warmer air overspreads the region and
skies remain mostly clear. The aforementioned High Wind Warning
will expire by 11 AM, but don`t be surprised if winds linger
just under or even around high wind thresholds during the early
afternoon hours as pressure gradients just barely decline below
the level we`d like to see for high winds. And with an even
stronger wind event right on the heels of this warning, this
will be a temporary respite at best.

&&

.LONG TERM UPDATE...No major changes to the below reasoning
outside of a bit better model confidence in the later portions
of the forecast period as both major ensembles expecting the
ridging to be a bit stronger and promote generally warmer
temperatures and higher winds. The forecast reflects this with a
mix of rain and snow Wednesday through early Thursday as arctic
airmass may not be able to make it quite as far south.
Otherwise, high wind watches have been issued for our expected
multi-day event, including some mountain zones as the potential
for gusts 80-95 in some of these zones remains well within the
realm of possibility. Casper to Craig pressure gradients near
the surface of over 110 meters would most assuredly guarantee
not only high winds, but could bring extreme winds with very
powerful wind gusts not out of the question. The question at
this point isn`t will we hit high enough winds, but rather how
high will they go. And although periods of lull are anticipated,
with the expectation that gusts will remain around or just under
high wind criteria Tuesday evening into early Wednesday before
another shot of stronger gusts occurs, there wasn`t enough value
in letting the watch end early and issuing a second
watch/warning set of products for Wednesday afternoon through
early Thursday.

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 415 AM MST Sun Dec 7 2025

There may be a brief break in the high wind potential Monday
afternoon or early evening, but wind parameters begin to ramp up
dramatically during the evening hours. A much more powerful upper
level low will race in from the northwest on the nose of a 150+ knot
jet streak aloft. While its track should remain well to our north,
it will send MSLP and low-level height gradients surging. By
daybreak Tuesday, LREF median 700-mb winds exceed 65 knots over much
of southeast Wyoming. The GFS indicates Craig to Casper 700-mb
height gradients reaching 90 meters, which is a fairly exceptionally
high value. The GFS continues to run a little hot compared to most
other models, but this is still work a discussion. The model
consensus scenario still supports a mid to upper tier wind event
beginning late Monday evening and continuing through the day
Tuesday. We will also be located in the right exit region of the
upper level jet streak, which supports large-scale descent and
should help get strong winds down to the surface. Forecast soundings
show a fairly strong inversion between about 600 and 700-mb, which
is just a little above mountain top height for our area. Winds
fairly constant through and above the inversion layer suggest the
potential for vertically propagating mountain wave activity which
could enhance surface winds in the immediate lee of the Snowy and
Laramie ranges. Confidence is very high in high winds for the wind
prone and adjacent zones, even though we are still a few days out
still. This event has the potential to be an upper tier event,
meaning the wind prone areas could see gusts of 90 mph, although
confidence in reaching that mark is medium at this time. For now,
the official forecast was nudged towards the NBM 90th percentile to
get a mention of 75 to 80 mph, which has fairly good confidence.

A strong stationary frontal boundary will be stalled underneath the
jet stream aloft during much of the week ahead. It`s exact position
is expected to waffle back and forth, surging southward behind the
passage of shortwave troughs aloft, and then pressing northward
again in between shortwaves. Once the first shortwave trough axis
passes through Tuesday afternoon or evening, we will see the
boundary push southward. Pressure gradients will begin to fall off
slightly Tuesday evening into early Wednesday as a surface high
traverses over the western edge of the High Plains. Even so, the GFS
has high wind parameters minimizing early Wednesday morning at
values still supportive of marginal high winds for the Arlington/Elk
Mountain area. If the surface high manages to push up against the
Laramie range, we should see a break in winds at Bordeaux and the I-
80 summit, but Arlington could continue straight on through. At the
same time, the front pushing southward will bring stronger
frontogenesis into the area, kicking up chances for rain and snow
showers Tuesday night into Wednesday morning.

Most models then show the ridge amplifying once again Wednesday
morning, nudging the frontal boundary back to just northeast of the
area, and sending wind speeds surging once again. This second period
for wind on Wednesday into Wednesday night has lower confidence, but
currently has similar high-end potential to the earlier event Monday
night into Tuesday. The more the ridge amplifies, the higher wind
speeds could get during this period.

After Wednesday, model consensus collapses. While essentially all
ensemble members agree that we will remain in an active northwest
flow pattern, there are subtle differences in the strength and
amplitude of the upper level ridge over the western CONUS. Just a
small difference in this feature would shift the location of the
stalled frontal boundary by a few hundred miles, and lead to wildly
different outcomes for our local weather. The weaker ridge scenario,
generally supported by the GEFS, would allow for the arctic front to
push right up against the Laramie Range by Thursday morning, sending
temperatures plummeting for both Thursday and Friday. In addition,
intense frontogenesis is expected to remain along the stalled
frontal boundary, supporting moderate snowfall wherever this ends
up. If the front stalls over our area, there is the potential for
significant snowfall with strong forcing stalling for a day or two
over the same area. The other scenario features a slightly more
amplified ridge, and is generally supported by the ECMWF ensemble.
This would lead to much warmer and drier conditions for most of the
area, but also come with an extension of the high wind event into
Thursday. The colder scenario would lead to highs in the teens and
snow on Friday for most of the High Plains, while the warmer
scenario would mean highs in the 40s or even 50s and strong winds.
Ensembles are split nearly 50/50 at this time, so the official
forecast splits the difference. Past experience suggests that
the arctic front usually wins in these scenarios, but that is
not enough to push the needle just yet. We would like to see a
little bit more model consensus before committing to one
scenario or the other.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1027 AM MST Sun Dec 7 2025

VFR conditions expected over the next 24 hours. CIGs will
generally be at mid to high levels or clear by tomorrow morning,
with SCT to BKN skies dominating most sites. Winds breezy most
sites at 10-25 knots, with KLAR and KCYS seeing much stronger
speeds in the 30-40 knot range both today and again tomorrow.
Also expecting low level wind shear most sites between 04-15Z
with speeds up to 50 knots.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from late Monday night through Tuesday
     afternoon for WYZ101-102-108-119.
     High Wind Watch from Monday evening through late Wednesday
     night for WYZ103.
     High Wind Watch from Monday evening through late Wednesday
     night for WYZ104-105-107-109-113>115-117-118.
     High Wind Warning from 11 PM this evening to 11 AM MST Monday
     for WYZ106-110-116.
     High Wind Watch from Monday afternoon through Thursday morning
     for WYZ106-110-116.
NE...High Wind Watch from late Monday night through Tuesday
     afternoon for NEZ019-020-054-095-096.

&&

$$

SHORT TERM...CG
LONG TERM...CG/MN
AVIATION...CG