487
FXUS65 KCYS 250001
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
501 PM MST Sun Nov 24 2024

.KEY MESSAGES...

- Snow shower activity will continue to progress from northwest
  to southeast through the evening hours.

- Another round of snow showers and colder temperatures may
  impact holiday travel on Tuesday through Wednesday. Heavy
  mountain snowfall is increasingly likely.

- Periods of strong winds will be possible for the wind prone
  areas of I-25 and I-80 on Thursday onwards.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 300 PM MST Sun Nov 24 2024

The stretch of warm weather has come to an abrupt end today as a
storm system slowly slides across the area today. Current satellite
imagery shows plentiful cloud cover along a jet streak stretching
from southern California across the Great Basin and towards our
area, with an enhancement over eastern Wyoming associated with a
potent vorticity maximum. The latest mesoanalysis shows significant
frontogenesis along the 700-mb trough axis, producing a band of
heavy snow stretching roughly from Cheyenne to Chadron at this
time. Meanwhile, moist orographic lift continues to produce snow
showers over the higher terrain and in the Laramie Valley with
low- level northwest winds. Overall, this is not expected to
produce much impacts for the High Plains, with a dusting to
perhaps an inch in grassy areas. Short-term model guidance shows
the frontogenesis exiting the region by around 8PM this
evening, which should ended the banded precipitation phase.
Marginal lift associated with the vort-max aloft may continue to
produce scattered snow showers behind the initial band of snow,
but this too should be concluded by the late evening hours. The
mountains and the I-80 corridor from the summit westward will
continue to pick up snow thanks to the orographic lift for a few
more hours, but drier air will work in this evening and shut
snow off there too. By midnight, a much drier airmass will be in
place over the area, ending any lingering snow showers and
leading to clearing skies towards Monday morning.

A transient ridge will try to build in over the area on Monday,
muting the impact of the post frontal airmass. Temperatures should
warm to near to slightly below average for this time of year and
plentiful sunshine. Breezy conditions are expected along and west of
the Laramie Range, continuing overnight in the wind prone areas.
However, the probability of high winds is low (20% or less). The
break will be short-lived though, as the nose of a powerful,
moisture-laden jet streak encroaches on the area Monday night.
Expect orographic snow to resume late Monday night in the mountains.
More details on this system can be found in the Long Term section
below.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 300 PM MST Sun Nov 24 2024


Primarily zonal upper-level flow overhead on Tuesday with a strong,
140kt jet. An upper-level shortwave trough is positioned just off
the coast of the Pacific Northwest, leading to zonal flow from
northern California eastward through western Nebraska. The upper-
level flow will be very moist, due to the remnants of an Atmospheric
River that will impact portions of the West Coast Sunday through
Monday. Forecast soundings from the GFS show deep moisture
throughout most of the atmosphere across the higher terrain, with a
saturated profile from the surface through about 250mb.
Southwesterly surface flow turns northerly behind a passing cold
front, leading to a prolonged period of very moist, upslope flow
across the mountains. Additionally, isentropic upglide will be
present across the mountains for much of this period, further
supporting ample rising motion. The strong, rising motion in a moist
atmosphere will continue through the day Tuesday and Wednesday,
likely coming to an end early Thursday morning. Current forecast
snowfall accumulation is right around 2 feet for the Sierra Madres
with over a foot in the Snowies. Atmospheric River events such as
these tend to do very well in the mountains, frequently producing
over 2 feet of snowfall during the duration. Winter Storm
Watches/Warnings will likely be needed for the mountains in the
coming days, but held off on any headlines for now due to the
ongoing Winter Weather Advisories for the same area. Snowfall totals
in the lower elevations are expected to be in the 1-2 inch range at
this time. Temperatures cool down significantly with the passing
cold front Tuesday, with highs in the 40s and 50s dropping to highs
in the 30s for Wednesday and Thursday.

Once this system finally pushes out of the region Thursday morning,
northwesterly flow returns to the upper-atmosphere through Friday
morning. Snow chances come to an end Thursday afternoon, with partly
cloudy skies returning for the remainder of the period. With the
cooler northwesterly flow aloft, surface temperatures will remain on
the cool side, featuring highs in the 30s Thursday, Friday, and
Saturday for the entire region. December looks to start out fairly
close to average for much of the region, with little to no
precipitation expected outside of the highest terrain. Strong winds
look to return Friday and Saturday in the typical wind prone regions.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 459 PM MST Sun Nov 24 2024

Scattered snow showers continue across a large portion of the
forecast area, leading to IFR and MVFR conditions due to poor
visibility. Expect improving conditions this evening for
terminals west of the Laramie Range, while terminals to the east
will likely not improve until later tonight. Snow showers will
wrap up this evening, however, low stratus will persist for
terminals east of the Laramie Range. Expect MVFR CIGs through
about midnight at most western Nebraska terminals.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until midnight MST tonight for WYZ112-
     114.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...AM
AVIATION...SF