711
FXUS65 KCYS 121208
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
508 AM MST Wed Feb 12 2025

.KEY MESSAGES...

- Light snow will taper off this morning as bitter cold
  temperatures continue. Widespread wind chills between 10 and
  30 below zero are expected through Thursday morning. Cold
  Weather Advisories have been issued.

- Strong winds and the potential for travel impacts due to
  blowing snow will return to the wind prone areas tonight and
  may spread to adjacent zones Thursday afternoon. High Wind
  Watches have been issued.

- Heavy mountain snowfall returns in force Thursday night and
  will continue into the weekend. The lower elevations will see
  chances for snow Friday and Saturday.


&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 500 AM MST Wed Feb 12 2025

Active weather will continue for the next few days. Arctic air
is firmly entrenched across the area this morning with
temperatures generally between -10F and 10F. Northerly
breeziness along I-80 from Cheyenne to Rawlins is pushing wind
chill values down to between -15F and -25F. While some zones are
hitting Cold Weather Advisory criteria, this isn`t expected to
last long. Radar shows snow echoes gradually trending downward
this morning as the weak isentropic lift wanes and drier air
moves into the cloud layer from the west. Expect this to
continue for the remainder of the morning. Skies will remain
mostly cloudy this morning, but partially clear in some areas
this afternoon. Meanwhile, another vort-max riding around the
back edge of the departing trough will swing across the area
this afternoon. This feature will also be accompanied by a
stratospheric intrusion, pushing the dynamic tropopause height
down to around 500-mb by the late afternoon over southeast
Wyoming. This will force steep low-level lapse rates, so the
little push of lift from the positive vorticity advection aloft
should kick off another round of marginally convective type snow
shower activity lasting through the evening. Mechanical forcing
is limited with no major frontal systems moving through, but a
locally intense snow shower is certainly possible in this
thermodynamic environment.

Much drier air will move into the area behind this final vort-
max late this evening, leading to mostly clear skies by midnight
across the area. While winds will be on the increase around the
Rawlins area, light winds look like a good best east of I-25.
The greater uncertainty is in the Laramie and Saratoga valleys,
where a shallow surface inversion may prevent the winds from
mixing to the surface and thus lead to much colder temperatures.
While the increasing winds will bring warm-air advection, the
starting point will be quite cold, leading to bitterly cold wind
chills as the wind first starts to pick up. The combination of
the wind picking up to the west and nocturnal inversions to the
east boosts confidence in seeing wind chill values under -20F
across a good portion of the I-80 corridor and Nebraska
panhandle. As a result, a Cold Weather Advisory has been issued
for tonight. The next shift may need to consider expanding this
advisory, but this was done for the highest confidence zones at
this time. Rapid warm air advection should bring a quick end to
the dangerously cold wind chill values by Thursday late morning.

The increasing winds tonight will also mean the return of the
potential for High Winds. Initially, we will see a more WNW
orientation of the 700-mb flow behind the departing trough
tonight into Thursday morning, although the surface wind will be
W to SW, shifted by a south to north MSLP gradient. 700-mb
height and MSLP gradients are marginal, but fairly consistently
favorable for low-end high winds in the Arlington/Elk Mountain
area. Confidence is a little lower for the I-80 summit and the
Bordeaux area. For the former, the surface wind direction and
orientation of the surface pressure gradient is not ideal, but
700-mb winds over 50-knots give enough confidence for a watch.
For the latter, models show a surface trough just west of I-25,
which may prevent strong winds across the central Laramie Range
from reaching the highway. Through the day Thursday, this wind
event will transition to a southwest flow aloft event as a
powerful trough moves inland from the Pacific. The substantial
southerly component will bring a break in the strong winds to
the I-25 corridor and central/southern Laramie Range, but begin
to increase winds in Carbon county. LREF median 700-mb winds are
now at 60-knots over KRWL Thursday evening, signaling strong
support for High Winds. The Elk Mountain area in particular will
also be favored Thursday afternoon and evening even as we may
see winds weaken slightly for Arlington and points eastward.
With the strong signal aloft, decided to issue another High Wind
Watch for central/northern Carbon county beginning Thursday
afternoon and continuing until Friday morning. The second part
of this wind event has some uncertainty regarding the mountain
wave potential, and whether winds will spill back into the I-25
corridor. Current probabilities are around 50% for I-25 near the
Converse/Natrona county line, but generally 30% or less for
Glendo and points southward. That being said, a strong mountain
wave off of the northern Laramie Range may still be able to
produce brief strong winds in other parts of I-25, but
confidence is too low to expand the watch just yet.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 500 AM MST Wed Feb 12 2025

A train of shortwaves off of the Pacific will continue the
active weather pattern into next week. The next in this series
will be a powerful, broad trough moving in on Thursday into
Friday. This system will be pushing an atmospheric river into
the west coast on Thursday, and it`s remnant moisture will push
across the Rockies on Thursday night. NAEFS mean IVT values are
exceeding the 90th percentile of climatology Thursday night into
Friday morning with strong southwest flow and a nearly saturated
column between 700-mb and 300-mb. As a result, expect a period
of heavy mountain snowfall to begin Thursday night and last into
Friday as this powerful trough swings through. A Winter Storm
Watch will likely be needed soon for at least the Sierra Madre
mountains, which are strongly favored in southwest flow events
like this one. The Snowy Range is a bit more uncertain, since
this flow regime is not ideal there. Light to moderate snow
accumulation can also be expected for the lower elevations of
Carbon county due to the abundant moisture and favorable
synoptic lift.

Expect a lull period sometime during the day Friday as the
trough axis swings through the area, but then mountain snowfall
will resume Friday night or Saturday as the back-edge of the
synoptic trough pulls through. The second phase will be a more
northwest flow aloft regime, which tends to be more favorable to
the Snowy Range. While the back edge of the synoptic trough
moves through, another arctic surface high will push down across
the High Plains Friday night. This will knock temperatures back
down to well below average for Friday night and Saturday. Most
model guidance shows this frontal boundary getting hung up along
the Laramie Range, and not quite making it into Carbon and
Albany counties. The combination of shallow low-level upslope
flow, frontogenesis associated with the front catching on the
topography, and QG lift should expand light snow out out into
the lower elevations, at least in Wyoming. Confidence is a
little lower for accumulating snow reaching the Nebraska
panhandle, since the frontal dynamics will be concentrated
closer to the mountains. Another thing to watch for Saturday
will be the potential for snow squalls. Very steep lapse rates
and decent boundary layer moisture over the I-80 corridor from
the summit westward may favor some stronger snow shower
activity, but it is uncertain still at several days out.

The trough will move off to the east on Sunday, allowing warm
air advection to return to the area. However, the strong upper
level low will stall and spin over the upper Midwest area for
much of the early part of next week, meaning that the arctic
high on its western flank will also not be far away at any point
during the week. After a break in mountain snow Sunday, moist
northwest flow looks likely to return Sunday night into Monday,
ending the lull in snow. Most guidance shows a shortwave
rotating around the west side of the broad upper level low
around the Monday into Tuesday timeframe, which would likely
push the arctic front back to the Laramie Range and plunge
temperatures back below average, while also increasing the
potential for low elevation snow accumulation. The details on
this event are highly uncertain at this lead time, but in the
vast majority of ensemble members, this is another largely light
snow event for the Plains.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 500 AM MST Wed Feb 12 2025

Light snow is trending down this morning across the area, but
periods of IFR will remain possible (50% chance) through mid
morning. MVFR CIGs are expected to prevail this morning, with
gradual improvements through the afternoon and evening. A second
round of snow showers is expected to develop in Wyoming this
afternoon. There is about a 30% for a snow shower to impact each
WY terminals, and if this occurs, expect brief IFR conditions.
This is handled with PROB30 groups at this time. Expect clearing
skies this evening winds winds increasing near KRWL.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Cold Weather Advisory from 8 PM this evening to 8 AM MST
     Thursday for WYZ110-113-115>119.
     Winter Weather Advisory until 11 AM MST this morning for
     WYZ117>119.
     High Wind Watch from Thursday afternoon through Friday morning
     for WYZ104-109.
     High Wind Watch from this evening through Thursday morning for
     WYZ106-116.
     High Wind Watch from this evening through Friday morning for
     WYZ110.
     Winter Weather Advisory until 2 PM MST this afternoon for
     WYZ116.
NE...Cold Weather Advisory from 8 PM this evening to 8 AM MST
     Thursday for NEZ002-003-019>021-054-055-095-096.
     Winter Weather Advisory until 11 AM MST this morning for
     NEZ054-055.

&&

$$

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