743
FXUS65 KCYS 101155
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
455 AM MST Fri Jan 10 2025

.KEY MESSAGES...

- Winds will increase in the wind prone areas today with
  elevated to marginally high winds expected.

- More widespread strong winds are expected this evening through
  early Saturday which may lead to another round of impactful
  blowing snow. High Wind Watches and Warnings are in effect.

- Heavy snow returns to the mountains tonight into Saturday,
  with scattered snow showers expected elsewhere. A few locally
  intense snow squalls may be possible in the lower elevations.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 430 AM MST Fri Jan 10 2025

Mostly clear skies are in place over the area this morning as
yesterday`s storm system pushes off to the southeast. Today`s
weather will be dominated by a transient ridge moving across the
area, but cloud cover associated with the next storm system is
already apparent on GOES satellite imagery moving into the Pacific
northwest and the northern Rockies. We`ll be off to a chilly start
today, especially in the valleys west of the Laramie Range where
temperatures have fallen below 0F. However, rapid warm air advection
aloft will commence quickly, and mix down to the surface by the
afternoon hours. 700-mb temperatures are currently analyzed around
-12 to -14C, but ensemble median values warm to above 0C by this
evening. As a result, highs today should recover to within about 5F
of seasonal averages. Most of the area should also enjoy mostly
sunny skies today, but cloud cover will rapidly increase after about
mid afternoon as the next system approaches.

A strong surface high pressure remains entrenched west of the
mountain barrier this morning, and surface pressure will be falling
over the plains today as the surface trough associated with the
approaching upper level trough moves to the south and east. As a
result, gap winds will be on the increase corresponding to the
increasing low-level pressure gradients. Elevated to marginally high
winds still look on track for Arlington and Bordeaux during the
daytime hours today, with breezy conditions elsewhere across the
area. We may still see some patchy blowing snow, but this will not
be to the extent we saw yesterday.

The wind event will transition from a surface driven even to a more
strongly forced synoptic wind event after sundown today as the next
trough approaches from the north. 700-mb height gradients rapidly
increase between 00z and 06z as 700-mb winds increase to above 60-
knots over parts of southeast Wyoming. For the wind prone areas,
winds speeds will be marginal for high winds during the day, but
should increase after dark with potential for a few 70 to 80 MPH
gusts in the gap areas. Winds will also increase over the I-80
summit, so had the confidence to upgrade to a High Wind Warning
there as well. In addition to the typical wind prone areas, this
event will have the potential to spill out into adjacent zones
including central/northern Carbon County, and the I-25 corridor from
Douglas to the CO state line. First to discuss Carbon county, the
overall WNW to NW wind direction aloft is not a typical wind
direction for high winds at Rawlins and Muddy Gap, but a strong
south to north oriented surface pressure gradient flowing out of the
strong surface high over northwest Colorado will orient surface wind
vectors in a more favorable SW to NE direction. The direction aloft
reduces confidence slightly, but with 700-mb winds around 50-knots
and the strong surface gradient, decided to expand the High Wind
Watch to include Rawlins and Muddy Gap. Moving east to I-25, the
main uncertainty here is the position of the surface trough and how
far east we will observe mountain wave breaking. Most guidance shows
the surface trough right along I-25, which would normally reduce the
chance of winds getting out of the wind prone areas. However,
forecast soundings indicate a very strong inversion at around 700-mb
which is near the top of the Snowy Range and just above the top of
the Laramie Range. This can often be a good indicator of the
potential for mountain wave breaking leading to significant
acceleration on the leeward slopes. In addition, the cold front
passing through the area should help to pull the surface trough east
of I-25, at least temporarily. Taken together, this gives enough
confidence to issue a High Wind Watch for eastern Platte and central
Laramie counties. We`ll also need to watch Converse county, but the
WNW orientation of the flow aloft may not be great for mountain wave
breaking off the northern Laramie range and into the Glenrock to
Douglas corridor. Therefore, left that zone out for now. The
mountain top inversion appears around 03z and is gone by 12z, so
expect winds to peak around 06z to 09z and then weaken once the
surface front is through the area. Models also indicate our area
will be in the right exit region of a jet streak through about 09z
Saturday, which is favorable for descent and stronger surface
winds. The last piece of uncertainty with this event is the
blowing snow. While temperatures will be warm today with
plentiful sunshine, the weak early January sun angle may not be
enough to form a crust. Therefore, at least patchy blowing snow
looks like another good bet with this wind event, but there is
some uncertainty if the crust can finally form today.

Lastly, the next system will also bring another round of snow
showers to the area. The upper level trough will once again be
fairly disorganized and split with multiple disconnected lobes of
vorticity pushing through between Friday evening and Sunday morning.
As a result, it will be difficult to get widespread low elevation
accumulation snow. However, moist upslope flow returns to the
mountains late Friday evening, with near saturation in the cloud
layer and strong westerly winds. W to WNW flow in the 700 to 500-mb
layer is favorable for both the Snowy and Sierra Madre ranges, and
thus we will be looking at a period of fairly heavy snowfall
beginning late Friday evening and continuing into Saturday morning.
Snowfall rates may approach 1 inch per hour at times in the higher
elevations. After about 18z Saturday, forecast soundings still show
excellent saturation through the column, but significantly colder
temperatures above the surface will likely result in less efficient
snow production and thus a reduction in snowfall rate. Due to the
period of heavier snowfall rates, decided to upgrade to a Winter
Storm Warning for both mountain zones through 06z Sunday. However,
decreasing winds and lighter snowfall rates for the last 12
hours of the Warning (and continuing for another 12-18 hours
after) may not be quite at Warning criteria impacts, so this
could possibly be replaced with an advisory after the period of
heavy snowfall has concluded rather than one long warning.

The lifting profile is fairly muddy for the lower elevations, mainly
from disorganized frontogenesis and vorticity advection aloft. In
addition, westerly to northwesterly low level winds will undercut
precipitation over much of the High Plains. Still, expect to see
scattered snow showers showing up on radar beginning late this
evening and continuing through Saturday morning. A modestly quick
pressure rise behind the initial cold frontal passage may lead to
some locally intense snow showers initially early Saturday morning,
but the thermodynamic profiles are not favorable for snow squalls.
This changes by midday Saturday however, where forecast soundings
show a fairly unstable airmass setting up with steep 700 to 500-mb
lapse rates between 7 and 8 C/km and fairly good moisture aloft. The
mechanical forcing Saturday afternoon is not quite as strong, but
there is still enough to watch for some snow squall activity during
the day Saturday and possibly into Saturday evening. Most of this
snowfall looks to be convective/showery type rather than stratiform,
so widespread accumulations will be tough to come by. Still, areas
such as the Arlington/Elk Mountain area, and the US-20 corridor from
Douglas to Chadron will need to be watched since these zones can
extract a little more snow thanks to favorable orographics in
northwest flow. No low elevation winter headlines were issued with
this update, but will need to monitor in the next few forecast
packages.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Issued at 430 AM MST Fri Jan 10 2025

Sunday...In the wake of the shortwave trough aloft passage, brisk
northwest flow aloft will continue with enough low and mid level
moisture for orographic snow showers over the Snowy, Sierra Madre
and northern Laramie Ranges, and plenty of cloud cover elsewhere.
700 mb temperatures near -13 Celsius will yield maximum temperatures
in the 20s to lower 30s.

Monday...Continued cold with broad troughiness aloft over Wyoming
and northwest flow aloft, along with 700 mb temperatures near -14
Celsius. Drier with limited low and mid level moisture.

Tuesday...Northwest flow aloft strengthens, strengthening the low
level flow as well, and producing a windy and cold day. Somewhat
warmer temperatures expected as 700 mb temperatures rise to near -10
Celsius. The atmosphere looks relatively dry and with limited lift,
precipitation seems unlikely.

Wednesday...A warming trend develops as cloud cover lessens in the
wake of the passing shortwave trough aloft, and a ridge aloft
develops overhead, and with 700 mb temperatures near -4 Celsius
yielding maximum temperatures in the mid 30s to mid 40s, except for
near 30 degrees over Carbon County near Rawlins.

Thursday...Continued relatively mild compared to earlier in the
week, as the flow aloft turns northwest, and 700 mb temperatures
near 3 Celsius yield maximum temperatures from the mid 30s to mid
40s.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1017 PM MST Thu Jan 9 2025

Skies will remain clear through the TAF period. The pressure
gradient will remain tight over KRWL as the surface high pushes
against the incoming shortwave. The gradient remains relaxed the
further east in the Panhandle so KAIA, KCDR, and KSNY are not
expected to see the strong winds compared to the Wyoming
terminals.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from 2 PM MST this afternoon through late
     tonight for WYZ104-109.
     High Wind Warning from 8 AM this morning to 11 AM MST Saturday
     for WYZ106.
     High Wind Watch from this afternoon through Saturday morning
     for WYZ107-118.
     High Wind Warning from 8 AM this morning to 2 PM MST Saturday
     for WYZ110.
     Winter Storm Warning from 8 PM this evening to 11 PM MST
     Saturday for WYZ112-114.
     High Wind Warning from 2 PM this afternoon to 11 AM MST
     Saturday for WYZ116-117.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...RUBIN
AVIATION...MM