853
FXUS65 KCYS 081730
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1030 AM MST Wed Jan 8 2025

.KEY MESSAGES...

- Strong winds are expected to return to the wind prone areas
  today. High Wind Warnings are in effect.

- Areas of blowing and drifting snow are expected today, mainly
  in Carbon and Albany counties, but this may extend into areas
  of the High Plains where snowpack is deeper.

- Another round of light snow showers will move into the area
  this evening into early Thursday. Accumulations are expected
  to be light, between a dusting and an inch or two, with the
  exception of higher amounts above about 7500 ft in elevation.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 330 AM MST Wed Jan 8 2025

Current GOES satellite imagery shows mainly clear skies over the
area, along with fairly widespread snow cover and light winds. Dry
air is apparent in the water vapor channels in much of the
atmosphere, associated with subsidence over the area. In fact,
modeled precipitable water values this morning below 0.05" are
approaching climatological minimums at what is already the driest
time of the year. This has created an ideal environment for
efficient radiative cooling, and bitterly cold temperatures across
the area. Several locations have dropped 10-15F below the coldest
model guidance, since models struggle to handle these shallow
inversions. The coldest temperatures so far are in the Laramie
Valley as is typical, where sensors have reported lows between -15
and -20F so far. For the official forecast, dropped temperatures
significantly below the minimum of model guidance in the typical
cold spots (Laramie, Saratoga, Baggs/Dixon, Alliance, etc) through
about 15z this morning.

Heading into the daytime hours, we will start off with sunny skies
as a transient ridge shifts over the area and yesterday`s shortwave
trough drops off to the southeast. The ridge will be very
short- lived though, as the next trough is already moving across
Montana, expected to arrive in our area this evening. The upper
level trough is supporting a surface low over the Canadian
Prairie, from which a surface trough extends southwestward to
the front of the Rockies. As the pressure trough drifts to the
southeast, we will see pressure falls through the day today and
thus an increase in the cross barrier MSLP gradient thanks to a
strong surface high over the inter- mountain west. While 700-mb
gradients and wind speeds aren`t all that impressive, models
show strong surface gradients including a 3- 5mb MSLP gradient
across both the Snowy and Laramie ranges (even in the more
conservative NAM model). These pressure gradients are typically
enough to support high winds in the typical gap areas, even in
the absence of strong synoptic forcing or strong winds aloft.
In-house guidance also indicates a 50-80% probability for high
winds near Arlington and Bordeaux. As a result, there was
sufficient confidence to upgrade the High Wind Watch to a
Warning for today.

In addition to the winds, the recent snowpack remains fairly
untouched by warm temperatures and sunshine across much of the area.
This will set the stage for blowing and drifting snow today. Much of
the I-80 corridor has seen 4-10 inches of snow over the last week,
with sunshine and above freezing temps both being sparse. Snow
should still be quite blow-able today. Winter Weather
Advisories for blowing snow remain in effect for the I-80
corridor from just west of Cheyenne towards Rawlins. The main
uncertainty with this is the expectation for fairly abundant
morning sunshine and also a chance for temperatures to get close
to freezing, at least below about 7500 ft in elevation. This
leads to some uncertainty in how severe the blowing snow will
be, but there is enough confidence in blowing snow to lead to
slick spots and patchy visibility reductions.

Cloud cover will increase quickly this afternoon as the next
shortwave trough drops down from the north. This system looks like
yet another splitting trough, with two lobes of vorticity separating
as they pass over the area. As has been the trend with these
systems, the disorganized nature will limit snowfall potential, but
we still expect fairly widespread radar echos over the area due to
vorticity advection aloft and the favorable left exit region of a
jet streak positioning itself over the area this evening. Mid to
upper level moisture will recover quickly this afternoon, and mix
into the boundary layer with virga and then snow showers this
evening. Overall expectations are for another minor snow event with
scattered to numerous light to moderate snow showers moving through
the area beginning around sundown and continuing through early
Thursday. Accumulations will be most in areas favored by NNW winds,
which includes of course the mountains (especially their northern
slopes), the Pine Ridge/US-20 corridor, and the north side of the
Cheyenne ridge (north of the I-80 corridor from Cheyenne to Sidney).
The most likely scenario right now is a widespread dusting to an
inch or two, with 2-4" and an isolated 6" or so on favored aspects
above about 7500` in elevation. This probably won`t be widespread
enough to warrant its own advisory, but did decide to extend the
Winter Weather Advisory issued for blowing snow through 6AM Thursday
to include the light snow potential tonight along the I-80 corridor,
as patchy blowing snow may also continue through the night.

As we head into Thursday, another cold airmass will settle into the
area in the wake of this storm system. This time though, it will
unfortunately come with stronger winds as most model guidance shows
40-knot 700-mb winds over the High Plains Thursday. As a result,
expect strong NNW winds to kick up once the cold front passes
through early Thursday morning, leading to a cold and windy day over
the plains Thursday. Highs in the 20s combined with winds gusting
over 40 MPH will produce apparent temperatures in the single digits
to lower teens even midday. Most of the support aloft will be over
the High Plains, and the wind direction will not be very favorable
for the wind prone areas, so confidence in any High Winds is low
with 700-mb winds generally below 50-knots. Still, it will be quite
cold and unpleasant, with possibly some patchy blowing snow concerns
on top of it, assuming the snow doesn`t have time to crust over
today.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 330 AM MST Wed Jan 8 2025

Friday...Northerly flow aloft continues with slowly rising heights
and thicknesses. With 700 mb temperatures near -2 Celsius, it will
not be as cold as Thursday, with high temperatures from the upper
20s to mid 30s. It looks dry with lack of lift and relatively dry
low and mid levels.

Saturday...The next shortwave trough aloft moves into western
Wyoming, and with adequate low and mid level moisture, we expect to
see scattered to numerous snow showers, most numerous west of I-25,
and over the northern Laramie, Snowy and Sierra Madre Ranges.
Strengthening low and mid level gradients suggest windy conditions
in the afternoon.

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 and Sierra Madre
Ranges, and plenty of cloud cover elsewhere. 700 mb temperatures
near -15 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...As the main trough aloft moves to our south, northerly
flow aloft continues overhead, limiting the warming trend, though it
will be slightly warmer as 700 mb temperatures rise to near -7
Celsius, yielding maximum temperatures from the 20s to mid 30s.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1030 AM MST Wed Jan 8 2025

Shortwave is moving through the area tonight into tomorrow
morning. Looking at the High-res ensembles there is some
uncertainty as to the timing of precipitation since there is a
little dry air to overcome first and each models some timing and
intensity differences. Overnight the ceiling will drop into the
MVFR category general around 2000ft after 06z. There may some
occasional lower level ceiling as snow falls tonight but the
prevailing ceilings look to be MVFR.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 8 PM MST this evening for WYZ106-110.
     Winter Weather Advisory until 6 AM MST Thursday for WYZ109-110-
     115>117.
NE...None.

&&

$$

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