Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 082241
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
341 PM MST Thu Dec 8 2016

.SHORT TERM...(This evening through Saturday)
Issued at 330 PM MST Thu Dec 8 2016

It is a cold day across the region with highs in the single digits
to mid teens.  Warm air advection will occur through tonight with
700 mb temps rising from around -13C at 00Z to -6C by 12Z.  This
means that low temps will be reached early in the night with warming
toward sunrise, especially across southeast WY.  The 850mb CAG-CPR
gradient increases to around 45 meters by 12Z tomorrow with the 700
mb gradient up to 700 mb.  With the WAA and stable layer around just
a few thousand feet off the sfc, believe that high winds will be
possible in Arlington, so went ahead and upgraded to a High Wind
Warning.  With the sfc trough right along the Laramie Range, this
pattern does not favor the southern Laramie Range or Bordeaux areas.

Still expecting orographic snows across the Sierra Madre and Snowy
Ranges to begin this evening and continue into the early afternoon
on Friday.  The winter weather advisory looks good with 6-9 inches
expected.  The GFS does show light snow continuing through Saturday
morning, but accumulations are expected to be only a few inches.
Temps will warm through Saturday with highs returning to around
normal.  After decreasing through early Friday night, the llvl
gradient will ramp up again between 06-12Z Saturday.  With the 850
mb CAG-CPR gradient increasing to around 75 meters by sunrise
Saturday, mid shift will likely need to consider High Wind Watches
for the wind prone areas through early Saturday aftn.  The next
round of mtn snows will begin by late Saturday with the upper jet
overhead and lots of Pacific moisture streaming into WY.

.LONG TERM...(Saturday night through Thursday)
Issued at 330 PM MST Thu Dec 8 2016

The active progressive long wave pattern will continue through
the long term. Forecast challenges will focus on potential for
high winds, snowfall and arctic cold air mass.

Strong westerly low-mid level flow with large height gradients,
may point to potentially low-end high wind event for the southeast
WY wind corridors Sunday and Sunday night. Shortwave energy and
decent moisture plume will produce widespread snowfall across the
CWA Saturday night and Sunday morning, with orographic snows for
the mountains Sunday night and Monday. Blowing snow will be a
concern for I-80 travelers during this time. Mountains may measure
up to 8 inches or more snow, with 1 to 3 inches for the valleys
and plains. Temperatures will be near seasonal normals.

There are model differences during the Tuesday-Wednesday period
regarding the impact of another arctic air mass intrusion. The
GFS backdoors the arctic front as far south as the I-80 corridor
whereas the ECMWF keeps it across the northern CWA. Trended closer
to the GFS with highs in the teens and low 20s Tuesday, with a
slow temperature moderation beginning Wednesday as the front
lifts northeast of the CWA with zonal flow aloft into Thursday.
Snowfall will primarily affect the higher terrain, with the
exception of light snow for the plains late Tuesday and again
Wednesday night and Thursday with some passing shortwave energy
aloft.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1033 AM MST Thu Dec 8 2016

VFR with cloud ceilings increasing/lowering from west to east
this afternoon and evening. Snow develops tonight west of the
Laramie Range, especially over the mountains and Carbon County.
Expect MVFR to IFR conditions in snow at RWL and SAA airfields
after 06Z, with mountain obscurement. West-southwest winds will
increase with gusts 25 to 35 kt from RWL to LAR late tonight.
Higher wind speeds will be possible over and near the Snowy/Sierra
Madre ranges, as well as at Arlington and Elk Mountain. Light snow
or flurries will be possible late tonight at CDR.

&&

.FIRE WEATHER...
Issued at 330 PM MST Thu Dec 8 2016

Windy conditions will develop across the district this weekend,
with the highest speeds expected in the wind prone areas of
southeast Wyoming. No fire weather concerns are expected though
due to cool temperatures and high humidities.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 6 PM MST Friday for WYZ112-114.

     High Wind Warning from 10 PM this evening to 11 AM MST Friday
     for WYZ110.

NE...None.
&&

$$

SHORT TERM...ZF
LONG TERM...MAJ
AVIATION...MAJ
FIRE WEATHER...RJM



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