Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 181732

Area Forecast Discussion
National Weather Service Cheyenne WY
1032 AM MST Mon Dec 18 2017

.SHORT TERM...(Today and tonight)
Issued at 200 AM MST Mon Dec 18 2017

We continue with High Wind Warnings for the higher wind prone/gap
wind areas of southeast Wyoming today as the Craig to Casper
gradient remains elevated through 18Z before tapering off. Right
now the Arlington WYDOT sensor has periodic gusts to 50 mph, with
Vedauwoo slightly less. Thinking is that these winds will see an
uptick after daybreak before tapering off this afternoon. Less
confident about Bordeaux along I-25 as they are closer to the lee
side trough, but any eastward migration of that trough today and
Bordeaux will see gusts to near 60 mph. Main impact messaging
today will feature potential for blowovers of light high profile
vehicles and trailers where westerly crosswinds could result in
loss of vehicle control. Otherwise, other than wind impacts today
for some areas of SE Wyoming, a somewhat mild period expected
through Tuesday with no precipitation expected. Highs today and
Tuesday will range from the 20s mountains to mid 30s lower
elevations across the western CWA with 40s and 50s east onto the

.LONG TERM...(Wednesday through Sunday night)
Issued at 245 AM MST Mon Dec 18 2017

Active. Medium to long range models continue to show the pleasant
weather typically associated with early fall coming to an end by
the middle of the week. Wednesday will be the last decent day for
a while with mild temperatures in the 50`s to near 60 across the
eastern plains and even near 45 to 50 degrees across the western
valleys including Rawlins and Laramie.

By Thursday, models in very good agreement showing the first of a
series of impressive cold fronts moving southeast across Wyoming.
The associated upper level trough is forecast to dig south and
attempt to form a closed low near the four corners region and move
slowly east. Will have to monitor this system closely for heavier
snowfall amounts as some ensemble members and 00Z Canadian model
are hinting towards a less progressive solution and more snowfall.
Current, looks like a light to moderate snowfall event for the
area including the western Nebraska panhandle. Impacts due to blowing
and drifting snow are possible as well with north to northeast
winds between 15 to 25 mph with higher gusts expected. Do not
anticipate the system to stall for a considerable amount of time,
mainly because of the major push of arctic air expected by late
in the week and during the holiday weekend.

Models indicate one of the more impressive arctic airmass
intrusions seen in this area over the last 5 to 8 years arriving
as early as late Friday and likely remaining over or near the
region through Christmas day and even into the middle part of next
week. Strong arctic front will rapidly move southward across the
plains and intermountain west with 850mb temperatures lowering
between -15c to -25c, which translates to surface temperatures in
the single digits across the lower elevations and single digits
below zero across the higher terrain. Overnight lows will be very
tricky due to the potential for a surface snow pack in addition to
lower than normal confidence in cloud cover. Record low
temperatures are possible this weekend through early next week. As
the arctic front arrives late Friday and Friday night, a strong
jet stream and upper level trough will dig southward and stall
near the Great Basin region. There is concern of a prolonged
snowfall event Friday night through Saturday, with considerable
snow amounts possible, especially further south. Although QPF
amounts aren`t all that impressive, models tend to struggle with
arctic air masses, thus this bares watching since dynamic forcing
appears to be impressive across all models given surface-700mb
Omega and moderate-high frontogenesis along with upslope flow. The
other concern is very cold to potentially bitter cold
temperatures depending on if/how much the surface snowpack
influences surface temperatures. Bitter cold wind chills appear
likely across the area with widespread values between -15 to -30,
even during the day, with even lower values expected at night and
early morning hours. High temperatures in the single digits and
teens may be too optimistic as it stands right now, although did
lower highs a few more degrees this weekend.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday morning)
Issued at 1030 AM MST Mon Dec 18 2017

VFR with considerable high cloudiness. Strong west-southwest surface
winds with gusts 30-40 KT southeast WY terminals. For western WY
terminals, expect gusty west winds 22-28 KT. This evening, west-
northwest winds diminish to light across western NE, but remain
breezy across southeast WY, with gusts to 25 KT. There will be
periods of mountain wave turbulence, especially this afternoon.


Issued at 200 AM MST Mon Dec 18 2017

No precipitation expected through Wednesday with a slight warming
trend expected, although not warm enough to result in critical
relative humidity values for fire weather concerns. Breezy today
but with RH values above critical concerns. Much
colder temperatures expected by Thursday with an increased chance
of wetting widespread snowfall.


WY...High Wind Warning until 5 PM MST this afternoon for WYZ106-110.

     High Wind Warning until 5 PM MST this afternoon for WYZ116-117.



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