Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 140210
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
710 PM MST Wed Dec 13 2017

.UPDATE...
Issued at 703 PM MST Wed Dec 13 2017

Early evening water vapor loop showed a positively-tilted upper
trough progressing southeast into western and central WY. Large
scale lift on left-front quadrant of upper jet producing a large
area of mainly light snow, with the leading band approaching the
Laramie Range. Low levels were moistening with snow reaching
the surface. Reduced visibilities in falling snow and patchy fog
along with slick/snow-covered roadways west of Cheyenne will
create hazardous travel as temperatures fall into the teens and
20s tonight. Accumulations will range from 1-2 inches higher
valley to 3-5 inches Snowy/Sierra Madre mountains by Thursday
morning. East of the Laramie Range, snow accumulations will be
negligible. Updated zone forecast and grids to reflect current
trends.

&&

.SHORT TERM...(This evening through Friday)
Issued at 205 PM MST Wed Dec 13 2017

The prolonged quiet weather pattern appears to be coming to an
end over the next 24 hours as a Pacific upper level trough is
forecast to push into south and eastern Wyoming by tonight. Much
colder temperatures are expected tonight with temps likely
lowering into the teens to low 20`s. Highs on Thursday will
struggle to reach freezing across most of the region as 700mb
temperatures will be between -10c to -15c. Some light snow is
forecast with this event, mainly tonight after sunset, until early
Thursday morning. Upper level flow is very progressive, so expect
the trough axis to be through the area by sunrise Thursday. Light
snow accumulations are possible, mainly near and west of
Interstate 25. However, snowfall will be fighting some very dry
air near the surface with dewpoints hanging around in the single
digits to lower teens. There is very little moisture advection in
the llvls, so temperatures will have to lower substantially for
snowfall to reach the ground. Thus, forecast snow accumulations
will generally be less than one inch outside the mountains. The
more favorable areas to see some travel impacts (icy or
slick/partially snow covered roads) will be around the Snowy Range
including Elk Mountain towards Laramie and the I-80 summit.
Limited travel impacts will be possible as far east as the
Nebraska border, but with very dry air in place, not very
confident with that outcome at this time. Will hold off on any
Advisories because of limited confidence, although the I-80 summit
may need an advisory for the morning commute depending on
visibilities and snowfall rates. 3 to 6 inches of snow expected in
the higher mountains, but with light winds, impacts should be
minimal.

The upper level trough will rapidly move southeast into the
central and southern plains for Thursday with snow ending early.
Winds will pick up across the plains under northwest flow, with
gusts up to 50 mph possible east of Interstate 25. For Thursday
night and Friday, winds will become the primary concern as models
are indicate a potential strong wind event for the wind prone
areas. After a thorough analysis of gradients, dynamics, and past
events, decided to hold off on a High Wind Watch for the wind
prone areas. The primary reasons are the lack of llvl subsidence,
north to northwest winds just off the surface, and relatively weak
gradients at 700mb. The primary jet stream will be well to the
north during this time. Having said that, will have to monitor
the Bordeaux area between Wheatland and Cheyenne as 850 mb
gradients will increase significantly tonight due to a strong lee-
side trough just east of the Interstate 25 corridor. High res
models show the location of this lee side trough in a favorable
position for locally strong winds through Friday. For now,
increased wind gusts between 50 to 55 mph for these locations.
Otherwise, Friday will be warmer with highs in the 50`s across the
high plains due to downslope flow, and in the 30`s to low 40`s
further west.

.LONG TERM...(Friday night through Tuesday)
Issued at 230 AM MST Wed Dec 13 2017

Strong winds likely Friday night especially out by Arlington. GFS
700mb winds across eastern Carbon/western Albany Counties right
around 55kts. Craig to Casper 850/700mb height gradients both
above 60mtrs til about 06Z Saturday off the GFS. May stick around
a little longer than what the high wind guidance is showing as
GFS 700mb winds stay above 50kts til 12Z Saturday. Whats causing
these strong winds is a cold front approaching from the west and
northwest.

This front begins to produce snow showers Saturday morning as it
sags across southern Wyoming. Snow begins out across Carbon and
Converse Counties as early as 12Z Saturday, becoming fairly
widespread during the morning hours. Did undercut temperatures for
Saturday as GFS 700mb temperatures fall to -12C by the afternoon.
Would think all precip would be in the form of snow. Could be
looking at some accumulations across the mountains. GFS QPF over
the north Laramie Range approaching .5 inches as well as the Snowy
Range. Later shifts will need to consider the need for winter
headlines. Fairly fast movement of the front through the area with
snow ending from north to south by early Sunday morning. This may
limit snow accumulations some as most of the snow will fall
during the day.

Going to stay cold as 700mb temperatures stay well below freezing
til at least Wednesday next week. That will likely be our next
high wind event as GFS 700mb winds again increase to 45-50kts.

Long term guidance continues to show an arctic boundary sagging
south towards the end of next week that we need to be watching
closely. GFS 700mb temps down as low as -20C across Converse and
Niobrara Counties next Thursday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 435 PM MST Wed Dec 13 2017

Western Nebraska terminals...VFR CIGS lowering to 5000 FT AGL
through Thursday. Gusty northwest winds of 25 to 38 KT. VCNTY
light snow showers at Chadron early Thursday morning.

Southeast Wyoming terminals...CIGS/VIS lowering to MVFR to IFR
Rawlins to Laramie in light snow and fog from 02Z to 14Z. MVFR CIGS
with VCNTY light snow showers at Cheyenne. The mountains will be
obscured. CIGS/VIS improve to VFR after 14Z. North-northwest winds 8-
15 KT tonight, increasing to 20 to 30 KT after 16Z.


&&

.FIRE WEATHER...
Issued at 1251 PM MST Wed Dec 13 2017

No fire weather concerns this week and through the weekend. A series
of Pacific cold fronts will move southeast across the area resulting
in periods of snowfall, colder temperatures, and higher humidities.
It will be windy through at least Saturday with gusts between 45 to
55 mph possible on Friday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...MAJ
SHORT TERM...TJT
LONG TERM...GCC
AVIATION...MAJ
FIRE WEATHER...TJT



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