Area Forecast Discussion
Issued by NWS Cheyenne, WY

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919
FXUS65 KCYS 070358
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
858 PM MST Tue Dec 6 2016

.UPDATE...
Issued at 843 PM MST Tue Dec 6 2016

Dangerous wind chills are looking more likely tonight for portions
of southeast Wyoming and the northern Nebraska Panhandle. Numerous
sites are already experiencing wind chill values in the 0F to -10F
range, so expect a widespread area to easily achieve adzy criteria
by sunrise as the arctic front blasts south and sfc winds increase
again by 12z or so. Thus, the main change tonight was to introduce
a Wind Chill Advisory for several areas which are not currently in
a winter highlight.

The Winter Weather Advisory is in good shape as snow has developed
along the I-80 corridor. Roads are becoming slick and snow covered
and visibilities are poor per WYDOT webcams. Radar trends indicate
snow could begin any time over the southern NE Panhandle. The most
recent IR satellite loop shows very cold cloud tops, so should see
some precipitation enhancement. However, the snow type will change
quickly in the next few hours as colder air filters in. This could
significantly impact total snow, but will leave the forecast as-is
for now.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 149 PM MST Tue Dec 6 2016

Very cold airmass has settled over the area today with afternoon
temperatures struggling to reach the mid to upper teens across
most of southeast Wyoming. Current water vapor loop shows the next
disturbance aloft moving over the Great Basin region this
afternoon. This feature will be our next snow producer across the
area...mainly along the I80 corridor. Winds have begun to switch
into the east and northeast over southeast Wyoming as a surface
low develops west of the Rocky Mountains...and an arctic high at
the surface drifts southward into Montana. Current Radar loop
shows some echoes aloft begin to form across the mountains of
Colorado. Based on short range models and ensembles...believe this
trend will continue through this afternoon. Upslope winds will
provide some moisture...but it is awfully dry in the boundary
layer with relatively humidities at the surface currently between
20 to 40 percent. Believe it will take some time for the air to
saturate...but snow is still expected to develop around
Laramie...Cheyenne...and the I-80 summit between 4 PM and 7 PM.
This snow will spread north and eastward through the night. Areas
of snow will likely not extend too far north of the Platte River
valley as the arctic airmass begins to move into the region...so
locations north of a line from Rawlins to Alliance Nebraska may
not see much snow accumulation at all. Current forecast looks
good...but added the Snowy Range...Snowy Range foothills...and SW
Platte county including Wheatland and Chugwater to the Winter
Weather Advisory for 2 to 4 inches of snow with travel impacts and
4 to 8 inches in the mountains. Thought about upgrading the
advisory to a Winter Storm Warning for the I-80 summit...but
maximum dendritic growth will be difficult due to the very cold
temperatures so decided against it. Travel conditions will still
be hazardous through Wednesday morning. In addition...winds appear
to be a little higher than previously expected so can not rule out
north to northeast gusts around 25 to 30 mph across the area.
Blowing snow threat still appears minima...but drifting snow may
be a concern well into Wednesday.

The arctic front will arrive late tonight and early Wednesday
morning. Temperatures tonight may remain in the teens for a while
before the bitter cold air enters the region after midnight. Would
not be surprised to see readings below zero around dawn across
most of southeast Wyoming and northern Nebraska. Wind chill
temperatures for the morning commute will likely range between -15
to -25 across the forecast area in light snow.

For Wednesday...snowfall will rapidly end from northwest to
southeast in the morning leaving flurries across the area in the
afternoon. High temperatures will be in the single digits...with
maybe low teens across western Nebraska. Will have to monitor
model trends with low temperatures Wednesday night/early Thursday
due to clearing skies and a snowpack in some areas. Can not rule
out lows near -25 to -35 in the more sheltered valleys due to
light winds. Believe most locations will range between -15 to -5
by early Thursday with extremely cold wind chill readings. Will
likely need Wind Chill headlines early Thursday morning for feel-
like temperatures between -25 to -40 across the area. Temperatures
will be 5 to 10 degrees higher on Thursday...but still in the
teens for highs. With a south wind...may need fog and/or low
stratus over the Cheyenne and Pine ridges into Thursday night.

.LONG TERM...(Friday through Tuesday)
Issued at 230 PM MST Tue Dec 6 2016

A generally west to west-northwest flow aloft will hold over
the region through the period. After a chilly morning Friday
the cold surface airmass retreats east and allows warmer air
to spread back over the CWA Friday. Temperatures will remain
on the mild side Saturday. This flow pattern will also bring
periodic snow showers to the mountains as moisture rides up
the facing slopes. Scattered snow showers should spread into
the plains late Sat as an upper shortwave scoots east across
the region with a push of colder Canadian air accompanying it.
Cool and mainly dry Sunday and Monday with a few snow showers
still possible over the higher mtns.  The next push of colder
air may arrive by Tuesday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 433 PM MST Tue Dec 6 2016

Snow will increase in coverage and intensity tonight along the I80
corridor between RWL and SNY. The heaviest snow will occur between
Arlington and Kimball. IFR/LIFR conditions should become sustained
at KCYS/KLAR shortly, followed by KRWL by 02z and KBFF/KSNY during
the 05z-07z time frame. Snow will diminish by mid Wed morning, but
low ceilings are likely to persist at most terminals thru at least
18z Wednesday.


&&

.FIRE WEATHER...
Issued at 149 PM MST Tue Dec 6 2016

No Fire Weather concerns this week as accumulating snow and
bitter cold temperatures are expected. Relative humidity will
remain high with excellent overnight recoveries. Slightly warmer
temperatures are expected by the end of the week before the next
cold front pushes through the region this weekend.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Wind Chill Advisory until 11 AM MST Wednesday for WYZ101-102-104-
     105-107>109-111-113.

     Winter Weather Advisory until 9 AM MST Wednesday for WYZ117>119.

     Winter Weather Advisory until 6 AM MST Wednesday for WYZ106-110-
     114>116.

NE...Wind Chill Advisory until 11 AM MST Wednesday for NEZ002-095-096.

     Winter Weather Advisory until 9 AM MST Wednesday for NEZ054-055.

&&

$$

UPDATE...CLH
SHORT TERM...TJT
LONG TERM...RE
AVIATION...CLH
FIRE WEATHER...TJT



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