Area Forecast Discussion
Issued by NWS Cheyenne, WY

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206
FXUS65 KCYS 060901
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
201 AM MST Tue Dec 6 2016

.SHORT TERM...(Today through Wednesday night)
Issued at 200 AM MST Tue Dec 6 2016

Today...Cold airmass will reside across our counties with high
temperatures only in the teens west of I-25 and in the upper teens
and 20s east of I-25. Next surge of colder air along an arctic cold
front will move into our northern counties this afternoon. Shortwave
moves across western Wyoming and western Colorado this afternoon,
spreading increasing snow coverage across our western counties west
of I-25. With coordination with our neighboring offices, we have
decided to start a winter weather advisory for snow at mid afternoon
for the Laramie Valley and southern Laramie Range.

Tonight...Models showing the negatively tilted shortwave moving
across northern Colorado with a decent 500-300 mb and 700-500 mb
layer quasigeostrophic forcing along with upslope flow and near
saturated airmass producing light to moderate snow in a band
primarily along the Colorado state line and I-80 corridor from
Laramie to Cheyenne to Kimball and Sidney. In coordination with our
neighboring offices at Boulder and North Platte, we have issued a
winter weather advisory for snow for our zones beginning this
evening from Cheyenne to Sidney. Based on QPFs and snow ratios,
expect to see some 2 to 4 inches at lower elevations with 6 to 8
inches over the southern Laramie Range. Fortunately, winds will be
relatively light during the snowfall event. Snowfall will likely be
enhanced along the baroclinic band of the arctic cold front.

Wednesday...Progressive shortwave will help light snow to come to an
end in the early morning hours as the moist layer becomes more
shallow and upward vertical motion wanes. However, it will continue
to be bitterly cold based on 700 mb temperatures near -20 Celsius
along with some cloud cover to inhibit warming.

Wednesday night...Clearing skies and light winds along with freshly
fallen snowpack and an arctic airmass will promote bitterly cold
temperatures and have gone closer to the colder guidance lows,
especially at Laramie where this valley location really bottoms out
with arctic airmasses, fresh snow and clearing skies.

.LONG TERM...(Thursday through Monday)
Issued at 200 AM MST Tue Dec 6 2016

A bitterly cold start to Thursday with morning temps below zero
for most locations and wind chills around the minus 20 degree mark
or lower. Heights will begin to build on Thursday as a low
amplitude ridge builds in from the west, so high temps should warm
some through the day although will still be mainly in the teens.
Shortwave energy sliding east across the northern Rockies on
Thursday night will deepen the leeside pressure trough, rapidly
strengthening the sfc pressure gradient across southern Wyoming.
West-southwest winds will increase in response mainly along and
west of the Laramie Range, becoming strong at times early Friday
morning for this area. Likely to see overnight lows happen early
on in the night, with rising temps through the remainder of the
night. Light winds should prevail across the plains making for
another very cold night here. As winds strengthen, will see
orographic snow develop over the mountains on Thursday night,
continuing through at least Friday night in this pattern. The good
thing is that temperatures will warm back up to more seasonable
values for Friday.

Models then differ somewhat on the pattern through the weekend as
GFS brings through a rather potent shortwave trough while the ECMWF
maintains weaker energy in strong zonal flow through Saturday night.
Resultant chances for snow will differ, so kept slight chance PoPs
except for the mountains where orographic forcing will likely keep
snow ongoing here. Overall snow chances will diminish Sunday and
Monday as drier air filters in from the West. Temperatures through
early next week will remain seasonable.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1022 PM MST Mon Dec 5 2016

VFR conditions are expected through Tuesday morning. Ceilings will
begin to lower during the afternoon as snow begins to develop over
the mountains and the I80 corridor from KLAR to KSNY. MVFR will be
possible over KLAR/KCYS by mid afternoon, with IFR cigs/vsbys more
likely after 00z as snow becomes more widespread. Heavier snowfall
may not move into KBFF and KSNY until late in the TAF period.

&&

.FIRE WEATHER...
Issued at 200 AM MST Tue Dec 6 2016

No concerns based on projected humidities.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory from 6 PM this evening to 9 AM MST
     Wednesday for WYZ117>119.

     Winter Weather Advisory from 3 PM this afternoon to 6 AM MST
     Wednesday for WYZ115-116.

NE...Winter Weather Advisory from 6 PM this evening to 9 AM MST
     Wednesday for NEZ054-055.

&&

$$

SHORT TERM...RUBIN
LONG TERM...RJM
AVIATION...CLH
FIRE WEATHER...RUBIN



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