Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 110524

Area Forecast Discussion
National Weather Service Cheyenne WY
1024 PM MST Sat Dec 10 2016

.SHORT TERM...(This evening through Monday)
Issued at 230 PM MST Sat Dec 10 2016

The afternoon GOES water vapor loop continued to show an intense
and moist Pacific upper jet moving across the Intermountain West
and Rockies. Considerable mid/high cloudiness dominated the region.
Snow was increasing across northwest CO ahead of an fast-moving
disturbance over southern ID/northern UT. This snow will begin to
affect the Snowy/Sierra Madre mountains late this afternoon, and
especially tonight into Sunday morning when orographic lift will
be maximized. Storm total accumulations are slightly higher than
WPC, with 12 to 22 inches, heaviest over higher west-facing slopes.
For the surrounding valleys and plains, snow accumulations will
be minimal, generally less than half inch through Sunday. The
CAG-CPR 850/700mb gradients range from 60-70 meters through
early this evening then decrease. Gusty west to southwest winds
will continue for the I-25 and I-80 wind corridors until that
time. Gusty winds of 25-35 kt will produce areas of blowing snow
over Snowy/Sierra Madres, which will create dangerous winter
weather conditions tonight.

Snowfall over the southern mountains will diminish appreciably
Sunday night and Monday as the orographic flow and available
moisture decrease. Another one to two inches will be possible.
Elsewhere, it will be dry with below normal temperatures. Will
need to monitor the potential for a marginal high wind episode
affecting the I-80 wind corridor between Laramie and Rawlins
late Sunday night and Monday morning. The CAG-CPR 700mb gradient
peaks near 63 meters between 06Z and 12Z Monday.

.LONG TERM...(Monday night through Saturday)
Issued at 200 PM MST Sat Dec 10 2016

Monday night/Tuesday...Fast and moist northwest flow aloft will keep
snows ongoing across our Snowy and Sierra Madre Ranges Monday night,
with snows spreading eastward across much of southeast Wyoming
Tuesday afternoon along a cold frontal boundary. Colder temperatures
Tuesday based on thicknesses.

Wednesday...Arctic type air mass resides east of the mountains,
keeping maximum temperatures in the teens, with 20s further west.
With a deeply moist airmass, upslope and some overrunning of the
cold dome, light snow looks likely across much of the forecast area,
especially over southeast Wyoming.

Thursday...Chance of snow continues over our southern counties with
adequate low and mid level moisture and upslope southeast winds.
Slightly warmer based on thickness changes.

Friday...Models similar in showing the next Arctic cold front
surging southward across our counties with bitterly cold
temperatures in the afternoon and highs struggling in the single
digits and teens. Widespread snow expected in the moist atmosphere,
combined with moist overrunning of the cold dome along with
localized upslope east of the mountains.

Saturday...Areal snow coverage decreasing with a decrease in low and
mid level moisture, though high temperatures still bitterly cold and
only in the single digits with a 1036 mb surface high over the
Dakotas and 700 mb temperatures near -18 Celsius over our counties.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1021 PM MST Sat Dec 10 2016

Followed latest HRRR guidance for the 06Z TAFs. Trouble areas look
to be west of the Laramie Range at KRWL and KLAR where IFR
conditions could materialize in light snow. Other area of concern
is KCDR in the northern Panhandle. 11-3.9U satellite imagery
showing some low clouds near the Pine Ridge. HRRR does not show
this. Kept low MVFR conditions there for a couple hours after 06Z
with clearing afterwards. Sunday should be VFR for all locations.


Issued at 150 PM MST Sat Dec 10 2016

The active weather pattern will continue through next week as a
strong Pacific jet stream resides overhead. It will be windy the
through Monday, especially along and west of the Laramie Range.
The Snowy and Sierra Madre mountains can expect significant
snowfall tonight and Sunday morning with the passage of a potent
weather disturbance. The snow will taper off Sunday afternoon and
night. A series of weather disturbances interacting with a arctic
front will bring accumulating snow and very cold temperatures to
the districts.


WY...Winter Storm Warning until 6 PM MST Sunday for WYZ112-114.



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