Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201732

Area Forecast Discussion
National Weather Service Cheyenne WY
1032 AM MST Sat Jan 20 2018

Issued at 910 AM MST Sat Jan 20 2018

Decided to break out the southern Panhandle for later onset of
winter storm conditions. Latest guidance showing the southern
Panhandle really not snowing until around 08Z tonight. Starting
the warning at 06Z tonight seems a little better than 5PM this
afternoon. Also extended this part of the warning into Sunday
night as this area will be the last to clear from the wrap around
moisture. Updates sent.


.SHORT TERM...(Today through Monday)
Issued at 439 AM MST Sat Jan 20 2018

Overnight water vapor loop showed the upper trough axis across central
NV and CA, with southwest flow advecting Pacific moisture from the
Desert Southwest into the central Rockies. Primary surface low was
forming over southeast NV, with a quasi-stationary boundary extending
into central UT and south central WY. Precipitation was developing
to the north of the front from north central WY into western SD.

The next 24 to 48 hours will be focused on the winter storm which
will impact the entire CWFA. Most of the numerical models are in
agreement with the intensity and track of the upper low, with the
ECMWF trending somewhat slower. The general track of the low is
across the Four Corners later today and tonight, and into southeast
CO Sunday morning. Surface low center over southeast corner of CO
Sunday morning deepens as it tracks east across KS Sunday afternoon
and evening. Impressive QG lift, north-northeast low level upslope,
cold air aloft, and plentiful moisture will all come together to
produce widespread significant snow accumulations beginning later
this morning north and west of a Douglas-Laramie line, spreading
south and east to a Lusk-Cheyenne line by late afternoon, and into
the western NE Panhandle this evening. Timing and snowfall amounts
were tweaked to match with our neighboring WFOs, with heaviest
weekend totals of 12 to 24 inches over the North Laramie, Snowy
and Sierra Madre mountains. Lower elevations can expect 6 to 10
inches, with up to a foot along the Pine Ridge. Upgraded the Winter
Storm Watches to Warnings, including the Laramie Valley where
outlying areas near the mountains could receive 6 to 8 inches.
Saratoga and the Upper North Platte Valley may be shadowed moreso
than the Laramie Valley, and left the Winter Weather Advisory
alone. Besides the snowfall, gusty northeast winds of 30 to 40 mph
will produce areas of blowing and drifting snow over much of
southeast WY and western NE Sunday. The result will be significant
travel impacts including Interstates 25 and 80. The snow will
taper off from northwest to southeast beginning late Sunday
afternoon and continuing Sunday evening. Blowing snow will be a
concern for travelers as winds will be slow to diminish. Snow
showers will linger over the Snowy and Sierra Madre ranges Monday.
Elsewhere, it will be dry.

After the recent warmth, it will be much cooler today with highs
in the 30s and 40s. Much colder Sunday as 700mb temperatures
plunge to -14C, with highs in the 20s. Slightly warmer Monday with
highs into the mid 30s. It will be blustery Monday across much of
southeast WY with west-northwest winds gusting as high as 45 mph.

.LONG TERM...(Monday night through Friday)
Issued at 439 AM MST Sat Jan 20 2018

Models bring a weak shortave down the northwesterly flow aloft
late Monday which may kick off some snow showers in the higher
elevations of Carbon and Albany counties, but right now this
system doesn`t look overly significant. We note a modest warm-up
midweek, then another storm system progged to move in from the
west late in the week when we will see our next chance of snow.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1032 AM MST Sat Jan 20 2018

A period of deteriorating and poor aviation conditions upcoming
through this TAF cycle. KRWL has already reported snow, with
lowering CIGS at KCYS. This trend will continue through the rest
of the afternoon, as snow slowly spreads northwest to southeast
across the region. The main adjustments for this issuance was to
delay the onset of the heaviest snow at KCYS and western Nebraska
airifields by 2-5 hours. Otherwise, expect snow to continue
developing and become heavy at times, especially overnight and
Sunday morning. As a result, expect widespread LIFR conditions at
most sites. This storm will also cause mountain obscurations
through the period as well.


Issued at 439 AM MST Sat Jan 20 2018

No fire weather concerns through next week. A winter storm will move
through the region resulting in widespread snowfall, gusty winds,
and cold temperatures. Slightly warmer temperatures are expected
for the middle of next week, followed by cooling late next week.


WY...Winter Storm Warning from 5 PM this afternoon to 6 PM MST Sunday
     for WYZ108-118.

     Winter Storm Warning from 2 PM this afternoon to 2 PM MST Sunday
     for WYZ101-102-106-107-115>117.

     Winter Storm Warning from 11 PM this evening to 11 PM MST Sunday
     for WYZ119.

     Winter Storm Warning until 2 PM MST Sunday for WYZ103>105-

     Winter Weather Advisory until 2 PM MST Sunday for WYZ113.

NE...Winter Storm Warning from 5 PM this afternoon to 6 PM MST Sunday
     for NEZ002-003-019-095-096.

     Winter Storm Warning from 11 PM this evening to 11 PM MST Sunday
     for NEZ020-021-054-055.



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