Area Forecast Discussion
Issued by NWS Cheyenne, WY

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821
FXUS65 KCYS 181020
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
320 AM MST Thu Jan 18 2018

.SHORT TERM...(Today through Friday)
Issued at 320 AM MST Thu Jan 18 2018

Overnight water vapor imagery depicted a upper level ridge axis
from southern CA to central MT with prevailing northwest flow
aloft. A surface trough extended from central MT to south central
WY. Modest pressure gradient was generating gusty westerly winds
of 40 to 50 mph across the wind corridors including Arlington,
Bordeaux and I-80 summit. Skies were partly to mostly cloudy
and temperatures ranged from the mid 20s to lower 40s.

Main short term concern will be high winds for the southeast WY
wind prone areas.

Upper ridge of high pressure will shift east across the CWFA today
and be located over the eastern Great Plains and Midwest Friday.
Northwest flow aloft will back to the southwest and increase tonight
and Friday. The Craig to Casper 850mb gradient peaks near 65m by
12Z Friday. The 700mb gradient is weaker, peaking near 55m by
Friday morning. The GFS 700mb winds ramp up to 50 to 55 kt at 12
Friday along the Laramie Range, as well as across the I-80 corridor
in Carbon County. At this time, confidence is high for high winds
to develop late tonight across the north Snowy Range foothills,
and upgraded the High Wind Watch to a Warning. The central/southern
Laramie Range/foothills and southwest Platte County will remain in
a High Wind Watch as confidence is moderate. The day shift may
upgrade these zones to Warnings based on the 12Z models. The
700/850mb gradients decrease after 18Z Friday, with winds subsiding
through the afternoon.

Above normal temperatures will prevail today and Friday, especially
the plains with highs in the 50s to lower 60s. Lows tonight will be
in the 20s and 30s.

.LONG TERM...(Friday night through Wednesday)
Issued at 320 AM MST Thu Jan 18 2018

The upper ridge is progged to shift to the east of our area by
Friday with an increasingly active southwesterly upper flow ahead
of the next system setting up. The models still project a
deepening upper trough passing across the central Rockies Saturday
Night into Sunday with a deepening surface to midlevel low
forming up over eastern Colorado during the day on Saturday. This
scenario will likely bring a foot or more of snowfall to the
mountains of southern Wyoming this weekend with several inches of
snowfall across some of the lower elevations, especially north of
a Cheyenne to Chadron line. We expect to see travel impacts due
to snowfall to begin out across the western areas Friday night
spreading east of the Laramie Range by Saturday afternoon. As the
upper trough moves out to the east on Sunday, expect snowfall to
diminish with the exception of some lingering snow showers into
Monday across the Sierra Madre and Snowy Ranges.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 320 AM MST Thu Jan 18 2018

VFR. SCT to BKN high to mid level clouds with CIGS above 15000 ft
AGL. West to southwest winds will become breezy at KCYS and KRWL,
with occasional gusts from 25 to 30 kt. Winds will be stronger
over and near the mountains.

&&

.FIRE WEATHER...
Issued at 320 AM MST Thu Jan 18 2018

Dry and warmer conditions will prevail today beneath an upper level
ridge of high pressure. Strengthening surface and low level pressure
gradients will result in rather strong and gusty winds for the wind
prone areas of southeast Wyoming late tonight and Friday morning.
Fire concerns will be low today and Friday with minimum relative
humidities between 20 and 30 percent. A strong Pacific low pressure
system and cold front will produced widespread accumulating snowfall
and colder temperatures this weekend. Seasonably cool temperatures
will prevail early next week with chances for mountain snow showers.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning from midnight tonight to noon MST Friday for
     WYZ110.

     High Wind Watch from late tonight through Friday morning for
     WYZ106-116-117.

NE...None.
&&

$$

SHORT TERM...MAJ
LONG TERM...JG
AVIATION...MAJ
FIRE WEATHER...MAJ



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