Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 170512
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1012 PM MST Sat Dec 16 2017

.UPDATE...
Issued at 933 PM MST Sat Dec 16 2017

WYDOT webcams showing pretty low visibilities between Cheyenne and
the I-80 Summit this evening. With easterly upslope winds
continuing, see no reason for this fog to dissipate. So went ahead
with a Dense Fog Advisory through the overnight hours. Updates
have been sent.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 225 PM MST Sat Dec 16 2017

Early afternoon water vapor loop depicted a sharp upper level trough
axis from western MT southward through the Great Basin and offshore
of southern CA. Pacific mid/upper level moisture was advected
northeast from the Desert Southwest northeast across north-central
Rockies and Great Plains. A quasi-stationary surface boundary extended
from IA across southern NE, northeast CO to a low center over
southwest WY. Isentropic lift on left-quadrant of upper jet and
low level post-frontal upslope developed areas of light snow north
of the front from central to northeast WY. Accumulations were
mostly confined to grassy surfaces, with roadways slick in spots.

The immediate short term period (Tonight and Sunday morning) will
be most challenging weatherwise in regards to precipitation type,
especially for the southeast WY plains into much of the western NE
panhandle. The latest numerical model guidance follows recent trends
with a mid/upper closed circulation near northern Baja CA, effectively
splitting energy to the north and east by Sunday. Occasional light
snow will continue through early Sunday morning across portions
of central and eastern WY. Additional accumulations of 2 to 4 inches
are likely, with totals of 3 to 6 inches, heaviest over the mountains.
Winter Weather Advisories remain in effect until 6 AM Sunday.

Across much of the southeast WY plains and western NE panhandle,
the 12Z GFS and NAM forecast soundings continue to indicate the
potential for light freezing drizzle, with average ice accumulations
around .05 inch. Deep layer saturation (1500-2500 ft) between 0C
and -10C in moist post-frontal low level upslope regime from early
evening through late tonight, when colder low levels support a
mix or changeover to light snow. Saturated low levels and light
winds will also support fog formation late tonight through early
Sunday morning. Potential travel impacts will be the fog and mixed
wintry precipitation, especially along and east of I-25 from
Wheatland to Cheyenne, and I-80 from Laramie to Sidney.

Precipitation will end after mid-morning Sunday as main energy
departs to the north and east of the CWA. It will be seasonably
cool Sunday with highs in the 30s and lower 40s outside the
mountains. West-southwest winds will become breezy along and west
of the Laramie Range Sunday afternoon. West-northwest flow aloft
and downslope west winds Monday will promote milder temperatures
with highs in the 40s to lower 50s east of the Laramie Range. A
pretty tight surface pressure gradient will develop along and west
of I-25 by Monday morning, with wind prone areas like Arlington
and Bordeaux approaching high wind criteria. CAG-CPR 700/850mb
height gradients rise to 50-60m after 09Z Monday and remain elevated
through Monday afternoon, before decreasing Monday evening. A
potential mitigating factor is that 700mb winds never exceed 45 kt.

.LONG TERM...(Tuesday through Saturday)
Issued at 225 PM MST Sat Dec 16 2017

Mild and mainly dry weather will be over the CWA Tuesday through
most of Weds as the upper flow gradually turns southwesterly ahead
of an upper trough that will move over the western CONUS on Wednesday.
This upper trough will bring big changes to the weather over the
area beginning late Weds as an Arctic cold front drops south across
the CWA Weds evening bringing much colder air. Accumulating snow
will also spread across the CWA behind this front Weds night and
continue into Thursday as energy in the system closes off an upper
low around the 4 corners area. Models in good agreement with this
scenario then diverge as the GFS cuts off the upper low and drops
it south across Az through Friday while the EC reopens it and swings
the energy across the southern Rockys into the southern plains.
Either way it appears the snow will taper off across the CWA late
Thursday as the cold and drier surface high builds into Wyoming.
Another shortwave then rotates across the area late Friday into
Saturday bringing some snow showers and another surge of arctic
air. Looks like an extended period of cold conditions as a large
upper trough sets up over much of the country keeping this area
under deep northerly flow through at least next weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1007 PM MST Sat Dec 16 2017

Forecast concerns for the 06Z aviation forecasts deal primarily
with the airports east of the Laramie Range and out in the
Panhandle. Easterly upslope winds creating lower conditions
already this evening at KCYS and KSNY and should continue through
12Z. KCDR and KAIA have had problems going down. Do think they
will eventually go down to IFR and this thinking supported by
latest HRRR forecast. KBFF though does not have a favorable wind
direction, northeast wind direction usually means MVFR at KBFF.
Thinking they may go more southeast which would be conducive to
lower ceilings though after 10Z or so.

&&

.FIRE WEATHER...
Issued at 225 PM MST Sat Dec 16 2017

No fire weather concerns for the foreseeable future with minimum
relative humidities above 25 percent. There will be some measurable
precipitation in the form of snow and freezing drizzle tonight and
early Sunday morning. A period of breezy to windy conditions with
milder temperatures Monday through Wednesday will be replaced by
much colder and snowier weather late next week with the passage of
a strong Arctic cold front.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 6 AM MST Sunday for WYZ101-103-104.

     Dense Fog Advisory until 6 AM MST Sunday for WYZ116>118.

NE...None.
&&

$$

UPDATE...GCC
SHORT TERM...MAJ
LONG TERM...RE
AVIATION...GCC
FIRE WEATHER...MAJ



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