Area Forecast Discussion
Issued by NWS Cheyenne, WY

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379
FXUS65 KCYS 240002
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
500 PM MST Fri Feb 23 2018

.SHORT TERM...(This evening through Saturday)
Issued at 247 PM MST Fri Feb 23 2018

Light snow is spreading eastward across south central Wyoming
at this hour with lowering ceilings and virga from Denver to
Cheyenne. Models continue to shift this area northeast this
evening on the cyclonic shear/mid level deformation side of the
jet. There are banding structures in this area of snow, but so far
not producing more than light snowfall. Models have been jumping
around with possible heavier snow bands, and seem to be settling
on a band on the south edge of this area, most likely in Colorado
but possibly moving toward the southeast corner of Wyoming before
the forcing diminishes later this evening.

Meanwhile the main area of lift will develop across northeast
Colorado this evening and expand as it moves into western and
central Nebraska by Saturday morning. There has been pretty good
consistency on the heart of this band being from Colorado toward
southwest Nebraska, with the western panhandle on the edge of
this. A slight northward shift would bring a threat of more
substantial snowfall to I-80 in the panhandle, but most likely the
area from Sidney westward will only see 1-3 inches of snow. For
Cheyenne and the rest of the lower elevations of southeast
Wyoming, we will again be on the edge of all of this and while
there is a good chance of a little snowfall, amounts will likely
be less than 2 inches. The mountains will see steadier light
showers, but there won`t be much wind for orographic enhancement.

Winds should kick up behind the trough early Saturday. Models
slowly ramp up the surface winds during the morning, but the
models show 40 knots at 9000 ft agl by 15z, and as much as 53
knots over Cheyenne at that height by midday. The wind may come
more suddenly than guidance indicates. Saturday`s wind will be
driven by the low level gradient and subsidence behind tonight`s
trough and looks marginal for the wind prone spots and the east
slopes out to about Cheyenne.

.LONG TERM...(Saturday night through Friday)
Issued at 247 PM MST Fri Feb 23 2018

There will be a lull in the winds Saturday night as the gradients
decrease, then they will come up again Sunday as the gradients
tighten in response to the jet axis shifting past us. Sunday will
be driven more by mixing with a window for mountain wave
enhancement midday through the afternoon. While strong winds may
be more widespread, the real high wind potential may be limited
more to the wave prone spots like Arlington, Bordeaux, and
elsewhere along the immediate east slopes of the mountain ranges.
In these places the winds could be even stronger than Saturday, up
to about 70 mph. The wind should diminish overnight with less
mixing, some decrease in the gradients, and flow going more
southwesterly.

The high mountains should see some orographic snow again ahead of
the wave Saturday night, then diminishing to a few light showers
after that.

For the coming week, we will mainly be in a mild southwest flow of
moderate strength. There continues to be good agreement on a
shortwave about Wednesday, with a tendency to take most of the
energy into the southern Rockies or shear it apart. It is probably
still enough for some increase in the wind and a little orographic
snow, but not much chance for any precipitation of consequence.
There may be some increase in wind and approaching moisture on
Friday from upstream troughing. For now a chance of mountain snow
appears sufficient. Most of the GFS ensembles are further west
than the operational run with the large scale trough, though a
fair number do show some kind of shortwave approaching in the
Fri/Sat time frame that would bring more wind, some cooling and a
chance of light snow.

&&

.AVIATION...(For the 00Z TAFS through Saturday afternoon)

Two primary concerns for aviation operations over the next 24 hours.
The first will be a light snow event that will impact the region
from west-east this evening and through most of tonight. IFR
conditions are expected between now and 09z from KCYS eastward into
all western Nebraska terminals.  KLAR will likely miss out on the
bulk of the snow potential, but may still have a period of MVFR
between 00-06Z. By 12z to 15z Saturday morning, most of the snow
will likely come to an end, with very windy conditions expected late
Saturday morning through Saturday afternoon across all southeast
Wyoming and western Nebraska terminals where gusts over 45 knots can
not be ruled out.

&&

.FIRE WEATHER...
Issued at 247 PM MST Fri Feb 23 2018

Strong winds are expected Saturday and Sunday, but minimum
humidities on the plains will be above 30%. Less wind is expected
after that.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...High Wind Watch from Saturday morning through Sunday evening for
     WYZ106-110-116-117.

NE...None.
&&

$$

SHORT TERM...WFOCYS
LONG TERM...WFOCYS
AVIATION...CAH
FIRE WEATHER...WFOCYS



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