Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 152114
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
214 PM MST Fri Dec 15 2017

.SHORT TERM...(This evening through Sunday)
Issued at 220 PM MST Fri Dec 15 2017

The primary forecast concern into this weekend will be a Pacific
cold front and associated upper level trough which is just now
moving into Idaho and Montana this afternoon. This system will
impact southeast Wyoming and the northern NE panhandle on Saturday
through Sunday morning. For now, localized strong winds across the
wind prone areas have started to subside. Can not rule out another
brief period of winds gusting to 60 mph this evening as llvl
pressure gradients increase shortly after sunset, before the
surface cold front begins to moving southward into central
Wyoming, thus eliminating the wind threat. Ahead of the front,
overnight lows tonight should be a little warmer compared to last
night with readings in the 20`s to mid 30`s, warmest along and
just east of the Laramie Range. Clouds will increase around or
shortly after midnight, preventing late night radiational cooling.

For Saturday, surface cold front will move southeast and then
stall somewhere across central Wyoming and the northern Nebraska
panhandle. Where this front stalls, before accelerating southward
as the upper level system splits, early Sunday will be key in
determining snow amounts and travel impacts Saturday and Saturday
night. The best dynamic lift, including difluence aloft associated
with the jet stream which will be co-located with the surface
front, should be across central Wyoming and extend northeast into
Converse and Niobrara counties. Most models agree that light to
moderate snow will fall in areas north of Rawlins, extending
northeast towards Douglas and potentially north of Lusk. These
locations have the best chance to see snow accumulations over two
inches and lower visibilities for much of the afternoon through
Saturday night, so issued a Winter Weather Advisory for snow from
northern Carbon county, northeast into the northern Laramie Range
and Converse county. There is a chance that central Carbon county,
Platte county, and Niobrara may be included in this advisory, but
confidence is low with regards to travel impacts at this time as
the system is expected to split southward, limiting dynamic
forcing over this area. Further south, the main concern will be
freezing fog along the I-80 summit and freezing drizzle with some
light snow across the eastern plains along I-80 and the lower
North Platte River valley. Model soundings show a classic drizzle
sounding with saturation occurring from the surface up to 700mb
Saturday night through Sunday morning behind the cold front.
Although precipitation amounts will be light, will have to watch
for some minor icing on area roadways in addition some light snow
accumulation. Most models are showing light QPF amounts as well
where drizzle might be a little more steady in intensity. Can not
rule out a Winter Weather Advisory for a combination of snow and
freezing drizzle south of the North Platte river valley towards
the Colorado border, especially along the ridges.

Precipitation will gradually come to an end from north to south
on Sunday. Temperatures will be cold through the day with mostly
cloudy skies. Highs in the 30`s will be common with a few areas
further north remaining in the 20`s through the afternoon.

.LONG TERM...(Sunday Night - Thursday)
Issued at 312 AM MST Fri Dec 15 2017

A slightly warmer and drier weather pattern is expected to develop
into early next week. The GFS/ECMWF are in good agreement w/strong
WNW flow aloft on the southern fringe of a strong mid level short-
wave/clipper tracking across the Northern Plains Mon-Wed. Pressure
falls to the north should support strong low-lvl gradients and the
potential for high winds in the typical wind corridors. Low chance
for any precipitation early in the period with the bulk of any mid
level energy well to the north of the CWA. Snow chances increasing
Wed-Thu with a cold fropa associated w/a rather strong disturbance
digging into the Great Basin mid-week. Current guidance suggests a
fast-moving/progressive system with limited snow amounts. However,
both the GFS/ECM hint at a low closing off over the 4 Corners late
in the week. If this does occur, any notable differences in timing
or location could significantly impact sensible weather across the
CWA. We will need to closely monitor this system over the next few
days as it has at least a small potential to impact holiday travel
across the region.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1004 AM MST Fri Dec 15 2017

Winds will be the main concern for ops at area airports for this
TAF cycle. West/southwest wind gusts of 25-35 knots can be
expected at all southeast Wyoming & western Nebraska Panhandle
airports through 00z. Winds will relax and become much less gusty
tonight. Otherwise, high-clouds will slowly increase and gradually
lower over the next 24 hours. Even so, expect clouds to be mid-
level, remaining well in the VFR category.

&&

.FIRE WEATHER...
Issued at 126 PM MST Fri Dec 15 2017

No Fire weather concerns this weekend and next week as colder
temperatures are expected with a few chances for accumulating
snowfall.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 9 PM MST this evening for WYZ106-110-116-
     117.

     Winter Weather Advisory from 6 AM Saturday to 6 AM MST Sunday
     for WYZ101-103-104.

NE...None.
&&

$$

SHORT TERM...TJT
LONG TERM...CLH
AVIATION...CAH
FIRE WEATHER...TJT



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