Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 101746 AAA
AFDCYS

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1046 AM MST Sat Dec 10 2016

.UPDATE...
Issued at 1039 AM MST Sat Dec 10 2016

High winds never materialized early this morning across the north
Snowy Range foothills and cancelled the warning by 730 AM. Gusty
winds of 35 to 45 mph will continue today along I-80 between
Cheyenne and Rawlins. The last 24 hours of SNOTEL data indicated
1 to 3 inches of new snow over the Snowy/Sierra Madres. Snow is
still expected to increase in coverage and intensity this afternoon
over the mountains west of the Laramie range. Lowered max temperatures
for Torrington, Lusk and Chadron based on trends and 12Z MOS guidance.

&&

.SHORT TERM...(Today through Monday)
Issued at 336 AM MST Sat Dec 10 2016

Strong zonal flow pattern will continue across the Lower 48 into
early next week as a 130 knot H3 jet resides overhead. The
associated sensible weather impacts include strong winds within
the WY wind corridor and favorable upslope flow along the west
slope of the Rockies.

Winds have been slow to develop across the Arlington zone this
morning with only a few gusts in the 40-50 mph thus far.
Presumably, this is a result of a deeper colder air mass existing
over the plains east of the quasi-stationary front. No area where
this boundary is more clear than across Natrona/Converse counties,
where at 09z Casper observed a temperature of 34F, while Deer
Creek about 30 miles away sits at 7F. Latest models continue to
point to this front retreating northeast this morning, which
will allow tighter llvl gradients to lift northeast into Carbon
County. Although CAG-CPR llvl gradients have stepped down
slightly compared to a day ago, progd values of around 60 meters
continue to suggest at least a marginal high wind event near
Arlington this morning. The NAM even suggest that these high winds
may linger some into the afternoon. Will leave the timing of High
Wind Warning alone for now, but day shift may need to consider an
extension depending how winds evolve this morning. As for
temperatures, have boosted highs today along the I-25 corridor
where adiabatic downslope warming/Chinook develops.

By this afternoon, attention turns to the next slug of Pacific
moisture evident on satellite pix early this morning, interacts
with the hier terrain. Snow will increase in intensity, especially
Saturday night as embedded shortwave races through WY. Inherited
Winter Storm Warning is on track with likely 1-2 feet of
additional snow falling across the Snowy and Sierra Madres.
Highest amounts will be on west facing slopes. This will be mainly
a mountain event, with lower elevations of Carbon/Albany Counties
likely only receiving 1-3 inches. Areas east of the Laramie Range
will likely see less than inch, with many places seeing little if
any accumulations. Cold front will push thru Sunday morning,
cooling highs back into the 20s west/30s east.

Aside from light snow chances lingering in the mountains Sunday
night& Monday, expect dry conditions elsewhere. Temperatures
will remain cool with highs on Monday only reaching into the 20s
west/30s east.

.LONG TERM...(Monday night through Friday)
Issued at 336 AM MST Sat Dec 10 2016

Medium range models in better agreement for early to mid next
week as widespread snowfall looks increasing more likely beginning
late Tuesday. Models continue to indicate progressive west to
northwest flow aloft over the area with an arctic cold front at
the surface moving southward over the high plains. This front is
expected to push southward into Colorado and then stall...while
the front also stalls over the mountains of Carbon county. In
addition...a 170 knot jet max will develop along the
area...colocated with an area of strong frontogenesis as the
arctic front remains stationary along the Front Range Tuesday
night. Models show the jet axis will be perpendicular to the llvl
arctic front...which is favorable for strong warm air advection
aloft and overrunning precipitation. The GFS and ECMWF are in
better agreement for Tuesday and Tuesday night showing widespread
snowfall with strong overrunning at 700mb. This event may linger
into late Wednesday/early Thursday after a brief break Wednesday
afternoon as the arctic front shifts position slightly. Will have
to monitor this system over the next few days since heavy snowfall
is possible given the pattern and strong low to midlevel forcing.
This tends to be our second most favorable pattern for heavy
snowfall across southeast Wyoming...behind the typical `Colorado
Low`. Current model soundings are showing favorable profiles for
maximum dendritic growth in the typical growth zone aloft...with
relatively warm and moist air over very cold arctic air near the
surface. Increased POP on Tuesday and Wednesday with values over
60% across the eastern plains. The mountains will do very well
initially with this system...with feet of snow possible over the
Snowy and Sierra Madre ranges by Thursday.

For late in the week...the GFS continues to hint at a major winter
storm for the area as a deep but slow-moving trough ejects
eastward from the eastern Pacific Ocean. The ECMWF shows this
feature as well...but does not show it digging far enough south to
pick up the additional subtropical moisture. However...all models
show what could be the coldest air of the season pushing into the
forecast area on Friday and remaining in the area for next weekend
as a strong arctic front reinforces the bitter cold air across the
plains and even the intermountain west. Even model consensus which
tends to have a warm bias for temperatures in the extended...shows
highs on Saturday struggling to reach zero degrees during the
daytime. Kept POP between 30 to 50 percent during this period with
the mountains over 70 percent. Although the specifics are tough to
pin down this far out in the forecast period...it looks like an
active weather pattern for next week and into next weekend with
very cold temperatures. Thankfully...winds appear to be relatively
light through next week and into next weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1027 AM MST Sat Dec 10 2016

Wyoming TAFS...VFR at Cheyenne. VFR prevails at Rawlins and Laramie
with occasional MVFR. Winds gusting 25 to 35 knots with localized
gusts to 42 knots through 21Z.

Nebraska TAFS...VFR.

&&

.FIRE WEATHER...
Issued at 336 AM MST Sat Dec 10 2016

The active weather pattern will continue into next week as a
strong jet stream resides overhead. A windy few days expected,
especially for areas along and west of the Laramie Range. Medicine
Bow Mountains will see significant snowfall later today and
especially tonight from a fast moving storm system. Snow will
lessen Sunday and Monday, with high temperatures falling into the
20s west/30s east. The active pattern will continue next week with
a series of weather systems bringing accumulating snow and arctic
temperatures back into the region.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Warning until 6 PM MST Sunday for WYZ112-114.

NE...None.
&&

$$

UPDATE...MAJ
SHORT TERM...CAH
LONG TERM...TJT
AVIATION...RUBIN
FIRE WEATHER...CAH


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