Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 090924
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
224 AM MST Fri Dec 9 2016

.SHORT TERM...Today through Sunday night

Synopsis...A progressive weather pattern remains in place across
the CONUS. Arctic air that has been in place in the CWA has begun
to modify with a Pacific airmass this morning. Temperatures
across western portions will continue to warm through the early
morning hours. Areas east of the Divide will be slower to respond
and warm by mid to late morning. The PFJ will shift northward
today, in response to a shortwave ridge. This ridge will flatten
by 06z Saturday, as a new jet max enters far western WY. This jet
max will slowly sag southward over the Great Basin through 12z
Sunday. Ample amounts of Pacific moisture will stream over the CWA
throughout the forecast period.

Frigid temperatures remain in place for much of the forecast area
this morning, with temperatures slowly beginning to warm across
the far west. Eastern portions of Sweetwater County are the
warmest, ranging from the upper teens to lower 20s. The arctic
airmass will continue to erode away across the area this morning,
as a warm front makes its way over the state today. At the same
time, persistent bands of snow will move over western portions
today through Saturday. Periods of moderate to heavy snow can be
expected tonight and Saturday. Winter Storm Warnings and Winter
Weather Advisories are in effect across the west through noon
Saturday. Expect 1 to 2 feet in the mountains, with 10 to 16
inches in the western valleys. Please see these products for
additional information.

East of the Divide will stay dry today, under mostly cloudy to
overcast skies. Gusty west to southwest winds will occur today
through Saturday, across the Absaroka and Wind River Mountains and
over the Wind Corridor from the Red Desert to Casper. These winds
will become strong and possibly damaging tonight through 12pm
Saturday. Windy conditions will be more widespread Saturday,
spreading over southern and western portions. Winds across the
area will begin to subside late Saturday afternoon into Saturday
evening, as the cold front sweeps through the state.

Light to moderate snow will continue across the west Saturday
night into Sunday behind the cold front. Amounts will be
significantly less, due to the bulk of energy and lift further to
the south with the jet.

.LONG TERM...Monday through Friday

Active week upcoming with high latitude ridge holding over the
Eastern Pacific and a cold northerly flow dominating Western Canada.
Farther south over us a mid level wly flow will continue with
embedded shortwaves keeping snow potential across the west. It looks
like we`ll start with the Arctic front along our nrn border or into
the far north Monday morning. This Arctic front will drop south
across all areas east of the divide Monday night into Tuesday with
bitter cold temps and some light snow or flurries. Temps will trend
downward Monday into Tuesday in the north and Tuesday into Wednesday
for the south half of Central Wyoming as this arctic air settles
across the area. What happens after that is looking very interesting
as ridge rebuilds offshore creating another period of steep nly flow
across Western Canada and possibly the Northern Rockies. The ridge
may fold over enough to create moist wly flow underneath this ridge
and possible even a developing upper low that moves in off the Ern
Pacific and into the Great Basin later Thursday night through Friday
(at least). Under this scenario...areas east of the divide would be
entrenched in arctic air and upslope while dynamics from this system
to the west overrun the cold dome with dynamics (strong qg
forcing/cyclonic nose of the jet and upglide) to produce a
significant snowstorm for parts of Central Wyoming later Thursday
night through Friday (and possibly beyond). The Euro takes a little
longer to develop this scenario starting next Friday but not peaking
until next weekend vs the GFS which starts it as early as late
Thursday night and peaking it Friday before improvement (at least in
snowfall not the cold on Saturday). Details are uncertain in this
complex flow pattern but the bigger picture at least supports arctic
air and upslope east of the divide with some energy either moving in
from the west or nw. Either way, a cold and unsettled period with
occasional snow out west and a possible snowstorm east of the divide
to end the medium range period.

&&

.AVIATION.../12Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions are expected through the period. Wind will increase
KRKS to KCPR after 14Z and then a gusty wind will continue through
06Z Sat. The wind will increase even more after 06Z at KCPR airport.
Isolated snow showers will occur this afternoon and tonight.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Widespread snow will occur through tonight. MVFR to IFR
conditions will prevail for most of the period. Local LIFR
conditions are possible at times. KRKS will see light snow at
times through 13Z. Expect mountains to be obscured. A gusty
wind will occur by 16Z at KRKS airport and this gusty wind will
continue through tonight.

&&

.FIRE WEATHER...

Fire Danger low for the foreseeable future as cold unsettled
conditions and general troughiness aloft continue over the region.
Several upper level disturbances will bring significant snowfall
west of the Divide through this weekend. Strong wind may be possible
across portions of northern and central Wyoming late tonight
through Saturday morning.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late tonight through Saturday morning for
WYZ003-019-020.

Winter Weather Advisory until noon MST Saturday for WYZ016-026.

Winter Storm Warning until noon MST Saturday for WYZ001-002-
012>015-023>025-027.

&&

$$

SHORT TERM...LaVoie
LONG TERM...Skrbac
AVIATION...AR
FIRE WEATHER...LaVoie



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