Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 110945

Area Forecast Discussion
National Weather Service Riverton WY
245 AM MST Sun Dec 11 2016

.SHORT TERM...Today through Tuesday night

Snow continues to end across much over the area this morning,
especially across southern Lincoln and Sweetwater counties.
Residual snow bands have formed over eastern ID and now moving
over NW portions. The cold front quickly swept through northern
and central portions, reaching CPR and RIW earlier Saturday
evening. The boundary will become stationary across southern
portions this morning.

NW flow aloft will reestablish itself across the region, as this
latest winter system ejects over the Plains this morning. The nose
of the PFJ will reenter NW portions of the forecast area today,
allowing for the stream of Pacific moisture to return as well.
This area will be on the cyclonic side of the jet, as the main jet
max stays to the SW over southern OR/ID and northern UT tonight
through Monday night. This, in conjunction with favorable upslope
flow, will provide enough lift and instability to bring another
round of snow to western portions. Snowfall amounts and rates will
be considerably lower across the area, however, advisory amounts
are possible in the Tetons and in Jackson Valley. Gusty west to
southwest wind will exacerbate the hazard through this time. This
activity will dwindle to light showers Monday night, as another
arctic boundary makes its way into northern WY Monday night.

Dry and cold conditions will occur for much of the forecast area
today and Monday. Temperatures will be at or below freezing across
much of the area. Gusty west to southwest winds will return
through the day today, from southern Lincoln eastward over the
Red Desert and into Natrona County. As mentioned earlier, these
winds will also effect western portions. This will be mainly in
response to the higher level winds over the higher terrain.

As precipitation and winds subside across the area Monday night
into Tuesday, another arctic boundary will begin to enter the
forecast area. This boundary will quickly enter northern portions
of the forecast area Monday afternoon and settle over areas east
of the Divide Monday night. Single digit to subzero temperatures
will return to northern and central portions and in the teens in
southern portions Monday and Tuesday night. As with all these
boundaries, this will be the major forecast challenge through the
next few days.

.LONG TERM...Wednesday through Sunday

An Arctic airmass will be in place Wednesday with the coldest air
east of the Divide. Wednesday looks to be mainly dry with chances
of snowfall confined to the southwest. By Wednesday night,
isentropic lift and some right entrance region jet support should
result in widespread snowfall breaking out across much of the
west. This pattern remains in place Thursday ahead of a trough
pushing into the West Coast with snow continuing over the west. In
addition models are showing a warm front trying to push northward
across much of the area. At 700mb temps warm considerable to -1C
to -5C by 18Z Thursday from the -10C to -15C in place 18Z
Wednesday. Areas west of the Divide will see the warmer air and
the typical wind corridor into Casper should see the Arctic
airmass retreat. However not confident Arctic air mass will be
scoured out across the central basins especially the northern/central
Wind River Basin, and the eastern Big Horn Basin. Also the strong
warm air advection over the Arctic airmass could result in cloud
cover and some light precipitation. Have gone below MEX MOS
guidance on Thursday in the typical trapped basins, and have kept
a mainly dry forecast east of the Divide.

A broad trough pushes into the Interior West Thursday night/Friday as
another Arctic blast slides southward across the Cowboy State
Friday. Looks like the snowfall across the west should shift east
of the Divide late Thursday night into the Day Friday. In the end
it looks like significant snowfall is a good probability over the
west Wednesday night through Friday with some chance east of the
Divide late Thursday night into Friday night. The western valleys
could get warm enough Thursday to change snow to rain before
changing back to snow Thursday night with the front. For now will
keep the precipiation mainly snow.

The trough axis is progged to push east of the area by Saturday
with a mainly northwest flow expected Saturday and Sunday with
precip chances mainly confined to the northwest/northern mountains
those days.




Local MVFR conditions due to lower ceilings will occur until 12Z.
Otherwise, VFR conditions will prevail through tonight. A gusty
southwest wind will occur at KCPR from 19Z through 12Z Monday.


Snow in southern WY will end toward 12Z. Expect MVFR/IFR conditions.
Then VFR conditions will occur across southern WY by 14Z. KRKS
airport will have a gusty wind during much of the day into the
evening. Otherwise, scattered light snow will continue in the
western WY mountains through tonight. The KJAC terminal site will
see snow at times through tonight. MVFR conditions will prevail
after 20Z at the airport.



Fire Danger will continue to remain low for the foreseeable
future as cold and unsettled conditions prevail over the region.
Persistent snowfall will return across the west tonight into
Monday. Another arctic boundary will invade areas east of the
Divide Monday night into Tuesday. Gusty winds will continue in the
higher elevations and wind prone locations today through Monday,
before subsiding Tuesday.


Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Monday for WYZ012-013.



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