Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 050624 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1124 PM MST Sun Dec 4 2016

.UPDATE...Added 06z aviation discussion.

&&

.AVIATION.../06Z ISSUANCE/

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

A cold front will continue to push south and east across the area
overnight with areas of LLWS ahead of and along the cold front.
Areas of MVFR/IFR -sn will follow the cold front mainly along and
east of a vicinity KCOD-KWRL-KCPR line through 18z Monday with snow
diminishing Monday afternoon. Areas of MVFR ceilings may linger in
this same area through Monday evening. Gusty northwest winds will
develop behind the cold front Monday, especially vcnty KDUB-KRIW.
Clear skies or possibly some thin, mainly translucent lower clouds
LCL MVFR ceilings, will prevail 06z-12z Tuesday along with
diminishing wind.

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

Areas of MVFR/IFR -sn, widespread mountain obscurations and LLWS
ahead of a cold front were along and south a a vicinity KJAC-KCOD
line at 06z. Drier air and improving conditions were occurring
behind the cold front north of this line.

This line of MVFR/IFR -sn and mountain obscurations will continue to
push south and east with the back edge (drier air) pushing south of
Interstate 80, KRKS, 13z-15z Monday.  Clearing skies and gusty west
to northwest winds will follow the cold front Monday morning.
Surface high pressure will build into western Wyoming Monday evening
with winds diminishing and mostly clear skies.

&&


.PREV DISCUSSION... /issued 200 PM MST Sun Dec 4 2016/

Short Term...Tonight Through Monday

Snow falling across the west and into the northern mountains
with strong pacific jet and significant mid level moisture
plume extending west to the west coast.  Cold front shows
up well on surface pressure analysis in northern Idaho panhandle
southwest to northwest California with pressure rises behind
front. Should continue to see significant snowfall with moderate
to heavy snow in the west mountains with strong ridgetop winds.
the cold front is expected to progress east at a good clip
entering the northwest and north 06z to 09z with snow along and
post frontal across the north through Monday morning spreading
south. Have issued winter weather advisories for the Bighorn
basin...the Bighorn mountains and johnson county from midnight
to noon.  also front will show up across the southwest late
tonight with snow spreading east west across sweetwater county
with potential for heaviest snow to line up with interstate 80.
snow and blowing snow should have adverse impact on travel across
I 90...I 25 and I80 through the morning. Wind River basin and
Natrona county look to miss most of the snowfall...but jet
dynamics may spoil that. Cold temperatures will move in thorough
the day with some early morning high temperatures...then steady to
falling temperatures.

Long Term...Monday Night Through Sunday

Snow will taper off across the area lingering in the mountains
with rather low density light snow to flurries lingering through
monday night. most lows will fall below zero with some -15 to -20
lows in the west.  Low stratus and fog may hold up the cooling
in some valleys and basins. Lingering mountain snow may continue
on tuesday with clearing most areas and cold with single digit
highs much of the area. The east will warm first across eastern
Johnson and Natrona counties. A closed low to the southeast over
eastern Colorado may muddle up the clearing depending on how far
north and west the wrap around cloud shield and light snow
extends Tuesday night and Wednesday. May clip Natrona...Fremont
and eastern Sweetwater counties as well as southeast Johnson
county. Cold temperatures will be the main forecast challenge
through the week. Some more light snow should enter the west
Thursday afternoon increasing through the night and Friday with
some warming.  Windy areas may develop across the south and east
and over and near the east slopes of the Absaroka and Wind River
mountains making the temperature forecast even more of a
challenge in the windy areas could be 20 to 25 degrees warmer
than the trapped cold basins and valleys. Snow in the north and
west extends through Friday night and Saturday spilling over east
of the divide possibly on Saturday but should not be significant
there. The western mountains could do very well with snow
accumulation over a three day period. Some light snow may also
fall in the west on Sunday with more wind east of the Continental
divide.

Aviation.../00Z Issuance/

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

Areas of LLWS along the leeside of the mountains will remain in
place ahead of an approaching trough/cold front. These areas will
weaken as the cold front passes through the area. Surface southwest
wind at 20-30 kt with gusts 40-45 kt will prevail from South Pass-
50SM NE KRKS-KCPR through 06z Monday.

Also, peaks above 9kft along the Continental Divide will be mostly
obscured in -sn through 12z Monday, while peaks east of the Divide
will become obscured between 06z-18z Monday with the passage of
the cold front.

This cold front will plunge south through KCOD-KBYG around 07z
Monday and near a KRIW-KCPR line by 12z Monday. Areas of MVFR/IFR
-sn and gusty north wind will follow the cold front. Conditions will
rapidly improve to VFR by 18z Monday from KCOD-KRIW-KLND and over
KCPR between 18z Monday-00z Tuesday.

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

Snow, widespread IFR conditions and mountain obscurations are now in
place, except near KBPI and KRKS. MVFR and near IFR conditions will
continue to spread around KBPI this evening, and reach KRKS by 06z
Monday. Strong ridgetop winds continue as well, with LLWS continuing
to spread southward across the area.

IFR conditions will prevail vicinity and north of a KAFO-KPNA line
through 06z Monday, with a cold front driving areas of snow further
south and east along the Interstate 80 corridor 06z-12z Monday.

LLWS will subside behind the cold frontal passage, with
precipitation becoming more showery along with some improvement in
ceilings and visibility vicinity and north of a KJAC-KDUB line by
12z Monday. Conditions will continue to improve to VFR conditions
between 12z-18z Monday.

Fire Weather...

Snow falling across the west and into the northern mountains
with strong pacific jet and significant mid level moisture
plume extending west to the west coast.  Cold front shows
up well on surface pressure analysis in northern Idaho panhandle
southwest to northwest California with pressure rises behind
front. Should continue to see significant snowfall with moderate
to heavy snow in the west mountains with strong ridgetop winds.
the cold front is expected to progress east at a good clip
entering the northwest and north 06z to 09z with snow along and
post frontal across the north through Monday morning spreading
south. Have issued winter weather advisories for the Bighorn
basin...the Bighorn mountains and johnson county from midnight
to noon.  also front will show up across the southwest late
tonight with snow spreading east west across sweetwater county
with potential for heaviest snow to line up with interstate 80.
snow and blowing snow should have adverse impact on travel across
I 90...I 25 and I80 through the morning. Wind River basin and
Natrona county look to miss most of the snowfall...but jet
dynamics may spoil that. Cold temperatures will move in thorough
the day with some early morning high temperatures...then steady to
falling temperatures. With cold temperatures and strong inversions
smoke dispersal should be poor through mid week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MST Monday for WYZ003>011-
028>030.

Winter Weather Advisory until noon MST Monday for WYZ002-014-016-
023>025.

Winter Storm Warning until noon MST Monday for WYZ001-012-013-
015.

&&

$$

SHORT TERM...C.Baker
LONG TERM...C.Baker
AVIATION...AEM
FIRE WEATHER...C.Baker


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