Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 160947
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
247 AM MST Fri Feb 16 2018

.SHORT TERM...Today through Sunday night

Synopsis...A brief reprieve today, as the pattern resets. NW flow
will continue over the area today, increasing clouds across the
state. Moisture will return across NW portions this afternoon, as
the nose of the jet moves over western portions later tonight. the
jet will remain over the area, as the next longwave trough digs
once again over the PACNW by Sunday, turning the flow aloft to the
SW. This will continue through Sunday night, as the upper low
settles over the Northern Great Basin.

Discussion...Low stratus clouds continue across western portions,
as mid to high level clouds begin to enter the area. Patchy
fog/stratus south of the Owl Creeks is beginning to break up and
move west of Boysen Reservoir and NNW of Riverton. If it holds
together, this fog could affect commuters near Pavillion/Missouri
Valley Road.

A busy weather pattern through the forecast period. Snow will be
possible over Yellowstone and the Tetons this morning, before
increasing in intensity and coverage this afternoon. The best
chances for snow across the west today will be from this
afternoon into the evening hours, becoming light and steady
tonight into Saturday morning. Additional moisture will align with
better jet dynamics into Saturday afternoon, bringing the next
wave of moderate to heavy snow. This will continue through the
overnight hours Saturday night and become light and steady Sunday.
Snowfall amounts of 4 to 8 inches (locally higher amounts of 8 to
12 inches in the Tetons/SW Yellowstone) will occur through
tonight, with 1 to 3 inches in the Jackson Valley. A Winter Storm
Watch will be issued this morning for snow for these areas on
Saturday and Sunday.

Wind will also quickly return this morning across the
Absarokas/Cody Foothills, east slopes of the Wind River Mountains
and the Wind Corridor. These winds are expected to stay below
criteria this afternoon/evening, but will remain high through the
overnight hours tonight into Saturday. Gusty winds will also
develop across western and southern portions this afternoon and
continue through much of the overnight period. As high winds
return Saturday afternoon for the Wind Corridor, strong winds will
develop over this area (including Casper Mountain), as well as
the Upper Wind River Basin, east slopes of the Wind River
Mountains and Lander Foothills. These winds will continue through
the overnight hours, before gradually subsiding before sunrise
Sunday. Winds will then decrease from north to south across the
area Sunday, as a cold front progresses through the state. Gusty
winds will continue across the south, as snow begins to fall early
Sunday afternoon. These winds will also subside, as the front
continues southward.

Snow will fall east of the Divide Sunday, as the front makes its
way through the area. This will mainly be due to overrunning flow
aloft and low level upslope flow. Snowfall amounts will be light,
with general amounts of 1 to 2 inches and 2 to 3 inches over
Johnson and Natrona counties.

.LONG TERM...Monday through Friday

Overview...An Arctic airmass will settle over the area  Monday
and Tuesday with highs in the single digits and teens across the
north and central basins, and sub-zero lows expected across most
of the area Tuesday and Wednesday morning with readings near 20
below possible along the lower river valleys in the Big Horn and
Wind River Basins. Areas of light snow will persist across the
area Monday into Monday night with drier conditions prevailing on
Tuesday. Temperatures will moderate but remain below normal
Wednesday through Friday; dry weather is expected on Wednesday
with chances of snow returning to the northwest Thursday and
Friday.

Discussion...Upper air pattern on Monday to feature longwave
trough extending from Hudson Bay to the southwest U.S. flanked
by strong ridges over the eastern Pacific and SE U.S./Bermuda.
At the surface, Arctic high pressure will be over Montana with
surface front extending W-E across the Central Rockies into the
Central Plains. Heavier bands of snow along the front (across
southern/east central Wyoming on Sunday) should propagate south
into northern Colorado/SE Wyoming on Monday, but light snow will
linger in baroclinic zone across mainly the SE 2/3rds of Wyoming
through the day Monday. Upper trough axis will translate across
the area Monday night and Tuesday with H5 -38c cold pool over
area, and Arctic high pressure sliding from Montana into NE
Wyoming; resulting in some clearing and efficient radiational
cooling. Models/MOS show a colder trend over the last few runs,
and we have continued to adjust the SuperBlend defaults downward,
the coldest readings of -20F to -25F around Greybull and Worland
Tuesday morning.

Strong Hudson Bay low that has been a feature for much of 2018 so
far is forecast to lift NE and weaken Tuesday and onward with
upper trough stretching and weakening to the SW. Both GFS and
ECMWF show this trough splitting on Wednesday. However, ridges
over the eastern Pacific and SE U.S. will remain. A series of
shortwaves will continue to dive down front side of eastern
Pacific ridge Thursday through Saturday, reloading upper trough
across the Rockies. However, Arctic source region over the
Canadian plains will continue to modify in the absence of strong
Hudson Bay low, so cold fronts with these systems will not have
quite the punch as the Sunday-Monday front.

&&

.AVIATION...For the 12Z TAFS through 12Z Saturday

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

VFR conditions will be over the valleys through 18z.  Increasing
MVFR, areas of IFR, ceilings and visibility in areas of snow over
the western mountains in Teton, Lincoln and Sublette counties
will obscure mountains above 9000 feet through 12z. After 18z,
MVFR ceilings and visibilities reduced in snow and mist will be most
probable at KJAC, with brief IFR conditions possible. After 20z
KPNA and KBPI may also experience MVFR ceilings at times,
especially from 02z to 12z Saturday. Increasing winds at mountain
top level will result in areas of LLWS. Surface west winds at
20-25kts with gusts of 35 kts will prevail across the I80
corridor.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Lower elevations will have prevailing VFR conditions. Over and east
of the Continental Divide down to 8500 feet in the Absaroka and Wind
River mountains after 20z will have areas of MVFR, local IFR, ceilings
and visibility reduced in snow obscuring mountains above 9000 feet
through 12z. Increasing winds at mountain top level will also result
in areas of LLWS.  Areas of LLWS will develop vicinity along the east
slopes of the Divide, vicinity KCOD-KDUB-KLND.

&&

.FIRE WEATHER...

A new storm system will approach the west today, with another
chance of snow, mainly this afternoon into tonight. Any snow East
of the Divide would be restricted to northern Wyoming and be
mainly light tonight into Saturday morning. Snow will continue
across the west Saturday and Sunday, before developing east of the
Divide early Sunday morning. Snow will then be widespread across
the state Sunday, gradually diminishing during the day Monday. Winds
will quickly increase this morning in the normally windier
locations and possibly become strong this afternoon. these winds
will continue through the overnight hours tonight and increase
across the west. These winds will continue through the day
Saturday, becoming strong by the afternoon across central portions
of the state. Strong winds will continue through the overnight
hours Saturday night, before subsiding Sunday morning, ahead of a
cold front. Mixing and smoke dispersal on Friday will range from
poor in the basins to good in the west and normally windier
locations.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Saturday afternoon through late Saturday
night for WYZ015-018>020-022.

Winter Storm Watch from Saturday afternoon through Sunday
afternoon for WYZ001-002-012-013.

&&

$$

SHORT TERM...LaVoie
LONG TERM...Meunier
AVIATION...Baker
FIRE WEATHER...LaVoie



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