Area Forecast Discussion
Issued by NWS Riverton, WY

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284
FXUS65 KRIW 201058
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
358 AM MST Mon Feb 20 2017

.SHORT TERM...Today through Wednesday night

A shortwave ridge will build over the area today, in the wake of
Sunday`s trough. This trough did briefly produce a mountain wave
of 70 mph earlier Sunday evening, as it passed over northern
portions of the Cody Foothills. This ridge will be in place
through Tuesday, before it begins to flatten and the longwave
trough currently over the eastern Pacific moves over the Pacific
Northwest. A 100-120kt PFJ remains progged to enter and remain
over the area during this time. Much of the forecast area will be
on the anticyclonic side of this jet, which will allow these winds
to translate to the surface. A large area of downward motion is
also progged, mainly over the Absarokas and Wind River Mountains.
This jet will also bring an abundant supply of moisture, which
will allow for snow to return to the western mountains this
afternoon. This activity will continue through the day Tuesday,
before subsiding late Tuesday night. A cold front will move
through the area during the day Tuesday, shifting the wind and
snow further south by Wednesday. Snow will remain possible across
the west, but will be on the light side. Snow will also spread
east of the Divide Wednesday night, as a leeside low begins to
develop along the cold front. Instability showers will also occur,
as the upper level low passes over northern and central portions.

Winter Storm Warnings have been issued across the west through
Tuesday night. H7 temps are not progged to be as warm as last
night`s 00Z runs, so rain should not fall in the mountains.
However, the valleys are expected to remain above freezing,
especially during the day. Although periods of snow will be
possible, rain is expected to mix during the day today and
Tuesday, which will keep accumulations low. This will change back
over to all snow Tuesday night, behind the cold front. Snowfall
accumulations of 1 to 2 feet in the higher elevations remain
unchanged. Winter Weather Advisories may be issued by Tuesday for
the western valleys, including the Upper Green River Basin.

High Wind Warnings remain in effect for the Cody Foothills and are
now in effect for the Wind Corridor. High Wind Watches remain in
effect for the Lander Foothills and now include Hot Springs County
for late this afternoon into Tuesday night. Strong winds have
subsided over the Cody Foothills in the wake of the trough, but
may return later this afternoon. There is a bit of uncertainty
with strength and timing of the winds over the Cody Foothills and
Hot Springs County. The WRF depicts 40-50 kt in these areas, from
22-23Z Monday to 15Z Tuesday. Considering the location of the PFJ
and area of upper level convergence, these areas may need
warnings. Confidence is a bit higher for the Lander Foothills, as
southwest winds of 50+ kt BL winds occur over the Wind River
Mountains through the period. Winds will subside across the area,
as the cold front sweeps through Tuesday night.

.LONG TERM...Thursday through Monday

Models continue to struggle on the details of the storm system
Thursday. The 00Z GFS has much less defined mid-level circulation
which results in much less snowfall for the system than ECMWF or
Canadian models. Regardless, the GFS still shows some low-level
upslope developing which would be still conducive some snowfall. Thus
will keep the likely PoPs ongoing east of the Divide with an
emphasis on snowfall amounts in typical upslope areas such as Lander
and Casper. The main system exits the area Thursday night, but a
weak system on its heels will keep chances of light snow going
across much of the area Friday.

A couple of weak systems could impact the area Saturday and Sunday,
but the forecast area seems to be bypassed with the main energy with
chances of snow mainly over the mountains and far west. Models then
show a more robust system possibly impacting the area beginning
Monday especially over the west.

Temperatures will be below average through the extended.

&&

.AVIATION.../12Z ISSUANCE/

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

A few snow showers in the western mountains overnight. Snow will be
developing over the western mountains this morning and the snow will
continue through tonight. MVFR/IFR conditions will occur in and near
the mountains. KJAC airport will see snow starting around 15Z and
expected through tonight with MVFR to IFR conditions. KPNA and
KBPI airports will see VCSH by 19Z with snow developing at these
sites between 00Z and 05Z Tuesday. KRKS airport will have VFR
conditions through the period.

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

VFR conditions will prevail at the terminal sites through the
period. A gusty wind will occur at KCPR airport after 15Z and this
gusty wind will continue through tonight.

&&

.FIRE WEATHER...

Busy weather across the area today and Tuesday, with Winter Storm
and High Wind Warnings in effect. Gusty winds will impact much of
the area today and Tuesday, with strong winds expected later this
afternoon across the Wind Corridor, the eastern slopes of the
wind River Mountains and over the NW mountains. These conditions
will continue through the day Tuesday, before subsiding late
Tuesday night. Snow will also return across the west today, with
periods of heavy snow through late Tuesday night as well. Please
refer to the aforementioned Warnings for further details.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from noon today to 5 AM MST Wednesday for
WYZ001-002-012-014-015-024.

High Wind Warning from 5 PM this afternoon to 11 PM MST Tuesday
for WYZ019-020.

High Wind Watch from this afternoon through Tuesday evening for
WYZ005-018.

High Wind Warning until 5 PM MST this afternoon for WYZ003.

&&

$$

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



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