Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 230604
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1105 PM MST Sun Jan 22 2017

.Short Term...Tonight Through Wednesday

Snow increasing significantly across northern Utah and southern
Idaho one track to spread across southwest and west Wyoming close
to model timing.  Southwest winds increasing across the higher
western mountains.  Have issued winter storm warnings tonight
through Monday for Teton and Lincoln counties. With lower snowfall
amounts expected in Sublette...and western Fremont counties
and across Yellowstone National Park, have gone with winter
weather advisories tonight through Monday for these areas.  Snow
expected to move into the Absaroka mountains later tonight and
have issued a winter weather advisory there which extends through
Monday night as the mountains will also receive snowfall as the
flow turns to the north. The turning to the north will be
associated with a cold front moving south from Montana with
snowfall shifting to areas east of the continental divide moving
into the far north Monday afternoon spreading south across central
and eastern areas Monday night and Tuesday morning. May need
winter weather advisories in the Big horn mountains...in and
around the Owl Creek mountains including Thermopolis. Advisories
may also be needed Monday night into Tuesday morning for Fremont
and Natrona counties with the greatest snowfall expected in the
Lander foothills and around Casper and Casper Mountain. Colder air
will spread across the west starting monday night then across the
east Tuesday and Tuesday night. snowfall will likely linger across
the western and northern mountains being rather light through
Wednesday. cooling will continue through Wednesday with possible
development of fog in the valleys and basins. Lows in the single
digits to -15 in the northwest possible Tuesday night and
Wednesday morning. low clouds and fog may complicate the
temperature forecasts through the period. Models still have their
differences with the NAM preferring more precipitation in the
south with the GFS preferring central Wyoming. They do come into
better agreement later Monday night and Tuesday morning. The GFS
prefers snowfall behind the Montana cold front, while the NAM
focuses on jet dynamics in the southwest.

.Long Term...Wednesday Night Through Sunday

Still have the trough and cold air aloft over the area Wednesday
night and Thursday. Models hint at some very light snow over the
mountain peaks through Thursday afternoon. Otherwise dry Thurs at
the lower elevations. Cold day for all places with the cold air
aloft over the region. Then Thursday night the upper ridge moves
toward the region with warming temperatures aloft. Dry weather is
expected Thurs night and temps will head below zero in all the snow
covered valleys and basins with a clear sky expected. This sharp
ridge axis will keep a northerly flow aloft over the area Friday and
Saturday. A more northwest flow aloft is forecast for next Sunday.
Dry weather for Friday through Sunday. The main forecast issue will
be the developing basin/valley inversions and the likelihood of fog
formation in the favored locations Thursday night through Sunday.
The sounding profiles get more favorable with time. Once the fog or
stratus forms, it may take much of the day to break up, or in some
cases it may not break up at all. As temps warm in the mountains and
mixed locations, the valleys and basins will remain cold with lows
at night below zero. However, the fog or stratus will prevent those
places from getting extremely cold, but it hampers temps rebounding
during the day if it does break out later in the day.

&&

.Aviation.../06Z Issuance/

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

Widespread MVFR/IFR conditions in snow and low ceilings are expected
through the taf forecast period. Looks like the most intense snow is
expected before 15Z Monday, and then pickup again after 21Z Monday
at most terminals. There could be period of time when VFR conditions
occur, but confidence is relatively low. The best shot at low VFR
conditions would be at RKS today. Mountains will be obscured with
IFR/LIFR conditions. Breezy south to southwest wind expected across
central/eastern Sweetwater County through 00Z Tuesday. A cold front
is expected to push south across Sweetwater County with winds
becoming northerly between 03Z and 09Z Tuesday.

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

VFR conditions are expected through 21Z Monday with considerable mid-
level cloudiness with some snow shower activity. However the brief
nature and light intensity of any snow shower activity precludes
prevailing any snow through about 21Z Monday. This will quickly
change between 21Z Monday and 03Z Tuesday as frontogenesis occurs
across northern Wyoming and pushes south across the area, and as a
mid-level circulation pushes across central Wyoming tonight.
Widespread IFR/MVFR conditions are expected to develop in snow and
low ceilings across much of the area between 21Z and 03Z. Windy to
Strong southwest wind is expected across southeast Fremont into
Natrona County through mid-morning Monday with some low-level wind
shear noted at KCPR and KLND. Wind fields are expected to weaken by
early afternoon in those areas with the aforementioned cold front
sliding south across much of the area between 21Z and 03Z switching
winds to the north. Mountains will be obscured at times through 21Z,
but will be obscured all of the time after sunset in IFR/LIFR
conditions.

&&

.Fire Weather...

Snow increasing significantly across northern Utah and southern
Idaho one track to spread across southwest and west Wyoming close
to model timing.  Southwest winds increasing across the higher
western mountains.  Have issued winter storm warnings tonight
through Monday for Teton and Lincoln counties. With lower snowfall
amounts expected in Sublette...and western Fremont counties
and across Yellowstone National Park, have gone with winter
weather advisories tonight through Monday for these areas.  Snow
expected to move into the Absaroka mountains later tonight and
have issued a winter weather advisory there which extends through
Monday night as the mountains will also receive snowfall as the
flow turns to the north. The turning to the north will be
associated with a cold front moving south from Montana with
snowfall shifting to areas east of the continental divide moving
into the far north Monday afternoon spreading south across central
and eastern areas Monday night and Tuesday morning. May need
winter weather advisories in the Big horn mountains...in and
around the Owl Creek mountains including Thermopolis. Advisories
may also be needed Monday night into Tuesday morning for Fremont
and Natrona counties with the greatest snowfall expected in the
Lander foothills and around Casper and Casper Mountain. Colder air
will spread across the west starting monday night then across the
east Tuesday and Tuesday night. snowfall will likely linger across
the western and northern mountains being rather light through
Wednesday. cooling will continue through Wednesday with possible
development of fog in the valleys and basins. Lows in the single
digits to -15 in the northwest possible Tuesday night and
Wednesday morning. low clouds and fog may complicate the
temperature forecasts through the period. Models still have their
differences with the NAM preferring more precipitation in the
south with the GFS preferring central Wyoming. They do come into
better agreement later Monday night and Tuesday morning. The GFS
prefers snowfall behind the Montana cold front, while the NAM
focuses on jet dynamics in the southwest. Mixing heights will be
low with stratus...fog...snowfall across the area with poor smoke
dispersal through mid week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 11 PM this evening to 5 AM MST
Tuesday for WYZ002.

Winter Storm Warning from 5 PM this afternoon to 5 PM MST Monday
for WYZ012-013-023-024-027.

Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Monday for WYZ001-014-015-025-026.

&&

$$

SHORT TERM...C.Baker
LONG TERM...AR
AVIATION...WM
FIRE WEATHER...C.Baker


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