Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 220614
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1114 PM MST Sat Jan 21 2017

.Short Term...Tonight Through Tuesday

A weak short wave low pressure trough extending from
western Montana...south through south central Idaho
into northeast Nevada...spreading light snow and clouds
across western Wyoming will continue to do so as it moves
east through the night. snowfall will continue through
Sunday but will be on the light side with 4 to 6 inches across
the mountains...locally up to 9 inches in the Tetons.  The
western valleys should see 2 to 3 inches with local 5 inches
in the northern Star Valley. Some light snow will spread east
into Sublette and Sweetwater counties.  As the trough passes
through western and central Wyoming Sunday...snow will lighten up
in the west and spread across the north east of the divide but
should be light. The valleys and basins should continue to see
areas of fog tonight and Sunday morning.  Clouds...snow and fog
will all work to keep temperatures warmer by 3 to 6 degrees most
areas. Brief partial clearing will occur in the west before
a second and stronger low pressure trough now moving inland
on the Pacific Northwest coast moves in rapidly behind the first
trough. By late Sunday afternoon snow from the second trough is
expected to enter the far west. Snowfall will spread east in the
western mountains across the Continental Divide into the eastern
foothills of the Absaroka and Wind River mountains Sunday night
and Monday. A winter storm watch was issued for Sunday night and
Monday. At this time most areas have advisory snowfall amounts
forecast. The Teton, Wyoming and Salt river ranges are expected to
see 10 to 15 inches of snow. West to southwest winds will increase
over the east slopes and foothills of the Absaroka and Wind River
mountains and through the wind corridor from Rock Springs to
Casper. The winds will cause warming but are not expected to blow
out the Wind River and Big Horn basins where cold air is trapped.
The south and east should warm nicely Monday before a cold front
from Montana moves south into northern Wyoming spreading south by
late afternoon and early evening. The snow will be decreasing
significantly or ending in the west as it spreads across north,
central and east Monday night and Tuesday spreading 1 to 3 inches
across the area. Temperatures will fall Monday night and Tuesday
as the cold front sweeps south. Modelwise GFS and NAM pretty
divergent with precipitation with the NAM hitting the south
southwest while the GFS follow the cold front. SREF a nice
compromise.

.Long Term...Tuesday Night Through Saturday

The major longwave trough will continue to bottom out over the
Desert Southwest Tuesday evening, as the associated surface low
ejects out over the High Plains. The GFS has this feature emerging
over the NE Panhandle, while the NAM is further S over N KS. Light
snow will gradually subside east of the Divide Tuesday night, while
minor NW flow occurs across NW WY. This light snow over the NW
mountains will continue into Thursday, as a ridge of high pressure
builds into the Canadian Rockies. Part of the ridge axis will
fold over the Northern Rockies Friday, followed by a minor
shortwave trough. This trough will bring a slight chance for snow
across the far northern mountains. With this ridge fold over,
there looks to be sufficient warming aloft for fog formation in
the valleys and basins. This ridge could turn into a blocking
high, as the GFS & ECMWF prog a h5 588dm height center by 12Z Sun
off the CA coast and a 1048mb surface high in the Great Basin.

&&

.Aviation.../06Z Issuance/

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

There could be continued stratus over the far western valleys
between 020-040 KFT AGL, but soundings support this deck scattering
out between 12Z and 15Z. If this deck does scatter out near sunrise
there will be a chance for some low-lying fog, boundary layer
moisture is marginal. Otherwise VFR conditions are expected through
about 21Z with mainly mid/high level cloudiness. This will quickly
change between 21Z Sunday and 03Z Monday as snow breaks out in a
moist southwest flow ahead of a storm system. MVFR/IFR conditions
are likely tonight in low ceilings/visibilities across much of the
area. Mountains will be obscured much of the time after 21Z.

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

Once the mid-level deck of clouds push away from the area between
09Z and 13Z, patchy fog could occur with some IFR/MVFR restrictions.
Currently not expecting fog to be as widespread as Saturday morning,
and will only have VCFG during the morning for the basins.
Otherwise, VFR conditions with some mid-high level clouds. Windy
southwest surface flow will occur across SE Fremont into Natrona
County including KCPR terminal. Tonight, an approaching storm system
will rapidly increase mid-level cloudiness and have some low-level
wind shear in spots including KCPR and KLND. Some models have some
precipiation spilling east of the Divide tonight, but will keep the
precipitation along and west of the Divide for now.

&&

.Fire Weather...

A weak short wave low pressure trough extending from
western Montana...south through south central Idaho
into northeast Nevada...spreading light snow and clouds
across western Wyoming will continue to do so as it moves
east through the night. snowfall will continue through
Sunday but will be on the light side with 4 to 6 inches across
the mountains...locally up to 9 inches in the Tetons.  The
western valleys should see 2 to 3 inches with local 5 inches
in the northern Star Valley. Some light snow will spread east
into Sublette and Sweetwater counties.  As the trough passes
through western and central Wyoming Sunday...snow will lighten up
in the west and spread across the north east of the divide but
should be light. The valleys and basins should continue to see
areas of fog tonight and Sunday morning.  Clouds...snow and fog
will all work to keep temperatures warmer by 3 to 6 degrees most
areas. Brief partial clearing will occur in the west before
a second and stronger low pressure trough now moving inland
on the Pacific Northwest coast moves in rapidly behind the first
trough. By late Sunday afternoon snow from the second trough is
expected to enter the far west. Snowfall will spread east in the
western mountains across the Continental Divide into the eastern
foothills of the Absaroka and Wind River mountains Sunday night
and Monday. A winter storm watch was issued for Sunday night and
Monday. At this time most areas have advisory snowfall amounts
forecast. The Teton, Wyoming and Salt river ranges are expected to
see 10 to 15 inches of snow. West to southwest winds will increase
over the east slopes and foothills of the Absaroka and Wind River
mountains and through the wind corridor from Rock Springs to
Casper. The winds will cause warming but are not expected to blow
out the Wind River and Big Horn basins where cold air is trapped.
The south and east should warm nicely Monday before a cold front
from Montana moves south into northern Wyoming spreading south by
late afternoon and early evening. The snow will be decreasing
significantly or ending in the west as it spreads across north,
central and east Monday night and Tuesday spreading 1 to 3 inches
across the area. Temperatures will fall Monday night and Tuesday
as the cold front sweeps south. Low mixing heights with low
clouds, snow and fog should produce poor smoke dispersal through
Tuesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday afternoon through Monday
afternoon for WYZ001-012>014-023>025-027.

&&

$$

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


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