Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 160817
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
117 AM MST Wed Jan 16 2019

.SHORT TERM...Today through Friday night

Another weak shortwave will bring light snow over the southern half
of the forecast area this morning. These showers will become more
focused over the far west after sunrise, as a second shortwave moves
over the area from the Great Basin. Snowfall amounts will remain
light, as these showers gradually end tonight. Will keep a SPS in
effect for now. Despite mid level temperatures cooling a bit and
inversions weakening, skies will be mostly cloudy to overcast over
areas west of the Divide and the Wind River Basin. Thus, will
keep highs today cold and below guidance once again.

A shortwave ridge will build over the ID/WY border tonight, as
another Pacific storm system moves onshore. Pacific moisture from
this system will make its way over western portions by Thursday
morning. There is enough isentropic lift and orographics for
steady light to moderate snow through the day Thursday and into
Thursday night. A heavier burst of snow could arrive by sunset,
as yet another shortwave moves over the area. Some of this snow
could also track over northern and central portions late Thursday
night into Friday morning. A second shortwave, that will track
further south, will track over southern portions with a burst of
snow over these areas after midnight. Any amounts will remain
light, as models are now progging most of the energy with this
shortwave to dig further south over the Four Corners by 12Z
Friday. Winter weather advisories will likely be needed by
Thursday morning, with snowfall amounts ranging between 2 to 4
inches in the western valleys, southern Lincoln County and Upper
Green River Basin, 4 to 8 inches in the Salt/Wyoming Ranges and
the Gros Ventres. 6 to 12 inches will be possible in the Tetons. 1
to 3 inches will be possible across the rest of Sublette County
and Sweetwater County.

As this system becomes organized over NM/TX Panhandle during the day
Friday, the flow over WY will turn more NNW. This will dry things
out over the Cowboy state and any snow activity will be isolated to
widely scattered and mainly occur over NW portions. We could see a
return to the frigid temperatures in the basins going into the
weekend, as inversions could return. Temperatures in the single
digits to the teens look likely Friday night.

.LONG TERM...Saturday through Wednesday

00Z Deterministic models have trended to a less amplified ridge
for Saturday over the Rockies ahead of a Pacific trough
approaching the West Coast. The flow is now projected to be more
westerly which will support a chance of snow across the far
northwest and west Saturday into Saturday night. The Pacific
trough is progged to push into the Pacific Northwest/N. California
coast around 00Z Monday, and then quickly track across the
Intermountain West and Rockies in the Sunday night and Monday time
frame, before digging into the Plains late Monday into Monday
night. The ECMWF is about 12 hours faster in the evolution of this
trough. The associated cold front with this trough is expected to
pull across the forecast area late Sunday night into Monday.
Currently it appears snow will break out over the west/northwest
late Sunday into Sunday night, and spread east of the Divide late
Sunday night into Monday. The snow looks to end from northwest to
southeast late Monday into Monday night. In wake of this system,
Tuesday looks to be mainly dry, before models show a quick moving
shortwave trough pulls southeast out of Canada into the northern
Rockies on Wednesday.

Temperatures look to be seasonal to slightly below average for
much of the extended.

&&

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

Areas of MVFR, local IFR conditions in -SN will continue to spread
north across Interstate 80, to a vicinity and south of a KAFO-KBPI-
KCPR line by 12z Wednesday.  This will be the northern extent of
this line of lower ceilings and -SN east of the Continental
Divide, with increasing westerly winds bringing in drier air in
the afternoon. The forecast keeps these conditions south of KCPR
and KLND, but keeps MVFR conditions over KRKS through the
afternoon.

West of the Continental Divide, these MVFR, and local IFR
conditions in -SN, are expected to continue to spread north
through Wednesday afternoon with widespread snowfall and mountain
obscurations expected along and west of the Divide by 21z
Wednesday. Drier air will then work in overnight Wednesday night
with decreasing areas of snowfall, but areas of lower MVFR and IFR
ceilings are expected to linger through 12z Thursday.

&&

.FIRE WEATHER...

Light snow will continue across the south and west this morning,
eventually becoming more focused to the far west by this
afternoon. Gusty southwest winds will continue over the Wind
Corridor and spread to zone 277 today. These winds will continue
over the Corridor tonight and include zone 279 Thursday. Light to
moderate snow will continue over the far west Thursday, spreading
across southern portions Thursday afternoon. Light snow will occur
east of the Divide Thursday night into Friday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...LaVoie
LONG TERM...Murrell
AVIATION...Meunier
FIRE WEATHER...Hattings


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