Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 092040
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
140 PM MST Tue Jan 9 2018

.SHORT TERM...(This evening through Friday) Issued at 127 PM MST
Tue Jan 9 2018

...Winter Storm (zone 12) and Advisories (rest of western Mountain
Groups) until noon Wednesday...

...High Wind Warning for the Wind Corridor until midnight tonight...

Imagery now finally shows something different...a broad ridge across
most of the CONUS with bookend trofs along/down each coast. The west
coast trof also has a closed low in its base, rotating into/through
srn California representing the southern portion of the West`s split
flow as it directs moisture and PVA towards wrn/swrn WY. Another
area of moisture due to arrive this evening is headed toward
wrn/nwrn WY from the PAC NW and through ID/wrn MT. Frontogenesis is
now just beginning to start over ern OR/WA and wrn ID. The SFC has
low pressure entering and building across much of the wrn CONUS with
high pressure being pushed off to the south and east. However,
weakening high pressure still remains in place across swrn WY as
hints of a cold front forming to the west begin. Precipitation, both
rain and snow, is continuing to increase and stream into wrn/ WY
this afternoon.

Heavier precipitation has now arrived across the west with moisture
streaming into the area from out of NV/UT/ID. Temperatures are
slowly warming  and may possibly mix with snow or turn back to rain
for a brief period in the valleys this afternoon as WAA continues
under modest ridging. Then the next and much more significant
moisture arrives tonight, flowing into the region more from the
Pacific Northwest. Again, expecting rain chances below 6500-7000 ft
to continue with the changeover back to snow at valley locations not
a sure thing until sometime early Wednesday morning (precip type
largely dependent on just how saturated the lower atmosphere
is...which it is now...and if wet bulb temperatures can push to or
below 0 deg C). This time period with the warmer atmospheric
temperatures also coincides with the heaviest QPF...with as much as
a half inch possible from this evening to around sunrise Wednesday.
This could correlate to as much as 4 to 10 inches during that
(roughly) 12 hour period...with 2 to 4 inches preceding and another
1 to 3 through Wednesday morning. All told, by  noon Wednesday, the
area is expecting total snowfall amounts ranging from 8 to 18 inches
(locally to 20 inches) in the Teton and Gros Ventre mountains; 4 to
12 (locally to 16 inches) in the surrounding mountains; and 1 to 4
inches in the area valleys (especially with the rain/snow/rain/snow
thing going on). Brisk south to southwest winds will also continue
west of the Divide with this weather system, especially from this
afternoon through Wednesday morning with gusts ranging from 35 to 55
mph at times across the favored ridges/passes. Lingering areas of
light snow showers or flurries will remain over portions of
western/northwestern WY Wednesday night through Thursday morning.
Then, the next winter weather system will begin to make itself known
Thursday afternoon...continuing into/through Friday morning. This
system will also race into/through the region out of the PAC NW,
taking just enough of a northern track to keep the better moisture,
precip, and snow across wrn/nwrn WY and MT. Another set of Winter
Weather Advisories (possible zone 12 Warning) for Thursday afternoon
into Friday morning.

East of the Divide: Flat ridging aloft, WAA and lee side troffing
will keep the region dry with strong winds blowing across the usual
spots...particularly across the Wind Corridor. High wind conditions
will continue into night across the Wind Corridor as the SFC P Grad
also continues strong and combines with relatively strong winds just
off the surface (brought down to SFC adiabatically. However, it will
remain mostly dry east of the Divide until Wednesday morning when
the eastern FA begins to feel the effects of the passing upper trof
and an associated strong cold front pushing south across the region.
Areas of snow will come mainly along and behind the front late
Wednesday morning, through the afternoon and into the early part of
the evening. There could be some areas of rain or rain mixed with
snow in the morning/early afternoon, particularly over portions of
Johnson and Natrona Counties. Although snow amounts look to remain
mostly below Advisory level criteria. increased impacts for a few
hours will be possible east of the Divide as post frontal gusty
north winds combine with areas of light snow. This will hold
particularly true for locations in the Cody Foothills and portions
of Johnson and Natrona Counties with slick/icy roads and reduced
visibilities into Wednesday evening. Will continue to evaluate.
Thursday afternoon and night, strong winds will return to the
foothills and Wind corridor once again...a prelude to the next
winter system due to arrive later Thursday into Friday. High Wind
Warnings will be possible once again...including the Cody Foothills
this time around. However, this next system will take just enough of
a northern track to keep most moisture, precip, and snow across
wrn/nwrn WY and MT..perhaps impinging on the Bighorn Mountains, but
not leaving much across lower elevations. Some small chances for
light snow will exist across eastern Johnson and Natrona counties
Friday.

.LONG TERM...(Friday night through Tuesday) Issued at 127 PM MST
Tue Jan 9 2018

Synopsis...The region will be dominated by NW flow aloft, as a
longwave ridge remains centered over the WCONUS. This ridge will
begin to break down by Monday night. In the meantime, a Pacific
system will be exiting the forecast area Friday night, with
lingering snow across the north. A second, possibly Arctic system
will could bring another shot of snow east of the Divide Sunday
night into Monday.

Discussion...Lingering snow will occur across the northern and
western mountains Friday night, gradually ending Saturday morning.
Conditions will be mainly dry Saturday and Sunday, with partly to
mostly cloudy skies and generally light winds. Isolated to scattered
PoPs will be possible across northern and central portions east of
the Divide Sunday night into Monday as the cold front moves through
the area. Temperatures will remain seasonal for through the period
as well.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday afternoon)
Issued at 127 PM MST Tue Jan 9 2018

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

MVFR ceilings with relatively brief IFR restrictions from visibility
in snow are expected at times this evening and tonight across the
western valleys. Snowfall will remain widespread however, heavier
snow is expected to overspread western Wyoming between 12Z and 18Z,
as a cold front begins to pull across the region.

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

The main concerns tonight will be LLWS and turbulence on the lee
side of mountain ranges, and the very windy southwest wind between
South Pass and Casper. These winds will gradually subside,
especially near KCPR into Wednesday morning. Otherwise, VFR
conditions with considerable lee enhanced clouds are expected.
Mainly light winds are expected in the central basins. Snow will
begin after 12Z Wednesday near KCOD, as the cold front moves
southward. Snow will then spread southward with the front and impact
KCPR and the central basins between 17Z and 19Z and continue through
the rest of the period. Snow will also end from north to south, with
VCSH expected after 21Z at KCOD and KWRL.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 127 PM MST Tue Jan 9 2018

Today through the end of the work week, a couple of strong winter
weather systems will impact the forecast area with periods of heavy
snow and blustery winds at times west of the Divide; strong winds
will continue through tonight across the usual windy spots east of
the Divide; light snow and gusty north winds will combine on
Wednesday. As much as 1 to 2 feet of snow will be possible in the
mountains of the west above 7500 feet through the day Wednesday with
more to follow later Thursday and Friday. Areas east of the Divide
will see much lighter accumulations. Smoke dispersion will be fair
to good today and good to very good on Wednesday.


&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MST Wednesday for WYZ001-002-
014-015-024.

High Wind Warning until midnight MST tonight for WYZ019-020.

Winter Storm Warning until noon MST Wednesday for WYZ012.

&&

$$

SHORT TERM...Braun
LONG TERM...LaVoie
AVIATION...LaVoie
FIRE WEATHER...Braun



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