Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 091107
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
407 AM MST Tue Jan 9 2018

.SHORT TERM...(Today through Thursday night) Issued at 327 AM MST
Tue Jan 9 2018

The main trough axis that will be delivering substantial snowfall
to far Western Wyoming through Wednesday morning is now aligned
along the Pacific coast. Patchy light snowfall is breaking out
ahead of this approaching Pacific trough across far Western
Wyoming. As the morning wears on, deeper Pacific moisture will
feed into the area with snowfall picking up in intensity. The
flow off the Pacific will be mild enough to mix the snow with or
change the snow to all rain in the western valleys this afternoon.
Then the precip will change back to all snow in the western
valleys this evening. The heaviest snowfall in the far west will
occur tonight after midnight as the associated cold front
approaches the area. By 12 noon Wednesday, we could see up to 20
inches of snowfall across local areas of the Tetons along with 4
to 12 inches across the rest of the mtns and 2 to 4 inches in the
western valleys.

In the meantime, as the cold front pushes south across Wyoming,
the snowfall will spill east of the divide late tonight before
sliding south to encompass central Wyoming east of the divide by
Wednesday afternoon. 9 to 10mb 3 hourly sfc pressure rises are
progged behind this front across our eastern zones, so we are
expecting up to 50 mph wind gusts along with the snowfall across
the lower elevations of Johnson County Wednesday. This will lead
to considerable blowing and drifting snow, especially across
Northern and Central Johnson County where the precip will be all
snow as opposed to southern Johnson and Natrona Counties where
the precip will briefly begin as rain later in the day as the
front progresses south. Stronger winds are progged for Johnson
County. 2 to 3 inches of snow should fall in the lower elevations
of Johnson County with 4 to 8 inches across the adjacent Big Horn
mtns during this time frame. We have an SPS addressing this.

As for winds ahead of this approaching storm system, we have a
high wind warning for zones 19 and 20 for today and this evening.
Thursday night also appears to be a very windy night ahead of yet
another approaching winter storm when 60 knot 700mb west winds
are progged along with 6 700mb height lines progged across Wyoming
spaced part every 30 decameters. We decided not to address strong
winds in the Cody Foothills for today and tonight based on the
fact that no strong Gap flow or mtn wave signatures exist during
that time frame.

.LONG TERM...(Friday through Tuesday) Issued at 327 AM MST Tue
Jan 9 2018

System out near the dateline gets kicked out by a system just
entering the wrn Pacific and moves quickly se across us on the
front side of our mean ridge later Thursday night and into the
start of the medium range period (Friday). This system will also
interact with a cold front sliding south out of Canada increasing
the chance of snow across our far nrn/nern zones Friday besides
the west and mountains. Ridge starts to sharpen up over the
weekend with a fairly strong/cold system dropping south into the
nrn high plains. Model timing varies considerably but most of the
guidance shows a fairly strong cold front and surface high
dropping south into MT or ND Sunday night or early Monday with a
decent cold front pushing south across areas east of the divide
Sunday or Sunday night with some strong/gusty north wind and
possibly a quick shot of light snow, especially for our far
nrn/nern zones. Brief ridging behind this system Monday night into
Tuesday before a weakening (most likely) system moves through
right at the end of this period. A chance of snow mainly in the
wrn mtns (mainly) with this system with some gusty west to
southwest wind ahead of and with trough passage causing some brief
basin/valley mixing.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 327 AM 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 today across the western valleys. The
best chance for widespread snowfall looks to occur in the morning
with more showery activity in the afternoon. However, heavier snow
is expected to overspread western Wyoming between 03Z and 09Z as
cold front begins to pull across the region.

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

The main concerns today and tonight will be LLWS and turbulence on
the lee side of mountain ranges, and the very windy southwest wind
between South Pass and Casper. Otherwise VFR conditions with
considerable lee enhanced clouds are expected. Mainly light winds
are expected in the central basins.

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

&&

.FIRE WEATHER...
Issued AT 327 AM 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 heavy snow and
blustery winds at times west of the Divide; strong winds today
through tonight across the usual windy spots east of the Divide;
and light snow and gusty north winds on Wednesday and Friday east
of the Divide. As much as 1 to 2 feet of snow will be possible in
the mountains of the west above 7500 feet through Wednesday with
more to follow later Thursday and Friday. Smoke dispersion will
poor to fair for most locations today, 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.

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

Winter Weather Advisory until noon MST Wednesday for WYZ014-015-
024.

Winter Storm Warning until noon MST Wednesday for WYZ012.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Murrell
FIRE WEATHER...Lipson



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