Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 281734
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1034 AM MST Mon Nov 28 2016

.AVIATION.../18Z ISSUANCE/

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

The coverage of snow over the west will continue to decrease in
coverage this afternoon and evening, but occasional MVFR/IFR
conditions will continue in snow shower activity through the
evening. After midnight and through the day Tuesday, lower
elevations should be mainly dry, with a continued chance of snow
showers/flurries over the mountains. A cold front is likely push
south southwest across much of eastern Sweetwater County between 04Z
and 07Z...shifting winds from the west northwest to north northeast
at KRKS. However this should be brief with winds shifting back to
the west northwest between 10Z and 14Z. Breezy northwest winds will
impact KBPI and KPNA tonight and Tuesday. Mountains will be obscured
much of the time.

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

Windy to strong northwest wind is expected to develop late this
afternoon into the early evening over much of the area, and last
through the day Tuesday. The strongest wind will be across northern
Johnson, and the northern Big Horn Basin. Models showed LLWS
impacting the WRL terminal late tonight into the day Tuesday. Areas
of snow showers developing over the mountains pushing southeast into
the lower elevations will be possible through tonight with the best
chance between 00Z and 07Z Tuesday. Although MVFR/IFR conditions
will be possible with this snow shower activity, confidence is not
high enough to prevail any terminal with those conditions. On
Tuesday, snow showers will be mainly confined to the northern
mountains especially over the Big Horn Mountains. Northern
Mountains will frequently be obscured.

&&

.PREV DISCUSSION... /issued 319 AM MST Mon Nov 28 2016/

SHORT TERM...Monday through Wednesday night

With the differences in the weather across the area, we will again
divide the discussion into west and east of the Divide.

West of the Divide...The snow banding developed today and developed
with a vengeance with more snow than we expected. Over a foot has
fallen across some of the higher peaks. Across the valleys though,
totals were not as impressive. Radar shows banding continuing and
with west to northwest flow continuing today, we will hoist a
winter weather advisory across the Tetons, Star Valley and the
Wyoming Range. Amounts may reach warning criteria in some areas,
but since there is not much wind and most accumulation would be
above pass level, impacts would be minimal. In addition, with the
banded nature of the snow, amounts could vary widely over a short
distance. As we head toward this evening, 700 millibar flow will
turn more toward the north and shut off the snow for the most
part.

From Monday night through Tuesday night, some shortwave ridging will
build across the area and bring mainly dry conditions across the
area with only a few higher elevation snow showers. Another trough
will approach the area on Wednesday but does not have a ton of
moisture to work with. Snow shower coverage is expected to increase
but amounts are not expected to be substantial at this time.

East of the Divide...And leftover flurries should be coming to an
end with the majority of the day largely dry. Temperatures will
continue to be cool but still fairly close to normals for this time
of year. There will also continue to be a gusty wind across the
favored areas. However, as flow turns more northerly the strongest
winds will shift toward areas favored by that flow, like the Wind
River Basin, northern Bighorn Basin and especially northern Johnson
County. And this leads to...

The strongest winds will be across northern Johnson County tonight
and into Tuesday. A tight height gradient across the state with
ridging to the west and an upper level low over the northern Plains
will strengthen 700 millibar winds to as high as 60 to 65 knots
across that area. MOS guidance also shows sustained winds over 40
mph at times later Monday night and into much of Tuesday. With
this, we will hoist a high wind watch for northern Johnson County.
In addition, the upper level low will wrap some moisture back into
eastern portions of the area. Most of any snow should remain north
and east of Wyoming, but there could be a few snow showers with
the wrap around. This should end Tuesday morning as the low slowly
moves away to the east, but gusty winds will remain through the
day.

Wednesday at this point looks to be a dry day as shortwave ridging
moves into the area. Cooler air will move in however so Wednesday
will be the coolest day of the period with many locations struggling
to reach the freezing mark. With light winds expected to return to
the valleys, inversions may make a return.

LONG TERM...Thursday through Monday

Synopsis...A storm system will dive south, west of the divide
into the Great Basin Thursday and Thursday night; bringing
chances of snow across the west, with some upslope snow
developing mainly along the east slopes of the Wind River Range. A
strengthening northwest flow behind this system will bring the
possibility of high wind across the Absaroka Range and Cody
Foothills Friday night and Saturday, along with chances of snow
showers across the west through the weekend. Temperatures will
remain mostly slightly cooler than average Thursday and Friday
with a gradual warming trend over the weekend.

Discussion...Medium range guidance has lined up with further
eastward track of next upper low, dropping it south across Idaho
into northern Utah on Thursday, descending across Arizona on Friday.
This track, especially with models showing H5 low closing off over
Idaho, is favorable for the development of NE upslope flow along the
east slopes of the Wind River Range, Lander Foothills.  GFS shows
the low level upslope pattern developing moreso than ECMWF. Areas of
snow should decrease north to south early Friday morning as low
drops into the desert SW.

The concern on Friday night and Saturday shifts to the northwest as
anticyclonic 160 knot jet noses into western Montana.  This is a
favorable setup for high wind and mountain wave activity along the
east slopes from Clark to Dubois.  Strengthening westerly flow will
also bring milder Pacific air into the area, though protected
valleys and basins will likely remain on the cool side under
inversions.

Both ECMWF and GFS are now showing an Arctic airmass knocking on our
northern border on Monday, the GFS faster to shove the front across
the area by Monday evening.  Consensus (slower) solution paints
windy and mild conditions ahead of the front Monday afternoon with
chances for snow showers across the west and far north.  We may be
in the deep freeze across the area by Tuesday, December 6th.  Stay
tuned and check the antifreeze.

FIRE WEATHER...

Snow showers will continue across the west today. Mainly dry
conditions will be found east of the divide with a gusty wind
developing in the afternoon across many areas. High winds are
possible across the northern Big Horn Basin and Johnson County
Monday night and into Tuesday. Mixing and smoke dispersal will be
generally good in the basins and good elsewhere.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon for WYZ012-
023-024-027.

High Wind Watch from this evening through Tuesday afternoon for
WYZ010.

&&

$$

SHORT TERM...Hattings
LONG TERM...AEM
AVIATION...WM
FIRE WEATHER...Hattings



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