Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 112040

Area Forecast Discussion
National Weather Service Riverton WY
140 PM MST Mon Dec 11 2017

.SHORT TERM...This evening through Thursday

Clipper passing off to the east is just close enough to kick up the
north winds a little in nrn Johnson county today but that`s about
it. Otherwise, another day of fairly steep inversions with areas of
fog out west. Don`t see a lot of change for tonight with potential
for some more fog in especially the Jackson valley northward with
only a few strands in the Star Valley. A little around Boysen is
likely also. On Tuesday, we warm back up (for the non-inversion
areas) behind our current system with temperatures likely similar
to the weekend with 50s in the mixed areas east of the divide
(where snow is non-existent) with generally 40s elsewhere. Next
system dives east of us Tuesday night/early Wednesday with an
increase in north wind again across nrn and ern Johnson County
with maybe a period of 30-45 mph winds. Another system close on
this one`s heels moves right through us Wednesday afternoon or
evening with more clouds, some mountain flurries/very light snow
and possibly a few flurries/sprinkles across the lower elevations
across the nrn/ern zones as a weak frontal boundary turns winds
n-ne. Yet another clipper drops just east of us on Thursday for
another round of gusty n-nw wind east of the divide and possibly a
period of very light snow in mainly the Bighorns. Otherwise, just
some colder air, clouds and gusty wind.

.LONG TERM...Thursday night through Monday night

Synopsis...The massive ridge that has been in place over the
Western CONUS for the past two weeks will begin to breakdown at
the beginning of the forecast period. A shortwave trough will
rapidly dive southeastward over western Canada and into the
Northern Rockies by Friday evening. Strong NW flow will be place
over northern WY by Monday, with the right front quad of the jet
possibly impacting Johnson County and the Bighorns with strong
winds. The jet will impact this area through the night Monday,
before a ridge begins to build over the PACNW going into Tuesday.

Discussion...Conditions will be dry through the day Friday. The
signal for windy to possible high wind conditions over the
Absarokas and Cody Foothills Thursday night into Friday continue
today. Models continue to show an 8 to 11C temperature gradient in
this area, which should induce at least some strong gap flow
winds, with models hinting at some mountain wave activity as well.

A cold front associated with the shortwave will quickly move
through the state Friday night. Isolated snow showers will begin
over the NW mountains late Friday afternoon, becoming scattered
and spreading across western portions Friday night. At the same
time, isolated to scattered snow will develop across the north.
This trend will spread southward east of the Divide after midnight
Saturday, as the cold air behind the front makes its way into the
state. Snow will continue through the day across all but southern
portions, before becoming confined to the NW. Isolated to
scattered snow will continue Saturday night through Monday night
for this area, with the potential for significant snowfall.


.AVIATION...For the 00Z TAFS through 00Z Wednesday

VFR conditions will occur through the period. Patchy fog or low
clouds are possible in the far west valleys and Yellowstone National
Park area again Tuesday morning after 10Z. Areas of MVFR/IFR
conditions are expected due to low ceilings and visibility until



Fuels remain at critical levels across over portions of Fremont,
Johnson and Natrona Counties.

Fairly steep temperature inversions will remain the next couple days
for the central and western basins/valleys with little afternoon
mixing. However, the lee of the mountain ranges (Absaroka, Wind
River and Bighorns), Cody Foothills and lower elevations of Johnson
County will see warm afternoon temperatures Tuesday along with min
rh`s in the teens. Breezy to locally windy areas will be noted in
generally these same areas. On Wednesday, a disturbance will slide
southeast across the area with a period of very light mountain
snowfall or flurries in mainly the northern mountains with areas of
gusty northwest wind.





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