Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 162007
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
107 PM MST Tue Jan 16 2018

.SHORT TERM...(This evening through Friday)
Issued at 101 PM MST Tue Jan 16 2018

Ridge axis is ovhd attm with steep basin/valley inversions keeping
areas of low clouds in early this afternoon. Otherwise, it is sunny
day with milder temps in the foothills and mountains than the
trapped basins/valleys. Weak, shearing system over Idaho will
continue to weaken as it passes through from west to east later
tonight through Wednesday morning with nothing more than a period of
mid and high clouds. Perhaps the highest peaks out west could see a
few flurries. Otherwise, inversions will remain strong with only the
increasing clouds out west trying to temper them. In the central
basins, clouds come in later so the redevelopment of some fog/low
clouds will likely return. Temperatures are struggling under these
inversions and both highs today and tomorrow (along with lows
tonight) were lowered quite a bit. Winds start to develop some
tonight and Wednesday in the wind corridor so that should help that
area warm some. Winds start to increase quite dramatically in the
Cody Foothills Wednesday night into Thursday morning as pressure and
thermal gradient increase for increasing gap flow potential. Jet
passing to the north is fairly far north but could provide some aid
late. Otherwise, upstream soundings (over the park) show a hint of
an inversion just below H7 and cross sections out to 12z Thursday
hint at some mtn wave potential. Not confident enough for a high
wind watch yet but it`s worth watching. Increased the winds
dramatically over the 3-6kts given form the forecast builder and now
we have some 25-35kt winds with gusts around 45-50kts. Best window
seems to be 06-18z Thursday. Will issue an sps and let the next crew
give it another look. We then look towards our next significant
storm for later Thursday night through Friday and into the first day
of the extended. The first part of this system is just south of the
Aleutian chain while the second piece is much further west our
between 160-170E. The Aleutian system ends up being the `north half`
while the second system ends up being the piece of energy that digs
the `south half` of this system. Frontal and jet (potential right
entrance region) forcing moves into the far west late Thursday night
to get the snow going in the nw mtns with a mixture in the valleys
early on, especially in the Star Valley where any pcpn may stay rain
until the front comes through Friday morning. Some heavy snow will
likely develop over the nw mtns as the front moves in and the jet
set up favorably for a few hours. Good dendritic growth temps show
up to late Thursday night/Friday morning and lapse rates also steep
Friday morning so dynamics will work on a favorable atmosphere. Will
likely need advisories across a big portion of the area for this
system. Strong sw winds blow ahead of this system across the wind
corridor and we`ll probably have to watch the LND foothills too
Thursday night/Fri mrng. Basin inversions should hopefully break
ahead of this system Thursday for some `milder` temps. High winds
are quite possible in the wind corridor also and possibly around the
Lander foothills Thu ngt/Fri mrng. Most of the snow potential
east of the divide holds off until Friday night. More on that from
my partner below...

.LONG TERM...(Friday night through Tuesday)
Issued at 101 PM MST Tue Jan 16 2018

By Friday night, the splitting trough will be approaching the
Rockies with most of the energy relegated to the southern jet over
the Desert Southwest. The GFS is still the fastest with this Pacific
trough with the ECMWF and the Canadian lagging behind. For now have
only painted the southwest portion of our CWA with any significant
snowfall for Friday night. Right rear quad dynamics, a good Pacific
stream of moisture, weak QG forcing, and a Pacific frontal system
will help enhance the precip. Light snowfall will continue into
Saturday across the CWA once we are on the cyclonic side of the jet.
Snow will end from northwest to southeast Saturday night. The GFS
has the trough axis over us by 00Z Sunday and the Canadian and ECMWF
by 06Z Sunday. Temperatures will be colder underneath and behind
this weather system but not bitterly cold as the airmass behind this
trough will be of Pacific origin. Then by Sunday morning, snow will
be developing in the far west as a ripple in the west northwest flow
approaches the area. Snowfall will increase in the west Sunday night
and continue into Monday as isentropic lift gets underway. Then
after the trough passes on through, light snowfall will continue in
the far west through Tuesday from moist nw upslope Pacific flow. Dry
conditions are expected east of the divide after Saturday night.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday afternoon)
Issued at 101 PM MST Tue Jan 16 2018

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

Erosion of MVFR/IFR ceilings are occurring over the Upper Green
River Basin as well as along the Jackson Valley. Light snowfall
falling out of low clouds will linger over the Jackson Valley
through around 21Z this afternoon. The fog is dissipating in Afton.
Any improvement is expected to be temporary with steep inversions
remaining over the area through the day. Once fog and low clouds
lift this afternoon, a weak upper level disturbance and some mid-
level clouds should decrease the extent of the redevelopment of fog
and lower ceilings from 06Z Wednesday through 18z Wednesday,
although patchy fog could still form at KJAC and at KBPI.

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

Satellite fog imagery is indicating the dissipating of an area of
MVFR/IFR cigs across the Wind River Basin that will likely vanish by
21Z this afternoon. Once these ceilings dissipate this afternoon,
VFR conditions with unlimited ceilings are expected to prevail
through 18z Wednesday. Fog, low cloud redevelopment should be
hindered as a weak upper level disturbance moves across the area.

&&

.FIRE WEATHER...
Issued AT 101 PM MST Tue Jan 16 2018

Dry weather will continue through Thursday. Inversions will continue
over the western valleys with areas of fog around. Inversions are
also likely over portions of the basins east of the divide. A gusty
breeze will develop across the southwestern wind corridor from the
Red Desert through Natrona County tonight and Wednesday and increase
further Thursday and Thursday night. Winds will also increase
significantly along the Cody Foothills later Wednesday night through
Thursday morning. Mixing and smoke dispersal will be generally
poor.

&&

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

&&

$$

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



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