Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 010557
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1055 PM MST Wed Nov 30 2016

.SHORT TERM...Tonight Through Saturday

Cloudy out west with periods of flurries/very light snow in a gentle
but moist upglide pattern. This overall pattern continue through
tonight but wind flow becomes very weak with little orographic help.
On Thursday, our weak, splitting upper trough moves into the west
and then moves east of the divide Thursday night. Fighting the
overall bigger scale ridge over the area so don`t expect anything
significant with this system but still enough moisture and
shallow, brief upslope in spots for chance pops east of the divide
in the mountains/near higher terrain and upslope favored areas.
local WRF NMM is picking this up with a bands of light snow dropping
south with the mid level low Thursday night. Kept our going
forecast with little change. Still got a shot of some light
accumulating snow in the Lander Foothills. Probably only a skiff
to an inch max but some light snow/flurries in spite of the
weakening system. System moves quickly away Friday before a
strengthening nw flow develops with isentropic upglide returning
to the west Friday night into Saturday. Main challenge was how far
south (in the west) this pattern will develop. GFS is further
south affecting most of the far west, NAM further north affecting
mainly the northwest half of the far west. Leaned towards the GFS
for now as even the NAM is showing moist upglide further south
just not as much qpf. Otherwise, east of the divide, still
watching some wind potential in the Cody Foothills Friday night
and Saturday as nw flow increases, thermal gradient increases and
even some hint of a mountain top inversion. Flow is a little too weak
(20-25kts) for significant mountain wave but could still some decent
gap flow with possible help from weak lee side mountain wave. Will
obviously have to watch the temps up there Friday night as evening
lows are quite likely before winds start. Any lulls though would
cause erratic temperature swings overnight.

.LONG TERM...Saturday Night Through Wednesday

Upper level weak ridging on Saturday will give way to moist
northwest flow by Saturday night allowing for light snow once again
across northwest WY.  As shortwave trough digs into the Pacific
Northwest early Sunday, mid level flow expected to shift to the west
during the day, providing for more favorable orographic forcing
along the far western slopes.  Temperatures Sunday will be the
warmest of the period with mid level warm air advection.  Sunday
night will see the flow shift more to the southwest as the fast
moving trough drops into Idaho and the initial cold air push moves
into northwest WY early Monday morning.  Looks like snow will become
more widespread Sunday night into Monday morning, especially across
western WY, southwest WY, and evening northern WY.  This pattern
should provide more needed snowpack to the western mountains.

Later Monday, the secondary push of cold air comes south out of
Canada and Montana, bringing with it -18 to -20C 700 mb temps.  Both
the GFS and ECMWF show scattered to widespread light snow through
early Tuesday. Today, the models have flopped compared to yesterday.
The 12Z GFS is now closing the 500 mb low late Tuesday over northern
UT and SE ID to provide a good opportunity for heavy upslope snow
across much of central WY.  Yesterday, the ECMWF provided the same
solution. However, the ECMWF today has split the northern and
southern streams and is not favoring upslope, much like yesterday`s
GFS. Have again broad-brushed the chance of precipitation forecast
during the period and will wait for more concrete details.  Guidance
still shows very cold air moving into the area Tuesday and
Wednesday, with some highs not getting out of the teens, and lows
likely to drop into the -5 to -15 F range Wednesday morning.

&&

.AVIATION.../06Z ISSUANCE/

.AVIATION.../12Z ISSUANCE/

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

VFR conditions are expected through mid-afternoon Thursday with
considerable mid-level cloudiness and light winds.  Late Thursday
afternoon into the overnight a weak disturbance will result in
isolated to locally scattered snow showers. The best chance for any
snow will be in favored upslope locations such as Lander and Casper,
but models show upslope flow shallow, weak and brief resulting in
low confidence. Trended towards the GFS/MAV MOS instead of the very
pessimistic ceilings of the NAM/MET MOS guidance.

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

Inclement weather will continue with -SN/Flurries continuing on and
off through the forecast period especially for the far west. The
main impact will be at KJAC where periods of MVFR ceilings are
likely. Elsewhere the -SHSN chances will be less and will have
mainly VCSH to account for this activity. Winds will be mainly light
through the period. Mountains, especially over the far west, will be
obscured much of the time.

&&

.FIRE WEATHER...

Some light snow or flurries will continue at times in the far west
through Thursday with a dry period east of the divide. A chance of
flurries or very light snow will develop Thursday night into early
Friday east of the divide, mainly in and near the mountains. Smoke
dispersal will generally be poor to fair. More significant snow and
wind is expected in the west Sunday with windy to very windy
conditions east of the divide.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Skrbac
LONG TERM...McDonald
AVIATION...WM
FIRE WEATHER...Skrbac



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