Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 060944
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
244 AM MST Tue Dec 6 2016

.SHORT TERM...Today through Thursday night

Imagery still shows general longwave across the CONUS with a few
embedded shortwaves riding through the flow over the central and
west. Nose to the jet has drifted swd and is now moving into the
central and southern Rockies. SFC has building high pressure across
WY with the main cold front well south of WY into the central Plains
and another secondary cold front moving through the northern and
adjacent high Plains into northeastern WY. Another weak nearly
stationary front exists from extreme north central WY through
southwestern MT. Light snow showers/flurries are accompanying this
last feature...perhaps getting just into the far northern FA over
the border.

This morning, post frontal very cold temperatures will be left along
with a few flurries or ice crystals...mainly near the aforementioned
stationary front. Lows this morning will range from around 25 below
in the high western mountains to the single digits in the basins and
lower elevations east of the Divide. Marginal WC conditions over the
Upper Green River Basin Monday night/early this morning should
become just very cold by sunrise. SPS covers this area. Otherwise,
as the cold front/stationary front moves south across WY today, more
snow chances will return to the mountains near the Divide and then
southern WY by late afternoon/evening as a quick moving disturbance
moves across this same area. Expecting generally between a half inch
and two inches of light snow between this afternoon and Wednesday
morning with north winds 5 to 15 mph across Sweetwater county. With
rather flat NW flow continuing over the rest of the FA...small
chances for light snow showers or flurries west of the Divide will
continue periodically through Wednesday evening. Very cold daytime
and overnight temperatures will also continue with both Wednesday
and Thursday mornings bottoming out as much as 25 below west of the
Divide and over the Bighorn mountains...along with below zero
reading east of the Divide too. Thursday morning, with 1040 plus
high pressure building into the FA, will be the coldest start to a
day this week or this winter so far for much of the FA...especially
for locations east of the Divide.

Thursday and Thursday night, Arctic turns Pacific and WAA begins
with the approach of a ridge from the west. EPAC moisture will also
increase in this modest ridging regime, flattening as a shortwave
moves through the ridge with terrain forced precipitation by
Thursday evening ushering in a period of rather prolonged snow
chances out west.

.LONG TERM...Friday Through Tuesday

Warm advection/upglide snowfall continues out west ahead of an
approaching shortwave with its origin in the Bering Sea area. This
strong open wave trough will likely blast across the area on
Saturday with more cold advection and more continued snow out
west. A swath of snow/snow showers will also likely move through
with the cold front, especially across the far north and south.
The GFS/GEM are stronger/further south with this system than the
Euro which would result in a much better chance of some snow east
of the divide. Snowfall out west could end up being quite
significant again by later Saturday with this new multi-day snow
event. Areas east of the divide, may see enough gradient Friday to
mix many of the basin areas out to provide some relief from the
cold the next couple days. A fast westerly flow will continue
through early next week with embedded shortwaves. Off and on snow
will continue out west. Arctic front may return across the north
on Tuesday with dynamics from the west overrunning it. Will have
to watch this scenario with time as good snowfall can occasionally
hit the far nrn lower elevation areas in this pattern. With snow
flying out west every day, there will likely be some impressive
snowfall amounts in the western mountains by this time next week.

&&

.AVIATION.../12Z ISSUANCE/

The main issue through 18Z will be patchy fog or stratocu mainly
over the far western valleys. Otherwise mainly SKC with light winds
through mid morning. This will quickly change this afternoon into
the early evening as a quick moving upper wave, and a reinforcing
cold front combine for increasing cloudiness and some snowfall. The
best areal coverage/duration of snowfall is expected over Sweetwater
County including RKS terminal where MVFR conditions are expected to
develop around 22Z and last through much of the period.  MVFR to Low
VFR ceilings are expected to develop across much of the area east of
the Divide shortly after sunset with some flurries/light snow
especially in favored upslope areas such as CPR/LND.

&&

.FIRE WEATHER...

Fire Danger low for the foreseeable future as cold unsettled
conditions and general troughiness aloft continue over the region,
with building strong high pressure at the surface will keep winds
generally on the light side and RH levels on the high side. Later
today and tonight, precipitation chances will increase across
southern WY near and north of a cold front and under the passage of
a quick moving upper level disturbance. Perhaps an inch or two of
snow will be possible in this otherwise rather dry regime. Next
significant chance for precipitation will occur Thursday evening
across the western mountains as a relatively moist Pacific system
heads into the region, beginning a period of snowfall that may last
for several days.

&&

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

&&

$$

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



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