Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 071134

Area Forecast Discussion
National Weather Service Riverton WY
434 AM MST Wed Dec 7 2016


Steady light snow is falling across much of western and central
Wyoming on both sides of the Continental Divide. enhanced snowfall
is occurring along upslope favored areas, namely the Lander
Foothills and along the adjacent East Slope of the Wind River
Mountains behind the sfc cold front that slipped through earlier
this evening. A vort max tracking east in northern Wyoming, and
another tracking east from southeast Idaho into western and
central Wy as well as southern stream energy from the system
delivering snow to southern Wyoming and Colorado are all
contributing to this quick lived snow event along with a fair
share of QG forcing. Temperatures are below the ideal range for
favorable dendritic growth so the snowfall has been fine with a
low water content, so it is slow to accumulate but very effective
in lowering visibility. 2 to 4 inches are expected in the
mountains and an inch or less elsewhere with most of the snow
ending from northwest to southeast later this morning. Dry and
cold weather can be expected for the rest of the day today. A
temperature inversion will become established tonight and milder
air aloft advects then in from the west southwest. Expect patchy
fog to form tonight in the basins and valleys late tonight and
Thursday morning.

Then our attention turns to the strengthening Pacific westerly
flow along with strong isentropic lift that will result in the
onslaught of a period of steady snow beginning around mid
afternoon Thursday in the far west. The snowfall out west will
increase in intensity Thursday night as the upslope component for
western Wy increases as h7 winds increase to 50 knots by Friday
morning from the influence of the jet max aloft. Steering winds
will slowly back from WNW to WSW through just after 12z Saturday
morning before the next open troughpa Saturday morning. The
snowfall will be fairly widespread across the west Thursday night
but will then favor upslope areas of the west Friday as we lose
the isentropic lift component. Then Friday night, snow, heavy at
times continues over much of the west with more isentropic lift
helping out once again. West southwest winds will increase from
South Pass to Casper during the duration of the storm. Expect
strong winds to accompany the snow event with white out
conditions quite possible along South Pass. East of the divide,
temps will break freezing Friday along the southern wind corridor.
One side note, Natrona County will likely see blowing snow from
the 7 inches of snow that fell there Monday by Friday morning
before temps break freezing in the afternoon.


Winter storm will likely be continuing out west through at least the
morning hours Saturday as strong shortwave moves across the area.
Snow will likely continue though into the afternoon with cold, nw
flow especially around the Tetons and other nw flow favored areas.
Jet will also be in favored location just south of the area. GFS/GEM
continue to be stronger and faster with this shortwave Saturday.
GFS/GEM would give a lot better chance of snow showers east of the
divide Saturday than the Euro and forecast is siding that way.
Northwest flow will likely remain the remainder of the medium range
period with a persistent upglide pattern keeping occasional snow
going out west. Timing of shortwaves passing off to the north and
into the west in this flow are quite a bit different in each model
so low confidence on this. Forecast is siding towards a blend with
the Canadian/Arctic front returning to the north Tuesday with cold
advection continuing into Wednesday for steadily falling temps for
the north half. Will still have to watch energy coming in from the
west and overrunning this cold dome for some occasional snow in the
north in this pattern. The far south will likely see some snow
showers as individual shortwaves pass across the area early next



Ongoing MVFR and some IFR conditions in snow and low ceilings early
this morning will improve from northwest to southeast this morning.
The slowest terminals to improve will be CPR, LND and RKS terminals,
but even those terminals should see ceilings lift to VFR albeit LVFR
between 17Z and 20Z. Light snow/flurries will end between 15Z and
18Z with dry conditions expected for the remainder of TAF period.
Winds will be mainly light through the forecast period. This evening
skies are expected to become mostly clear with the main concern
being the potential for fog development at most valley locations
since boundary layer moisture will be high, light winds, and some
warming aloft allowing surface inversions to strengthen.



Light snow or flurries will end later this morning. Dry and cold
weather will occur this afternoon through Thursday morning. A
temperature inversion along with valley and basin fog is expected
for late tonight and Thursday morning. A potential for
significant snowfall is slated for far Western Wyoming from late
Thursday afternoon through Saturday morning. Temperatures will
moderate toward the end of the week. West southwest winds will
increase toward the end of the week, especially along the east
slope of the Continental Divide and along the southern wind
corridor from South Pass to Casper. Very low mixing heights with
light to calm winds will produce poor smoke dispersal through
Thursday morning.


Winter Storm Watch from Thursday afternoon through Saturday
morning for WYZ001-002-012>014-023>025.



LONG TERM...Skrbac
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