Area Forecast Discussion
Issued by NWS Riverton, WY

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886
FXUS65 KRIW 051752
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1052 AM MST Mon Dec 5 2016

.PREV DISCUSSION... /issued 829 AM MST Mon Dec 5 2016/

UPDATE...dropped winter storm warnings and advisories across the
west as snow has ended and wind has decreased.  issued winter
weather advisory until 4 mp mst for natrona and southern fremont
counties.  continued advisories in the bighorn mountains eastern
johnson county...hot springs and washakie counties until noon.

SHORT TERM...Today through Wednesday night

Updated to cancel the winter storm warning for the northwest. Snow
has finally decreased or ended. There is still some strong wind
at peak level but that can be handled by an sps.

Imagery shows longwave trof across most of the CONUS this morning
with nearly COL still over nrn Mexico. Embedded shortwave and nose
of jet just starting to enter nwrn WY but with dry air entraining
into nwrn/wrn WY too. The upper flow is nearly zonal and moisture on
nose of jet is now into ncntrl WY and scntrl MT with another area of
moisture pushing into/through swrn WY. Light to moderate snow is
occurring roughly between these two areas ATTM...with embedded
isolated stronger snow showers...at the interface of the cold dry
air with that of the warmer moist...coming into the Teton area.  SFC
has developing low pressure from ern MT to nern WY with a cold front
extending from nern MT into the nwrn corner of WY and a SFC trof
from ern MT into ern WY. High pressure across the srn Rockies is
keeping the SFC P GRAD tight over srn to scntrl WY...along with
gusty, sometimes strong, SW winds.

Dry very cold air is currently starting to work its way into the FA
from the north and west...with lighter areas of snow and flurries
replacing the heavier snow that the region had earlier. As upper
level cold air snow showers are breaking out over portions of west
central WY as described above, especially over/near the Tetons, will
keep the WSW going for now...but it may have to be taken down later
this morning before it is scheduled to expire as the dry Arctic air
continues into the region...pushing southward slowly as it does.
Timing would seem to put this somewhere between 3am and 5am.
Otherwise, strong winds have mostly cutoff over the northern zones
as a cold front begins to move through...with breezy/gusty to
occasionally strong winds drifting south over portions of southern
to central WY. Strongest winds currently located across zone 19 from
srn Fremont county into western Natrona...with slightly less wind to
the south of this area.

Today, the main upper trof axis and strong cold front (Arctic) will
move through the FA...with snow chances finally moving through
the rest of the area east of the Divide...albeit chances look to be
rather light for the most part. Current WWAs are mostly impact
based for the morning commute with most areas not likely to see
Advisory amounts of snow...but when combines with potentially gusty
SFC winds could cause hazardous driving conditions.

Later overnight tonight/by sunrise Tuesday, post frontal very cold
temperatures will be left along with a few flurries or ice crystals.
Lows Tuesday morning will range from around 25 below in the western
mountains to the single digits in the and lower elevations basins
east of the Divide. Tuesday will be cold post frontal with flurries
around but skies trying to clear out except near the southern border
region which will reside just on the northern side of the cold front
and where small chances for snow will continue to exist. With rather
flat NW flow continuing over the rest of the FA...small chances for
light western mountain snow showers or flurries will continue
periodically through the end of the forecast. Very cold daytime and
overnight temperatures will also occur with Wednesday morning
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 may 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.

LONG TERM...Thursday Through Monday

Cold nw flow starts the medium range period with cold air remaining
entrenched east of the divide. West of the divide, flat ridge and
warm advection begins moving in which will result in another
significant snowstorm for the far west Thursday into Friday. Strong
isentropic upglide develops quickly on Thursday for increasing
snowfall that will likely continue into Friday. Flow flattens
quickly on Friday as system approaching the PacNW coast sends out a
series of shortwaves. Strengthening wly flow and jet streak dynamics
will then come into play later Friday into Saturday with
considerable blowing snow from all the fresh snow that will have
fallen across the west. As our cold surface high pushes off to the e
the increasing wly flow and developing lee side low will result in
increasing wly surface winds east of the divide for most areas east
of the divide Friday. The front may try and linger over the nern
corner into Friday. Gradient will likely be strong enough to mix out
any basin inversions, especially for areas that don`t have much snow
cover. A quick swath of snow showers will likely impact some areas
east of the divide with these shortwaves at some point later Friday
into early Saturday, mainly across Sweetwater County and in the far
north. Favorable wly flow continues out west the remainder of the
weekend and into early next week with persistent to off and on snow,
especially with the jet near or just south of the area. Plenty of
snow will pile up in the west during this period while the east
remains mostly dry with periods of gusty to strong wind but not as
cold.

AVIATION.../18Z ISSUANCE/

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

Improving MVFR-VFR conditions across all locations except KCPR.
Expect IFR conditions to continue through 20z, as the last bout of
snow clears the area. However, MVFR ceilings will remain through
03z Tuesday. Mountain obscurations will continue through the
period. Gusty northwest winds will occur this afternoon,
especially vcnty KDUB- KRIW- KCPR. Clear skies or possibly some
thin, mainly translucent lower clouds LCL MVFR ceilings, will
prevail 06z- 12z Tuesday along with diminishing wind.

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

Some clouds will linger behind the front this afternoon, partially
obscuring mountains above 8500 feet. Otherwise clearing skies and
gusty west to northwest winds will be in place this afternoon.
Surface high pressure will build into western Wyoming this evening
with winds diminishing and mostly clear skies.

FIRE WEATHER...

Fire Danger low for the foreseeable future as cold unsettled
conditions and general troughiness continue over the region, keeping
RH levels on the high side. Precipitation chances will mainly begin
shifting east of the Divide Today. Some mountains of the west have
seen up to 18 inches of snow already and may see up to two feet of
snow locally before all is said and done. 3 to 10 inches possible in
the western valleys. The lower elevations east of the Divide will
only see a trace to an inch for most areas with some foothill areas
and portions of Johnson county perhaps getting 1 to 3 inches through
the rest of today. Gusty, sometimes strong, southwest winds ahead of
a strong cold front will drift across south central to southern WY
and continue to blow 10 to 20 mph gusting 30 to 40 mph with a few
favored locations between South Pass and western Natrona county
seeing gusts of 40 to 60 mph at times.  Monday will see widespread
good to excellent smoke dispersion by this afternoon...mostly due to
brisk west to northwest winds across the forecast area.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MST today for WYZ005>011.

Winter Weather Advisory until 4 PM MST this afternoon for WYZ019-
020.

&&

$$

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



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