Area Forecast Discussion
Issued by NWS Riverton, WY

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477
FXUS65 KRIW 041753
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1053 AM MST Sun Dec 4 2016

.PREV DISCUSSION... /issued 245 AM MST Sun Dec 4 2016/

SHORT TERM...Today through Tuesday night

Imagery shows deep trof across central to west central CONUS with
modest ridging over the far west and WY under NW flow aloft. Main
trof axis stretches from central Canada down through the
high/central Plains to northern Mexico where a nearly COL resides in
the base of the trof. 100 kt plus jet located from northern WY to
the PAC NW. Strong upstream shortwave trof coming into BC/northern
WA...due to hit the WY area tonight. Modest moisture now advecting
through the PAC NW and starting to pile up into NW WY. Light areas
of snowfall across portions of west central to northwestern WY
associated with this increasingly moist flow. SFC has high pressure
across the Great Basin into southwest WY with a N/S trof located
through the northern and central Plains. Decent P GRAD keeping gusty
(at times) wind around the area east of the Divide.

Today, the upper NW flow starts to flatten and will become nearly
zonal by tonight in response to the approaching (now Gulf of AK)
trof and associated strong 100+ kt jet. This will also allow
additional EPAC moisture back into the western/northwestern FA along
with increasingly good upper level forcing and a significant
increase in precip chances across the western zones...especially
from later this afternoon into Monday morning. Warning amounts of
snow now look likely over the Teton...Gros Ventre mountains and
across portions of YNP and the Jackson Valley. A Winter Storm
Warning is now in effect for those previously mentioned areas.
Advisory amounts of mountain/valley snow will surround this
area...including the Salt, Wyoming, Wind River and Absaroka
mountains in addition to the the Upper Green River foothills. Also,
both the surface pressure and wind fields will respond to the
increasing favorable upper level flow pattern as mid/upper wind flow
becomes nearly normal to the mountain ranges and the surface P GRAD
tightens as lee troffing deepens across central WY. Breezy to
occasionally strong winds are expected over some of the FA`s
foothills and the wind corridor this afternoon and evening. SPSs
address these winds.

Monday, the main upper trof axis and strong cold front (Arctic) will
move through the FA...with snow chances finally moving east of the
Divide...albeit on the light side. Then, later overnight Monday/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 20 below in the western mountains to the single
digits to near 10 deg in the 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 or
flurries will continue periodically. Very cold daytime and overnight
temperatures with Wednesday morning bottoming out as much as 25
below west of the Divide and over the Bighorn mountains.

LONG TERM...Wednesday through Sunday

The polar express remains parked over the region Wednesday as high
temps will struggle to warm up with those 700MB temps around -20
degrees C. It looks to be dry Wednesday all areas. Another cold
night Wed night as a mostly clear sky will allow decent radiational
cooling. The GFS has warming 700MB temps late Wed night while the
ECMWF model is still very cold. If the GFS comes true, then temps
will rise after midnight in some areas. If the ECMWF is correct,
then a quite cold night will occur. This has been the scenario
for 4 nights now. I will lean toward the colder solution but not
as cold as Tue night. Plenty of time to watch this. The onset of
precip in the west with the warm advection pattern starts Thursday
afternoon and continues into Friday. The GFS is faster tonight
with the next weather system and cold front, as it has it passing
through the region Friday afternoon. The ECMWF model is Friday
night with the cold front. Dry east of the divide Thursday and
Thursday night with breezy to windy areas. High temps will
moderate Thurs in all areas, but some of the colder valleys will
not warm up as much as the inversion will hold. It is Friday when
the inversion should break everywhere and milder highs all areas.
Will add a precip chance to southern WY Friday afternoon ahead of
the front. Fri night looks to have the cold front pass by and a
chance of some snow in most places along with brisk wind. Some
snow showers in the mountains on Saturday in the NW flow aloft.
Cannot rule out some snow showers in the lower elevations Saturday.
A little cooler high temps Saturday for the region. Next Sunday
will see some light snow in the far west once again and mostly dry
elsewhere. High temps will be near seasonal values.

AVIATION.../18Z ISSUANCE/

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

Areas of LLWS are in place ahead of an approaching trough/cold
front. All areas, except KLND, will weaken by 06z Monday. Surface
southwest wind at 20-30 kt with gusts 40-45 kt will prevail from
South Pass- 50SM NE KRKS- KCPR 12z Sunday-06z Monday.

Also, peaks above 9kft along the Continental Divide will be mostly
obscured in -sn through 12z Monday.

A cold front will plunge south through KCOD-KBYG around 07z
Monday and near a KRIW-KCPR line by 12z Monday. Areas of MVFR/IFR
-sn and gusty north wind will follow the cold front.

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

Snow, widespread IFR conditions and mountain obscurations are now
in place, except near KBPI and KRKS. MVFR and near IFR conditions
will spread around KBPI this afternoon, and reach KRKS by 06z
Monday. Ridgetop winds continue to increase, with LLWS continuing
to spread southward across the area.

IFR conditions will prevail vicinity and north of a KAFO-KPNA line
through 06z Monday, with a cold front driving areas snow further
south and east along the Interstate 80 corridor 06z-12z Monday.

LLWS will subside behind the cold frontal passage, with
precipitation becoming more showery along with some improvement in
ceilings and visibility vicinity and north of a KJAC-KDUB line by
12z Monday Conditions will continue to improve to VFR conditions
between 12z-18z Monday.

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 occur
across western WY through today and tonight before shifting east of
the Divide later Monday. The mountains of the west may see up to a
two feet of snow locally with 3 to 10 inches possible in the western
valleys. The lower elevations east of the Divide will only see a
trace to an inch with some foothill areas perhaps getting a couple
of inches through the day Monday. Otherwise increasingly gusty,
sometimes strong, winds will be the only real impact to fire weather
as southwest wind of 10 to 20 mph gusting 30 to 40 mph today grows
to gusts of 40 to 60 mph at times today and tonight over the western
mountains...Cody foothills...and across the wind corridor from
eastern Sweetwater, through eastern Fremont and across Natrona
counties. Smoke dispersion will range from poor this morning and
tonight to fair/good for a few hours this afternoon over northwest
WY and portions of south central WY. Monday will see widespread
good to excellent dispersion...mostly due to brisk winds across the
forecast area.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MST Monday for WYZ002-014-015-
023>025.

Winter Storm Warning until noon MST Monday for WYZ001-012-013.

&&

$$

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



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