Area Forecast Discussion
Issued by NWS Riverton, WY

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671
FXUS65 KRIW 160845
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
145 AM MST Sat Dec 16 2017

.SHORT TERM...(Today through Monday night) Issued at 138 AM MST
Sat Dec 16 2017

Light patchy snowfall is showing up in the northern and western
zones as the next upstream trough axis begins to make landfall.
West Yellowstone is now down to 2.5 miles in light snow.
Ahead of this trough, a mid level circulation will track from
Northwest Wyoming today and then will drift southeast and set up
shop over Southeast Fremont County tonight where we could see
localized exaggerated snowfall amounts in the Green Mtns tonight,
potentially including Jeffrey City. The energy from this weather
system will split as it approaches today and tonight which will
keep snowfall amounts in the low end advisory range on both sides
of the divide. The northern stream energy will eject a Montanian
cold front toward Wyoming today with the southern stream energy
helping to pull the cold air south resulting in frontogenesis
today. The southern stream energy will eventually reach way down
to the Desert Southwest tonight. The transition from the bulk of
the snowfall falling over areas west of the divide to east of the
divide should occur between 12Z and 18Z today as the frontogenesis
and low level upslope flow become established east of the divide.

Then once this weather system exits off to the east and the snowfall
exits off to the southeast tonight, a fast west northwest flow will
quickly resume within the rapidly tightening gradient. West
southwest winds will increase from RKS to CPR beginning Sunday
afternoon, along with strong west winds at the higher elevations. A
sfc lee side trough will become established Sunday night coupled
with right front quad dynamics, so Sunday night should be especially
windy. A weak shortwave trough within the fast west northwest flow
will produce some patchy light snowfall in the western and northern
mtns Monday and Monday night. These winds should continue through at
least Monday night. A slight warm up is expected early next week.

.LONG TERM...(Tuesday through Saturday) Issued at 138 AM MST Sat
Dec 16 2017

Shortwave ridging to start the period before things begin to take
a dramatic change. Some light upglide snow may fall in the nw
mtns on Tuesday but otherwise, it`ll be a dry day. Things start to
change Wednesday as cold system over the Bering Sea moves into
wrn Wyo. Models vary some on the track (between wrn UT and wrn WY)
but they all point towards a sewd moving fairly strong trough
swinging se across our forecast area Wednesday through about
Thursday morning with a period of snow for much of the area.
Digging/splitting system will likely limit the amounts out west
with generally sub-advisory amounts for most areas. East of the
divide, the system may briefly close off to our sw enhancing the
snowfall for a few hours. 00z GFS is the most open with the system
with a quick shot of mainly sub- advisory amount type snowfall
except perhaps around Casper. The GEM and Euro are showing the
potential for a brief closed low with a better chance of some
advisory amounts in central WY Wednesday night into Thursday
morning. Mean ridge strengthens out near 140W so pattern
definitely supports this and its just a matter of the details.
Snow for sure with amounts determined a little further down the
road. Cold to bitter cold behind this system Friday morning with
sub-zero readings in the cold, clear, light wind areas. A very
cold system will then drive straight south out of Canada Friday
night and spread bitter cold air across us through Saturday with a
period of light snow with frontal passage. Models showings
1040-1045mb surface high setting over us later Saturday leading to
an even colder Saturday night.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning) Issued
at 138 AM MST Sat Dec 16 2017

A cold front will push widespread MVFR, areas of IFR, ceilings
and visibilities in -SN, from northwest to southeast across the
area this morning, with these conditions expected to prevail
vicinity and north of a KEMM-KLND-KCPR line by 18z Saturday. Areas
further south are expected to gradually fill in with areas of
MVFR-IFR -SN in the late morning and afternoon, including KRKS.
Conditions will then gradually improve from northwest to southeast
03z-12z Sunday; however, areas of fog or lower LIFR/IFR ceilings
will likely linger over the basins and valleys drier air moves in
aloft.

&&

.FIRE WEATHER...
Issued AT 138 AM MST Sat Dec 16 2017

A Pacific low pressure trough moving south and east from British
Columbia and the Pacific northwest will spread snow into the
western mountains late this evening. The heaviest snow is expected
in the Tetons. East of the Continental Divide windy areas are
expected along and near the east slopes of the mountains as clouds
spread east combining for warmer low temperatures. Snow will
continue across the western mountains through Saturday. The trough
and a Canadian cold front will move into the north along and east
of the Continental Divide by early morning with snow and areas of
blowing snow spreading south into central and east central
Wyoming through the morning Saturday. Snow will continue Saturday
night tapering off late. Temperatures will cool significantly
Sunday and Monday. Clearing will occur Sunday and Sunday night
with some fog possible in the valleys. A weaker Pacific trough
will bring areas of snow to the northwest and west on Monday.
Windy areas will again develop along and near the east slopes of
the mountains with warming in the foothills.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 AM early this morning to 5 AM MST
Sunday for WYZ002-003-005>011-014-015-017>020-022-025.

Winter Weather Advisory until 5 PM MST this afternoon for WYZ012.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Meunier
FIRE WEATHER...Lipson



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