Area Forecast Discussion
Issued by NWS Riverton, WY

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867
FXUS65 KRIW 100844
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
144 AM MST Sat Dec 10 2016

.SHORT TERM...Today through Monday night

Zonal flow pattern returns over the forecast area, as shortwave
ridge slides eastward over the High Plains. A new jet max is
establishing itself in this flow, as evidenced by WV/IR imagery
and extending into the eastern Pacific. The arctic airmass
remained in placed east of the Divide Friday and continues to do
so this morning. Wind prone locations, from South Pass eastward to
southern Natrona County mixed out with gusty SW winds Friday and
continue to blow this morning. Oddly enough, the arctic air has
not eroded along US 20/26 in central Natrona County to Casper yet.
The wind finally made it to southern portions of Casper after 06z
this morning and caused the temperature to jump 30 degrees in 10
minutes. This wind is expected to finally reach the rest of the
county by 10z this morning.

Periods of moderate to heavy snow is expected across the west
today, as a 110 to 120kt jet max moves over the area. At the same
time, WAA will finally erode the arctic airmass out of central and
northern portions east of the Divide. Snowfall totals remain on
track today, with additional amounts of 3 to 6 inches in the
western valleys and 5 to 10 inches in the mountains. Snow will
continue tonight into Sunday morning, but only 1 to 2 inches is
expected in the valleys and 2 to 4 inches in the mountains.
Temperatures will rise into the middle 30s in the Green River
Basin for a few hours this afternoon, which will allow for a
rain/snow mix. This brief rise will be in response to strong 35-50
kt WSW flow off the Salt/Wyoming Ranges, generating downsloping
winds into the Basin.

A cold front associated with this system will quickly pass through
the forecast area tonight, entering northern portions shortly
after 00z Sunday. The boundary will clear much of the state by
12z. Due to this speed and the apparent lack of moisture, have
removed much of the isolated PoPs east of the Divide. As drier air
infiltrates the forecast area behind the front, moist upslope
flow will continue across the west. Persistent snow showers will
be in place Sunday afternoon through Monday night. Additional
warnings and advisories might be need for the Tetons, Gros Ventres
and the western valleys.

.LONG TERM...Tuesday through Saturday

An Arctic airmass will already be in place Tuesday. A colder shot of
Arctic air is expected to push south into the area in the Thursday
and Friday time frame as a 1040 MB surface settles into the area
with 700 MB temps between -20C and -25C on Saturday. If this is
correct high temps on Saturday will struggle to get above 0F across
central and northern Wyoming. Although the bitter cold/well below
average temperatures especially east of the Divide are not in
question, timing/location/intensity of snowfall in the extended
remains a challenge.

On Tuesday it looks like most of the snowfall will be across the
southwest/southern sections of the area spreading east and northeast
possibly impacting much of the area Tuesday night into Wednesday as
some isentropic lift and right entrance upper jet dynamics
overspread the forecast area. Some areas mainly across the
southwest/south including the interstate 80 corridor could see some
significant snowfall. However confidence is not real high at the
moment. In wake of the secondary blast of Arctic air, light to
moderate snowfall is expected to impact much of the area in the
Thursday thru Saturday time frame, but not continuously. Global
models have various scenarios, but have trended with the heaviest
snowfall farther south and east of the area compared to last night,
but the set up still remains favorable for -SN at times in the Arctic
airmass.

&&

.AVIATION.../12Z ISSUANCE/

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

VFR conditions are expected through 00Z. A gusty southwest
wind will occur at KCPR and this will continue into this
evening. Isolated snow showers will occur in the north today.
A cold front pushes into northern WY after 00Z with a wind
shift to the north or northwest. This wind shift will reach
central WY by midnight tonight. Scattered light snow
will develop over the north this evening with local MVFR
conditions and this snow may move into central WY after 07Z
Sunday.

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

Snow will occur in the west through 00Z. MVFR to IFR
conditions will prevail with local LIFR conditions. Gradual
improvement will occur in the west through after 00Z as the
snow decreases and ceilings and visibilities rise. KRKS will
remain VFR until about 00Z, then MVFR to IFR conditions
will develop there as snow moves into that terminal site.
The snow is expected to linger through 12Z Sunday in southern
WY including the KRKS airport.

&&

.FIRE WEATHER...

Fire Danger low for the foreseeable future as cold unsettled
conditions and general troughiness aloft continue over the region.
Several upper level disturbances will bring significant snowfall
west of the Divide through this weekend. Strong wind may be possible
across portions of central Wyoming late through this afternoon.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 PM MST this evening for WYZ019-020.

Winter Weather Advisory until 6 PM MST this evening for WYZ016-
026.

Winter Storm Warning until 6 PM MST this evening for WYZ001-002-
012>015-023>025-027.

&&

$$

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



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