Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 111707
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1007 AM MST Wed Jan 11 2017

.UPDATE...Cancelled Winter Weather Advisory for Natrona County.

.SHORT TERM...Wednesday through Friday night

Given the contrast in the conditions between the regions...we will
again divide the discussion between West and East of the Divide.

West of the Divide...More of the same this morning as moderate to
heavy snow continues to fall across the mountains with totals
continuing to pile up. However, there is some light at the end of
the tunnel finally. The models show the worst of the snow ending
this morning across the western mountains as the best upper level
forcing moves away to the east. However, light to moderate snow will
continue through much of the day with light snow lingering into
tonight as ridging finally moves into the area. Winds should also
gradually decrease tonight as the pressure gradient weakens and mid
level winds finally decrease. Meanwhile, the models are also
consistent in bring snow into much of Sweetwater County, including
the I-80 corridor this morning as the system shifts to the east.
All in all, the highlights still look good and we will continue
them as is for now.

There is now an interesting wrinkle for Thursday afternoon into
Thursday evening as a piece of energy moves northeastward into the
area out of the Great Basin. Both the NAM and GFS spread some
precipitation across Sweetwater County, although with different
placement. For now, we favor the GFS and have increase POPS across
Sweetwater County. This is a fast moving system though and will move
out of the area Thursday evening. Dry and chilly weather will then
dominate from Thursday night to Friday night as high pressure builds
in.

East of the Divide...There are some nasty conditions across northern
Wyoming as snow continues to fall with a good amount of wind with
the cold advection behind the cold front. It will continue to move
south through the morning. This will make high temperatures tricky
with the cold advection behind the front as many may occur in the
morning. In addition, we hoisted winter weather advisories across
Johnson and Natrona Counties. Although snowfall amounts may not reach
criteria, the combination of snow and blowing snow will make
traveling difficult. Both the NAM and GFS show a period of brief
period of upslope across southern Natrona. And this maximizes
around the morning rush hour in Casper. This snow should move out
this afternoon. Elsewhere, expect only light snow with little
accumulation. After the front passes, the wind should finally end
as well so we will likely cancel the warning for Natrona County
with issuance this morning and the others may be cancelled later
this morning. All snow should end by this evening with mainly dry
conditions through Thursday morning.

There is a small wrinkle for late Thursday and Thursday evening as a
bit of energy moves in from the southwest. The NAM does have a brief
period of snow moving toward southeastern Fremont County and into
Natrona County. The GFS keeps everything further south however. For
now, we kept this area mainly dry but it will have to be watched.
However, anything that does fall would be light since the system is
a fairly quick hitter and would be gone by late Thursday evening.

High pressure will then bring dry conditions from Thursday night
through Friday night. In addition, winds should remain light. The
question now is for high temperatures. In some of the areas more
prone to inversions, it will turn cold again. However, it should not
be as cold as past weeks. For one, the air mass is not as cold.
Also, in areas where new snow did not fall, there will be the
reduced albedo from older snow. So, it will be chilly but nothing
too bad for the middle of January.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

Synopsis...A storm system will lift out of the southern Rockies
into the central Plains Sunday and Monday.  West and Central Wyoming
will remain in the cool, dry air north of this system.  The next in
a series of Pacific storms will begin to push onto the west coast
Tuesday and into the Interior West on Wednesday.  Snow is expected
to spread back into western Wyoming on Wednesday with windy, dry and
mild conditions developing east of the Continental Divide.

Discussion...Both GFS and ECMWF have southwest upper low over the
Baja Peninsula Saturday morning, with the usual spread developing
lifting the upper low out into the southern/central high plains
Sunday and Monday.  Clouds and precipitation with this system will
stay well south and east of our area.  GFS and ECMWF have
differences with weak northern stream energy splitting into the
backside of this system or overshooting it across Canada.  Either
way, little if any precipitation is expected with this northern
stream energy with mainly some passing mid and high clouds.  Winds
will be generally light over the weekend allowing some cold air to
remain trapped in lower central basins and western valleys where
high temperatures will struggle to break the 20 degree mark, while
"banana belt" locations in the foothills see low to mid 30s by
Sunday.  Strong zonal cross-Pacific jet stream along 40-45N will
bring the first in the next series of storms onto the west coast
Tuesday.  Both GFS and ECMWF show at least the first major storm
digging southern California and the desert SW late in the week.
Beforehand, difluent westerly flow should bring the next shot of
significant snowfall and wind into the west on Wednesday in what
looks to be shaping up of another prolonged stormy period for
western Wyoming just beyond this extended period.

.AVIATION.../18Z ISSUANCE/

West of the divide...KBPI/KJAC/KPNA/KRKS Terminals

Periods of snow in the west through 09Z. KJAC airport will stay
in the snow through 06Z Thurs. MVFR to IFR conditions will occur
through the period. The other terminal sites in SW WY will see
improving conditions between 20Z and 23Z with VFR conditions
prevailing after 23Z. Expect a gusty wind at many terminal sites
through 02Z. Mountains will be frequently obscured. Please see
the individual terminal forecasts for more details.

East of the divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

In northern WY areas of snow will occur through 06Z with MVFR to
IFR conditions prevailing. Conditions will be improving in the
north after 06Z as the snow ends and cloud heights rise. Expect
VFR conditions in most places by 06Z Thurs. For the central
area...scattered light snow will continue through 22Z with MVFR
to IFR conditions occurring. KCPR terminal will see the snow from
18Z until 22Z with improving conditions toward 00Z. KRIW and KLND
should remain VFR, but some lower clouds could develop which
would lead to MVFR CIGS until 20Z. Mountain snow will continue
along the continental divide with some obscuration also expected
through the period. Please see the individual terminal forecasts
for more details.

.FIRE WEATHER...

Periods of snow will fall across much of the area today as a cold
front drops southward across the area, with the heaviest snow
falling in the morning. Temperatures East of the Divide will turn
colder behind the cold front. Wind will subside East of the Divide
by this morning and West of the Divide tonight as upper level winds
weaken. All in all, fire danger will remain low today.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon for
WYZ028>030.

Winter Storm Warning until 11 PM MST this evening for WYZ001>003-
012>015-023>027.

Winter Weather Advisory until 11 PM MST this evening for WYZ004-
016.

&&

$$

SHORT TERM...Braun
LONG TERM...AEM
AVIATION...C.Baker
FIRE WEATHER...Hattings



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