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Area forecast discussion 
National Weather Service Riverton Wyoming
242 PM MST Friday Nov 21 2014

Short term...tonight through Saturday

An interesting period. Relatively benign conditions expected
during the early evening hours tonight...with a significant winter
system expected to impact the area during the overnight hours.
Snowfall will begin before midnight...with activity and coverage
increasing through the overnight hours. Snowfall rates will
increase during the morning hours...especially between 0500 and
1100 Saturday. Snowfall associated with this system will continue
impact the area through Saturday...with activity spreading to
include portions of southern Wyoming during Saturday afternoon.

Snowfall accumulations will be significant across the western
mountains and valleys...with some lesser amounts expected across
southern Wyoming. Storm storm will continue into Saturday
night...with further details on total accumulations covered in the
long term section.

Long term...Saturday night through Friday
the shortwave ejected by the low spinning around in the Aleutians
will be crossing over our area around 06z Saturday night. After the
frontal passage...the snowfall intensity will begin to diminish in the west.
However...many areas of Sweetwater County should see 2 to 3 inches
of snowfall Saturday night from the combination of the asct strong
Pacific cold front and q vector forcing. The Wind River basin almost
always manages to receive at least just under an inch of snowfall
whenever central Wyoming receives the brunt of the 700mb cold punch
from a classic catafront like this one as will be the case Saturday
night...so increased probability of precipitation there as well as added corresponding snow
accumulations there. Johnson County is another area to watch for
more precipitation since that area will be closer to the 700mb low tracking
east southeast across North Dakota but the precipitation there will start
as rain so not expecting too much in the way of accumulating snow
there. Snow totals of 3 to 5 inches will be common in the Big Horn
Mountains

As far as snow totals go in the far west...by 12z Sunday morning we
will be looking at 12 to 20 inches in The Tetons...Gros Ventre
Range...salt and Wyoming ranges and along and west of The Divide in
the Wind River mountains along with local amounts of up to 26 inches. The
Jackson Valley can expect 8 to 12 inches with the highest amounts
expected in the north. The Star Valley will see 6 to 8 inches.
Southern Lincoln County will see 2 to 4 inches around Opal and 4 to
8 inches around Cokeville. Yellowstone Park will have snow totals
ranging from 16 inches in the southwest to 3 to 8 inches in the
north including Mammoth. The absarokas will see up to a foot in the
west to mainly 3 to 10 inches in the east. Areas just west of DuBois
will see 4 to 8 inches with 1 to 3 inches just to the east. The
Upper Green river foothills will see around 9 inches at Bondurant to
5 inches in Pinedale. The Upper Green river basin will see 3 to 6
inches.

Post frontal 7 to 8mb...3 hourly surface pressure rises will keep
Post frontal northwest winds up through middle day Saturday in the 30 to 40
miles per hour range with gusts to 50 miles per hour in the Wind River basin and northern
Johnson County. High temperatures Sunday should be 10 to 15 degrees
colder Sunday than on Saturday most areas. Occasional snowfall will
continue through Sunday and into Monday in the far west as a series
of vorticity lobes parades within the northwest flow with one distinct
shortwave coming through Monday. On Tuesday the jet stream will pass
overhead as it slides off to the northeast of US which will pull in
more moisture from the northwest continuing the light to
occasionally moderate snow in the northwest mountains except for some
continuing light snow in the Big Horn Mountains...areas east of The
Divide will dry out after Saturday night. For the rest of the
week... Tuesday night through Thursday...the GFS and the Euro have come
more in line with a high pressure ridge building in from the west
with a gradual warming trend. However...with the ridge axis still to
the west and continuing northwest flow...will keep low probability of precipitation in the northern
and central mountains for this time frame. Would rather wait for more run
to run model consistency before we would be willing to tone down the
mountain probability of precipitation any more than they already are...especially since this is a
sensitive time of the month given all the Holiday travelers.

&&

Aviation.../00z issuance/

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

VFR conditions expected across the area through the evening
hours...though periods of breezy wind will be found at kcpr and
areas southwest of the terminal. The approaching winter system
will cause wind shear over many of the terminals after 08z this
evening. Low level wind shear has been included at the majority of
terminals...except at kcpr. Upper level wind will still be strong
at kcpr as well...but with already strong surface wind
present...the change between speeds at The Levels is not as
significant. Snowfall will spread across the area through Saturday
afternoon...with most terminals seeing at least vcsh by 20z. Kcpr
is the exception with snowfall anticipated to begin after 00z
Sunday.

West of The Divide...kbpi/kjac/kpna/krks routes

A fairly messy aviation period ahead for areas west of the
Continental Divide. Snowfall associated with the approaching
winter system will being to impact the western mountains and
valleys after 03z this evening...with prevailing -sn anticipated
after 09z at kjac. Snowfall will intensify and increase in
coverage through the overnight hours...with most terminals seeing
prevailing snow by 14z Saturday. Additionally...strong wind shear
will be present at the terminals...especially after 09z this
evening. Expect increased turbulence over the western aviation
passes...and also mountain obscuration with low clouds and ongoing
snowfall.

&&

Fire weather...

A significant winter weather system will begin impacting western
Wyoming this evening...and snowfall spreading to include much of
the forecast area by Saturday afternoon. Winter weather highlights
area in effect. Additionally...strong wind will develop across the
wind corridor between Rock Springs and Casper...and will also
develop in the Cody foothills. High wind warnings are in effect
for much of the day Saturday for these locations. Fire weather
behavior is expected to remain minimal under this system...with
the best dispersion rates occurring during the storm. Please see
the appropriate weather highlight product for additional
information regarding the winter storm and high wind
warnings...and the Winter Weather Advisory.

&&

Riw watches/warnings/advisories...
High Wind Warning from 6 am Saturday to midnight MST Saturday
night wyz003-019-020-022.

Winter Weather Advisory from 3 am Saturday to 6 am MST Sunday
wyz016-026-027.

Winter Storm Warning from midnight tonight to 6 am MST Sunday
wyz001-002-012>015-023>025.

&&

$$

Short term...branham
long term...lipson
aviation...branham
fire weather...branham

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