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Area forecast discussion 
National Weather Service Riverton Wyoming
310 am MST sun Dec 21 2014

Short through Tuesday night

An active period across western and central Wyoming. Snowfall will
be abundant across the area...especially across the western valleys
and mountains where snowfall will be heavy at times. Some areas
seeing heavier snowfall will also see increased wind...creating a
hazardous combination of snow and blowing snow. Winter storm
warnings are currently in effect...and will remain through Monday
morning for some locations. Snow totals by Monday morning could be
up to 2 feet across the western mountains with 5 to 10 inches
possible across the western valleys.

As this push of moisture moves through will bring an
increased gradient across the forecast area. High winds are
expected across the Interstate 80 corridor and the Green Mountains
and rattlesnake range this morning...and will continue through the
late evening hours. High wind warnings have been issued for these

The northwest flow associated with this pattern is also favorable to
creating strong wind across the Wind River basin. Model guidance
suggests an increased probability of stronger wind developing late
tonight through Monday morning...with a high wind watch issued.
Similarly...a high wind watch has been issued for northern Johnson
County through Monday afternoon.

Conditions will start to improve Monday evening across the area as
high pressure builds across the area. Some lingering snowfall will
be found across the eastern third of the forecast area...but is
expected to have generally moved out of the area by Tuesday evening.
Some additional moisture in the ridge may create some light snowfall
across the western mountains Wednesday morning...though model
variation decreases confidence in this development for now.

Long term...Wednesday through Sunday

Wednesday looks dry for most places with breezy areas. Some moisture
moves into northwest Wyoming in the afternoon for some light snow developing
in the mountains. The GFS/European model (ecmwf) have gone drier for Wednesday in the
west and if this trend continues then precipitation chances will Need
More trimming. The incoming weather system impacts the west and
north Wednesday night. Then the system moves through the rest of the
area Christmas day. Decent cold frontal passage will put a chill
in the air along with some light snow. Snow amounts look to be 1
to 4 inches for the lower elevations with locally higher amounts
possible. Expect drifting snow in open areas. Still have a few
days to watch the track of this weather system and refine snowfall
amounts. The light snow ends late Christmas night in most places.
Friday looks cold with mountain light snow showers with not much
if any snow showers at the lower elevations. The GFS and European model (ecmwf)
models differ with the weather pattern Saturday and Sunday. The
GFS brings another weather system into the north Saturday night
along with another cold front. Then this cold air boundary sags
south next Sunday into central Wyoming. The European model (ecmwf) model does not show
this weather system at all Saturday night and Sunday. Thus the
European model (ecmwf) is dry Saturday and Sunday. Have leaned toward the drier
European model (ecmwf) model but have left low chance of snow in the western and
northern mountains for now. As for temperatures...Christmas evening will be
slightly milder...then colder Christmas day and Friday. Slightly
milder Saturday and Sunday.


Aviation.../12z issuance/

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

A trough of low pressure will move across Montana on Sunday and into
the northern plains Sunday evening. A strong jet stream 120-130kts
will cross the Wyoming Divide south of this system this morning
causing areas of MDT-severe turbulence...low level wind shear over shallow Central
Basin inversions...and strong surface winds from vicinity South Pass to
vicinity kcpr and south along the Interstate 80 corridor. The Lee
slopes along The Divide and Bighorn range will likely remain mostly
obscure above 9kft. A strong northwest flow will follow this system Sunday
night with isolated-scattered MVFR/IFR snow showers possible along the front
across the lower elevations Sunday evening...especially vicinity and
east of Interstate 25. Drier air prevail northwest-southeast but gusty northwest surface
winds will remain after 06z Monday.

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

Widespread IFR...areas of LIFR conditions in snow/blsn with mountain
obscured are expected vicinity and north of a kevw-20sm SW klnd line
through 0z Monday. Strong Mountain top winds will also result in areas
MDT-severe turbulence and low level wind shear. A cold front will push south across
the area Sunday afternoon...and across Interstate 80 00z-03z
Sunday. The front will spread numerous MVFR/IFR -shsn south along
I-80...vicinity krks...through Sunday evening. Drier air and improving
conditions will prevail behind the front from northwest-southeast 06z-12z Monday.


Fire weather...

Ongoing wintry conditions will keep snowfall and areas of strong
wind across western and southern Wyoming today. Wind is expected to
increase across central and northeastern Wyoming later tonight with
potentially strong wind lingering through Monday afternoon. Winter
storm and high wind warnings are currently in effect...along with a
couple areas in a high wind watch.


Riw watches/warnings/advisories...
High Wind Warning until 2 am MST Monday wyz027>030.

High Wind Warning until 11 PM MST this evening wyz019.

Winter Storm Warning until 11 am MST Monday wyz002-014-015-025.

Winter Storm Warning until 8 am MST Monday

High wind watch from this evening through Monday afternoon

High wind watch from this evening through Monday morning

High Wind Warning until 11 am MST this morning wyz003.



Short term...branham
long term...Arkansas
fire weather...branham

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