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Area forecast discussion 
National Weather Service Riverton Wyoming
220 PM MST Monday Dec 22 2014

Short term...tonight through Tuesday

As the main trough moves east and undergoes rapid cyclogenesis in
the plains there will still be a few hours of steep lapse rates as
the cold pool settles at the back edge of the trough...followed by
some wrap-around moisture and short wave that will bring another
round of light snow to the eastern third of the forecast area
tonight. It will also be accompanied by some strong northerly wind
with gusts over 50 miles per hour expected around Buffalo this
evening...elsewhere expect steady 15 to 25 miles per hour winds across much of
the area between sunset and midnight with wind and snow tapering off

On Tuesday...high pressure builds into the west with another snow
shower or two possible along the i25 corridor and in the bighorns.
Elsewhere...clearing and cool with decreasing winds.

Long term...Tuesday night through Monday

A transient ridge builds in for Tuesday night for dry weather
everywhere. Then on Wednesday...the next upstream shortwave trough
will approach the Pacific coast. As a result...light snow will break
out in northwest Wyoming. As the trough approaches...the gradient
will retighten and a resultant surface Lee side trough will develop
with southwest winds increasing from the Green Mountains to Casper. The
trough will then dig over the Great Basin Wednesday night with the
west southwest difluent upslope flow increasing in the far west out
ahead of the approaching trough for increasing snowfall in the far
west. 12z Thursday is where the GFS and the European model (ecmwf) begin to diverge.
The GFS closes the trough over southern Idaho and tracks it east
across southern Wyoming...roughly along the northern Sweetwater
County border during the day Thursday. This is a great track for
widespread snowfall across the entire County Warning Area Christmas day given the
favorable position of the left front quadrant of the jet along with
ample q vector forcing and weak isentropic lift from the north. The
surface prognosticated by the GFS...will form over southeast Colorado and
provide the low level upslope flow and resultant snowfall with the
help from the seeder feeder action from the clouds above...linked to
the aforementioned upper air dynamics. This track is farther south
then the northern Wyoming Route of the previous GFS run. The Euro
keeps the trough open and focuses most of the energy to the south
over The Four Corners area. Will keep likely probability of precipitation everywhere Thursday for
now. Snowfall will slowly decrease Thursday night as the trough
moves east. Except for isolated mountain snow showers on Friday due to a
vorticity lobe in the northwest flow on the backside of the main trough...expect
mainly dry conditions. On Saturday the flow will back to westerly
again resulting in increasing upslope snowfall for the far west in
response to a trough moving onshore to the Pacific northwest. The
snowfall will intensify Saturday night as the trough digs over the
Pacific northwest. For Sunday and Monday...the GFS and the Euro are
surprisingly similar in carving out the trough over the Great Basin
and attempting to close it off over the vicinity of slc. The GFS places
the 700 mb low right over central Wyoming Sunday. Have high chance probability of precipitation over the much
of the County Warning Area Sunday and Sunday night. This will be a colder system
than the Christmas storm with -20c 700 mb temperatures being pulled southwest
from the Hudson Bay area behind the asct cold front Sunday nt. Temperatures
will warm Wednesday with above freezing temperatures in many areas east of The
Divide ahead of the snow event on Christmas evening. Then expect
seasonably cold temperatures through Sunday. Behind and after the cold
system Sunday night...many areas will struggle to get above the
teens Monday.


Aviation.../00z issuance/

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

Areas of gusty northwest wind will remain...especially vicinity
kdub-kriw and kbyg...through about 12z Tuesday with decreasing winds
thereafter. -Shsn activity will remain mostly isolated-widely scattered with
more numerous shsn obscuring the surrounding mountains through 03z. Some
wraparound moisture is expected to push back from an upper low over
the plains into eastern Wyoming tonight...and this may bring more
widespread MVFR/IFR -sn as far west as the Bighorn range to vicinity addition to the gusty 15 to 30kt winds...gusts over 45kt
expected near byg...could also result in areas of blsn.

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

An upper level disturbance will dive south along The Divide early in
the period...exiting south of Interstate 80 this evening. Snow
shower activity will eventually wane...improving conditions
following the disturbance this evening. Clearing conditions and
diminishing wind will follow overnight with most areas expected to
remain fog free due to the continued northwest breezes...however
conditions will eventually become calm enough in the far western
valleys for some patchy fog...especially near kjac.


Fire weather...

Gusty wind will remain across much of the forecast area...with wind
weakening during the overnight hours. Temperatures will continue to
cool Tuesday before experiencing a slight warming on Wednesday. Snow
showers will taper off after sunset with a period of snow lingering
across casepr dispatch past midnight. The next approaching trough
will move into northwestern Wyoming Wednesday afternoon and
evening...and will spread east through the overnight hours into
Christmas day.


Riw watches/warnings/advisories...


Short term...Allen
long term...lipson
fire weather...Allen

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