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Area forecast discussion 
National Weather Service Riverton Wyoming
1125 PM MST Wednesday Dec 24 2014

Update...updated to add 06z aviation discussion.

Short term...tonight through Friday night (issued at 307 pm)

The long awaited Christmas snow storm is nipping at our heels with
snow already falling in central Idaho as of early Wednesday
afternoon. This swath of initial snow will overspread the western
mountains by midnight tonight with some snow beginning to wrap
around into northern Wyoming shortly thereafter. There will be a
good burst of snow with the front as it drops south into the
region overnight...through the Bighorn Basin early in the morning
and then through the central basins around sunrise Christmas
morning. There will be several hours of steady snow behind the
front as reinforcements arrive as the upper wave arrives along
with a good swath of strong qg forcing. Layer temperatures all the
way through the atmosphere will be favorable for dendritic growth
as the front passes as well as several hours afterward. However
the atmosphere becomes cooler and dryer after 21z Thursday which
will increase snow ratios but it will end the favorable heavy snow
environment. Winds will be fairly steady with the front at 15 to
25 miles per hour in most areas with some gusty spots...specifically around
Cowley and Buffalo as well as through Wind River Canyon early in
the morning. The gusty wind will then shift to South Pass and into
eastern Sweetwater County...likely impacting travel along
Interstate 80 east of Rock Springs through the day. In
addition...there will be significant blowing snow through Johnson
and Natrona counties in the afternoon as north winds become steady
at around 25mph.

The snow will taper off Christmas night into Friday with snow
clearing from north to south. Winds will be relatively light on
Friday with skies remaining partly to mostly cloudy as another
weak disturbance swings south. It will not result in much
additional snowfall but there could be some flurries in the
mountains with this wave. It will however keep the temperatures
from bottoming out behind the exiting system Friday night.

Long term...Saturday through Thursday
by 00z Friday...the snowfall will slowly begin to end from north to
south and decrease in intensity somewhat...except in the upslope
favored regions east of The Divide. The middle level circulation will
still be over southwest Wyoming at 00z Friday along with lingering
isentropic lift from north to south as well as lingering sufficient
qg forcing. We will begin to lose these dynamics after few hours
after sunset as the system pulls away to the southeast.
the 700mb level low moves off to our south over Colorado...the winds
will back to the northeast at 700mb and increase for enhanced
upslope flow in areas like Lander...Casper and Thermopolis. After
06z...the snowfall should decrease everywhere. On Friday...except
for some lingering isolated mountains flurries...expect dry weather. Dry
weather is expected Friday night. On Saturday...light snow will once
again develop in far West Wyoming as a vorticity lobe in the northwest
flow on the backside of the main exiting trough tracks across the
area and the increasingly difluent northwest flow increases the upslope
potential. This upslope flow will increase Saturday night with snow
increasing in the west as the just east of the
Aleutians...dives southeast and deepens over the Pacific northwest.
By Sunday morning...the trough will begin to close off along the
Washington Oregon border and snowfall will become more widespread
and intensify over the west. By Sunday afternoon...snowfall will
begin to spill over east of The Divide with the 500 mb and the 700 mb low
over Idaho per GFS solution. Then the energy with the asct trough will
split with the Idaho low tracking down to Vegas by 12z Tuesday. By
this time the trough axis will be overhead. The northern stream
energy will pull down the coldest Arctic air of the season into
Wyoming Tuesday. Bitter cold advection will accompany the front
Mon/Tuesday. In the meantime...chance probability of precipitation are in the forecast for most
areas with the approaching trough for Sunday night with likely probability of precipitation
in the west and Big Horn Mountains snow will decrease Monday as the
energy splits. As alluded to above...Tuesday will feature steady or
falling temperatures after a frigid morning with very cold wind
chills as the Post frontal air works its way into the north as a
1048mb surface high drives the frigid air south. Temperatures will continue to
drop Tuesday night as skies clear and winds diminish. Warm air
advection aloft is expected from the northwest Wednesday which will only
strengthen the cold air inversions in the basins and valleys for
Wednesday morning with some areas east and west of The Divide seeing
temperatures as low as 30 below or a little colder. The Euro...on the other
hand...unlike yesterdays diverging away from the GFS solution
which is to bring a shortwave trough through Sunday and another one
Tuesday. Am ignoring this solution at this time but...if nothing
another reason to save the coldest air for Wednesday morning in the
forecast after the coldest wind chills occur Tuesday morning. The
GFS is advertising 700 mb temperatures of -28c with the Euro showing -32c
moving into Wyoming from the north at 700 mb on Tuesday.


Aviation.../06z issuance/

An upper level low pressure system will dig southeast across the Great
Basin into The Four Corners area Christmas and Thursday night.

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

A cold front and IFR -sn areas of LIFR snow will spread south across
northern Wyoming to a vicinity klnd-kcpr line between 8z-14z. The front
will also be accompanied by gusty north winds 15-20kts with gusts to
30kts and areas of blsn. These conditions will persist across most
areas through 00z Friday...followed by gradually improving conditions
and diminishing wind from northwest-southeast Thursday night. The last areas to
show improvement will be upslope favored areas vicinity klnd-kcpr where
areas of IFR ceilings and visibility in -sn may persist through 12z Friday.

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

Conditions will continue to deteriorate overnight with widespread
IFR -sn...mountains obscured...prevailing vicinity and north of a
kevw-kbpi-kpna line by 12z. A northeast flow will bring downslope
winds and improving conditions across the northwest to vicinity kjac
and kpna around 18z...but IFR conditions will likely persist to the
south and west.

Along the Interstate 80 corridor and vicinity krks...a cold front and
increasing NE winds will bring widespread IFR -sn/blsn 15-18z
today. These conditions will likely persist well into Thursday
night with some improvement possible early Friday morning.


Fire weather...

Widespread snow and gusty wind through early Friday morning. Basins
and valleys will quickly become trapped on Friday night with very
poor smoke dispersion. On Saturday...milder air moves in along with
some stronger winds that will encourage some mixing. Another snow
storm is possible on Sunday into Monday.


Riw watches/warnings/advisories...
Winter Storm Warning from 5 am to 11 PM MST Thursday

Winter Weather Advisory until 8 PM MST Thursday

Winter Weather Advisory from midnight tonight to 8 PM MST
Thursday wyz003-004-006-008>011-016.

Winter Weather Advisory from 8 PM this evening to 8 PM MST
Thursday wyz025-026.

Winter Storm Warning from midnight tonight to 8 PM MST Thursday



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

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