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Area forecast discussion 
National Weather Service Riverton Wyoming
313 am MST Sat Dec 20 2014

Short term...today through Monday night

An interesting forecast period with a lot of potentially
significant weather developing. A nearly zonal flow pattern is
expected this morning and afternoon...with some moisture increase
snowfall coverage a bit across western Wyoming.

A Pacific storm system will continue to move through the
intermountain west Saturday evening...with snowfall increasing
across western Wyoming. Additionally...an increased temperature
gradient is expected between Cody and the eastern portion of
Yellowstone National Park. Additional mountain wave signatures are
being demonstrated in current model analysis...with very strong
wind possible around the favored Clark area. A high wind watch is
in effect for this location.

Snowfall is expected to increase tonight and persist through
Monday morning. Snowfall is looking to become heavy at times with
significant accumulations possible across the western mountains
and valleys. The Winter Storm Watch remains across the area.
Additional locations look favorable for the need of winter weather
advisories...and snowfall totals should be able to be finalized
during the upcoming shift. Simultaneously...models continue to
indicate the potential for very strong winds across Sweetwater
County and into the Green Mountains and rattlesnake range Sunday
afternoon and evening. Another high wind watch has been issued to
account for these potentially strong winds.

Snowfall and wind is expected to weaken Monday as northwest flow
works to build over the area...with most activity waned by Monday
afternoon. Some light snowfall will remain possible across areas
east of The Divide late Monday as the system continues to move
into the Central Plains.

Long term...Tuesday through Saturday

Quite the model divergence tonight between the GFS and the European model (ecmwf).
The models start out pretty close through Tuesday with transitory
ridge moving across the area after our sun/Monday snow and wind event.
European model (ecmwf) has some moisture and upglide in the pattern already Tuesday
so some chance probability of precipitation remain...especially by Tuesday night. After
that...the changes become very significant. The 00z GFS allows the
Christmas evening/Christmas day system to come in at a higher latitude
with less ridging behind it. This solution has a southeastward moving
disturbance coming through with the main effect being strong to high
wind across the lower elevations and a quick shot of snow...mainly
in the mountains. The European model (ecmwf) on the other hand is still showing this
system dropping into the pacnw and then the northern Great Basin as
ridging develops quickly behind it. The system then closes off over
southern Wyoming on Christmas day with widespread snow spreading eastward and
southward Christmas evening into early Christmas day. The development of this
system is still a combination of the current trough over the
central Pacific accompanied by at least a 170-180kt jet and a
pocket of cold air pouring south in the Kamchatka region. A strong
trough with very cold air behind this system and a lead
disturbance are responsible for the upstream ridge along the coast
that the European model (ecmwf) especially shows. The GFS has a similar scenario
but diverges slightly not allowing the ridge to build upstream as
much. That makes all the difference in the world for our sensible
weather. Sticking closer to the more consistent European model (ecmwf) and usually
better Euro in this scenario which means little change for our
going forecast. Cold behind this system...especially with the
European model (ecmwf) solution Friday night. Another potential southeastward cold moving
shortwave may move into the northwestern parts at the end of this period.
This system is actually the one expected to pump some warm air
into our ridge along the coast and aid in the digging our
Christmas system.

&&

Aviation.../12z issuance/

Increasing moisture per models into western Wyoming on Saturday.
Snow will increase and become moderate to heavy by Saturday
night...especially in the mountains and possibly near kjac. This
will result in mountain obscuration for all mountain ranges.

Outside of the Wind River basin of central Wyoming...VFR conditions
can be expected through Saturday east of The Divide. With a
tightening pressure gradient winds will increase
Saturday...especially across the southwestern wind corridor from
krks to kcpr. Any fog at klnd and kriw should begin to left around
15 to 16z and be fully lifted by 19z.

&&

Fire weather...

A plethora of potentially adverse weather conditions are possible
over the next couple days. Strong wind may develop across the Cody
foothills late this evening...and continue into Sunday morning.
Meanwhile...snowfall will intensify across the western mountains
and valleys...and will persist through Monday morning. Snowfall
may combine with gusty wind at times. As the system continues to
impact the state...strong wind is possible to develop across Green
Mountain and rattlesnake range...along with the Interstate 80
corridor through Sweetwater County. These strong winds and
combined snow and wind will make for generally unfavorable burn
conditions.

&&

Riw watches/warnings/advisories...
high wind watch from Sunday morning through Sunday evening
wyz019-028>030.

Winter Storm Watch from this evening through Monday morning
wyz002-014-015-025.

Winter Storm Watch from this evening through Monday morning
wyz001-012-013-023-024.

High wind watch from this evening through Sunday morning wyz003.

&&

$$

Short term...branham
long term...skrbac
aviation...hattings
fire weather...branham

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