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Area forecast discussion 
National Weather Service Riverton Wyoming
341 am MST Sat Nov 28 2015

Short through Monday night

Imagery showing lw trough over the Continental U.S. With deep trough pinching off
over the western Continental nearly cut-off low. Smaller satellite cyclone
now located at around 130w 38n...orbiting counter clockwise for at
least a short period...fujiwara effect-like...the primary low.
Embedded vorticity maximum spinning currently spinning into northestern
Utah/swrn/cntrl Wyoming...showing high deformation and overriding a weak
surface front...kicking off light snow activity over the area. Other than
weak surface front/boundary poking into southwestern Wyoming...the rest of the region
has general high p north over Wyoming and Montana...into the plains...with weak
Four Corners cyclone sitting nearly stationary at this time.

Today...patchy fog this morning east of The Divide. Strong upper vorticity maximum
currently traveling up the western Colorado border toward southwestern Wyoming...showing
good upper deformation and overriding front. Low to middle level
dynamics/moisture over the southwestern forecast area will continue to produce light to
moderate snowfall across this region through the morning. Quantitative precipitation forecast and cold
temperatures will combine to produce widespread areas of 2 to 4 inches of
new snow through this morning. Therefore a Winter Weather Advisory has been
issued...continuing through the morning period for southern Lincoln and
Sweetwater counties. Then...upper vorticity energy rotates around the low
and back over the forecast area while some middle/low level moisture returns to the
eastern County Warning Area from the S/southeast and is forced into/under surface high p. Surface/middle
level frontal boundary also looks to try and lift northward into portions of
eastern/central Wyoming. Together this set-up will increase chances for light snow
across much of the area east of The Divide and over the southern zones...but
leave most of the west-central and northwestern mountains out of the precipitation zone
for now.

Tonight through Sunday...the Pacific SW mentioned above begins to move southeast
to vicinity of the deep closed low over the western Continental U.S. And is quickly
absorbed into the southern portion of the main upper low as it begins to
eject to the NE towards the northern plains in response to the approach
of another stronger deeper upstream trough. There will be two parts to
the evolution of this complex system. main up low ejects
northeastward...weak cyclogenesis will take place over central Wyoming...with some
wrap around moisture and slightly stronger upper level lift going on for
at least a short period...bringing areas light precipitation to portions of
central and eastern Wyoming through the day Sunday...expanding westward to the western
mountains overnight Sunday through Monday morning. the Pacific SW
is absorbed into the southern portion of the ejecting upper low...a
somewhat stronger secondary cyclone will form over/near the southern
rockies...tapping into moisture from the central and Southern Plains by
Sunday night...keeping most of that moisture from reaching into our
forecast zones...with only very light areas of snowfall/flurries
expected at this time over the eastern forecast zones through the end of
forecast period.

Long term...Tuesday through Saturday upper ridge will prevail over the area Tuesday through
Thursday bringing dry weather and milder temperatures to the
mountains and wind corridors while some lower central basins and
western valleys will be colder with areas of fog under steep
inversions. The next Pacific trough will split along the West Coast
Thursday with most of energy headed south of the area with only
isolated to scattered snow showers expected across the west with
this system.

Discussion...GFS and European model (ecmwf) are in better agreement with synoptic
pattern across the west through the period. Upper high currently
centered near Vancouver Island will expand across The Rockies Tuesday
and Wednesday advecting in much warmer and drier air in the middle-
levels with 700 mb temperatures 0c to -3c east of The Divide on
Wednesday. The biggest challenge will be temperatures...with no
available guidance showing the trapped cold air that will remain
under steepening inversions over snow-covered central basins and
western valleys. The inversions will be most pronounced over the
Wind River basin where highs will likely remain in the upper teens
to middle 20s though Thursday with lows 5 below to 5 above or generally
10f to 15f degrees below most of the guidance numbers. Steep
shallow inversions will also be favorable for fog formation...which
could become quite widespread and dense across the Wind River basin
and portions of the Upper Green river basin Tuesday night through
Thursday morning. On the other end of the spectrum...the warmer
areas will be along the Cody foothills and along the Lee slopes of
the Bighorn range where highs are expected in the lower to middle 40s
Wednesday and Thursday.

GFS is trending toward the more consistent European model (ecmwf) with next Pacific
trough splitting along the West Coast Thursday. That trend is
toward a slower solution with most of the energy headed south into
the SW U.S. And southern rockies. The main upper low is expected to
be near or south of The Four Corners region by 12z Saturday with the
associated surface cold front moving across the area Friday and
Friday night...bringing some light snow showers across the west and
stirring up the trapped cold air in the Wind River basin Friday
night and Saturday.


Aviation.../12z issuance/

An area of light to moderate snow will lift northward across the
area tonight into Saturday morning with widespread MVFR to occasional IFR
conds for 6 to 12 hours with the worse conds lasting 3 to 6 hours.
Some low clouds with MVFR conds will also affect an area around kcpr
and over the kbpi ahead of this disturbance before the snow starts.
The snow will move into krks at the start of this period and last
through about 18z. The snow and lower ceilings will become widespread as
far north as a kcpr to klnd to kriw and just south of kbpi after 09z
and last through Saturday morning. This area will then lift northward into
the northern taf sites. After 16z for kwrl and 21z for kcod. Snow will
also move into the kjac area by 00z Sunday with mountain obscuration most
of the day. Areas of fog will also occur in the west through 18z
Saturday. Scattered areas of light snow with local MVFR and mountain obscuration
will prevail through Saturday night along with patchy fog.


Fire weather...

Fire danger low for all locations through this weekend.
Increasing clouds across much of the southern zones today...with the
same slow moving system that brought snow to much of Wyoming over
the Thanksgiving Holiday returning to form another complex winter
weather pattern over the forecast area...with increasing chances for
light snow east of The Divide today....expanding to the north and
west through Sunday.


Riw watches/warnings/advisories...
Winter Weather Advisory until 11 am MST this morning for



Short term...Braun
long term...aem
fire weather...Braun

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