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Area forecast discussion 
National Weather Service Riverton Wyoming
530 am MST Thursday Nov 26 2015

dry slot moving into central areas from the SW has resulted in a
decrease in snowfall as better upper divergence and enhanced
snowfall migrating eastward roughly along the Laramie range early
this morning. Blizzard Warning across Sweetwater County has been
replaced with a Winter Weather Advisory for snow and blowing snow
through 5 PM.


Previous discussion... /issued 259 am/

Short through Saturday night

.Winter weather continues across the forecast area with assorted
warnings and advisories spread over western and central Wyoming...

Imagery showing large cold deep trough western Continental U.S. West/ closed portion of
low over eastern Nevada/western Utah. Strong jet...100 to 120 kts... stretches from
the base of the the NE over Utah and northwestern Colorado...across
central and eastern Wyoming and off into the northern plains. Secondary cumulonimbus tried to
fill in this gap earlier Wednesday evening...producing some brief heavier
convective-like precipitation for an hour or two...but then lost the ingest
of unstable air and has since returned to a stratiform pattern. the jet is still screaming overhead...there has been
some good prodigious lift at times...showing in infrared imagery as broken
transverse bands across central Wyoming...keeping light snowfall in
abundance east of The Divide. Additionally...shear deformation zone
west of the jet has kept better precipitation out west across the southern
border area...with decreasing lift to the north. Will drop advisory
for zones 1 12 and 13. Changed expirations time for zones 4 5 10
23 25 26 27 28 29 and 30 to 11 am local. Surface pattern has
evolved as foreseen...with a relatively strong surface cyclone over
northwestern Colorado and associated nearly stationary front backed into/from
the Front Range of Colorado...across southern Wyoming...and on into scntrl Idaho then
northward. Deepening high p...1035 at this time...building over Montana this morning.

Advisories/warnings...including Blizzard Warning across Sweetwater
County and i80...cover the entire forecast area through this afternoon. Heaviest
snows have followed the front into the region...with the foothills
and northern/northestern slopes of the Wind River mountains winning out so far
with 5 to 8 inches so far. Little snow has fallen across the western
mountains north of Jackson...with better accumulations to the south.
Additionally...Sweetwater County continues with light snow...but has
combined with northeast winds gusting 40 to 50 miles per hour at near blizzard like conditions at times.

Snow will then taper off through the rest of today...with flurries
across the eastern forecast area Thursday night...and clearing very cold skies across
the northwest. Stationary front should still be hanging around the
southern zones/border area with Colorado while the jet core remains very strong
over this same area...keeping chances for light snowfall around
Sweetwater County into Friday morning before sinking southward. Then...the
upper low looks to be in a favorable position to becoming nearly cut
off through at least Sat morning...maybe longer. While general surface high
p will still be in place across the forecast area...models now indicating a
chance for the front to return to the north...across the southern/southeastern
portions of Wyoming...while lift is generated above as the upper low ejects
and expands over Wyoming as southern branch jet streak moves into the southeastern
portion of the upper low and new moisture returns to the region from
out of the southern/Central Plains.

Long term...Sunday through Thursday

Synopsis...after a retreat to the southwest...our low pressure
system (the culprit of our white thanksgiving) will wobble slowly
northeast across Wyoming over the weekend and finally exiting into
the northern plains on Monday. Chances of snow will be mainly to
the north and east of the low pressure system...across north and
central Wyoming on Sunday with some lingering light snow across the
mountains and along and east of the I-25 corridor on Monday. A
ridge of high pressure will prevail Tuesday through Thursday
bringing a gradual warming trend to most locations...but snow-
covered central basins and western valleys will likely remained
cold...trapped under steepening inversions with areas of fog.

Discussion...medium range models have trended slower overall
ejecting the SW low out of the Great Basin into the northern and
Central High plains Saturday through Monday. Weaker upper low near
40n/140w early this morning is expected undercut ridge along the
West Coast and get caught up in a fujiwara interaction with Great
Basin upper low with a portion of this vortex expected to orbit
across northern Wyoming Sunday before being pulled southeastward
into new upper low spinning up across the northern plains. European model (ecmwf)
continues to paint a neater comma head of precipitation wrapping NE-
SW across northern and central Wyoming on Sunday around upper low
near the Black Hills. GFS shows a much messier pattern with
northern portion of this vortex located over SW Montana on Sunday.
European model (ecmwf) has shown better consistency with the complex fujiwara
interaction and placement of upstream ridge along the West Coast.
This consistency leads toward more weight being given to the European model (ecmwf)
in the Tuesday through Thursday timeframe...with broad ridge
building across the northern rockies with incoming Pacific energy
splitting across Canada and the SW U.S. This will result in a
mainly dry pattern...warmer air pushing in at mountain top level and
through the wind corridors. European model (ecmwf) advertises 700 mb temperatures to be near 0c
on Wednesday and for snow covered central basins...especially the
Wind River basin...this will likely result in steepening inversions
with fog formation. Temperatures were trended in this
direction...with the Wind River basin and lower Bighorn Basin
struggling to break freezing by Wednesday (that's being optimistic)
while surrounding areas with better mixing push into the 40s.

Aviation.../12z issuance/

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

Snow and low ceilings and visibilities will prevail over the region through
15z. In the north conditions will improve after 15z. In the central
section conditions will stay MVFR through 00z with some improvement
after 00z. VFR conditions should prevail by 06z Friday. Please see
individual terminal sites for more details.

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

Areas of snow with low ceilings and visibilities will occur through 15z.
Conditions will improve by 19z with areas of MVFR conditions
lingering until 00z. VFR conditions will prevail after 00z.
Krks will remain in MVFR conditions through 03z. Then VFR conditions
will occur after 03z. Please see individual terminal sites for more

Fire weather...

Fire danger low for all locations through this week/weekend. Expect
snow and blowing snow to continue today...but taper off through the
day...including blizzard-like conditions at times across Sweetwater
County and the I 80 corridor. Tonight will bring clearing skies
across the west and northwest...with bitterly cold low temperatures
Friday morning. Chances for some light snow will again be possible
across southern/southwestern and eventually eastern forecast area
later Friday night through the day Saturday....spreading to the
north/northwest Saturday night.

Riw watches/warnings/advisories...
Blizzard Warning until 11 am MST this morning for wyz028>030.

Winter Storm Warning until 5 PM MST this afternoon for wyz005-

Winter Weather Advisory until 11 am MST this morning for wyz003-

Winter Storm Warning until 11 am MST this morning for wyz026-027.

Winter Weather Advisory until 5 PM MST this afternoon for wyz002-


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


Riw watches/warnings/advisories...
Winter Weather Advisory until 5 PM MST this afternoon for wyz002-

Winter Storm Warning until 5 PM MST this afternoon for wyz005-

Winter Weather Advisory until 11 am MST this morning for wyz004-

Winter Storm Warning until 11 am MST this morning for wyz026-027.



short term...Braun
long term...aem
fire weather...Braun

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