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Area forecast discussion 
National Weather Service Riverton Wyoming
314 PM MST Thursday Nov 27 2014

Short term...tonight through Friday

WV imagery showing continuing ridge/trough pattern across the Continental U.S.
From west to east with split flow...trough over the ridge near
confluence of northwestern US and British Columbia...combining to give a strengthening
very strong jet across the northwestern states...headed this way.
Otherwise...northwest flow is in place across the forecast area with some modest middle
level moisture continuing to stream into portions of the western forecast area from
the epac...with orographic cloud and some light snow falling. At the
surface...a Lee trough/stationary front is forming and hanging about from
central Montana...through eastern Wyoming and into eastern Colorado.

Today through Friday...northwest flow continues but begins to flatten and sag southward
in response to the approach of the jet and SW trough off British Columbia coast. The
main event through the end of the forecast period will be the increasing
winds...particularly over the absarokas and adjacent Cody foothills
where high wind warnings have been posted for an extended period
lasting through the day on Sat. This is in response to greatly
increasing middle/upper wrly flow nearly perpendicular to the mountain
ranges...with excellent thermal and pressure grads developing
across the mountains and into the Lee side foothills. Kchj is
already blowing 45+g60+ and has been increasing all morning...while
kcrk is just started to mix and be affected with 20+g45+ winds.
Additionally...mountain wave production is proceeding in earnest
with highly turbulent signatures showing in all Sat imagery. This
will help to pin winds closer to the surface. Mountain waves and winds
extending into the foothills should start to reach maximum levels by
Friday morning with even kcod reaching (near) warning criteria
around noon or just after on Friday as downward momentum continues
to creep eastward. The Clark area will likely gust a 100 plus kts through
this period. Somewhat lesser...but still brisk wind will be found
across the usual wind corridor from southeastern Fremont...eastern Sweetwater
and east through Natrona counties. For aviation purposes...low level wind shear will reign
across most of the region east of The Divide in addition to those
regions/airports in more of the flatland basins west of The
Divide.

A minor part of this set-up has to do with the middle level moisture
streaming into the region from the west. This will only mean a
continuing presence/chance for light snow/flurries across some of
the higher elevation peaks of the west/northwest through out the period. It
does appear...however...there will possibly be a decrease in this
action during the daytime on Friday as this moisture gets undercut by
a dry lay in association with the stronger flow nosing into the area
at this time. Interesting but not terribly significant.

Long term...Friday night through Thursday

Pretty busy in the long term period...first there will be some
hefty mountain wave action in the Wind River mountains that will
likely come crashing into Lander and feet Washakie Friday
night...while strong wind will also be ongoing in the absarokas
and Cody foothills. Southwest winds will also be howling across
the southern wind corridor from Rock Springs to Casper. Even
though winds are supposed to stay below high wind criteria...it
could hit in a few of the favored spots and strong cross wind
along interstates 80 and 25 will make travel difficult for high
profile vehicles and light trailer traffic from Friday night
through early Sunday morning.

In addition to the wind...there will also be periods of heavy snow
across the western mountains with over a foot of new snow expected
over the higher terrain and up to 10 inches possible in the
Jackson Valley...but it will be hard to measure given the amount
of wind expected both at the valley level and in the mountains.
Expect a blustery day on Saturday with a lot of snow and blowing
snow through Teton and park counties. Travel will also be
difficult in these areas...especially over Teton and togwotee
passes.

Strong wind and snow are also expected in the Wind River
mountains...however...since the heavier snow is not expected to
impact near South Pass...we decided against including these zones
in the watch for now...but it is still important to stress that
tomorrow through Sunday will not be a good time to go into the
Wind River mountains...expect periods of heavy snow...wind gusts
over 70 miles per hour at times and near blizzard conditions at times west of
South Pass.

Snow will eventually spread east of The Divide as the cold front
associated with this system drops south early Sunday morning.
Moisture will be limited behind this front but the snow ratios
will be a lot higher so some spots could get an inch of snow out
of the front east of The Divide.

We will lose the upper level dynamics and the moisture by Sunday
evening but the right entrance region of the jet along with a low
to middle level convergence zone may keep some snow going across the
south into Sunday afternoon.

Models begin to diverge thereafter with significant differences
after Tuesday as the GFS digs a trough to the west and brings
widespread snow by Wednesday while the European model builds a Dry
Ridge into the area. For the official forecast...stuck very close
to continuity with generous slight chance probability of precipitation as there is too
much uncertainty to stick with any one solution.

&&

Aviation.../00z issuance/

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

VFR conditions will prevail across the forecast area. However...
areas of low level wind shear will be common through the period...especially
near/along the east/northeast slopes of The Divide...vicinity klnd-kcod
and also kcpr later this evening...where winds in the fl080-100
layer will be 260/40-60kts in addition to producing mountain waves locally.
Some of these stronger winds will push down the foothills into vicinity
kcod-kdub by 00z Friday...with a high likelihood of 50+kts gusts into
kcod by Friday afternoon...perhaps klnd by the end of the forecast.

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

Westerly winds at ridge-top level will increase to between 50-60 kts
today...resulting in mountain wave production and continuing
turbulence through the rest of the period. Low level wind shear will also be common
vicinity krks and kpna later this evening...kbpi and kjac joining the
club Friday morning/afternoon. This strong westerly flow will also
keep some higher mountains obscured in clouds and -sn with some local near
MVFR ceilings vicinity kjac.

&&

Fire weather...

Overall fire danger will be low...remaining low through the weekend.
Light snow or flurries will linger over the higher peaks of the
region today...particularly the northwest mountains while
temperatures warm east of The Divide as west winds as some
downsloping increases across the area. High winds will become a
threat east of the Continental Divide...especially in the Absaroka
Mountains and Cody foothills...from today through the day on
Saturday. However...relatively widespread snow cover and cool to
seasonal temperatures will help in keeping minimum relative humidity values well
above critical even with the wind. A return to winter expected this
weekend as another storm brings more snow to the west on
Saturday...to the east overnight Saturday and through Sunday
morning.

&&

Riw watches/warnings/advisories...
High Wind Warning until 5 PM MST Saturday wyz002-003.

High wind watch from Friday afternoon through Saturday morning
wyz018.

Winter Storm Watch from late Friday night through Sunday morning
wyz001-012-013.

&&

$$

Short term...Braun
long term...Allen
aviation...Braun
fire weather...Braun

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