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Area forecast discussion 
National Weather Service Riverton Wyoming
1148 PM MDT Sat Oct 25 2014

Update...updated to add 06z aviation discussion

Short term...tonight through Sunday (issued at 325 PM mdt)
the lead vorticity lobe within the approaching trough now over Idaho will
approach the County Warning Area tonight and track across the area Sunday. The pressure
gradient will increase as it does. It will get windy as a result.
Now to decipher where...when and how strong. As far as any mountain wave
potential...local WRF cross sections indicate that 60 knot winds
will surface onto the Cody foothills along with a mountain wave
signature...albeit without a critical layer. There is a Froude
number close to one correlating with the favored time frame late
tonight. As far as any contribution to gap flow...700 mb WRF temperature
gradient not overly impressive from ynp to the Cody foothills. So
will go ahead an upgrade the high wind watch for zones 2 and 3 to a
low end warning. The overall gradient at the surface to 700 mb is not that
tight but bl winds are prognosticated to be 35 knots from the Green Mountains to
Casper Sunday so these winds will probably be Special Weather Statement
worthy...especially for places like outer drive just south of Casper
for Sunday. In addition...unlike the last event...there will be no
significant hook up wind component from right front quadrant jet
dynamics to draw the stronger winds aloft to the surface so the winds
along the southern wind corridor should not get too crazy.

The increasing veering west-southwest flow will increase upslope potential
combined with weak instability west of The Divide tonight and Sunday
and will result in increasing rain and snow showers along with some
upslope in the Big Horn Mountains with the snow level dropping to 7000
feet in the Big Horn Mountains Sunday with rain turning to mostly all
snow by late afternoon in the west. Most snow accumulations will be
relatively light before increasing after sunset closer to the time
of the troughpa with elevations above 10k seeing an inch or two by
sunset Sunday.

Temperatures Sunday will run roughly 10 degrees cooler than today
given the cold air advection with the trough...added clouds and

Long term...Sunday night through Saturday
a passing wave of low pressure will keep showers...rain and
snow...over the western mountains and valleys Sunday night...which
will linger into Monday morning. Cooling temperatures will be
anticipated with much cooler 700 mb temperatures associated with the
passage of the trough. The trough axis will pass through the County Warning Area by
Monday afternoon...with building northwest flow anticipated over the
area. Lingering moisture will keep some shower activity across the
western and northern mountains into Tuesday morning.

A flattened ridge of high pressure will move over the area by
Tuesday afternoon...with generally dry conditions expected. This
transitory ridge will be short lived as another swath of moisture
moves through the intermountain west Tuesday afternoon and evening.
Models show that moisture could be stalled from reaching
northwestern Wyoming until late Wednesday morning...though some
isolated activity may be around earlier. A few showers will be
possible Wednesday afternoon...but the presence of shower activity
will be short lived.

A ridge of high pressure will amplify over the area on
Thursday...helping to taper off lingering showers...and providing
the potential for warming temperatures for the Holiday weekend. The
ridge of high pressure remains over the area through Saturday
evening...with another wave of low pressure moving through the
western United States...potentially impacting the state Sunday
afternoon and into Monday.


Aviation.../06z issuance/

West of The Divide kbpi/kjac/kpna/krks routes
a line of showers and thunderstorms will continue to push into far
West Wyoming ahead of a cold front through 08z. Brief MVFR
conditions in ts may occur at kjac during this period. Mountain
obscurations and areas of MVFR/IFR conditions in -ra/-sn will likely
linger behind the front from vicinity kafo-kjac and northward across
Yellowstone National Park through the day Sunday with snow levels
around 8kft. Further south...gusty west winds at 20-30kt will
develop along the Interstate 80 corridor behind the front Sunday
afternoon. A secondary disturbance will move into southwest Wyoming
Sunday evening spreading scattered-numerous MVFR/IFR -shsnra into vicinity
kpna-kbpi-krks primarily between 02z-08z Monday.

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

Areas of low level wind shear will be encountered...especially along the east slopes
of The Divide vicinity kcod-klnd...through 15z Sunday with winds at
fl080 25040kts. A cold front will push through in the late morning
hours with west winds mixing down to the surface at most terminals. An
upper trough lagging behind the front will push across the area
Sunday evening...spreading isolated-scattered -shra/shsn into central Wyoming
with local MVFR conditions and higher mountain obscurations.


Fire weather...
fire danger will be elevated for the rest of this afternoon...mainly
in the Lee of the mountain ranges over the basins east of The
Divide as minimum relative humidity values across these areas fall into the low teens
while winds increase out of the southwest to 15 to 25 miles per hour with
gusts of 25 to to 50 miles per hour at times. Sunday...moisture will increase
across the entire region with a better chance for showers across
the west and north...lowering fire danger once again.


Riw watches/warnings/advisories...
High Wind Warning from midnight tonight to noon MDT Sunday



Short term...lipson
long term...branham
fire weather...lipson

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