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Area forecast discussion 
National Weather Service Riverton Wyoming
330 am MDT Tuesday Oct 21 2014

Short through Thursday night

Swift southwest flow developing today ahead of an approaching short
wave trough. Wind speeds will be elevated but below warning
criteria...main impact will be to light trailers and high profile
vehicles...especially in Sweetwater County...southeast Fremont and
Natrona counties. Later on this evening...the stronger winds will be
found along the absarokas and into the Cody foothills...and upper
Wind River basin as the flow turns a little more westerly
..increasing the breaking mountain wave threat. Flow will likely be
too quick and southwesterly at first and will sail right over the
barrier but there could be some breaking action early in the
evening...especially in west Cody and around Clark as winds veer
more westerly. This is expected to be brief as the subsident left
entrance region of the jet sails by early in the evening followed by
some broad but weaker forcing for descent behind the exiting trough.

While there is limited moisture with this system there will be at
least some rain and snow...with any measurable precipitation
expected to be limited to areas along and west of The
Divide...including Sweetwater County. Downsloping east of The Divide
should limit any precipitation there. There will be some jet
energy..weak instability and some qg forcing helping to maximize the
potential of this wave so there could be a few showers make it over
the mountains. Recent model runs have taken the bulk of the energy
from this wave a little further north than previous runs...but probability of precipitation
largely left alone...with some trimming in a few areas but overall
not much change in continuity with previous forecast.

The models have also been backing off on 700 mb temperatures along and
behind the cold front. Snow levels will now be only as low as 8kft
across the western mountains lowering to 6.5kft this evening before
the system exits...higher as you head east. The front eventually
makes it all the way through the state tonight...with the cooler
airmass hanging around into Wednesday with some moderation on
Thursday. Thursday could also be a breezy day as a tug of War
between the cold but dynamic trough to our north and the ridge
poking in from the south. Overall it looks like the ridge will win
out in our area with rebounding temperatures expected.

Long term...Friday through Tuesday

Synopsis...unseasonably mild and mainly dry conditions will prevail
Friday and Saturday with record or near record highs possible across
central and southern Wyoming along with breezy/windy areas. A
trough of low pressure and associated cold front will push across
Wyoming on Sunday/Sunday night bringing chances of rain and snow
showers. A drier and cool airmass will follow on Monday.

Discussion...upper low over SW Alaska early this morning is forecast
to wobble southeast along the coast of British Columbia with upper trough
sharpening and becoming more upright off the West Coast on
Friday...pumping up ridge over the High Plains. Medium-range models
in good agreement up to this point. Model differences become more
apparent as the trough makes landfall along the coast Friday night
and Saturday with the GFS shearing lead shortwave NE across
Idaho/western Montana on Saturday while the European model (ecmwf) shows a slower
much more split trough pushing across The Rockies on Sunday. GFS
and ec respective ensemble means support slower overall progression
than GFS and not as much splitting as European model (ecmwf) deterministic guidance.
Forecast leans toward the ensemble means with cold front pushing
into the west late Saturday/Saturday night with increasing probability of precipitation
across the west and spreading across central areas Sunday and Sunday
night. Gefs mean keep the best chances of precipitation north and
west of Wyoming with downslope flow keeping chances of precipitation
low east of The Divide. Ensemble means also show heights rebounding
a bit quicker Monday as next trough approaches the northwest coast with
flat ridging over the area on Tuesday. Forecast keeps low probability of precipitation over
the mountains on Monday dry and seasonably cool temperatures across
the basins and valleys. Milder...above normal temperatures are
expected on Tuesday.


Aviation.../12z issuance/

Shortwave approaching southwest Idaho will swing across northwest
Wyoming today. Cloud cover and showers will be on the increase
through 18z today as best forcing and jet energy push across
northwest Wyoming. Mountain top obscurations will be widespread
through about 21z-23z before trough and associated cold front swing
east of kjac. Could still see a brief period of MVFR at kjac with a
heavier rain shower between 16z-21z today. Otherwise...kjac will
remain VFR with rapid clearing between 23z/Tuesday and 02z/Wed. Other
terminals will all be VFR through the period. Expect enough
instability and moisture to generate convection across southwest
Wyoming...mainly affecting kbpi...kpna...and krks terminals. Much
more isolated convection elsewhere. Larger issue east of The Divide
will be downslope winds near the mountains and foothills and gusty
south to west surface wind. Winds will be 15-20kts with gusts
25-35kts. Strongest wind will be during the midday hours at kcpr
where 25g35kts is likely. Trough passage between 00z-06z/Wednesday will
veer winds to the northwest with decreasing speeds after sunset. The
exception will be kriw where favorable northwest wind of 15-25kts is
likely through about 06z/Wed. All terminals will see rapid clearing
from the west beginning at 00z/Wednesday and continuing through 06z/Wed.


Fire weather...

Another unseasonably warm and windy day east of The Divide while a
storm moves into the west. Downsloping winds will limit chances for
wetting rain east...while more favorable flow will maximize
potential along west facing slopes along and west of The Divide.
There could also be some wetting rains in Rawlins dispatch area. A
cold front sweeps through tonight...changing the wind direction to
more westerly. The wind and the rain will clear out on Wednesday
with cool temperatures remaining. Temperatures rebound a bit on
Thursday as high pressure builds in from the south but it will be
another breezy day in spots with southwest winds of 15 to 25 miles per hour
common between Rock Springs and Casper and over the higher


Riw watches/warnings/advisories...


Short term...Allen
long term...aem
fire weather...Allen

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