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Area forecast discussion 
National Weather Service Riverton Wyoming
336 am MDT Thursday Aug 21 2014

Short term...today through Saturday night
showers and a few thunderstorms continue to erupt over eastern
Idaho as the vorticity lobe of the night moves on through central Wyoming
with the next vorticity following close behind over the Great Salt
Lake. As a result there will be plenty of residual showers across
western and northern Wyoming this morning. The last aforementioned
shortwave will track across western and central Wyoming today for
an early show of convection before a transitory ridge builds over
the area later this afternoon. After 00z Friday... precipitation will
diminish and will mainly be limited to the northwest. Then after
06z Friday...widespread showers will develop over western Wyoming and
will spread toward central Wyoming after 12z Friday once the negatively
tilted trough over the Pacific northwest closes off over Washington and
Oregon and the southwest flow ahead of this low increases from the
southwest. Fridays precipitation will be very dynamically induced on a
synoptic scale...first time in a while. Qg forcing is prognosticated for
northwest and southern Wyoming Friday with qg forcing spreading to Natrona
County Friday nt. The asct middle level circulation will track to Idaho
Friday night. Saturday looks to be a cool wet day as the main low
approaches the northern rockies. Expect left front quadrant dynamics
over northwest Wyoming Saturday once the 500 mb low reaches the Montana/Idaho border acdg
to the GFS with the NAM a little faster taking it to central Montana.
On Saturday the middle level circulation is prognosticated to track from ynp
to around Hot Springs County and then Johnson County by Sat evening.
To the north of the 700 mb low over the northern border of Wyoming...even
some wrap around isentropic lift is expected. Surface cyclogenesis is
expected over southeast Wyoming. Moist cooler air will wrap around behind
these circulations Saturday night as a strong cold front moves
southeast across Wyoming. Expect numerous showers and a few
thunderstorms to overspread the area Friday night and into
Saturday with the increasing gradient of shower activity running
south to north and then overspreading the whole state by late
Saturday. As the system gradually pulls out of the area Saturday
night...700 mb temperatures will drop to -1c in northwest Wyoming which will bring snow
levels there from 10500 feet Saturday morning to 8000 feet by 12z
Sunday. The higher peaks elsewhere will also see snow from any
lingering showers by 12z Sunday as well. Could see up to an inch
and a half of snowfall in the Big Horn Mountains by early Sunday
morning and an inch in the absarokas. High temperatures in most lower
elevations Saturday will struggle to get above 70.

Long term...Sunday through Thursday
saturday's storm system will likely be off to our east to start this
period with just a little bit of moisture and instability left
behind in the far northern mountains where some Sunday showers/isolated
thunderstorms are expected. Otherwise...a cool to mild day with
highs only in the 60s to lower 70s with 50s and some 60s in the high
country. The next major cool trough which originates from the far
western Aleutian low is expected to strengthen as it drops southeast into the
mean trough. Leaning towards the GFS/Gem idea again tonight and
not the European model (ecmwf) with its developing cutoff low over eastern California and Nevada on
Tuesday. There are a few GFS ensemble members that are leaning that way
but most keep the trough in the flow and over The Rockies region.
Worth watching but it would be unusual to see that strong of a
cutoff low in August that far south. So going with the GFS
idea...main impact during the day Monday with current timing is
the northwest and possibly a swath in the far southeastern zones but then Monday
night the associated middle level circulation may move right across
our forecast area bringing a good chance of showers and embedded
thunder Monday night and Tuesday morning in our central and eastern
sections. A warmer day east of The Divide on Monday along with
increasing wind ahead of this system. Inland trajectory should
bring down some pretty cool air for late August with snow levels
lowering to between 8k and 9k feet Monday night in our central and
northern mountains. Dry with a warming trend Wednesday through Thursday
as shortwave ridging moves in. Still below normal temperatures Wednesday
and then back to around normal by Thursday.

&&

Aviation.../12z issuance/
an upper level trough of low pressure will continue to lift
northeast across the northwest half of Wyoming into S cen Montana by midday
today. This system will continue to bring scattered rain showers/isolated thunderstorms and rain
mainly vicinity and northwest of a kemm-kdub-kbyg line. Activity will be
more isolated south of this line. Local IFR/LIFR fog and ceilings may
develop in the western valleys through 15z...mainly vicinity kjac-
kafo. Isolated-scattered thunderstorms and rain will Blossom in the same general area across
the west and north this afternoon. The atmosphere will quickly
stabilize from 0z-3z Friday with most shower and thunderstorm
activity moving NE into Montana and the northern plains. Local
IFR/LIFR fog may re-develop in the far west valleys after 06z
Friday.

&&

Fire weather...
showers will be prevalent this morning with thunderstorms short to
follow through middle afternoon,. There will be a break in the action
later this afternoon and evening as a short lived high pressure
ridge builds over the area except for northwest Wyoming where
scattered showers and thunderstorms will continue. A potent
Pacific storm system will approach the area later tonight with
numerous showers and a few thunderstorms spreading from northwest
to southeast later tonight through Friday and into Saturday.
Cooler weather will filter in behind this weather system with snow
levels lowering to 8000 feet in the northern mountains by late
Saturday night. High temperatures in most lower elevations by Saturday
will struggle to get above 70. Winds will be generally light
except for brief gusty winds near thunderstorms as well as during
the cold frontal passage later this week.

&&

Riw watches/warnings/advisories...none.

&&

$$

Short term...lipson
long term...skrbac
aviation...team Riverton
fire weather...lipson

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