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Area forecast discussion 
National Weather Service Riverton Wyoming
310 am MDT Friday Oct 31 2014

Short term...today through Sunday night
the high pressure ridge overhead will slide off to the east today
allowing the steering flow to back more to the southwest ushering in
warmer air with many lower elevations east of The Divide reaching
high temperatures in the 70s today and in the 60s west. The eastern County Warning Area
will eke out another nice day before clouds increase ahead of the
next weather system. The circulation now off the Pacific coast out
at 45n/135w is drawing subtropical Pacific moisture north into the
Pacific northwest out ahead of it with lightning breaking out over Oregon
within the last few hours. The energy from the associated trough
will split by Saturday as it encounters the ridge over the central
United States. The models seem more realistic with this approach
than previous runs that did not show as much of a split. Both the
Euro and the GFS show this split taking place. We will focus our
attention on the closer northern piece of energy. Southwest breezes
will increase today from rks to cpr ahead of the approaching Pacific
weather system. The trough will make landfall by Saturday. As it
does...clouds will increase from the west and showers along with
embedded isolated thunderstorms will break out in western Wyoming along
and west of The Divide with showers increasing during the afternoon
hours Saturday. On Saturday...a northern stream circulation will close off
over Nevada/Oregon. This 500 mb low will open but remain a focused area
of vorticity as it tracks across Idaho and into Montana Saturday
night and into Sunday. The 700 mb low will take a similar track with the
surface low tracking northeast across northwest Wyoming early Saturday evening. This
low will have a fairly well defined warm and cold front and a
resulting well developed comma cloud. Most of the more widespread
dynamics will come together Saturday night. Left front quadrant dynamics
will add to the lift Saturday night as the jet shifts to our south.
As this trough nears...southwest winds will continue to increase
from rks to cpr Saturday. When the asct cold front trailing from the Pacific
surface low...many other areas will see an increase in wind speed as
the winds veer Saturday evening with some areas seeing an 8mb 3
hourly surface pressure rise. Snow levels will drop to the valley floors
Saturday night out west. As the Pacific system moves northeast into
Montana...showers will become more prevalent in northern Wyoming and will
continue in western Wyoming Sunday from the lift more directly
associated with the low. The heaviest most widespread precipitation should
remain north of the area Sunday in Montana where the best wrap around
is entropic lift will be located. As this weather system continues to
pull away from US to the northeast Sunday night the showers will
diminish. After all is said and done...2 to 4 inches of snowfall is
expected in the western valleys by late Sunday and 4 to 8 inches in
the surrounding mountains. Only 1 to 3 inches are expected in the
Big Horn Mountains by Sunday night. As far as any potential
significant snow banding GOES east of The Divide...it now appears
that the Post frontal 700 mb northwest flow will be too weak to
support any with the main energy to drive these type of snow
showers to far north,

Long term...Monday through Friday

Weak but chilly northwest flow starts the period as the weekend trough
lingers over the area through the day but with most of the energy
split between a low near far northern ND and another over the desert SW.
Just some chilly air left behind with maybe a little residual
moisture in the mountains and southeast fringes to start the period. Monday
should be the coldest day and Monday night the coldest night before
we begin another warming trend through Thursday. Upper low and cold
air between Alaska and the Aleutian chain work together to develop
another trough over the eastern Pacific early next week that pumps up
the ridge over the West. Heights rise through at least Wednesday
night and middle level temperatures peak on Thursday. Highs generally
start out Monday in the 40s with mainly 30s in the mountains and
then warm on average 5 to 8 degrees each day through Thursday. If we
get enough mixing...Thursday could be even warmer with both the GFS
and European model (ecmwf) showing 700 mb temperatures between +6 to +10 east of The
Divide. A shortwave then rotates through the eastern Pacific trough and
the ridge may weaken enough to bring a chance of rain/snow across
the area on Friday. For now...mainly confining probability of precipitation to the mountains of
the west and north with a slight chance in the lower elevations of
the north. Not a lot of precipitation potential through the week but there
may be just enough flow and weak disturbances/moisture in the northwest
flow to allow some very light mountain showers/sprinkles/flurries at
times in the northwest mountains Tuesday and Wednesday. An increase in winds at
times in the mountains and normally windy areas with each passing
ripple but nothing too significant expected. We do get a little
better gradient on the GFS by Thursday and if this does occur then
like mentioned above temperatures could soar east of The Divide to
record territory. Too early yet to go that warm but just something
to watch over the next several days.

&&

Aviation.../12z issuance/
VFR conditions will occur through 12z Saturday. Expect some
afternoon wind at kcpr and krks terminal sites.

&&

Fire weather...
a ridge of high pressure is expected to last through Saturday
morning and bring mainly dry weather. A gusty southwesterly breeze
will develop today from the Red Desert through Natrona and
Johnson counties. The next potential round of precipitation may
be found beginning Saturday afternoon as an area of low pressure
approaches western Wyoming.

&&

Riw watches/warnings/advisories...none.

&&

$$

Short term...lipson
long term...skrbac
aviation...Arkansas
fire weather...lipson

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