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Area forecast discussion 
National Weather Service Riverton Wyoming
1105 am MDT Sat Aug 30 2014

Short term...today through Monday night

A broad digging trough across the pacnw will continue to move
east...affecting our weather through Sunday now...previous runs had
the trough moving through sooner and quicker without much of a
reinforcing shot on Sunday...now it looks like there will be several
waves and reasons to keep showers and storms going off and on into
Sunday evening. There will be plenty going on aloft to sustain
showers and storms well into the night as difluent flow ahead of an
approaching jet maximum transitions into the left exit region of the jet
dipping just to our south by Sunday evening...leading to a sustained
period of dynamic lift aloft. There will also be several waves of qg
forcing and broad lift associated with each approaching short wave
trough...the first of which will arrive into the west around noon
today and moving east through the afternoon. The next more potent
wave actually arrives late this evening into the west but is slower
to Rake across the forecast area...hanging around central and
eastern Wyoming through peak heating on Sunday...kicking out to the
northeast in the evening.

At the surface...we will have a low dropping south out of Alberta at
a fairly fast clip...arriving in northeast Wyoming by 18z...sharply
increasing the surface gradient...which will easily tap into the
swift southwest flow aloft leading to a rather windy day across the
forecast area...most noticeably across Sweetwater County and into
Natrona County. This southwest flow will draw up some warmer and
drier air from the southwestern u... in a relative
sense...there will be plenty of instability and middle level moisture
to work with for some thunderstorm activity this afternoon and
evening...however the storms across the south will likely produce
some strong wind gusts. Elsewhere...there will be some decent
instability and shear along and behind the front in addition to some
lowering freezing levels and decent middle level instability...which
could lead to some more organized storms capable of producing strong
to severe wind gusts and hail.

Also at the surface...the aforementioned cold front will work its
way across the area...roughly northwest to southeast but most of it
will become hung up along The Divide until reinforcements arrive
late tonight...cold enough for more snow above 8500ft in the
northern mountains. There will be some decent instability along the
front...with convective available potential energy roughly around 1000 j/kg even well into the
night...which could make for quite the light show east of The Divide
tonight.

Increased precipitation chances quite a bit for Sunday as the models
are actually in decent agreement on keeping showers and storms going
into Sunday evening. It will definitely be cooler across the
board...and will remain breezy across the south...so that part of
the forecast remains unchanged.

On Monday...we will be in the cooler back side of the trough with
lighter northwest flow. There are some hints at a disturbance trying
to zip by Monday late afternoon so some isolated probability of precipitation were included
for the afternoon but the timing on this could easily change before
then.

Long term...Tuesday through Saturday

Tuesday looks dry and seasonably warm. Wednesday appears to be
mostly dry as well and warmer...afternoon breeze in some places.
Some middle and high level moisture moves in which will result in some
afternoon clouds. Very isolated showers/storms for the mountains Wednesday
PM and early evening. The weather pattern gets more active next Thursday
through next Saturday. A southwest flow aloft with a trough of low
pressure over the Pacific northwest extending to the northern U.S and an
upper high over the SW desert states into the Southern Plains. The
GFS model gradually sags the trough toward northern Wyoming Thursday
through Saturday. The European model (ecmwf) model swings the trough across
northern Wyoming Thursday PM and night. It then leaves moisture around
Friday and next Saturday with weak disturbances nearby. On
Thursday the best chance of showers and storms will be over the northwest
third...say from Afton to Buffalo line. Isolated or nothing south
of this line. On Friday and Saturday the whole region could see a
shower or storm with the better chances over and near the
mountains and over the north where the trough is closest. A cold
front is forecast to move into the north Thursday and this front
moves south Thursday night into Friday. Best cooling over the
north Thursday and best cooling over the north and central Friday.
Slight moderation of temperatures next Saturday.

&&

Aviation.../18z issuance/

An evolving large upper level trough with several embedded
disturbances will push across the forecast area through the forecast
period. This system will destabilize the atmosphere and push a
couple of cold fronts south across north and central Wyoming this
afternoon and again Sunday morning. These boundaries will be the
focus for stronger thunderstorms this afternoon...primarily vicinity
and north of a kdub-kwrl-50sm S kbyg line. These storms will be
moving at a good clip...around 35 to 40 kts...with some of these
storms capable of marginally severe hail and wind gusts with brief
MVFR/IFR conditions. South of the front(s)...west-southwest winds at 20-30kts
will prevail in the afternoon along with isolated to widely scattered
thunderstorms. The main hazard with these storms will be erratic
outflow wind gusts near 45kts. Most of this activity will move east
into the High Plains this evening. The back side of this trough
will push into western Wyoming early Sunday morning spreading
scattered-numerous rain showers/isolated thunderstorms and rain and higher mountain obscurations across the
state...but primarily vicinity and north of a kjac-klnd-kcpr line.

&&

Fire weather...

Several waves of showers and thunderstorms are expected during the
next 48 hours...with increasing southwest wind ahead of it today and
persisting into Sunday. There will be elevated fire weather
conditions in the Sweetwater...Fremont...Natrona and southern
Johnson counties ahead of the cold front due to strong southwest
wind of 25 to 40 miles per hour with a brief period when rh's may drop into the
upper teens to low 20s. However...the cold front and even some
showers will move into the central part of the area by late morning
to around noon with higher rh's and some shower and isolated
thunderstorm activity. Other areas will have higher rh's but still a
period windy conditions and some thunderstorms.

&&

Riw watches/warnings/advisories...none.

&&

$$

Short term...Allen
long term...Arkansas
aviation...jb
fire weather...Allen

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