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Area forecast discussion 
National Weather Service Riverton Wyoming
334 am MDT Friday Aug 29 2014

Short through Sunday night

A fairly potent short wave is keeping some convection going into the
early morning hours...this wave will continue to trek east at
a fairly quick pace...exiting our forecast area and into eastern
Wyoming around noon. There will be some ridging behind the trough
with a weak short wave rotating atop the ridge...mainly affecting
Montana but supplying some broad lift to the northern zones...this
in conjunction with a cold front left behind by the overnight system
will supply some chance for a shower or thunderstorm across the
northern half of the forecast area. Instability is rather modest but
shear across the northern half of the state should be enough to
sustain some of the storms that pop up over the higher terrain long
enough to carry them into the adjacent foothills and basins.

There will be a break in the...relatively tame...action this evening
ahead of the next short wave that could begin to bring the next wave
of precipitation into the west as early as 12z Saturday. The latest
model runs have started bringing in this wave a little slower...but
kept previous slight chance probability of precipitation to cover the uncertainties in

The bulk of the energy from this broad...dynamic digging trough will
arrive during the day on Saturday. There will be plenty of dynamic
forcing aloft to support scattered to numerous showers with some
embedded thunder during the afternoon with some stronger storms
capable of producing marginal hail...strong gusty winds and some
torrential downpours later in the afternoon and into the evening.
Across the south...ahead of the surface cold front...there could be
some elevated fire concerns as the strong southwest wind aloft will
easily mix down to the surface during the day with gusts to 50 miles per hour
possible across Sweetwater County and possibly up into eastern
Fremont and Natrona counties...however the relative humidity will be
marginally low...likely only dipping into the upper teens during the
day. However...the air at the surface will be dry enough across the
south...combined with the swift unidirectional speed shear aloft...
to elevate the high wind threat with any storms that do form in this

Sunday...the trough continues to trek east across the
quickly depends upon which model you believe...but the eastern half
of the forecast area may still be on the more favorable front end of
the trough during the afternoon...which could support more
thunderstorms...while other solutions put western and central
Wyoming in the subsident back edge of the trough...which would make
for a much quieter day across the a few gusty winds.
Much cooler conditions for sure...with continued gusty west to west-southwest
winds across the area west of The Divide behind the exiting surface

Long term...Monday through Friday

On Monday...GFS and Gem models are drier than the European model (ecmwf) on Monday.
This is due to timing of a weather system in the northwest flow aloft. The
European model (ecmwf) model brings a system across the area while the GFS/Gem are
weaker and faster. For now will keep low chance of showers/storms in
the forecast which means leaning toward the European model (ecmwf) timing. Isolated
early evening showers/storms then decreasing clouds Monday night.
The flow aloft turns west for Tuesday and a dry day expected.
Wednesday looks mostly dry for most places. Some moisture is
forecast to move in Wednesday afternoon which would lead to isolated
showers or storms...mainly over and near the mountains. Flow aloft
backs to the SW for Thursday and Friday. More middle and high level
moisture is forecast to return to the region...more so on Friday.
In addition...a frontal boundary will be sliding south into
northern Wyoming Thursday afternoon and it looks to push south into
central Wyoming by Friday morning. Then the boundary may or may not
move back north Friday PM in the central zones...GFS says
no...European model (ecmwf) says yes. The frontal boundary will also be a focus for
showers and storms...not to mention the moisture return from the
SW. Will keep the better chance of showers/storms in the northwest and
north Thursday. Then isolated showers and storms for most places
next Friday. As for high temperatures...Monday will be mild and little
changed from Sunday highs. Warmer highs for Tuesday and Wednesday.
Then cooler for the northwest and north Thursday with a few degrees of
cooling central and south. Highs Friday cooler north and central
with slight cooling south.


Aviation.../12z issuance/

An upper level disturbance will continue to push east across Montana
early this morning. Scattered showers and isolated thunderstorms
are expected to be mostly east of the Interstate 25 corridor after
12z...but possibly extending south-southwest into vicinity kcpr through around 14z.

A couple of more ripples in the flow will push east across the
northern Great Basin...across Wyoming Friday afternoon and evening.
Mainly isolated -tsra/-shra will occur across most of the area with
possibly more scattered activity over the high terrain. This activity
will mostly dissipate shortly after sunset. The next Pacific system
will spread middle and high clouds vicinity and west of The Divide 06z-12z
Saturday along with isolated -shra.


Fire weather...

Another system will bring more isolated thunderstorms across the
northern half of the state. The main threat from these will be gusty
wind to 45 miles per hour and lightning. On Saturday...a stronger cold front
will sweep east across the area with increasing southwest winds
ahead of it and isolated to widely scattered thunderstorms along and
behind the cold front. Fire weather concerns will increase ahead of
the front for a few hours as min rh's drop into the teens and SW
winds blow 20 to 35 miles per hour with frequent gusts up to 50 miles per hour in the more
wind prone areas. However...all the recent rain may help keep the
min rh's above critical levels. We will continue to monitor.


Riw watches/warnings/advisories...


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

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