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Area forecast discussion 
National Weather Service Riverton Wyoming
1152 am MDT sun Aug 2 2015

Short through Tuesday night

WV imagery showing stagnant large scale pattern with ridge western
Continental U.S....trough eastern Continental U.S.. upper level center of this high located over southern
nm with a stronger embedded wave over surface heat low over Arizona. Surface has
weak high p across western Wyoming with pressure lowering to the NE over the
northern plains.

Today...hottest day of the period...with moisture starting to slowly
work its way into the forecast area from the south through the day...allowing for
low chance precipitation across favored High Mountain ranges later this
afternoon/evening. upper wave continues to slow a does look like most of the precipitation will still be held off to
the south until Monday as upper ridge axis rotates clockwise...with
axis centering over Wyoming while better monsoonal-like moisture feeds
across Wyoming accompanying what was the Arizona disturbance/now mesoscale convective vortex mentioned
above. Precipitable waters will be higher than normal...approaching an inch in some
locations...while storm motion will be variable around the
developing mesoscale convective vortex...some slow moving showers/storms that come and go
and some faster moving showers/storms that may train from southeast to northwest
over the Wind River mountains. Most forcing for Monday day and night
will come from this feature across western and central Wyoming...while Tuesday
will add a developing surface Lee cyclone into the mix...helping to
increase moisture and forcing across northern and northestern Wyoming through the end of
the forecast period. Take all these conditions together...and there will
some a possibility for isolated areas of flash flooding between
Monday evening and Tuesday evening. Hard to locate specific
locations at this time...however areas along and south of the Wind River
mountains and late Monday...and in and around the Bighorn Mountains
as the surface low spins up over eastern Wyoming by later Tuesday afternoon look
to be the most susceptible. Additionally...Monday afternoon the eastern
zones may see a brief period of isolated stronger storms with gusty
strong winds and small hail possible along with brief heavy rain.

Long term...Wednesday through Sunday

A couple of weather systems will move through the area Wednesday and
Thursday. Showers and storms will be the result both days with
greater coverage over and near the mountains and across the lower
elevations of the north. Isolated coverage for the central and
south zones. Friday looks mostly dry at this point with a ridge of
high pressure moving overhead. Moisture returns in a SW flow aloft
on Saturday into Saturday night. Widely scattered showers and
storms to develop and this activity could linger past midnight
Saturday night. Next Sunday looks unsettled at this time with a
weather system passing through the area. GFS model is more
aggressive with the system while the European model (ecmwf) model weakens the
system as it moves by. For now widely scattered showers and storms
for the west and north with lesser coverage central and south.
Plenty of time to watch this weather scenario unfold for next
weekend. Models are trending a little cooler with high temperatures
Wednesday and Thursday with cooler 700mb temperatures and more clouds.
Very warm to hot on Friday with more sunshine and warmer 700mb
temperatures. Then slightly cooler next Saturday and Sunday.


Aviation.../18z issuance/

Persistent high pressure will help to keep the majority of showers
and thunderstorms away from area terminals this afternoon. An
increase in moisture and instability may result in the initiation
of convection over the central mountains...which would potentially
increase turbulence across the aviation mountain passes. Shower
and thunderstorm activity will lag this evening across much of the
area with widespread VFR conditions. A stronger wave will move
towards southern Wyoming Monday morning...with cloud cover
expected to increase across southern and western Wyoming.


Fire weather...

Enhanced fire danger again today as hot and dry conditions continue
with minimum relative humidity values expected to remain in the low to middle teens
across the lower elevations...with upper teens and 20s over the
mountains. Expect generally light to moderate winds today from the
north to northeast at 5 to 15 miles per hour. Additionally...the very dry
atmosphere with higher pressure at the surface and aloft will allow
only isolated chance for precipitation across the higher mountain
ranges. Changes are on the way however...for both Monday and Tuesday
as an upper level disturbance drags along some good monsoonal-like
moisture into the region. Increasing chances for showers and
embedded thunder will begin Monday morning across the far southern
zones...with areas west of The Divide seeing first drops...spreading
into the central and northern forecast area Monday night...ending up
across eastern/northeastern Wyoming by Tuesday afternoon...lasting
into the evening. Fire danger will decrease both Monday and
Tuesday...except for a couple of possible fire starts due to
lightning strikes.


Riw watches/warnings/advisories...


Short term...Braun
long term...Arkansas
fire weather...Braun

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