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Area forecast discussion 
National Weather Service Riverton Wyoming
246 PM MDT Tuesday Sep 16 2014

Short term...tonight through Wednesday

An weak upper level disturbance that moved across Idaho this morning
and brought some showers will cross western Wyoming this afternoon
into this evening and bring a scattering of showers and
thunderstorms. There is not a lot of moisture with this system...but
with decent temperature/dew point spreads there could be some gusty
winds with any thunderstorm. Meanwhile...east of The Divide expect
mainly dry weather except for the Big Horn range. Gusty breezes this
afternoon across eastern portions of the County Warning Area should
subside tonight. Showers should also wane tonight but not end across
the higher elevations.

As for Wednesday the models have trended slightly drier as today's
shortwave shifts to the east. The moisture from olide will still be
to the south. The GFS has trended very dry and the European is
fairly dry as well. The NAM remains a bit more active but holds any
convection mainly over the higher elevations. As a result we reduced
probability of precipitation a bit for Wednesday afternoon and confined them mainly to the
higher elevations. As for temperatures...look for another warm
day...especially east of The Divide with highs rising well into the
80s in the lower elevations.

Long term...Wednesday night through Tuesday

The upper level ridge axis will be moving through early in the
period with largely benign sensible weather. There could be a weak
short wave rounding the top of the ridge early in the period
interacting with some modest instability producing a few showers and
storms early...otherwise fairly calm and mild conditions Wednesday night.

The warm up continues into Thursday with high temperatures near 90 in the
Big Horn basin but increasing clouds will likely counteract any
warming...especially with the lowering sun angle of middle September.
The broad scale lift associated with an approaching trough to our
west will encourage a few more showers and storms on Thursday that
will likely last well into Thursday night as an associated cold
front moves across the area. West winds will begin to increase
somewhat by this afternoon as the aforementioned Pacific trough
approaches from the west...tightening the gradient aloft as a Lee
side trough digs at the surface.

A splitting trough moves across the area on Friday keeping some
showers and storms going...especially east of The Divide where some
more moisture will encourage some more robust cape values. A
reinforcing shortwave may also aid in thunderstorm development and
coverage depending on when it arrives. Otherwise...looks like a dry
Saturday with a little more uncertainty as we go into Sunday.
Previous runs have had solutions that were completely out of phase
with one another but the latest 12z run of the European model (ecmwf) is beginning to
come into line with the GFS...that brings a closed upper low that
forms on the southern edge of that splitting trough we are expecting
to get the northern piece of this Friday...right into eastern Great
Basin. It is expected to slowly track east-northeast from Nevada/Utah
Sunday night into northeastern Colorado by Tuesday morning. It could
be peeling off some tropical moisture from the Baja California region early
next week...which could result in some decent rainfall potential but
given the recent uncertainty of model solutions lately...went very
conservative with probability of precipitation but at is stands looks like the
southern half of the state has the best chance of a wetting rain.

Aviation.../00z issuance/

VFR conditions will prevail through 00z Thursday. Isolated showers
and thunderstorms will continue over western Wyoming until about 04z
Wednesday. Gusty outflow wind to 35 knots is likely near storms. The
terminal sites west of the Continental Divide may be impacted by
showers or storms. More showers and thunderstorms are possible on
Wednesday after 19z but mainly across the mountains and west of The

Fire weather...

A ridge of high pressure will continue to control the weather into
Wednesday and bring warm and dry conditions to most of the area east
of The Divide. An upper level disturbance will move through western
Wyoming and bring isolated showers or thunderstorms this afternoon
and again Wednesday afternoon. A breeze will once again develop
across eastern portions of the County Warning Area but should remain
below critical levels.

Riw watches/warnings/advisories...


Short term...hattings
long term...Allen
fire weather...hattings

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