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Area forecast discussion 
National Weather Service Riverton Wyoming
349 am MDT sun Sep 21 2014

Short term...Sunday through Tuesday night

All is quiet right now across the County Warning Area after a
beautiful late Summer day on Saturday. However...things will be
changing over the next couple of days. The culprit will be an upper
level low now swirling over California that will move northward
toward the Great Basin through the day today. Clouds will be on the
increase from south to north today and with the clouds some showers
will begin to move in from the south. The models have slowed just a
bit so we slowed the arrival of any showers and thunderstorms as a
result. Overall placement for convection for today looked good
though with coverage mainly west of The Divide through the daylight
hours. With more sunshine...expect northern portions of the area to
have the warmest temperatures. By tonight the low will move into
Idaho with a deeper moisture plume moving northward across much of
the County Warning Area with chance for showers spreading across
much of the County warning. Again...continuity looked reasonable so
few changes were made. Models are consistent in bringing the
steadiest precipitation to the western mountains so probability of precipitation and quantitative precipitation forecast
were highest in this area. As for the chance of any of that white
stuff that starts with the letter S...with this system warm any now
should remain well above pass level.

The low then lifts turns east and moves across Wyoming. Both the NAM
and GFS place the low over northern Wyoming for Monday afternoon and
night. However...these lows are fairly fickle and hard to predict
the movement. Again...with continuity looking fairly reasonable few
changes were made. With the low heights most areas will have a
chance of showers and thunderstorms although pinpointing a more
active area is still difficult.

By Tuesday ridging will begin to build back over the area from the
south and west and bring a return to drier and more settled
conditions. The one exception may be over the western mountains
where a bit of residual moisture and a shortwave moving through the
flow may be enough for isolated convection to linger.

Long term...Wednesday through Sunday

High pressure ridge will be overhead to start the period...bringing
warm and dry conditions. This warm and dry trend will continue into
Thursday...with middle to upper 80s still possible across areas east of
The Divide. As the high pressure system exits to the east on
Thursday...breezy southwest winds will be possible...mainly across the
wind corridor from Rock Springs to Casper. These breezy winds could
even continue into Friday. Models still differ on the large trough
system for the weekend. GFS is slightly slower then recent
runs...but still progressive in nature...and brings
shower/thunderstorms across western Wyoming starting Friday
afternoon and evening. This slower timing will also allow for
continued warm temperatures in the 80s...east of The Divide on
Friday. GFS solution then indicates widespread precipitation on
Saturday. The best chances for precipitation will come across the a punch of dry air at the middle levels could limit
precipitation to only isolated thunderstorms for areas east of The
Divide. GFS solution then brings most areas showers and
thunderstorms on Sunday. European model (ecmwf) trended back towards a slower and
slightly more southern track. This would delay precipitation to
later Saturday...with the better chances coming on Sunday and into
early next week. Given the differences...put at least slight
chance of showers and thunderstorms in for the weekend across most
areas...with higher probability of precipitation across the west.


Aviation.../12z issuance/

Showers and thunderstorms will develop over southwest Wyoming after 19z.
This activity will spread northward into northwest and central Wyoming
after 06z Monday. Wind gusts to 35 knots will be possible with
storms. Local MVFR conditions will occur in southwest Wyoming after 00z
through 12z in areas of heavy rain and lower ceilings. Mountains may
be obscured at times in the west. Otherwise VFR conditions will
prevail over the region during the forecast period.


Fire weather...

An upper level low and associated moisture approaching from south
and west will result in increased cloud cover and some showers
spreading west of The Divide in Sunday afternoon. Showers will
spread across much of the area on Sunday night into Monday as the
low moves near and across Wyoming. Showers will end from southwest
to northeast on Monday night as the low exits. Ridging building from
the west will bring drier and warmer weather for the middle of next
week. All in weather concerns should remain fairly
minimal into early next week.


Riw watches/warnings/advisories...


Short term...hattings
long term...Anglin
fire weather...hattings

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