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Area forecast discussion 
National Weather Service Riverton Wyoming
313 PM MDT Thursday Sep 18 2014

Short term...tonight through Friday

Dirty ridge in place over Wyoming...with a couple of small shortwaves
moving through and cresting the ridge. Rain showers/thunderstorms and rain across Teton and park
counties from this morning on...continuing through the evening before
the ridge axis moves far enough east to bring small chances for
precipitation into central Wyoming overnight. Middle-day ml cape running between 750
and 1000 j/kg across the western zones...with 0-6 shear running around
25 kts late this afternoon...increasing into the evening
cape is decreasing however. West/ cloud decks running 8 to 10k
feet...strong outflow gusts of 45 kts will be possible along with very
small hail. Additionally...winds from both surface p gradient and those
aloft being brought down to surface are combining with locally low relative humidity
values to give red flag conditions across Natrona County.
Overnight...ridge continues to drift eastward as second SW rides through
and flattens the flow. This will in effect keep instability on the
higher side with at least isolated convective activity continuing
into the early morning hours before decreasing in coverage by
sun-up. Through the day Friday...split flow ensues with broad trough
swinging across the northern forecast area...and remnant of ridge across the
south...with weak surface front diving S across the forecast area as another small
SW gets squeezed through this flow/set-up with enough cooling aloft to
initiate and maintain some convection east of The Divide along and
south of the front mainly from southern Johnson through Natrona and brushing
eastern Fremont and Sweetwater counties. Storm Mode will carry through the
gusty wind possibility for another day with somewhat less cape
potential. Friday's set-up will also help in decreasing ambient wind
across the forecast area...with slightly cooler temperatures...higher relative humidity and less

Long term...Friday night through Thursday

Medium-range models are in fairly good agreement through the middle
portion of next week with only minor timing differences. Shortwave
will complete its transit of the forecast area Friday evening with
lingering convection across the east and north. Drier northwest flow
and surface high pressure will envelop the region Saturday with very
few clouds and temperatures falling closer to seasonal normals.
Elongated deformation axis trailing the departing shortwave will
stretch along our southern border Saturday before it wobbles north
again on Sunday. To the southwest...low will cut-off near Southern
California at the base of the long-wave trough Friday night into Saturday
before ejecting northwestward. This upper low will traverse the
Great Basin Saturday and begin to spread middle and high clouds into
the southwest early Sunday. Convection will become more scattered
Sunday afternoon across the southwest and slowly spread northeast
during the evening and overnight hours. 250mb jet may enhance
convection and coverage across the southwest centered around
00z/Monday. Both the GFS and European model (ecmwf) bring the upper low across the
forecast area Monday and Tuesday...with the GFS still trending
slightly faster than the European model (ecmwf). Could be some good rainfall across
the west from about Sunday evening through Monday afternoon before
the low begins to slide east favoring precipitation east of The
Divide. Snow levels will remain 10-11k no elevation snow
threat with this system. Looks as though convection will prevail
through at least Tuesday morning although the European model (ecmwf) lingers the
precipitation through the afternoon. Have trended precipitation chances
downward Tuesday afternoon but have lingered scattered low probability of precipitation
through the afternoon. Regardless...Monday will be the coolest day
of the period although it will be just slightly cooler than normal.
Temperatures then begin to warm Tuesday through the remainder of the
forecast period. Transitory shortwave ridge crosses the state should be a quieter and warmer day ahead of the next
large-scale trough taking shape over the Pacific northwest. This
trough will still be west of the forecast area Thursday and expect
strong southwest flow ahead of this digging system. Have bumped
temperatures for Thursday and may not have increased them
enough...but at this range will at least trend in that direction.
More interesting weather arrives next weekend as long-wave trough
pushes farther onshore.


Aviation.../00z issuance/

VFR conditions will prevail through the forecast period. A couple of
upper level disturbances will move into and through the region
tonight from the west...and then Friday east of The Divide. These
disturbances will help to produce scattered showers and storms along
the way...continuing and influencing terminal sites across the west
through 12z Friday....then again after 18z east of The Divide.
Outflow wind gusts to 35 knots will be likely with some of these
storms in addition to small hail. East of The Divide showers and
storms will be more isolated but will still be able to produce gusty
outflow winds at times. After 03z tonight showers will begin to
slightly increase in coverage in central Wyoming along with a couple of
isolated thunderstorms. The convective activity will then decrease
at/near all terminals by 12z...before increasing east of The Divide
by 21z. Brief forays of MVFR in low ceilings will be possible with any
of these storms through the forecast period.


Fire weather...

A trough of low pressure will move across the region late tonight and
Friday. This will allow a weak surface cold front to pass through
most of the forecast area Friday morning. The result will be higher
relative humidities Friday and Friday night. However...the airmass
dries out late Friday through Saturday. Sunday will also be dry with
some middle and high level clouds increasing over the south and west. weather concerns remain rather low across west
central Wyoming.


Riw watches/warnings/advisories...
red flag warning until 8 PM MDT this evening wyz280.



Short term...jb
long term...cnj
fire weather...kpl

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