Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 290833

Area Forecast Discussion
National Weather Service Riverton WY
233 AM MDT THU SEP 29 2016


As the 1030mb sfc high built southeast into the northern Plains
this evening, winds have increased from the southeast across many
areas, especially east of the divide. These winds will eventually
tap into moisture to our south over Colorado and Utah where showers
and thunderstorms have been erupting as our southeast winds continue
to veer to the southwest by later this morning. Some showers have
already made it into extreme western Wyoming including Afton. As
the ridge axis moves to our east, a weak trough will track
northeast across wyoming later today. Ahead of the trough, the
south southwest flow will become somewhat weakly difluent along
with negative lifted indices across the area with increasing capes
in NW WY today. We can expect a good chance of showers and a few
afternoon thunderstorms in western wyoming and in the mountains
today along with isold activity across Sweetwater County. Some of
these showers will linger past midnight tonight. There will be
enough lingering instability and moisture for some high mountain
isold showers and some thunderstorms on Friday afternoon and
evening. Then on Saturday, a closed low will swing around the
bottom of the next prominent long wave trough just off the Pacific
Northwest Coast, which in turn will significantly increase the
southwest difluent flow aloft over the area, in much the same
fashion last weeks storm system did, and will result in a frequent
succession of northeast moving showers and a few thunderstorms in
western Wyoming within a marginally unstable atmosphere late
Saturday afternoon and evening. Temperatures will remain rather
mild through the period as todays weather system will be warm and
the the next weather system on Saturday will at least start out


Mean longwave trough remains over the west through the period with a
slight progression ewd with time but still just overhead by
Thursday. This overall pattern favors our system next week to be
another strong and dynamic system with the potential for a
reasonably slow moving system Monday through Tuesday. Pattern would
seem to favor a system tracking from the Great Basin into the south
half of Wyoming later Monday into Tuesday. Some spread in the model
solutions do exist with the track as it moves over the state but
enough confidence to continue increasing pops during this period.
It has the makings of another significant rain and snow producer. In
fact the 00z GFS is even a couple degrees colder in the mid levels
which would make it interesting for the rain/snow line Tuesday
morning in Central Wyoming. This system originated north of
Kamchatka and is currently moving through Nrn AK so it may be a
little colder than our last one. So lead shortwave Sunday will
produce some shower/isold storms before things begin to
deteriorate from Monday into Monday night...continuing east of the
divide through Tuesday. Significant lower elevation rain (with
some brief mixture above 6K or so) and mountain snowfall very
possible. Models diverge more Wednesday into Thursday as to
whether we get some shortwave ridging with dry conds or the
potential for a faster wnw flow with an embedded disturbance or
two. Still mild Sunday to start this period then turning sharply
colder later Monday into Tuesday. Wednesday will likely be chilly
yet behind our system with some moderation Thursday.



The remnants of an upper level low pressure system will lift north
northeast across the eastern Great Basin and western Wyoming
Thursday.  This will bring isolated to widely scattered showers
across the southwest, to a vicinity KJAC-KRKS line by 12z. Scattered
showers and thunderstorms are expected to develop across the south
and west Thursday afternoon.  Thunderstorm activity is expected to
be most numerous along vicinity KPIH-KJAC line.  Occasional mountain
obscurations and brief MVFR conditions are possible vicinity and
west of the Continental Divide Thursday afternoon.  Some of this
activity will spread across north central Wyoming late Thursday
afternoon and Thursday evening - vicinity KCOD-WRL.  The system will
pull out into the northern plains early Friday morning with
decreasing clouds SW-NE overnight.



A rather warm upper level low pressure trough over Southern
California and the Desert Southwest will move north northeast
bringing increased mid and upper level moisture into western Wyoming
with some showers developing over the southwest mountains this
morning. The low will weaken significantly as it moves north but
will continue to pump up moisture into Wyoming with increased
clouds, showers and isolated thunderstorms today and Friday with
generally mild temperatures.


.RIW Watches/Warnings/Advisories...None.



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