Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 270453 AAA

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1053 PM MDT MON SEP 26 2016

.UPDATE...Added 06z aviation discussion.



VFR conditions with unlimited ceilings will continue to prevail
through 12z Wednesday under a ridge of high pressure.  Patchy fog or
isolated LIFR/IFR may partially obscure terrain in the western
valleys (vicinity KJAC-KBPI-KPNA) and the lower river valleys of the
central basins through 16z Tuesday. Light surface winds of 10 knots
or less will prevail across the area.


.PREV DISCUSSION... /issued 133 PM MDT MON SEP 26 2016/

SHORT TERM...Monday night through Tuesday

High pressure will control the weather through the period and bring
quiet weather. Southwest flow will increase a bit on Tuesday and
bring somewhat breezy conditions for favored areas. This will also
allow temperatures to rise several degrees above normal with highs
in the 80s a possibility for some of the warmer spots like
Casper, Worland and Greybull. Some patchy fog may form late
tonight near bodies of water, but should be patchy and burn off
relatively quickly during Tuesday morning.

LONG TERM...Tuesday night through Monday

High pressure will continue over the area from Tuesday night through
late Wednesday.  The ridge axis moves east of Wyoming, allowing the
southwest US low to move NE over the area on Thursday.  Max
temperatures Wednesday should push into the 70s as a result in the
lower elevations.  Cloud cover will increase from the SW Wednesday
night with winds becoming more south to southeast.  NAM is
suggesting isolated showers and thunderstorms before midnight while
the GFS holds off until after midnight.  NAM is then more bullish
with showers early Thursday before noon across western WY as the
shortwave trough moves from utah through western WY.  Surface winds
to become more southwesterly east of the divide Thursday as some
showers and thunderstorms may move off the mountains over the east
side of the basins.  Thursday afternoon looks good for moderate
showers and isolated thunderstorms across western WY.  Expecting the
convective activity to remain into the evening hours and increase to
scattered across the northern half of the state.

The shortwave trough then leaves the area Friday, but a large scale
trough is currently being forecast to move into the Pacific
Northwest.  Result will be upper and mid level SW flow over the
area, continuing the instability and mid level moisture over the
area.  Friday afternoon should again see slight chance of showers
and thunderstorms, mainly across western WY and Sweetwater County
into SE WY.  GFS and ECMWF in fairly good mesoscale to large scale
agreement up to this point in the forecast.  Both models then
suggesting a leading shortwave trough to move from Oregon into Idaho
and SW MT on Saturday.  This pattern should give rise to moderate
showers and isolated thunderstorms across western WY with slight
chance of showers in southern WY.  Temperatures to remain in 60s and
70s in the lower elevations heading into the weekend.  With the
large scale digging into western Oregon and eastern Idaho on Sunday,
atmosphere over WY will become more unstable and flow should bring
more moisture to western part of the state.  Sunday afternoon and
evening continue to see moderate showers over western WY. Colder air
associated with the system is forecast to move into the western part
of the state late on Sunday into Monday morning, transitioning the
bulk of the precipitation from rain to snow.  Snow levels at this
time are expected to reach 8000 to 7500 feet Monday morning in
western WY.  Monday should be largely wet and cool again with shower
activity over much of the area and max temps making it to about 60.
Details in the pattern over the weekend have moderate certainty at
this time due to differences with the energy rotating around the
large scale trough.  Regardless, October looks to start off cool and
wet with some nice snow in the western and NW mountains.


With high pressure in control of the weather through Monday, VFR
conditions are expected through 00Z Wednesday. However, patchy fog
may cause some local obscurations near bodies of water. Wind
should remain light to moderate through the period. &&


Fire concerns should remain low through Tuesday. High pressure
will build over the area and bring mild to warm temperatures along
with mainly light winds. Relative humidity will remain above
critical levels. Mixing and smoke dispersal will be poor to fair
today and largely fair on Tuesday.


.RIW Watches/Warnings/Advisories...


SHORT TERM...Hattings
LONG TERM...McDonald
FIRE WEATHER...Hattings is the U.S. government's official web portal to all federal, state and local government web resources and services.