Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 160457

Area Forecast Discussion
National Weather Service Riverton WY
1057 PM MDT Tue May 15 2018

.SHORT TERM...This evening through Friday
Issued at 125 PM MDT Tue May 15 2018

Next approaching upper low off the cntrl CA coast will be the kicker
to get our forever low over nrn NV to finally lift ne. The nrn part
of the low lifts and shears apart over MT while at least a piece of
the low gets absorbed into the new low which comes onshore tomorrow
and then moves across srn NV into srn UT Thu ngt into Fri. At the
same time, a shortwave over srn Canada aids in the development of a
surface high strengthening over srn Alberta/far swrn Saskatchewan on
Friday. This surface high combined with lowering pressures over srn
CO aid in decent frontal push with n-ne winds and upslope east of
the divide on Friday. Weak QG forcing moves up into srn WY late
Thursday night and then moves into Fremont and Natrona Counties
during the day Friday along with some weak, moist upglide. General
trend on the GFS is farther south with the upper low and thus weaker
overall forcing/lift. The NAM remains further north with better
overall forcing. They both however show general widespread shower
potential over Fremont and Natrona southwest to Sweetwater and srn
Lincoln Counties. They are both showing the front backing into far
wrn Sweetwater or srn Lincoln County with a heavier pcpn band along
the convergence axis. Backing up a few days, the ern fringes of our
NV upper low spits out a disturbance through the evening hours that
keeps convection going in our far west (lifting nwd) through the
evening hours. The remnant low lifts nnewd across ID on Wednesday
with sern edge of the trough axis impacting the nwrn zones with weak
forcing and cooler mid levels for a decent shot of showers and a few
tstms which may come off the Absarokas later in the day into the
evening. Weak boundary in nrn Johnson and potential weak vort may
aid in a few relatively stronger storms in nrn Johnson. As the new
low replaces the old one over NV, we see multiple weak disturbances
lifting ne across the area combined with mid level cooling proving
more shower/isold tstms chances the wrn and nrn half. Then back to
the Friday potential. Will increase shower potential further or as
much as I can. It`s not a huge rain maker this far north most likely
but enough to produce measure rain for a decent chunk of the area
and hamper outdoor activities at times Friday, especially by
afternoon and into Friday night.

.LONG TERM...Friday night through Tuesday
Issued at 125 PM MDT Tue May 15 2018

The system affecting our weather on Friday night is expected to be
drifting NE across Utah. Surface winds out of the northeast will
bring cloudy conditions and areas of rain, higher mountain snow east
of the Divide while showers and isolated thunderstorms develop west
of the Divide where airmass will be more unstable. This system
should shear across the central Rockies into the High Plains Friday
night and Saturday Low level E-NE flow will continue to keep cloudy
conditions with chances of showers along and east of the Divide,
with some afternoon/evening thunderstorms across the west. Chances
of showers will then decrease Sunday and Monday with a warming
trend. Another Pacific upper low is expected to move into the SW
U.S. Monday and Tuesday of next week and begin spreading showers and
thunderstorms northward into the area on Tuesday.


.AVIATION...For the 06Z TAFS through 06Z Wednesday Evening
Issued at 1057 PM MDT Tue May 15 2018

VFR conditions anticipated through the forecast period. Shortwave
associated with the broad and weakening upper low over southern
Idaho is continuing to produce isolated showers and thunderstorms
across portions of northwest Wyoming late Tuesday evening. Coverage
should decrease through the remainder of Tuesday night, but isolated
showers are likely to linger in far west Wyoming until sunrise. The
upper low will rotate slowly northeast and clip northwest Wyoming
Wednesday as an upstream kicker low moves onshore in California. The
best instability Wednesday afternoon will be north of a line from
Afton to Dubois to Cowley. There is a more favorable wind profile
for sustained convection Wednesday afternoon and evening. KJAC is
the most likely terminal to see showers/storms with KCPR, KRKS, and
KWRL the least likely.


Issued AT 125 PM MDT Tue May 15 2018

The best chance for showers and isolated thunderstorms will be
across the western mountains and in adjacent foothills through 03z
before gradually decreasing and ending from south to north. Wind
will remain light to moderate outside of any shower or thunderstorm.
Wednesday will be warm and dry east of the divide with many areas
seeing afternoon rh`s in the upper teens to around 20 percent. Areas
of 10 to 20 mph southwest wind will increase the fire threat. In the
northwest, scattered showers and isolated thunderstorms will return.
Smoke dispersal will be very good to excellent in the afternoon. A
more unsettled period is expected Friday and Saturday as a low
pressure system moves just to the south of the area with an
increased shower chance across most of the area.





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