Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 171621 AAA
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1020 AM MDT Thu Aug 17 2017

.SHORT TERM...Thursday through Saturday night

Over the next few days the pattern reminds me in a way of the classic
song "Spinning Wheel" by Blood, Sweat and Tears. One of the most
famous verses in that song is "What Goes Up, Must Come Down." In
this case, what will be going down into the first part of the
weekend is the chance of convection. We still have a couple of
showers out there this morning but they are light and will be gone
by morning. This afternoon a weak disturbance will brush by the
area to the north and east. It could get close enough to bring
some isolated convection to portions of the northern mountains and
more likely in Johnson County. Otherwise, with drier air moving
into the area a vast majority of the area will be dry. Friday at
this point looks like a typical late summer day with plenty of
sunshine and mainly dry conditions. By Saturday, a bit more high
level moisture may creep into the area from the south but any
showers would hold off until Sunday.

And what goes up will be the temperatures as heights and
thicknesses rise. With breezy conditions developing in some areas,
this will warm temperatures back close to seasonal averages today.
By Friday and especially Saturday, above normal temperatures will
be found across the entire area with summer making a return. Some
of the warmer spots like Greybull, Worland and possibly Casper
could see highs in the 90s by the first part of the weekend.

.LONG TERM...Sunday through Thursday

Eclipse Outlook on Monday...The latest medium-range model runs
have trended faster with a shortwave moving eastward across the
northern Rockies into the northern High Plains Sunday night-Monday
morning. Any shower activity is expected to be east of the area
by sunrise Monday with an overall clearing trend. The other
concern is a band of potential high cloud cover along a
deformation area across southern/central Wyoming. Models have
trended further south overall, away from the path of totality with
this high level moisture. Hopefully, these trends continue;
however, these trends are not well established at this point. The
other factor for clouds will be slight instability along the
Divide. Again, the overall trend has been favorable here, less
moisture, and instability across NW Wyoming (Jackson Hole and
Grand Teton National Park). The best chances of late morning
cumulus buildups and afternoon showers and thunderstorms will be
across the Wind River-Salt-Wyoming Ranges.

Backing up a bit to Sunday (Eclipse Eve?), the aforementioned
shortwave is expected to move east across Montana, encountered
the better moisture and instability along and east of a lee side
surface trough that will be across eastern Wyoming. Any showers
and thunderstorms across west and central Wyoming will be mostly
isolated and confined to the higher terrain.

Post-Eclipse (Tuesday through Thursday)...Upper ridge is expected to
amplify over the Rockies on Tuesday and slowly shift eastward
into the High Plains by Thursday. An upper low is expected to
shear from southern California into the Great Basin Wednesday and
Thursday, and bring increasing chances of showers and
thunderstorms mainly to western portions of Wyoming. Tuesday and
Wednesday are expected to be the warmest days of this period with
most highs in the 80s to lower 90s across the lower elevations.

&&

.AVIATION...18Z Issuance

VFR conditions will occur at the terminal sites through the period.
Isolated showers and thunderstorms will occur after 20Z in the
northern WY mountains until 03Z. Northern Johnson county may also
see isolated showers or a storm from 21Z until 03Z. Expect brief
mountain obscuration at times. VFR conditions are expected Friday.

.FIRE WEATHER...

Drier conditions will move into the area today as high pressure
builds across Wyoming. A weak upper level disturbance will
approach move across Montana this afternoon and may bring isolated
thunderstorms mainly to Johnson County. Breezy conditions will
develop across western and southern Wyoming this afternoon.
Although critical fire conditions are not expected...near critical
conditions may occur in portions of southwestern Wyoming. Relative
humidity will fall into the teens across many of the lower
elevations this afternoon. Mixing and smoke dispersal will range
from fair in some of the northern basins to very good to excellent
in portions of southern Wyoming.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Ross/Lipson
WEATHER...Hattings



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