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000
FXUS65 KRIW 180934
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
334 AM MDT Fri Aug 18 2017

.SHORT TERM...Friday through Sunday night

Tranquil weather looks to be the rule today as high pressure builds
across Wyoming today. This will mean dry weather for all areas along
with a good deal of sunshine. There may be a bit of a breeze at
times West of the Divide but nothing unusual for this part of the
country. It will be quite warm as well with many of the lower
elevations climbing into the 80s and some of the warmer spots such
as Greybull and Worland seeing highs returning to 90s. At this point
Saturday looks fairly similar although with maybe a few more clouds
in some areas. A bit of moisture will try to creep northward but any
thunderstorm would likely be confined to southwestern Sweetwater
County but even here they would be few and far between.

There will likely be a few showers and thunderstorms around on
Sunday as a weak shortwave moves towards the area. As usual, the GFS
looks wetter than the NAM. At this point, the best chance would be
across the western mountains but again coverage is expected to be
fairly sparse. There will be more clouds around as well with the
increased moisture into the area. Temperatures will remain fairly
similar, with above normal conditions and 90s possible in many of
the lower elevations East of the Divide.

.LONG TERM...Monday through Friday

For Monday, Eclipse Day, there remains a significant degree of
uncertainty in the forecast for sky cover late Monday morning. The
main upper air features will be a ridge over the NW U.S. over an
upper low off the southern California coast. A clipper system is
forecast to be diving south in NW flow across western Montana
Monday morning. One moisture source region for potential cloud
cover is seen this morning on IR satellite imagery in the form of
mid-level clouds stretching west to east from central California
into west central Utah. All models bring this moisture northward
into a confluent westerly flow across southern Wyoming. The GFS
brings and enhances this moisture further north, into the path of
totality between Jackson and Casper, than either the Canadian or
ECMWF. The preference here is for the stronger westerlies (and
less noisy ECMWF/Canadian) to keep this moisture/cloud cover
shearing ENE mostly along and south of I-80. The second source of
potential cloud cover will be ahead of the clipper diving south
across western Montana. There is general agreement in global
models to keep the clouds with this system north of the
Wyoming/Montana border through 18z (noon MDT) Monday with a dry
slot across NW Wyoming. Given the overall lack of consistency
among any model over the last 24 hours, confidence remains low to
stray either way from a partly cloudy (35-45 pct cloud cover)
along and at the time of totality. The satellite and forecast
trends with the clouds/moisture currently across western Great
Basin will be key.

After the big event, shortwave diving across Montana is expected to
encounter increasing CAPE across northeast Wyoming where a few
thunderstorms may initiate off the Bighorn Range before
strengthening across the High Plains.  Slight instability will
result in isolated weaker storms over the Western Mountains Monday
afternoon.  Winds on Monday will generally be light.

For the rest of the forecast period, upper ridge is expected to
amplify over the Rockies Tuesday and Wednesday as upper low drops
down from the Gulf of Alaska along the British Columbian coast. Weak
upper low off the southern California coast is forecast to shear NE
across the Great Basin/Central Rockies Thursday and Friday, bringing
more cloud cover and chances of thunderstorms mainly to the south
and west along with a cooling trend.

&&

.AVIATION...12Z Issuance

VFR conditions will occur at the terminal sites through the period.
Wind speeds of 12 knots or less will occur. No showers or
thunderstorms are expected across the region.
&&

.FIRE WEATHER...

Dry and warm conditions are expected across all of Wyoming today
with high pressure in control of the weather. A bit of an afternoon
breeze will develop West of the Divide, but will remain below
critical levels. Relative humidity will fall below 15 percent
across many of the lower elevations. Mixing and smoke dispersal will
range from fair in some of the basins to very good across southern
Wyoming.
&&

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

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Ross
FIRE WEATHER...Hattings