Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190500
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1100 PM MDT Fri Aug 18 2017

.SHORT TERM...(This evening through Saturday)
Issued at 307 PM MDT Fri Aug 18 2017

Shortwave ridge moving overhead today with a dry and warm day. The
ridge starts to flatten on Saturday as a weak shearing shortwave
approaches from the northwest. Very limited moisture and instability
is near the northern border late Saturday afternoon into Saturday
night but right now are sticking with a dry forecast up there. The
GFS is a little further south with a slight chance of showers/storms
showing up in the far nrn reaches Saturday evening. The only other
area that could possibly see something in the area is the far swrn
corner of Sweetwater where storms could form off the nearby Uinta
mountains. The approaches shortwave then weakens and gradually moves
across MT through Sunday while the flow begins backing to the wsw/sw
across our area. The end result will be some increase in mainly mid
level moisture with a few weak embedded disturbances and weak
instability. Isold afternoon and evening showers/storms are possible
west of the divide, mainly in and near the mountains. A few of these
weaker showers may linger overnight in the southwest over to the
Wind River Mountains.

On eclipse day, Monday, both the NAM and GFS is showing a weak
disturbance moving into the southwest during the morning and then
across the central portions after 18z. A band of clouds and isold
showers are quite possible from the Wind River Mountains southwest
to srn Lincoln County Monday morning. Clouds will be a concern for
most of the totality area with a general partly cloudy sky with
local mostly cloudy conds. Isold thunderstorms will move east of the
divide in the mid to late afternoon as heating returns following our
eclipse and the disturbance interacts with cold front sliding south
behind a second shortwave impacting points to the north, especially
Montana and the far nrn part of our area. Partly to mostly cloudy in
the afternoon east of the divide with those isold storms. There is
still some hope that the nrn moisture stays a little further south
and the nrn shortwave cloudiness stays north of the totality path
but as of now our partly cloudy forecast we`ve been carrying for
Monday morning still looks good. We`ll continue to monitor and it
may not be until Sunday until we can raise our confidence.
otherwise, expect warm to very warm conditions across Saturday
through Monday with only the clouds and isold storms trying to cool
us on Monday (plus the temporary cooling with the eclipse).

.LONG TERM...(Saturday night through Friday)
Issued at 307 PM MDT Fri Aug 18 2017

After the fun and frolic of the eclipse, we could see some lingering
thunderstorms over Johnson County Monday evening as the
clipper type shortwave bisects a cape gradient in that region. On
Tuesday expect warmer and drier conditions as a long wave ridge of
high pressure builds in from the west. The ridge should be overhead
by Wednesday. Wednesday will likely be the hottest day of the week
with low 90s common in the Big Horn Basin, well up into the 80s
elsewhere at the lower elevations east of the divide, and upper 70s
to low 80s west of the divide along with 60s in the mountains. Only
isolated late day thunderstorms are expected in the western mtns.
The threat of late day thunderstorms will increase Thursday and
Friday as the next upstream shortwave approaches the area. The
Canadian is the least aggressive with this feature as it keeps the
trough mainly in Canada (could be biased) with the trough axis
passing by to our north on Friday. The Euro and the GFS close it off
over Alberta with the GFS the most aggressive taking it down to a
544mb low with these latter two models tracking the trailing
shortwave across the CWA Friday night. Temperatures will gradually
cool off by a few degrees Thursday and Friday from the
showers/clouds as heights fall.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday evening)
Issued at 1100 PM MDT Fri Aug 18 2017

All terminals to be VFR through 06Z/Sun. Weather impacts will focus
on gusty west to northwest wind at KBPI, KPNA, and KRKS. Expect
winds to increase between 18Z-20Z/Sat with 12G22kts common. KCPR
will see 12-15kt winds most of the day on Saturday. A weak shortwave
will elongate along and near the Montana border late Saturday
afternoon and evening. Moisture and instability are very limited,
but a stray shower over the northern Absaroka Range cannot be ruled
out. Little more than some increased mid-high cloud at KCOD.

&&

.FIRE WEATHER...
Issued AT 307 PM MDT Fri Aug 18 2017

Elevated fire weather conditions will prevail the next couple days
due to very warm conditions and min rh`s in the teens. A west wind
will occur near and west of the divide Saturday at 10 to 20 mph at
times increase the fire weather conditions. Winds will increase
about another 5 mph on Sunday increasing the fire weather concerns
even more as the mountains and Sweetwater County see west winds of
15 to 25 mph with 5 to 15 mph east of the divide. Min rh`s on Sunday
will be quite low (even lower than Saturday) with 10 to 15 percent
quite common in the central portions (only slightly higher Saturday
but not by much).

&&

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

&&

$$

SHORT TERM...Skrbac
LONG TERM...Lipson
AVIATION...CNJ
FIRE WEATHER...Skrbac


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