Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 192221 CCA
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
147 PM MDT Mon Jun 19 2017

.SHORT TERM...(This evening through Thursday)
Issued at 145 PM MDT Mon Jun 19 2017

Imagery shows modest (dirty) ridge trof pattern. west to east,
across the CONUS. Upstream embedded ridge running shortwave that
extends from nrn California northward to BC will be headed toward
WY...arriving later Tuesday. The SFC has general high P building
across WY. No precipitation across the CWA today...or expected
through Tuesday morning.

Today through Tuesday morning: The FA is east of the ridge axis and
generally drying and stable under NW flow aloft and high P at the
SFC. Seasonal highs today with modestly gusty west to northwest SFC
winds west of the Divide this afternoon.

Tuesday afternoon dirty portion of the ridge axis arrives across the
wrn FA along with the leading edge of the aforementioned embedded
ridge running SW trof. Warmest day of the week expected.
Additionally, this will cause a modest increase in mid/upper
moisture, weak lee side troffing, increased instability, MLCAPE
values 500 - 800 J/kg, DCAPE of 800 - 1500 J/kg, and effective shear
of 15 to 25 kts. This pattern will produce a period of isolated to
widely scattered showers/thunderstorms moving quickly to the east at
30 to 40 kts with little rainfall and occasional lightning. The
showers and storms will likely produce strong gusty winds at
times...a few approaching or exceeding severe levels...especially
across northwest and northern WY from Yellowstone through the Big
Horn Basin...possibly as far east as Johnson county. They should die
off by or shortly after mid-evening.

Wednesday through Thursday: The Tuesday`s trof exits the region
while another pushes through the FA on its heels into the Plains
with upper flow turning back nwrly once again and drying. With
little moisture to work with, expectations are low regarding
precipitation production for most locations. However, the passage of
the second upper trof will allow for the formation and passage of a
clipper front east of the Divide by Wednesday afternoon and through
the evening. As P GRADS tighten in response to this passage, gusty
west to northwest winds will be expected west of the Divide and over
portions of central WY south of the front. To the north of the
front, a wind shift to the north behind the front is expected.
A very slight chance for a shower/storm will possible over
portions of northern Natrona and Johnson County - including the
Bighorn Mountains - Wednesday evening mainly along/near the front,
where the northerlies meet the westerlies. If they happen...fast
moving, little rain, wind and small hail producers likely.
Thursday will be dry and breezy west of the Divide...with gusty
winds expected once again across southern/southwestern WY and the
higher mountains. No precipitation expected.

.LONG TERM...(Thursday night through Monday)
Issued at 145 PM MDT Mon Jun 19 2017

Some significant model differences to start this period in regards
to how the energy with our upper low in the Gulf of Alaska affects
us Thursday night through Saturday. The problem is with the handling
of the low as it opens up and moves east towards us while at the
same time, we start to see rapidly building heights originating from
a upper high out near 40N/135W. The Euro keeps this energy/wave
along or north of the Canadian border generally while the GFS shows
it dropping more se into Montana and nrn Wyoming Thursday night into
Friday as it responds to building upstream heights. The second piece
of the upper low are both shown coming through Friday night and
Saturday (Euro is stronger with the center). The challenge is which
one will be right with the first piece of the upper low Thursday
night and Friday. The GFS 5-wave chart would at least support a
compromise with a chance of a stronger/further west wave like the OP
GFS. Will have to lean towards a middle of the road for now but we
need to watch it closely as it would be a lot windier ahead of it
and a lot cooler behind it Friday if the OP GFS is more right. They
are both quite cool Saturday with H7 temps dropping to below 0C even
into srn Fremont County (and a little of nern Sweetwater Count) at
some point Saturday (early on the GFS and later on the Euro). Either
way, quite cool for late June. Will also gradually play some
showers/slgt chance tstm spreading from the north during the day
Friday into the central zones later Friday night and Saturday. Since
the Euro has more with the second piece of the upper low, it`s
cooler weather lingers into Sunday with H7 temps still at or below
0C at noon on Sunday down to Fremont County. GFS has the ridging
building in more quickly behind it`s weaker second system.
Confidence low towards a rapid warmup with the strengthening ridge
near or off the coast so model blend (which is about half way in
between) is ok for now. Otherwise, dry behind these couple systems
early next week and warming up again, especially Monday and
beyond.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday afternoon)
Issued at 145 PM MDT Mon Jun 19 2017

VFR conditions will prevail across the region through Tuesday.
However, moisture in the flow around an upper ridge over the
Desert Southwest will allow for some gusty high based showers and
thunderstorms to develop by Tuesday afternoon and evening,
especially in and near the mountains.

&&

.FIRE WEATHER...
Issued AT 145 PM MDT Mon Jun 19 2017

All fuels are currently in green-up and below critical levels for
all locations. Fire danger remains low across all mountain
areas...but will continue somewhat elevated in the afternoons across
the lower elevations east of the Divide and over portions of
southern WY. These areas will generally coincide with relatively low
RH values and gusty winds through the end of the week. Winds will
gust out of the west to northwest 15 to 25 mph today...15 to 30 on
Tuesday...and 20 to 35 on Wednesday across this area of concern. No
appreciable precipitation expected today. Tuesday afternoon and
evening, fast moving high based showers and storms will be possible
on an isolated to widely scattered basis. These showers and storms
will produce little rainfall and strong winds...some gusting 40 to
60 mph at times well out ahead of the storms themselves. Lightning
should be on an occasional basis.

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Skrbac
AVIATION...Skrbac
FIRE WEATHER...Braun



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