Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231725

Area Forecast Discussion
National Weather Service Riverton WY
1125 AM MDT Wed Aug 23 2017

.SHORT TERM...(Today through Friday night) Issued at 200 AM MDT
Wed Aug 23 2017

Imagery shows an upper ridge/trof pattern across the CONUS from west
to east with a weak cut-off upper low still over cntrl California
coast and the Bermuda High also still intruding across sern/scntrl
USA. Large broad trof out over the EPAC/PAC NW just starting to
intrude on WA. Weak split flow is ending and weak monsoonal flow is
returning to the FA with circulation around the upper high and low
over the srn/swrn CONUS helping to move modest moisture back into WY
from the southwest. Winds quiet over WY with the nrn Branch of the
jet well to the north over Canada and a lesser srn Branch to the
south over the Desert SW and into the cntrl and srn Rockies.
Associated (with the srn jet) "upstream" disturbance located to the
south and west of WY from NV ewd through UT and into wrn CO. The SFC
has continuing heat induced low pressure over the Desert SW with a
weak slow moving front stretching from nwrn MT through ern WY and
then down along the Front Range before turning out into the southern
Plains. Weak high pressure lays to the west of this front. A few
light showers currently near the southern border with CO.

Today, will have the ridge axis directly over wrn WY with some
modest monsoonal flow along with the weakening shortwave disturbance
rotating into and across WY from the southwest. Additionally, expect
weak lee side troffing to develop ahead of/with the passage of the
upper level disturbance. It also looks like some minor frontogenesis
may occur as the upper wave passes through this afternoon/evening.
This will allow isolated (low elevations and foothills) to widely
scattered (mountains and over/near front) showers/thunder to blossom
in the afternoon and evening across much of the FA except for the
Big Horn Basin and Johnson county. With PWs still forecast to rise
significantly above average values - to around .75 inch for much of
the FA - and light steering flow aloft, locally heavy rain...most
probably with terrain tied mountain storms... will be possible in
addition to small hail and gusty winds...especially over the
southwestern quad of WY.

Thursday will have decent moisture/clouds remaining across the FA
with the upper level still over WY along with a weak front and SFC
trof just to the east of the front. As the upper ridge gets slowly
squeezed ewd toward the Plains, the embedded upper shortwave that
still remains over the FA will get an extra boost from a shortwave
ejecting out from the large PAC trof then moving into/through the
PAC NW. This combined upper level disturbance will then push through
nwrn WY, reinforcing and pushing the front through northern WY late
in the afternoon and through the evening. With relatively high PWs
remaining, but faster moving showers/storms, the region can expect
some chances for brief heavy rain, small small hail and gusty
wind...particularly across the northern third of WY.

Friday, flat ridging aloft, as both the srn upper high butts heads
with the large PAC trof, will also yield some modest split(ting)
flow over the forecast area. Moderate monsoonal flow from the south
will keep moisture over WY while an embedded upstream shortwave
associated with a weak upper jet intrusion moves into/through WY
from the west...producing another day of stronger showers/storms.

.LONG TERM...(Saturday through Wednesday)
Issued at 200 AM MDT Wed Aug 23 2017

Mean ridge holds over the Great Basin area through the period with
a generally dry and warm to very warm period. GFS is showing a
stronger shortwave on Saturday passing across the area, especially
north half, but the Euro is much weaker and further north. Based
on the longwave ridge location, the Euro solution will likely be
more correct. So bottom line, only a slight chance of nrn mtn
showers/storms Saturday afternoon and evening. Dry and warm/very
warm Sunday and morning with just a few/slim chance of a few late
day storms over the nrn mtns. GFS and a a lesser extent the Euro
is showing a little more energy moving se across the area Tuesday
with the isold coverage expanding swd into the central mtns but
that`s about it. Some hint of a stronger disturbance moving
through the high Wednesday but we`ll see as mean ridge is holding
strong. Little more coverage is in the forecast for now, mainly
in the nrn/cntrl mtns with isold in the far south but main weather
through the medium range will still be the very warm temps each
day. Highs will be in the 80s to occasionally lower 90s in the
Bighorn Basin with upper 70s to mid 80 west of the divide (lower
elevations) with generally 65 to 80 in the mountains/foothills.


.AVIATION...18Z Issuance

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

Mainly SKC conditions and light winds will continue over KCOD-
KWRL, with SCT-BKN mid to high level clouds now encroaching over
KRIW-KCPR. Isolated convection will be possible near KRIW and KCPR
this afternoon, but will end between 03Z and 06Z. The SCT-BKN band
of clouds will continue its movement toward the NE tonight,
leaving a mid level of SCT to BKN band by 12Z Thu.

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

A band of SCT-BKN mid-high level clouds continue surging
northward over the area, now impacting all terminals. Isolated
showers also continue to occur over the area, south of KJAC. These
showers will continue this afternoon, with additional
thunderstorms expected by 20Z. This convection is expected to
wane/decrease in coverage between 03Z-06Z Thu. Except for brief
MVFR/IFR conditions in convection, VFR conditions are expected. As
convection clears tonight, only SCT mid-level clouds will remain
into Thu morning, becoming FEW after 15Z Thu.


Issued AT 200 AM MDT Wed Aug 23 2017

Fuels at critical levels for most of the low elevation forecast
zones in addition to mountain zones above 8000 feet across western
and central Wyoming. (Please see the Fire Weather Forecast product
for more information). Elevated fire weather concerns for portions
of central and northern Wyoming today, mainly due to low RH values
expected by this afternoon along with occasional diurnal mixed out
wind gusts between 15 and 20 mph over the Wind River Basin. The dry
conditions will produce minimum RH values dropping into the mid
teens across the lower elevations, lower to mid 20s in the
foothills, and 25 to 35 percent in the mountains. Otherwise, both
today and Thursday will be cooler and more humid across much of
southern, central, and western Wyoming with isolated to widely
scattered chances for slow moving heavy rain producing
showers/storms...particularly over the mountains.




LONG TERM...Skrbac
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