Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240807
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
207 AM MDT Thu Aug 24 2017

.SHORT TERM...(Today through Saturday night)
Issued at 200 AM MDT Thu Aug 24 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/OR. Weakening split flow is ending and weak monsoonal
flow is returning to the FA between the upper high and low over the
srn/swrn CONUS helping to move modest moisture back into WY from the
south/southwest. Winds quiet over WY with the nrn Branch of the jet
well to the north over Canada and a lesser srn jet to the south over
the srn/cntrl Rockies. Modest upper level disturbance located across
western/cntrl WY. The SFC has continuing heat induced low pressure
over the Desert SW with low pressure and a weak front stretching
across nrn ID/MT. Mesoscale high pressure now over Yellowstone with
weak lee side troffing over cntrl and ern WY...driving some
occasionally gusty nw winds between the two. A few light
showers/thunder sprinkles across the area this morning.

Today will have decent moisture/clouds remaining across the
FA...increasing across the nrn FA...with the upper level wave still
over WY along with a 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 across/through the PAC NW, toward nw WY. This combined
upper level disturbance will then push through nwrn WY later this
afternoon, reinforcing the stationary front and then pushing the
front through northern WY through the evening. With relatively high
PWs will remaining and effective CAPE ranging between 500 and 1000
j/kg over the nrn/nwrn WY, faster moving showers/storms today will
produce some chances for brief heavy rain, small hail and gusty
wind...particularly across the northern third of WY.

Friday, flat ridging aloft, as the large PAC trof intrudes ewd with
a weak jet in tow, brings some modest EPAC moisture into the FA to
yield some isolated/widely scattered showers/storms across the nrn
half of WY with a few stronger storms possible over nwrn WY (CAPE
500 to 1000 j/kg, shear 25 to 35 kts). Marginally large hail (<=1
inch) and strong gusty winds possible for a short period. Saturday,
relatively flat  upper level flow becomes somewhat nwrly by
afternoon...with a SFC front moving south through the region e of
the Divide. Additionally, the FA drains of moisture with drying and
much more stability across WY. A couple mainly mountain
showers/storms will be all that`s possible.

.LONG TERM...(Sunday through Thursday)
Issued at 200 AM MDT Thu Aug 24 2017

Mean ridge holds over the Great Basin through the period with some
weakening for the second half of next week. GFS/GEM show more
erosion of the ridge Wednesday and Thursday of next week than the
Euro does and thus more pcpn/shower/tstm chances. Main weather
word much of this period will still be very warm to hot with above
normal temperatures continuing. Only by Thursday if the GFS/GEM
solution is more correct will we be down to more normal late
August temperatures. Sunday looks dry and quite warm with mid 80s
to around 90 east of the divide with upper 70s to mid 80s west.
Mountains will be mostly in the 65 to 80 range. Monday will be
about 3 to 5 degrees warmer east of the divide with very little
change west and mountains. There is a slight chance of a stray
shower/storm in the nrn mountains. Tuesday will see little change
in high temperatures with isold storms in the mountains. GFS/GEM
show some disturbances starting to move e/se across the area as
the ridge weakens leading to isolated to widely scattered
showers/storms across the north and west mainly. Further breakdown
of the ridge over our area by Thursday could lead to at least
isold storms just about anywhere along with temperatures
retreating to about normal.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday)
Issued at 200 AM MDT Thu Aug 24 2017

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

Isolated showers will linger this morning in the Wind River and.
Absaroka mountains. New shower and thunderstorm development will
occur after 18Z. This activity will be isolated across the central
and south. The activity in the north will be more scattered in
coverage. Showers and storms will be ending in the central and south
by 06Z Friday. In the north, showers and storms will linger through
09Z Friday. VFR conditions will prevail through the period. Local
mountain obscuration may occur at times.

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

Isolated showers and storms will linger this morning. New shower and
thunderstorm development will occur this afternoon and evening with
more coverage in NW WY than SW WY. The shower and thunderstorm
activity will decrease to isolated by 06Z Friday and end by 09Z Fri.
VFR conditions will prevail through the period. Local mountain
obscuration may occur at times.

&&

.FIRE WEATHER...
Issued AT 200 AM MDT Thu Aug 24 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). Relatively dry conditions east of the Divide
will produce minimum RH values dropping into the mid teens to lower
20s across the lower elevations, mid/upper 20s in the foothills, and
30 to 50 percent in the mountains. Otherwise, both today and Friday
will be cooler and more humid across much of central and northern
Wyoming with isolated to widely scattered chances for showers/storms
with small hail and gusty winds...particularly over northern
Wyoming on Friday.

&&

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

&&

$$

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



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