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908
FXUS65 KRIW 210535
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1135 PM MDT Wed Sep 20 2017

.SHORT TERM...(This evening through Thursday)
Issued at 303 PM MDT Wed Sep 20 2017

Next storm system is developing to our west with the main
circulation center over srn WA/far nrn OR early this afternoon with
developing baroclinic stretching from nrn NV into swrn ID. Another
cold and area of good cold air advection is also noted just off the
Pacific Northwest coast. These two combined with the building
upstream ridge will help with the development of a significant upper
low (closed off) over swrn ID by late Thursday afternoon. As the
ridge builds and folds over, the upper low moves very little into
Friday before beginning to shear later Friday. Part of the energy
shears off to the ne into MT while the srn portion lingers to our sw
through the end of this period (and likely longer!). Baroclinic
boundary begins impacting the west this evening and as the trough
strengthens Thursday to our west, strong QG forcing will develop
across the entire west with widespread rain and snow. The nrn piece
of the jet may also provide some right entrance region support
tonight in the nw. Snow levels are tricky but it looks like they
have a good chance of changing to snow for a while in the Jackson
Valley towards 12z with the potential for a couple inches of snow
Thursday morning. Further south, it may just stay rain or a mixture
in the Star Valley and Upper Green River Basin (maybe the higher
spots could see some brief changeover). Next crew can continue to
evaluate and see what the snow potential looks like, especially
around the Jackson Valley. It`s not out of the question, that a
quick 1 to 4 inches falls between Jackson and the Moose/Moran area
tomorrow morning. H7 temps only fall to -2 to -3C at h7 but
significant pcpn could be falling at sunrise and that may be just
enough to cool the lower levels enough for a changeover. Otherwise,
snow levels will stay rather high in the Salt/Wyoming Range over to
the south half of the Wind Rivers where snow levels may be up around
10K feet yet but lower to around pass level or a little lower
towards Togwotee Pass. Another area which could see significant pcpn
developing Thursday will be in our far nrn zones behind
strengthening cold front along with increasing QG forcing Thursday
into Thursday night and some jet streak support at times. It looks a
little too warm yet for COD to changeover to snow Thursday morning
but colder air filters in Thursday night. Strong QG forcing shifts
into our nwrn quadrant Thursday night with the best pcpn likely
occurring from YNP se to the northern Wind Rivers. A strong jet
couplet may also develop in the nwrn quadrant. Heavy wet snow is
likely during this period. The baroclinic boundary will also bring a
swath of showers into the swrn and western part of Central Wyoming
Thursday night. The ern sections will remain dry. As the low slowly
edges ewd and the cold front becomes better established east of the
divide Friday, the pcpn chances start to increase from west to east
across areas east of the divide (srn sections as pcpn continues up
north). It gets more tricky Friday night into Saturday as the trough
shears apart. How much energy will remain in the southern part of
the trough. The GFS and even moreso on the Euro, has a decent closed
circulation with another bout of precipitation that develops Friday
night into Saturday. The NAM only tries to wrap some moisture into
our far ern zones on Saturday. I would think with the upstream ridge
development there will be a decent srn low with a much better chance
of pcpn east of the divide Friday night and Saturday. The GEM also
supports this idea. Most of the precipitation in our central
sections between Riverton/Lander and Casper are generally too warm
for snow but areas above 6,000 to 6,500 feet could certainly seem
some snow Friday night into Saturday.

.LONG TERM...(Thursday night through Wednesday)
Issued at 303 PM MDT Wed Sep 20 2017

By the middle of the weekend, the mean trough over the western US
looks to be slightly positively tilted and just to the west of
Wyoming, though the primary areas of lift and precipitation are to
the east of the trough.  The characteristics of the trough are still
a bit sketchy this afternoon with all models splitting off a
southern area, each with varying intensities.  The Canadian Global
is the most bullish with a southern stream cutoff, and the GFS and
ECMWF are less so.  As a result, Sunday should continue to have cool
air over the area and a chance to slight chance of precipitation
southeast of a line from Buffalo to Kemmerer.  Rain is expected in
the lower elevations with snow above about 8000 feet.  The trough
axis should move eastward right over Wyoming by late Sunday night
shifting the focus of the precipitation into SE WY.  Monday though
should still see some mountain showers, particularly over the
Bighorns, Wind Rivers, and Absarokas.  Temperatures won`t warm much
on Monday with 700mb temps still around -3 to -5C.  The split flow
on Tuesday should result in clearing skies and slightly warmer
temperatures with a few high elevations showers.  700mb temps should
warm then on Wednesday to 1 to 3C as fold over ridging impacts the
northern Rockies.  Northern WY may see some slight showers due to a
weak shortwave trough in NW flow, but the Canadian and ECMWF have
this feature and the GFS does not.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1134 PM MDT Wed Sep 20 2017

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

IR satellite imagery and regional radar showing large precipitation
shield moving into far western Wyoming in southwest flow aloft.
Precipitation should increase through the remainder of the night
with KJAC gradually deteriorating to MVFR. Little doubt now that
KJAC should be all rain through the day Thursday and probably
through the forecast period. Mountain tops will be obscured across
western Wyoming through Thursday evening. Heavier rain will create
occasional mountain obscurations. Warmer model trends and
observations indicate snow levels could be closer to 10K feet MSL
through the day Thursday. KJAC southwest surface wind will veer to
the north during the mid-late afternoon hours Thursday. VFR to
prevail for most of the period at KBPI, KPNA, and KRKS. Possible
MVFR at KBPI and KPNA late Thursday afternoon as moisture slides
slightly east and ceilings lower. Tightening pressure gradient and
daytime mixing will lead to southwest 25G35kt wind at KRKS, while
KBPI and KPNA look to remain 10G16kts in the afternoon.

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

All terminals except KCOD to be VFR through the forecast period.
Precipitation will be generally confined to areas along and west of
the Continental Divide, with drier air in place across areas to the
east. However, northerly frontal push will provide a better chance
of MVFR ceilings at KCOD beginning late morning through the
remainder of the period. This frontal push provides northerly flow
in the Big Horn and eventually Wind River basins. Speeds of 10-15kts
will spread north-to-south in these basins during the late afternoon
and early evening. Besides KCOD, best chance for low-end VFR or MVFR
ceilings will be at KWRL beginning around 00Z/Fri. Occasional higher
gusts of 16-22kts possible with a secondary surge around sunset. At
KCPR, dry conditions with along with gusty southwest wind 15G25kts
are expected through about 23Z/Thu before secondary northerly surge
produces a north wind Thursday evening.

&&

.FIRE WEATHER...
Issued AT 303 PM MDT Wed Sep 20 2017

Fuels at critical levels for most of the low elevation forecast
zones east of the Divide and across southern Wyoming. (Please see
the Fire Weather Forecast product for more information).

A major storm system will begin to impact the region tonight with
the beginning of a rather prolonged period of precipitation across
western Wyoming with from 1 to 2 inches of liquid expected across
the western mountains through Friday. Heavy areas of snow also
expected at times in the mountains. Otherwise, winds are expected to
remain quite gusty out of the west to southwest today as strong
winds aloft transfer energy down near the SFC...gusting 30 to 55 mph
at times over much of the forecast area...especially across the
mountains. Minimum relative humidity today will generally range from
near 20 across the low elevations of central Wyoming to as much as
70 percent in the mountains. Min RH`s will be much higher Friday and
Saturday. Smoke dispersion in the afternoons will be fair to
excellent on Thursday...and fair to good Friday.

&&

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

&&

$$

SHORT TERM...Skrbac
LONG TERM...McDonald
AVIATION...Jones
FIRE WEATHER...Skrbac