Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 192008
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
208 PM MDT Tue Sep 19 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 206 PM MDT Tue Sep 19 2017

Unstable wly flow in the nw is leading to building convective snow
showers in the nw with even a few lightning strikes nearby. This
will likely lead to a period of heavier snow showers in the nw into
early this evening. Will keep the advisories going to cover a quick
shot of 1 to 4 inches in some areas. Strong jet and low level flow
will likely produce a period of strong wind in the Lander and Cody
Foothills into this evening. The next rather complex storm will be
developing over the Great Basin later Wednesday into Wednesday
night. The first part of this is system over the sern Gulf of Alaska
which helps to soften up the flow (dig a trough) over the Great
Basin Wednesday night and Thursday while another system rotates
westward from B.C., then off the PacNW coast and then into our
developing mean trough later Thursday into Thursday night. Even more
energy looks like it will drop swd into this developing mean trough
Thursday night. With the sharp upstream ridge forecast to develop
and the mean longwave trough holding through the weekend, this
system will be slow to move. For the short term, it adds up to a
significant weather make from Wednesday night onward. The first one
will likely impact the far west Wednesday night with a good shot of
snow above 8500 to 9000 feet initially but then lowering to at or
near the valley floor by sunrise Thursday. quick shot of 6 to 12
inches of snow could impact the Tetons swd to the Salt and Wyoming
range Wednesday night into about 15z Thursday with 1 to 3 inches
in the valleys. The Wind Rivers will also begin to get into the
action after about 03z Thursday. I think the far west will see a
break (lightening) of pcpn on Thursday but the Wind Rivers may
continue to see significant snow above 8500 feet or so. South Pass
looks like mostly rain Wednesday night and Thursday but Togwotee
and the higher elevations of the north half especially look rather
snowy. Trough dynamics increase late Thursday into Thursday night
with colder temperatures working in and increasing jet streak
support (even possible dual jet structure for the nwrn areas).
Cold front that works south Thursday will continue to usher in
colder air with rain quite possibly changing to snow in the Cody
Foothills late Thursday night/early Friday. The NAM even has a
cold air damming signature in the Lander Foothills. The bullseye
area for pcpn seems to be the Wind Rivers, Absarokas and nearby
foothills for this Thursday night and Friday period. More pcpn
will occur elsewhere in the Friday night and beyond period. More
on that below. Will be issuing a winter storm watch for the Wind
Rivers from 03z Thursday through Friday 18z. Other advisories will
likely be needed for the other mountain ranges with time. The
Absarokas may also need an eventual winter storm watch for
Thursday night onward. Busy short term!

.LONG TERM...(Wednesday night through Tuesday)
Issued at 206 PM MDT Tue Sep 19 2017

Large, longwave trough will be very slow to move over the weekend
and into the first part of next week. Earlier ECMWF runs had this
trough lingering into Monday, while the GFS was more progressive in
moving it east. However, the latest GFS has trended notably slower.

Jet axis and favorable dynamics will shift into central and eastern
sections of Wyoming on Saturday. Precipitation will decrease west of
the Continental Divide Friday night as energy lifts east and north.
Scattered to likely precipitation chances will remain along and east
of the Divide Saturday. Temperatures cool enough by early Saturday
that some basins around 5,000 feet plus or minus 500 feet could see
very light snow accumulations. Extensive cloud cover and ongoing
precipitation Saturday will keep temperatures in the 40s for most
locales, some 20-30F degrees below normal. Certainly the
unseasonably cold air will lead to additional mountain snow
accumulations through at least Saturday night and likely through
Sunday. While less organized Sunday, precipitation will continue
east of the Divide with similar conditions anticipated. The bulk of
the moisture and energy shifts more to our east on Monday with some
drying from the northwest especially later Monday. Low temperatures
Sunday night and again Monday night will need to be monitored as any
clearing could generate widespread sub-freezing temperatures.
Conditions remain cool Tuesday, although warmer than previous days.
Precipitation chances will be less, but if the GFS solution becomes
reality there would be a need for increased precipitation chances
Tuesday, especially across the south.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 206 PM MDT Tue Sep 19 2017

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

Conditions will be VFR through the forecast period, with only some
isolated MVFR lingering through early Tuesday evening around the
Bighorn Mountains. The area around the Bighorns will also see
occasional mountain top obscurations through 03Z/Wed before
conditions improve in and around the higher terrain. Gusty westerly
wind of 15-30kts will diminish between 00Z-03Z/Wed. Downsloping
westerly flow at KCOD tonight will necessitate LLWS. Wind shear
could be 45-50kts within the lowest 2K feet AGL after 06Z/Wed.
Southwest wind will increase between 17Z-20Z/Wed in response to the
next weather system digging into the Pacific Northwest and
Intermountain West.

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

Precipitation has slowed across southwest Wyoming this afternoon.
Conditions should improve to VFR between 22Z/Tue and 01Z/Wed.
Otherwise, conditions will be VFR through Wednesday. Only lingering
precipitation after 03Z/Wed will be across northwest Wyoming, where
light snow showers could persist until 09Z/Wed. Mountain tops will
be frequently obscured through 03Z-06Z/Wed, an until 09Z/Wed in the
Yellowstone region. Post-frontal westerly wind 15-30kts at KBPI,
KPNA, and KRKS along with the southwest wind of similar speed at
KJAC will diminish early this evening. The next weather system will
back the upper-level winds to the southwest on Wednesday with
moisture beginning to reach far west Wyoming during the afternoon.
Have trended toward MVFR at KJAC late Wednesday afternoon and
included VCSH at KBPI and KPNA.

&&

.FIRE WEATHER...
Issued AT 206 PM MDT Tue Sep 19 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).

The first of several storms is winding down west of the divide with
just scattered areas of mainly higher elevation snow into tonight
and Wednesday before the next major storm bears down on the region.
Many areas will remain windy into this evening with this system
passing through. Gusty to strong winds will continue on Wednesday in
many areas. Min rhs` will drop into the 20s east of the divide but
the west will remain moist with min rh`s from the 30s in the
southwest to 50s and 60s most other areas. The next storm will bring
significant precipitation into the west Wednesday night through
Friday with snow levels lowering to the valley floor in most areas
west of the divide. Even east of the divide snow levels will drop
into the foothills late Thursday night and Friday morning. It will
remain unsettled through at least this weekend and possibly into
early next week before any significant drying and warming returns.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 9 PM MDT this evening for WYZ001-
002-012-014-015.

Winter Storm Watch from Wednesday evening through Friday morning
for WYZ014-015.

&&

$$

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


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