Area Forecast Discussion
Issued by NWS Riverton, WY

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713
FXUS65 KRIW 082037
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
137 PM MST Thu Dec 8 2016

.SHORT TERM...Tonight through Sunday

Imagery shows general longwave trof still stretched across the CONUS
with upstream embedded modest shortwave ridge getting poised to
enter the FA from the west and open the door to WAA and increased
EPAC moisture as it does...along with a 90 to 100 kt jet. Several
upstream embedded shortwave trofs also exist that will have some
affect on the CWA over the several days. SFC has general high
pressure in control of much of the west including WY ATTM. Bitter
cold and isolated patchy HZ/FG around the region this morning...but
no precipitation.

The Arctic air currently in place begins to turn Pacific with WAA
beginning under the approach of the ridge from the west. EPAC
moisture will also increase in this modest ridging regime, ushering
in a period of rather prolonged snow chances out west...enough to
prompt a Winter Storm Warning for for much of the western mountains
and valleys with an Advisory in place for the Upper Green and Upper
Wind River basins from late this afternoon through Saturday morning.
Tonight both the moisture from the EPAC and the nose of the jet will
arrive across southwest WY first, lifting north across the west
through the overnight period. With forcing offered from the jet,
good veering profile with height and the terrain, light snowfall
this afternoon and early evening will be replace by moderate to
occasionally heavy snow overnight...decreasing in intensity back to
light snow through much of the day on Friday. Friday night, as the
main body of the jet drifts north a bit, into a favorable position
with respect to the entrance area of the non-cyclonic portion of
the jet, a relatively strong associated shortwave trof will move
into and through the ridge with good PVA going on by Friday evening
and continuing until the axis passing sometime Saturday morning.
This will be the time of greatest snowfall potential with periods of
heavy snow mixing with winds 15 to 30 mph gusting 35 to 50 mph at
times. Snow will then taper off in intensity through the rest of the
morning period but never really stop as the nose of yet another
strong portion of the jet and developing shortwave moves into the
region...with the exit portion of this jet helping to increase
snowfall across the western to southwestern zones Saturday night.
Advisory level snowfall will be likely from the Tetons south across
the Salts/Wyoming Ranges through Sunday morning. Additionally, the
I80 corridor will be affected by this system Saturday night through
Sunday morning with both wind and light snow also bringing possible
Advisory conditions to Sweetwater County during this time.

This system of storms will mainly impact areas west of the Divide
with little snowfall east of the Divide except for light areas of
snowfall around LL convergent boundaries such as late overnight
tonight/Friday morning over zones 19, 20 and 22 in a modest LL speed
convergence situation as southwesterly SFC flow begins (less than a
half inch of isolated to widely scattered snow). Increasing gusty
southwest winds (gusting 20 to 30 mph) Friday will also offer some
chance for blowing snow and some occasional reduction in visibility.

Saturday...another, and somewhat better, chance for snowfall east of
the Divide will occur from early Saturday morning through the
daytime period as both an upper level shortwave and associated SFC
front move across the region. Perhaps a half inch to and inch and a
half with this one. Then yet another chance for snow east of the
Divide beginning early Sunday morning and lasting through the rest
of the morning with perhaps a half inch or less for much of the
area, to 2 to 3 inches over Casper mountain. Winds could become
problematic by early Saturday morning and lasting through the
morning hours with possible high wind criteria met or exceeded in
the Cody foothills and across portions of the Wind corridor. This
could also mix with snow on the ground and cause more widespread
areas of blowing snow...with near zero visibility possible through
the morning hours on Saturday. South pass and highways 789 and 287
would be likely areas to be most impacted...perhaps all the way to
Casper. A High Wind Watch has been hoisted for these locations in
the meantime.

.LONG TERM...

Sunday night through Thursday

Riddles still about this afternoon in the long range forecast as the
models continue to have a spat about who will be correct. As a
result, this is causing much head banging in frustration, and not in
the good 1980s metal kind of way. The models are fairly consistent
through Monday, with some snow continuing west of the Divide and
areas east of the Divide looking mainly dry with seasonable
temperatures.

Monday night and Tuesday is where we have some divergence. Both
models show a cold front dropping across the area and at this point
little precipitation with it. The big question is about
temperatures. The GFS is especially cold with a piece very cold air
moving over Manitoba with some minus 40C 700 millibar temperatures.
The European also has the cold air, but keeps the heart of it
further north. The result would be somewhat milder temperatures over
Wyoming for the middle of next week. At this point, we split the
middle since uncertainty is high.

There is also some disagreement for precipitation for next week. The
European keeps some precipitation over the south while the GFS
shoves it further south and keeps most of Colorado and points
further south. For now, we went with slight POPS but not any higher
for now just to see if the models can come into better agreement.

The models have come into better agreement for later Wednesday and
into Thursday in showing some warm advection snow spreading
northward across the state. As a result, we have added some slight
POPS to that period. Temperatures will also begin to moderate a bit
with somewhat warmer air pushing into the area.

&&

.AVIATION.../00Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions are expected through the period although there will
be a decent amount of high level clouds through the period.
Winds will remain light in the basins through most of the period,
but winds will be picking up from the southwest over
the mountains and in the central wind corridor from SE Fremont
County into the Casper area. Before the surface winds can respond
there could be a period of time where wind shear will be present at
KCPR this evening. A few showers may fell after 08Z tonight in the
mountains outside of KCPR and KLND, otherwise dry conditions are
expected through 00Z Saturday.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Widespread snowfall is expected to breakout across west and
southwest Wyoming from now through 03Z. After 03Z, Persistent MVFR
conditions with occasional IFR conditions are expected through the
night at area terminals, except for KRKS where MVFR/IFR conditions
are expected to be more brief in nature. Mountains will become
obscured late afternoon into the early evening, and remain obscured
through Friday afternoon with IFR to LIFR conditions. The main
exception will be over South Pass where ceilings will be higher, and
confidence for continued snow is not as high. Periods of snow will
continue through 00Z Friday. At this point, the heavier snow looks
most likely before 12Z Friday and after 21Z Friday.

&&

.FIRE WEATHER...

Fire Danger low for the foreseeable future as cold unsettled
conditions and general troughiness aloft continue over the region.
Several upper level disturbances will bring significant snowfall
west of the Divide through this weekend. Strong wind may be possible
across portions of northern and central Wyoming late Friday night
through Saturday morning.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late Friday night through Saturday morning
for WYZ003-019-020.

Winter Weather Advisory until noon MST Saturday for WYZ016-026.

Winter Storm Warning until noon MST Saturday for WYZ001-002-
012>015-023>025-027.

&&

$$

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



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