Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 090541
Area Forecast Discussion
National Weather Service Riverton WY
1041 PM MST Thu Dec 8 2016
.SHORT TERM...Tonight through Sunday (Issued at 137 PM MST)
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
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
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
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.
EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
VFR conditions are expected through the period. Wind will increase
KRKS to KCPR after 14Z and then a gusty wind through 06Z Sat. Only
isolated snow showers are expected during Friday into Friday night.
WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
Widespread snowfall will occur through Friday night. MVFR to IFR
conditions will prevail for most of the period. Local LIFR conditions
are possible. KRKS will see light snow at times overnight but it
should end by 13Z. Expect mountains to be obscured.
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.
High Wind Watch from late Friday night through Saturday morning
Winter Weather Advisory until noon MST Saturday for WYZ016-026.
Winter Storm Warning until noon MST Saturday for WYZ001-002-