Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 111816
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1116 AM MST Thu Jan 11 2018

.SHORT TERM...(Today and tonight)
Issued at 332 AM MST Thu Jan 11 2018

The next Pacific shortwave now making landfall onto the Pacific
coast will be advancing southeast toward Wyoming in the northwest
flow today through tonight with the troughpa occurring around 12Z
Friday. A substantial amount of snowfall is expected in the far
west this afternoon, tonight, and Friday morning as a westerly
difluent upper flow, strong 50 to 70 mph upslope flow, and
isentropic lift all combine with the deep Pacific moisture to give
far Western Mountains and valleys advisory amounts of snowfall
with some local warning amounts in a relatively short period of
time. The snow will diminish in intensity Friday afternoon. Only
lighter amounts of snow will spill over the divide to the east,
mainly in Northern WY over the mountains. This will be a colder
weather system than the once on Tuesday night and Wednesday. 70
knot 700mb winds within the anticyclonic flow will surface along
the southern wind corridor tonight as well as along the foothills
along the east side of the Continental Divide. Strong northwest
winds can be expected in the Upper and Lower Wind River Basin
after midnight tonight once the winds veer at 06z Friday night.
Most of the snowfall will end Friday night with only spotty light
areas of snow expected in the northwest through Saturday night as
light Pacific moisture streams in from the northwest up and over
the building high to the west. Do not expect much in the way of of
fluctuations in temperatures through the period as a sfc front
osculates close to the divide.

.LONG TERM...(Friday through Wednesday)
Issued at 332 AM MST Thu Jan 11 2018

|Sharp ridge holds over the west to start this period but cold swd
moving shortwave and arctic front will move into the nrn zones by
Sunday afternoon and then move through most areas east of the divide
by Sunday evening. Sharply colder temperatures, gusty north wind and
a brief period of snow will be likely across our nrn and ern zones
with areas a little further southwest like Fremont County more
challenging. Big picture supports the best chances in our nern
corner and the majority of the guidance has a period of light snow
from the nrn and ern Bighorn Basin over to Johnson and south to
Natrona County and that seems reasonable for now. Much colder Monday
behind this system Monday with some lingering light snow or flurries
in our nern/ern quadrant. Strong surface high that comes down will
likely result in a cold day Monday east of the divide, especially
Johnson County and possibly Natrona County. Lowered highs some in
this area but not as much as they may need to be with time. Surface
high near 1050mb and far nrn trajectory of this system should bring
down some cold air that will likely not be flushed out that quickly
on Monday as the blend gave us. Cold night Monday night with a clear
sky and light wind and possibly some fresh snow cover east of the
divide. Inversions for the basins and valleys Tuesday with shortwave
ridging with some warming aloft for warming in the mountains. A
weakening shortwave is expected to skim the far west around Tuesday
night with a bout of light snow and gusty mtn top wind. The ridge is
slowly weakening as the second half of next week and that may lead
to a stronger system at the end of the period (next Thursday).
Models are obviously struggling with the pattern change as we
transition from the western U.S. ridge to a developing trough along
or just off the west coast late next week and beyond but the odds of
snow increase Thursday into Thursday night, especially across the
west with increasing wind in many areas. One side note, expect
some patchy fog in the basins east of the divide this morning.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday)
Issued at 1116 AM MST Thu Jan 11 2018

West of the Continental Divide...KJAC, KBPI, KPNA, and KRKS

Moist northwest flow aloft has reached far western Wyoming late
Thursday morning. Light snow already falling at KJAC with IFR/MVFR
conditions likely to persist through much of the period. The snow
will combine with wind early Thursday to further reduce visibility.
A disturbance will clear the area shortly after 12Z/Fri, but expect
enough moisture in northwest flow to continue intermittent snow
showers through the remainder of the period. Best chance for
IFR/MVFR conditions at the other three terminals will come between
08Z-14Z/Fri as the disturbance slides southeast through the state.
Expect west-northwest 25-40kt gusts at KBPI, KPNA, and KRKS
beginning early Thursday morning. Mountain tops will be obscured
across western Wyoming by 00Z/Fri and across much of the forecast
area by 06Z/Fri.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Wednesday`s snow left a moist boundary layer and subsequent fog and
low clouds at KWRL and KCOD this morning that have been slow to
erode. All terminals to be VFR by 20Z/Thu. Otherwise, the main story
will be gusty downslope winds and low-level wind shear, especially
near the foothill terminals of KCOD and KLND. Have added VCSH for
late tonight at most terminals in association with a disturbance
that will swing through between 08Z-14Z/Fri. Winds and lingering
snow showers should decrease between 15Z-18Z/Fri. Mountain tops will
be occasionally obscured after 08Z/Fri.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 332 AM MST Thu Jan 11 2018

Another storm system will be moving into western Wyoming later
today. Moderate to locally heavy snow will return to the west
from this afternoon through the day Friday. Expect strong
southwest to northwest winds over many areas tonight. Smoke
dispersion will be very good for many locations today and fair to
good both Wednesday and Thursday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 11 AM this morning to 5 PM MST
Friday for WYZ001-002-012>015-023>025.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...CNJ
FIRE WEATHER...Lipson



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