Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 230943
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
243 AM MST Mon Jan 23 2017

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT

Light snowfall has been occurring during the last several hours
underneath a strengthening backing difluent southwest semi upslope
flow along with fairly good isentropic lift as vort lobes track
northeast ahead of a strong closed low off the Pacific Northwest
Coast. In addition, left front quad dynamics appear to be
responsible for a light band of snowfall that broke off the main
area of snow to the southwest that survived the trek over the Divide
and is now moving east northeast and crossing over the Fremont
Natrona County line. Snowfall appears to be fairly significant
across southwest Wyoming attm via web cams and is likely asct with
the jet energy. This snowfall is having a tough time making it much
beyond far Western Sweetwater County during the past few hours but
has finally briefly reached RKS where the vsby currently is 1/2
mile. After the gradient weakens and the low weakens and propogates
eastward to the Wyoming Idaho border, QG forcing will keep the
snowfall going today. The mid level low will then jump the divide
this afternoon over to Central Wyoming east of the divide. At this
point, much of the snowfall will jump the divide by early this
evening. In addition, a cold front will dive south across Wyoming
tonight, adding sfc convergence to the mix. With the mid level low
expected to track east across the Wind River Basin and Natrona
County, as well as deepening upslope flow around the backside of the
exiting low, the Lander Foothills, east slope of the Wind River
Mountains, Green/Granite Mtns, Natrona County and Casper Mountains
should see the most snowfall east of the divide by the time the
storm winds down by late afternoon Tuesday. Extended the valid time
the winter weather advisories and warnings will be in effect for all
zones west of the Divide through noon Tuesday but kept the southwest
zones going through 00z Wednesday. Upgraded the Upper Green River
Basin Foothills to a warning as the backing sw to ssw flow is a
favorable flow for that zone as well as the west slope of the Wind
River Mtns which is already in a warning. The steadier snowfall
should get underway in Sweetwater County by 12z this morning and
extend til ))Z Wed. Since Sweetwater County will be south of the mid
level low, the airmass will likely become unstable as lifted indices
are progged to be minus 4 in western Sweetwater. This will result in
heavier periods of snowfall today for Sweetwater embedded in the
extensive area of lighter snowfall. Introduced advisories for areas
east of the Divide in the aforementioned areas including the Wind
River Basin, owl Creek Mtns, and Southwest Big Horn Basin for
tonight and Tuesday, ending from north to south Tuesday as a sharp
back edge to the boundary layer mstr tracks south southeast. After
the weather complex exits east, skies will begin to clear along with
cooling temps. Some snow showers will linger in the far west
Tuesday night through Wednesday night.

.LONG TERM...THURSDAY THROUGH MONDAY

Mean ridge is expected to build along/near the West Coast in the
medium range period. The main issue will be basin and valley
inversions and fog potential. The other thing to watch is models
showing a ssewd moving shortwave Thursday and again Monday. In
between, steep inversions in the basins and valleys with fog
potential and colder temps than most guidance, especially with new
expected snow cover from our Monday/Tuesday system. Lowered temps
some more but not as much as they may end up being with new snow of
4-8 inches across Fremont/Natrona County area on top of what is
still on the ground. There is some potential that the ridge axis may
be far enough west to allow some clouds to spill over the ridge and
temper the extremes but that remains to be seen. Otherwise,
abnormally cold January will continue in the Wind River Basin
especially where we sit 5th all-time right now in over 100 years.

&&

.AVIATION.../12Z ISSUANCE/

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

Widespread MVFR/IFR conditions in snow and low ceilings are expected
through the taf forecast period. Looks like the most intense snow is
expected before 15Z this morning, and then pickup again after 21Z
this afternoon at most terminals. There could be period of time
when low VFR conditions occur, but confidence is relatively low.
The best shot at low VFR conditions would be at RKS today.
Mountains will be obscured with IFR/LIFR conditions. Breezy south
to southwest wind expected across central/eastern Sweetwater
County through 00Z Tuesday. A cold front is expected to push south
across Sweetwater County with winds becoming northerly between 03Z
and 09Z Tuesday.

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

VFR conditions are expected through 21Z this afternoon with
considerable mid- level cloudiness with some snow shower activity.
However the brief nature and light intensity of any snow shower
activity precludes prevailing any snow through about 21Z this
afternoon. This will quickly change between 21Z this afternoon and
03Z Tuesday as frontogenesis occurs across northern Wyoming and
pushes south across the area, and as a mid-level circulation
pushes across central Wyoming tonight. Widespread IFR/MVFR
conditions are expected to develop in snow and low ceilings across
much of the area between 21Z and 03Z. Windy to strong southwest
wind is expected across southeast Fremont into Natrona County
through mid- morning Monday with some low-level wind shear noted
at KCPR and KLND. Wind fields are expected to weaken by early
afternoon in those areas with the aforementioned cold front
sliding south across much of the area between 21Z and 03Z
switching winds to the north. Mountains will be obscured at times
through 21Z, but will be obscured all of the time after sunset in
IFR/LIFR conditions.

&&

.FIRE WEATHER...

Basin and valley inversions will wash out today. Another extensive
snow event will lay down more accumulating snow on a well
established snowpack that is already in place over much of the area
today through Tuesday. Temperatures will become a bit cooler behind
the exiting weather system Tuesday night. Except for brisk southwest
winds from the Red Desert to Casper today where winds will gust to
45 mph in some areas at times, expect mainly light to moderate winds
through the next couple of days. Temperature inversions and valley
and basin fog will reestablish themselves by Thursday night and
Friday morning.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST Tuesday for WYZ028>030.

Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Tuesday for WYZ018>020-022.

Winter Storm Warning until noon MST Tuesday for WYZ012-013.

Winter Weather Advisory from 5 PM this afternoon to noon MST
Tuesday for WYZ005-007-017.

Winter Weather Advisory until 5 PM MST Tuesday for WYZ014-015-
026.

Winter Storm Warning until 5 PM MST Tuesday for WYZ023>025-027.

Winter Weather Advisory until 5 PM MST this afternoon for WYZ001.

Winter Weather Advisory until noon MST Tuesday for WYZ002.

&&

$$

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



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