Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 092023
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
123 PM MST Fri Dec 9 2016

.SHORT TERM...Tonight through Monday

Imagery shows general longwave trof still stretched across the CONUS
with embedded modest shortwave ridge starting to enter the FA from
the west...opening the door to WAA and increased EPAC moisture as it
does...along with a 90 to 100 kt jet. Other upstream embedded
shortwave trofs also exist that will have some affect on the CWA
over the next several days. SFC has general high pressure in control
of the region with a building nearly stationary trof draped from SE
WY up through NW WY. Cold air still in place to the north of this
front...relatively warm and breezy/windy to the south.

Pacific airmass and WAA entering the FA under the ridge from the
west, ushering in a period of rather prolonged snow chances out
west. A Winter Storm Warning continues for the western mountains and
valleys with an Advisory in place for the Upper Green and Upper Wind
River basins...now through Saturday afternoon as the
Warning/Advisory has been extended. So far, there has been between 1
and 6 inches of snow around the Warned area.

Tonight, 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 this evening and continuing until the axis passes through
sometime late Saturday afternoon and evening. Slower than thought
before...hence the extension of the Snow Products. This will be the
time of greatest snowfall potential with periods of heavy snow
mixing with winds 15 to 30 mph gusting 35 to 45 mph at times. Snow
will then taper off in intensity through the Saturday
afternoon/evening period from NW to SE. Additionally, the I80
corridor will be affected by this system Late Saturday afternoon and
Saturday night into Sunday morning with both wind and light snow
also bringing possible Advisory conditions to Sweetwater County
during this time. An SPS is in place for this activity.

This system of storms will mainly impact areas west of the Divide
with little snowfall east of the Divide. Best chance for light
accumulating snow east of the Divide will occur Saturday night to
Sunday morning...with up to 2 to 3 inches over Casper mountain and
3to 6 inches over the Bighorns. Otherwise, increasing gusty
southwest winds today (gusting 20 to 30 mph) will eventually,
slowly, work their way northward into central WY and lasting through
much of the day Saturday in a High Wind Warning situation across the
Wind Corridor. This could also mix with snow already in place on the
ground and cause areas of blowing/drifting snow...with near zero
visibility possible at times on Saturday if the snow remains
powdery. South pass and highways 789 and 287 would be likely areas
to be most impacted...perhaps all the way to Casper. A High Wind
Warning has replace the Watch for these locations...with blowing
snow wording.

Sunday and Monday, trof axis moves east of the FA Sunday along with
a brief frontal passage. Light snow chances, such as they were, end
east of the Divide. However, west of the Divide and across the
western mountains and valleys, brisk and relatively moist (EPAC)
westerly flow continues under the influence of a continuing strong
(100+ kt) jet and developing weak shortwave...with the jet/SW and
terrain features providing extended areas of lift, condensation and
precipitation (snow) to that region. Heaviest snowfall expected by
Monday morning under best forcing conditions. Advisory level (at
least) snowfall will be likely across the mountains and valleys in
the Sunday/Monday time period...with possible warning criteria in
the Teton/Gros Ventre zone.

.LONG TERM...Monday night through Friday

We continue to have the same problems as yesterday and that there is
not a lot of model agreement on the details of the forecast for the
middle of the week and beyond. There will be another cold frontal
passage Monday night and into Tuesday. The models are in fairly good
agreement in the front not having a lot of moisture to work with so
only some light snow or flurries are expected with the passage. The
models have come into better agreement in taking the core of the
cold air a little further east toward the eastern portion of the
country. It will still be cold, but not as cold as it could have
been.

This is where the agreement ends though. The GFS is much drier
following Tuesday night with dry but cold conditions dominating
across much of the area through the remainder of the work week,
which is a complete flip-flop from yesterday`s solution. The next
good chance for precipitation on this models is not until next
Saturday. Meanwhile, the European has a much wetter pattern. It also
still has the potential for a decent snowstorm with good warm
advection moving in for Thursday. Which model is right at this point
is anyone`s guess. So for now, we made only minor changes to the
long range pops to avoid flip-flopping. It does there is a decent
chance for some snow everywhere in the area next week, but details
and timing of it are very difficult if not impossible to nail down
at this point.
&&

.AVIATION.../00Z ISSUANCE/

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

VFR conditions are expected through the period. A southwest wind will
increase through the night at KCPR and continue through Saturday.
Isolated snow showers will occur late tonight and on
Saturday...mainly in vicinity of KCOD, KLND and KCPR.

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

Widespread snow will occur through Saturday. MVFR to IFR
conditions will be the rule for most of the period with the exception
of KRKS where conditions will be more variable. Local LIFR
conditions are possible at times. Expect mountains to be obscured. A
gusty wind will continue at KRKS airport and this gusty wind will
continue through tonight and Saturday. Some improvement in
visibility and ceilings may be seen after 21Z Saturday at some TAF
sites.
&&

.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 central Wyoming late tonight through Saturday
afternoon.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from midnight tonight to 6 PM MST Saturday for
WYZ019-020.

Winter Weather Advisory until 6 PM MST Saturday for WYZ016-026.

Winter Storm Warning until 6 PM 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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