Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 110502
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1002 PM MST Wed Jan 10 2018

.SHORT TERM...(Tonight through Saturday)
Issued at 230 PM MST Wed Jan 10 2018

...Winter Weather Advisories for most of the western mountains
and valleys from Thursday afternoon until 5 pm Friday...

Imagery shows a large ewd translating trof over the wrn CONUS with
the trof axis extending from swrn Canada to the sse into/through NM.
A broad ridge leads the way further to the east with its axis
roughly down the East coast. Locally, the trof axis is now moving
across the Continental Divide. Precipitation now ending west of the
Divide with a sharp cold front moving south over the region east of
the Divide. Winds are gusting with and behind the front 25 to 35 mph
while temperatures are dropping from 30s to lower 40s ahead of the
front into the lower teens and upper single digits behind the front.

West of the Divide: Heavier precipitation has now ended with areas
of light snow or flurries continuing through Thursday morning. The
upper levels will transition to NW flow behind the current exiting
trof with the next significant winter weather system (embedded
strong jet and associated low amplitude shortwave) beginning to make
itself known Thursday afternoon. Expect both moisture and clouds to
increase once again as a strong jet and associated embedded
disturbance move overhead. Heaviest precipitation...all of it as
snow this time even across the valley floors...will fall between
late Thursday afternoon and near sunrise Friday morning. By the
afternoon on Friday, the area should expect new total snowfall
amounts ranging from 4 to 12 inches (locally to 18 inches) in the
Teton and Gros Ventre Mountains. Even the valleys should see 3 to 6
inches of new snow with up to 8 inches locally. Brisk south to
southwest winds of 15 to 30 mph the rest of today (gusts 35 to 45
mph) and tonight will increase to 25 to 35 mph once again by late
Thursday afternoon, gusting 40 to 60 mph at times, with the
strongest gusts likely between late evening Thursday and mid-morning
Friday. Friday night through Saturday, the jet and embedded upper
wave move out of the FA with clearing skies east of the divide and
across southern WY, leaving cold temperatures in place.

East of the Divide: The effects of the passing upper trof and an
associated strong cold front pushing south across the region are now
being felt. Areas of snow are coming mainly along and behind the
front and have moved into the wind River Basin and points east
including Johnson and Natrona Counties where areas of light snow are
mixing with strong gusty north winds, causing slick roadways and
reducing visibilities to less than 2 miles at times. Thursday
afternoon and night, increasing brisk (locally strong) winds will
return to the foothills and Wind corridor once again...a prelude to
the next winter system due to arrive later Thursday into Friday.
Although winds may gust to "high" status at a few locations, most of
the area should remain below High Wind criteria. Therefore, no High
Wind Watch at this time. Additionally, the next system will take
just enough of a northern track (both jet and embedded wave) to keep
most moisture and snow across wrn/nwrn/nern WY..impinging on the
Bighorn Mountains and perhaps adjacent areas across Johnson County,
but not leaving much elsewhere over lower elevations. Friday night
through Saturday, the jet and embedded upper wave move out of the FA
with clearing skies east of the divide and across southern WY,
leaving cold temperatures in place.

.LONG TERM...(Saturday night through Wednesday) Issued at 230 PM
MST Wed Jan 10 2018

The period begins with northwest flow over the forecast area with a
ridge holding over the western U.S. A weak disturbance will brush
the far eastern zones but don`t expect much from this ripple. The
dry northwest flow persists through Sunday with a dry day in store
for the region. The trough to our east associated with a large
Hudson Bay low will be reinforced Sunday night and Monday morning
with a shortwave pinwheeling south through the upper-level flow.
This shortwave has the potential to cause a shift in our forecast
Sunday night and Monday depending upon how much it can backdoor to
the west along the east slopes of the Continental Divide. The GEM is
most bullish with the cold front and associated moisture, whereas
the ECMWF and GFS keep this front a little more to the east and with
less QPF. Given the ridge to our west and the projection that the
best shortwave energy should be east of Wyoming, am more inclined to
believe this front backdoors into Johnson and Natrona counties only.
In this scenario, Monday would be cooler in those counties, but the
much colder air would remain to our east. Areas west of the
Continental Divide will see no impact and will remain dry. The
western ridge then begins to slide east later Monday in response to
an incoming system in the Pacific Northwest. The ridge axis will
traverse the forecast area Tuesday with dry conditions and at least
seasonal temperatures. Some areas of the southwest and the east
slopes of the mountains could climb to above normal Tuesday and even
Wednesday. The Pacific Northwest energy will reach western Wyoming
Wednesday. Medium-range models all show at least some kind of
disturbance near/around/across Wyoming Wednesday. Right now, it
would appear the westerly flow would favor the western mountains and
have adjusted precipitation chances accordingly.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday)
Issued at 1002 PM MST Wed Jan 10 2018

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

Northwest WY will see VFR conditions through 16Z with local MVFR
ceilings. After 16Z, snow will be developing in NW WY and snow will
continue through Thursday night. KJAC airport will see MVFR
conditions around 19Z with snow developing around 21Z. IFR
conditions will likely develop at KJAC airport around 00Z and
continue through 06Z Friday. The mountains will be frequently
obscured Thursday afternoon and night.

VFR conditions will occur in southwest WY through 03Z Fri. Then KPNA
and KBPI will see MVFR conditions develop after 03Z Fri along with
light snow developing around 06Z Friday.

Please see individual terminal sites for more details.

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

Fog may occur at the KWRL and KLND airports until 16Z with local
MVFR/IFR conditions. Otherwise, VFR conditions will prevail through
Thursday night. Gusty wind conditions will return to KCPR Thursday
and increase in speed Thursday night.


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 230 PM MST Wed Jan 10 2018

Through Friday, one storm system exits the area tonight with the
next moving into western Wyoming later Thursday. Heavy snow has
ended west of the Divide with lingering areas of light snow or
flurries remaining over the mountains through Thursday morning.
Blustery north winds will follow a cold front east of the Divide
through this afternoon mixing with light snow at times. Moderate to
locally heavy snow snow returns to the west from Thursday afternoon
through the day Friday. 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 Thursday to 5 PM MST Friday
for WYZ001-002-012>015-023>025.

&&

$$

SHORT TERM...Braun
LONG TERM...Jones
AVIATION...AR
FIRE WEATHER...Braun



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