Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 200748
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
148 AM MDT Wed Sep 20 2017

.SHORT TERM...(Today through Friday night)
Issued at 150 AM MDT Wed Sep 20 2017

Imagery shows a broad deep longwave trof across most of the CONUS
with wrn Atlantic high pressure only affecting the coastal
eastern Sea Board (with the exception of the embedded Hurricane
JOSE currently east of the Virginia(s)). Zonal to southwesterly
flow aloft currently in place ovr WY with jet associated with the
low across the nwrn CONUS and through WY. SFC has SFC front
through the FA...lying well to the east and south over the Plains
and scntrl Rockies to the Desert SW.

Today (starting early this morning), one of the trof axes has moved
through the FA and into the nrn Plains along with a strong upper
jet. Drying will take place with precipitation amounts dropping off
significantly until a new batch of EPAC moisture pushes into WY once
again this evening as the entrance region of the jet approaches the
region. The influence of the jet will be strongly felt throughout
the forecast period. Otherwise, this will herald the beginning of
another period of significant precipitation of one sort or another
across western WY through at least Friday night (probably
longer)...spreading east of the Divide Thursday night. Moderate to
heavy mountain snow (as H7 temps range from -3 deg C to -7 deg C) is
expected above 8000 feet...with Warning amounts over the Teton, Gros
Ventre and Wind River Mountains beginning tonight and lasting
through Friday morning...possibly longer. Warnings/Advisories have
been issued for much of the western and central mountains. The
western valleys may have to be added at a later period when snow
levels get low enough...perhaps sometime Thursday night (maybe).
Some light snow may even be possible over the basins and valleys
east of the Divide and across southern WY by late Thursday
night...then again Friday night...as snow tapers off west of the
Divide.

.LONG TERM...(Saturday through Wednesday)
Issued at 150 AM MDT Wed Sep 20 2017

A longwave trough will slowly push east across the Rockies this
weekend into early next week. A couple of shortwaves pulling
northeast ahead of the mean trough will allow for periods of
valley rain/mountain snow mainly along east of the Divide
Saturday/Sunday from isentropic lift over the baroclinic zone
located southeast of the area, as well as dynamics associated with
each wave. West of the Divide will be a lot less active with
mainly only slight chances for precip. These waves could bring
some significant snowfall to central mountains/eastern slopes of
the Bighorn Mts, but confidence is not high.

The trough lingers into Monday which could result in the
continuation of the inclement weather mainly east of the Divide.
However have decided to trend PoPs lower Monday to mainly slight
chance east of the Divide for now.

Warmer, and mainly dry conditions are expected Tuesday and
Wednesday as the trough continues to weaken/exit the area with
possibly some shortwave ridging developing. Tuesday could see some
mountain snow showers over the northern mountains in the northwest
flow in the departing trough.

Temperatures will be below to well below average especially for
highs through Monday, but temperatures possibly returning to
seasonal levels by middle of next week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday night)
Issued at 150 AM MDT Wed Sep 20 2017

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

Conditions will be VFR through the forecast period over the lower
elevations.  Areas of mvfr ceilings and visibility in snow showers
will develop over the Absaroka and Wind River Mountains from 06z -
09z  Gusty west to southwest wind 15-30kts will develop between 15Z-
18Z/Wed. Wind shear could be 45-50kts within the lowest 2K feet AGL
over and near the east slopes of the mountains.

West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

Conditions will be VFR through 06z over the lower elevations.  Areas
of mvfr to ifr ceilings and visibility in snow showers will develop
over the mountains around 06z becoming widespread through 12z. Gusty
west to southwest wind 15-30kts will develop between 15Z-18Z/Wed.
Wind shear could be 40-45kts within the lowest 2K feet AGL over and
near the mountains.

&&

.FIRE WEATHER...
Issued AT 150 AM MDT Wed Sep 20 2017

Fuels at critical levels for most of the low elevation forecast
zones east of the Divide and across southern Wyoming. (Please see
the Fire Weather Forecast product for more information).

Today will be much warmer all locations and also much drier than on
Tuesday. Isolated to widely scattered light showers continue
periodically across northwestern WY. Then tonight, the beginning of
a rather prolonged period of precipitation across western Wyoming
begins with from 1 to 2 inches of liquid expected across the western
mountains through Friday. Heavy areas of snow also expected at times
in the mountains. Otherwise, winds are expected to remain quite
gusty out of the west to southwest today as strong winds aloft
transfer energy down near the SFC...gusting 30 to 55 mph at times
over much of the forecast area...especially across the mountains.
Minimum relative humidity today will generally range from near 20
across the low elevations of central Wyoming to as much as 70
percent in the mountains. Smoke dispersion in the afternoons will be
very good to excellent today...fair to excellent on Thursday...and
fair to good Friday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 PM this evening to noon MDT
Friday for WYZ001-002.

Winter Weather Advisory from 6 AM Thursday to noon MDT Friday
for WYZ024.

Winter Storm Warning from 9 PM this evening to noon MDT Friday
for WYZ012-014-015.

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Baker
FIRE WEATHER...Braun


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