Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 161727
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1127 AM MDT Sat Sep 16 2017

.SHORT TERM...(Today through Monday night)
Issued at 2 AM MDT Sat Sep 16 2017

Imagery shows deep amplified trof across the western CONUS with a
ridge over the eastern CONUS. The trof axis is currently located
down through western MT...eastern ID...and into UT. The SFC has low
pressure along and south of a cold front lying from Minnesota to
Kansas to northern Arizona. High pressure to the north of the front.
Isolated to widely scattered rain and high mountain snow showers
currently moving from south to north across Wyoming.

Today, the upper trof axis passes through Wyoming from west to east
with precipitation ending from south to north during the day.
Current Winter Storm Warnings and Advisories for the mountains of
western, central and northern Wyoming is set to expire a 18Z today,
however it could likely be expired earlier. Tonight, skies clear and
temperatures will fall to near or below freezing across all lower
elevations including basins, valleys and foothills. A Freeze Watch
has been issued for these locations.

Sunday, brief shortwave ridging aloft and high pressure at the
surface will allow for relatively stable, sunny to partly cloudy
skies with virtually no chance for precipitation and warming/drying
surface conditions. Sunday night will see near zonal flow aloft
before transitioning to southwest flow on Monday ahead of the next
seasonally strong upper level trof. With weak lee side troffing,
(dry) warming at the surface and relatively cold temperatures aloft -
dry adiabatic lapse rates will be strong enough to allow strengthening
winds above to get near the SFC...increasing SFC winds both Sunday
and Monday. Sunday will see southwest winds across the west in
addition to the Wind Corridor gust 20 to 30 kts...while on Monday
winds will increase a bit more and gust 25 to 45 kts out of the
southwest across much of the area. Isolated showers/thunder possible
across the western mountains Monday afternoon. Monday night, a large
trof and attending cold front approach the FA from the west...moving
into/through northwestern/western Wyoming late Monday evening then
overnight. Precipitation chances increase rapidly with what now
looks like another period of significant EPAC rainfall. Snow levels
will begin rather high at well above 10k ft...lowering to around
7.5k ft by sun-up Tuesday. ATTM...snowfall accumulation by sun-up
Tuesday morning looks to range anywhere from a trace at or below
7500 feet to as much as 3 inches (locally) above 9500 feet.

.LONG TERM...(Tuesday through Saturday)
Issued at 2 AM MDT Sat Sep 16 2017

Models are showing a base of a trough and associated cold front
sweeping east across the forecast area Tuesday. This system looks
to bring mountain snow/valley rain with even a few thunderstorms
possible. The best chance for precipitation will be across the
west/northwest. Snow levels could dip down to around 6000 feet
after frontal passage over the west, but keeping at 7500+ feet
over the east. Northwestern mountain zones could see near
advisory snow amounts with this system. In addition to the snow,
this system looks to be quite windy.

Precipitation looks to quickly taper off Tuesday evening, with
possibly some spotty western mountain snow possibly remaining
through the night in the moist westerly flow that remains in wake
of the base of the trough.

On the heels of this first system, an upper low over the Pacific
Northwest Tuesday night is progged to gradually dig into the Great
Basin Wednesday carving out a nice longwave trough across the
Western U.S. that should to stick around through the end of the
week.

On Wednesday, the forecast area should see flow backing to the
southwest which will lift a warm front northward across the area
with the best shot of any precipitation across the northwest.

By Wednesday night/Thursday models are showing frontogenesis
across the forecast area with the front gradually pushing east of
the area Thursday/Thursday evening as shortwave in the longwave
trough pulls northeast over the area. The strong baroclinicity,
upper jet dynamics at times should give the area a decent shot at
least some precipitation in the Wednesday night through Thursday
night time frame with models still struggling on the details.
Precipitation looks to be mountain snow, rain and snow western
valleys, and rain in eastern lower elevations.

Although the area will then be in a longwave trough Friday and
Saturday, much of the energy shifts south and east of the forecast
area with coverage/intensity of the precipitation looking to be
less/weaker.

Temperatures will be below average in the long term with the
warmest day on Wednesday in between systems.

&&

.AVIATION...For the 18Z TAFS through 18Z Sunday afternoon
Issued at 1118 AM MDT Sat Sep 16 2017

Areas of rain and mountain snow will continue to dissipate over
the north this afternoon, before ending by 03Z Sunday. Areas of
MVFR to IFR conditions will occur in the north along with frequent
mountain obscuration through 03Z. VFR conditions are expected
across the area through the rest of the forecast period. Mid to high
clouds will quickly move over the area by 06Z Sunday, which could
impede fog development. Have left it in KJAC forecast for now,
with KBPI/KPNA/KRIW being other possible locations.

&&

.FIRE WEATHER...
Issued AT 2 AM MDT Sat Sep 16 2017

Fuels at critical levels for most of the low elevation forecast
zones in addition to many mountain zones below 8500 feet. (Please
see the Fire Weather Forecast product for more information).

Today will be cool and relatively moist with precipitation ending
from south to north through the day. Winds will be rather gusty
across southern Wyoming...gusting 20 to 25 mph out of the west at
times. Sunday and Monday will see a warming and drying trend with
winds gusting out of the southwest across western and southern
Wyoming to 25 mph on Sunday...then increasing and gusting 25 to 45
mph at times over much of the forecast area. Relative humidity both
days, however, will generally range near or above 20 percent. Smoke
dispersion in the afternoons will tend from fair to very good today
and Sunday...then very good to excellent Monday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Watch from late tonight through Sunday morning for
WYZ003>006-010-011-013-016>020-023-025>030.

&&

$$

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



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