Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 281735 AAA

Area Forecast Discussion...Update
National Weather Service Riverton WY
1135 AM MDT Fri Oct 28 2016

.SHORT TERM...Today through Sunday night

Imagery shows a broad trof across most of the CONUS with a embedded
jet streak currently moving to the northeast through the flow near
northwest WY. Fast moving shortwave trof moving into/across
California and headed this direction. SFC has weakening high
pressure west to low pressure over the plains and a cold nearly
stationary front to the northwest/west of WY. A few light showers
near the northwest corner of WY.

The upper ridge continues to translate eastward, bringing much of
the FA under strengthening SW flow. Gusty southwest SFC winds
continue across Natrona county this morning. A few possible strong
wind gusts will be possible near most any east to northeast facing
foothill location early this morning with some mountain wave
production and a good P and T GRAD running along the
foothill/mountain interfaces...particularly in the Lander foothills.
Through the rest of the day today, significant changes begin to
occur in both the upper and lower levels as a vigorous and fast
moving shortwave approaches western WY before overspreading the
region late tonight through Saturday morning. Cyclogenesis will
rapidly take place across eastern ID/western WY lifting a warm front
up and across southern and western WY keeping snow levels quite
high to start with at above 10500 feet. EPAC moisture...with some
additional moisture from hurricane Seymour...will also accompany
this system with rain showers increasing quickly across the area
west of the Divide by mid-day Friday. Maximum convergence around the
developing cyclone and into western/northwestern WY`s terrain
features looks to occur this evening with the shortwave trof axis
moving through wrn WY beginning around midnight. The position of
cyclogenesis with respect to WY...the warm front...and the
subtropical origin of the air mass will keep temperatures elevated
with H7 temps only falling to just around/just below freezing late
tonight. Most accumulating snowfall will then be found generally
above 8500 feet over YNP...the Absaroka mountains...and the Wind
River and Teton mountains. As it currently looks fairly impressive
for a short period of time, especially considering the speed of this
system, 18 hr QPF values across Northwest WY, through Saturday
morning, could range between 0.5 and an inch of liquid. Still,
snowfall amounts should mostly remain at or below in advisory levels
under the warmth of this subtropical-type system. Have issued a
Winter Weather Advisory for the Teton, Gros Ventre, Absaroka, Wind
River, and Bighorn mountains as elevations above 8.5k ft could see 4
to 10 inches...with perhaps a foot in the Bighorns by mid-day
Saturday. Localized (flash) flooding or mud or rock slides could be
possible later this afternoon and evening in areas or below where
lower elevation snow still lingers in addition to steep sided
canyons. An SPS has been issued reflecting this possibility. As the
upper wave moves quickly eastward tonight through late Saturday
morning, the core of the SFC low looks to dissolve as the wave lifts
and opens further. the Big Horn Basin will see some good rainfall
before all is done, but not much across the low elevations to the
south. By Saturday afternoon, a rapid decrease in precipitation will

Saturday night, the next batch of EPAC moisture arrives ahead of the
next stronger and deeper shortwave trof with precipitation beginning
west of the Divide as deep cyclogenesis begins over
central/northeastern WY and southwesterly flow increases into/across
the region. The airmass with this system, while still relatively
warm, will start our colder than the first system. With most of the
moisture from this system heading across west central to southwest
WY then into central WY...the Tetons, Gros Ventre and Wind River
mountains will see the best in QPF and snow with the Wind River
mountains in the bullseye. 24 to 36 hr liquid precip in the
Wind River Mountains could range between 1 and 2 inches with perhaps
12 to 24 inches of new snow accumulating above 8500 feet between
Saturday night and Monday morning. Highlights of some kind will
most likely be necessary with this tricky weather system.

.LONG TERM...Monday through Friday

The Pacific cold front associated with a strong shortwave trough
should be through much if not all of the area by 12Z Monday. The
associated precipitation with this system appears to quickly exit
much of the forecast area early Monday. The exception will be the
northwest where a moist westerly flow will keep snow showers
ongoing for much of the day. Otherwise the main issue will be
strong to potentially high wind over central and southern areas.

Winds are expected to decrease rapidly in most areas between 5 and
7 PM for those planning on trick or treating.

Global models remain inconsistent beyond Monday making for a
relatively low confidence extended forecast. The 00Z model runs
tonight show a weak overall trough pushing across the Rockies
Tuesday and Tuesday night with some light precipitation over the
west. Models then show ridging building across the area Wednesday,
before another weak system and associated cold front could impact
the area Thursday and Friday. Overall no major systems expected
with mainly above average temperatures.




An area of rain and drizzle will continue to stream northeast
across western Wyomimg, most numerous at KJAC and points north
for the rest of this afternoon. Mountains will become
frequently obscured this afternoon. Snow levels are expected to remain
between 8500-9000 feet through the afternoon.  An upper disturbance
will push across the northwest this evening when conditions are
expected to continue to deteriorate vicinity and north of a KAFO-
KPNA line with areas of IFR conditions likely along with some
heavier areas of rain and higher mountain snow.  Precipitation will
decrease 08z-12z Saturday, but areas of lower MVFR/IFR ceilings and
patchy fog is expected to linger past sunrise Saturday.


A cold front will push south into northern Wyoming this afternoon
along with scattered showers, higher mountain obscurations and local
MVFR conditions.  MVFR, areas of IFR ceilings are expected to follow
the cold front to a vicinity KCOD-KBYG line by 00z Saturday and to
vicinity KCPR-KRIW line by 06z Saturday.  These lower ceilings are
likely to persist through 12z Saturday, especially vicinity KCPR
and KCOD.



Today, fire danger low most locations...low to moderate across
Natrona county. Minimum RH values will remain above 20 percent for
all of the forecast area with Natrona and Johnson counties seeing
the lowest RH values this afternoon along with gusty southwest wind
of 15 to 25 mph gusting to 30 mph at times. Significantly increasing
precipitation chances will occur across much of the region west of
the Divide today...continuing through Saturday morning.
Precipitation will begin to spread east of the Divide late this
afternoon and through early afternoon Saturday. Heaviest
precipitation will occur this afternoon through this evening with
from a half an inch to an inch of precipitation possible across the
northern half of WY by Saturday evening. More mountain precipitation
will be on the way again Sunday and Sunday night.


Winter Weather Advisory from 6 PM this evening to noon MDT
Saturday for WYZ001-002-008-009-012-014-015.



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