Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 170730
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
130 AM MDT Wed May 17 2017

.SHORT TERM...(Today through Friday night)
Issued at 109 AM MDT Wed May 17 2017

Imagery shows developing very complex upper level pattern with
longwave trof across the western 2/3s of the CONUS and embedded lows
over the southern Rockies and another more vigorous one moving
through the PAC NW with diffluent flow moving atop of wrn WY. SFC
has developing broad area of low P across WY with a deepening trof
over the ern border area of WY. Frontogenesis occurring over cntrl
ID/wrn MT. Increasing moisture and precipitation now pushing through
ID and into wrn WY this morning.

Today, the srn/cntrl Rockies low continues into/across CO/NM while
strong lee side cyclogenesis takes place once again from the High
Plains of sern CO into the cntrl Plains of KS/NE which will limit
moisture return into WY east of the Divide along with precip
chances...at least for a while as a "dry slot forms between this
exiting system and the next one on its way into the region from the
PAC NW. Upper level diffluence this morning continues to increase
west of the Divide with the approach of the PAC NW trof/closed low
system with attending increasing deep moisture while very strong
SFC/mid lvl frontogenesis also continues to take place across ID/wrn
MT. Lee troffing will begin just east of the Divide...a precursor to
further strong development expected through this morning.
Additionally, unseasonably very cold air along with the cold front
and terrain will combine with the increasing available moisture from
the EPAC and increase precip chances rapidly this morning. Snow
levels will initially begin above 9.5k ft...but quickly drop to
(near) valley floor level before sun-up this morning as H7 temps
drop to -7 to -10 deg C. A quick half to 2 inches of snow may be
found in portions of the Star/Jackson valleys through mid-day
today...with perhaps another inch or so through the rest of the day.
The western mountains should see anywhere from 4 to perhaps 12
inches by the end of day today...with heaviest amounts likely over
the Teton, and Gros Ventre mountains. A Winter Weather Advisory has
been issued for the western mountains...valid through the end of
today or tonight depending on location. The Bighorn mountains and
Absaroka mountains are within a potentially better snowfall area as
moisture gets circulated around a rapidly developing SFC low over
cntrl to nrn WY...pushing . back into these mountains. Advisory to
near Warning amounts will be likely for the Absaroka
mountains...with heaviest snow found over the northern portion of
the range. The Bighorn mountains are in the best location of
all...with warning amounts likely above 8k feet. Therefore, have
issued a Winter Storm Warning for the Bighorns and an Advisory for
the Absarokas with warning wording for the far northern portion of
the range. These highlights will begin this morning and continue
through Thursday morning across the Absarokas...end of day Thursday
for the Bighorns. By Thursday morning through Friday, H7 temps drop
to -5 to -8 east of the Divide...with surface temps dropping
accordingly to near or just above freezing levels at times. The area
east of the Divide will likely see some snow or rain mixed with
snow. May need to add additional highlights especially for portions
of srn/ern Fremont county...ern Sweetwater County (west of the
Divide) and all of Natrona county as a cold front plunges southward
across central, eastern and southern Wyoming Thursday in response to
very strong cyclogenesis again across the srn High Plains and into
the cntrl/srn Plains. Additionally, some good moisture return is
expected to reach back into ern/cntrl WY at this time around the
developing cyclone. This wx maker finally exits Friday...leaving
only areas of lingering and shrinking showers through Friday
evening.

.LONG TERM...(Saturday through Wednesday)
Issued at 109 AM MDT Wed May 17 2017

In wake of the major spring storm in the short-term, a north to
northwest upper flow is expected over the Northern Rockies for
much of the extended. Global models show one distinct shortwave
pushing southeast from the Pacific Northwest across the region
Saturday night/Sunday, and then another one dropping south out of
Canada into the northern High Plains on Monday. The system on
Monday will be the stronger of the two with a decent cold front
pushing south across the region. There will be decent chances of
convection both Sunday and Monday. The 00Z ECMWF is much faster in
sweeping out the moisture by Tuesday with dry conditions while
the Canadian/GFS continue to show enough moisture and instability
for some convection. For now will continue to have slight chances
over the east on Tuesday, but will have the west dry for now.
Another wave in the northwest flow could impact the area
Wednesday, but will keep chances for convection mainly over the
northwest and northern mountains and adjacent foothills for now.

Temperatures in the extended will be seasonal to slightly below
average for mid-late May, but nothing as cold as what occurs in
the short-term.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 109 AM MDT Wed May 17 2017

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

Rain and snow showers will occur through 18Z in most areas. Then
the focus of the rain and snow will be over northwest WY through
06Z Thursday, before drying moves into that location. The rest of
the region will see isolated rain or snow showers after 18Z through
12Z Thursday. Areas of MVFR/IFR conditions will occur through 18Z
with improvement to VFR conditions in all but NW WY after 18Z.
Mountains will be obscured in NW WY through the period.
Windy conditions will be common today in southwest WY through
00Z, then decreasing. Please see terminal forecasts for details.

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

Isolated showers will occur through 18Z. Showers will become more
numerous over northern WY by 18Z today with widespread rain through
06Z Thursday. Then drying will move into this area after 06Z Thurs.
Expect areas of MVFR/IFR conditions over the north by 18Z through
12Z Thursday. Windy conditions will occur in the central part of
WY today with the wind decreasing after 00Z. Please see terminal
forecasts for details.

&&

.FIRE WEATHER...
Issued AT 109 AM MDT Wed May 17 2017

Fire danger low across the forecast area through the week as all
fuels are currently in green-up and a late season winter storm is
set to push through the forecast area beginning today....continuing
into the Friday. Precip will be increasing west of the Divide this
morning...with snow levels reaching valley/basin floor level by sun-
up this morning. Significant precipitation is expected from this
developing system. Additionally, expect winds to become quite gusty
today around the region. Smoke dispersion fair to good afternoons
through the rest of this week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT this evening for WYZ012-
024.

Flood Watch from this evening through late Thursday night for
WYZ003>006-008-010-011-020.

Winter Storm Warning from 6 AM this morning to 6 PM MDT Thursday
for WYZ008-009.

Winter Weather Advisory until 6 AM MDT Thursday for WYZ001.

Winter Weather Advisory from 6 AM this morning to noon MDT
Thursday for WYZ002.

&&

$$

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



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