Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 180908
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
208 AM MST Sun Feb 18 2018

.SHORT TERM...(Today through Tuesday night)

Imagery shows a large, flat, broad longwave trough across the CONUS
with a weak, nearly cut-off low to the west of Baja and a deep
Arctic low/trough coming into/through the PAC NW. A large area of
baroclinic deformation aloft over ID/MT and into the Dakotas,
associated with a favorable position under the jet (entrance
region), also shows well in the WV imagery with cyclogenesis taking
place over nern WY. The SFC has brutal cold front stretching from
the cyclone (nern WY) back to the northwest all the way to nern WA
with a warm front off into the central Plains. Precipitation is
currently falling across most of ID and MT, pushing into/across west
central, northwest, northern WY. South of the front, the
precipitation is falling as both rain and/or snow with all snow
along and north of the front.

High Wind Warnings currently across portions of central and northern
WY should begin to fall off later this morning.

As today progresses, the first SFC portion of the cyclone slides
into the Central Plains while secondary cyclogenesis begins to take
place over eastern CO, helping to pull the Arctic cold front down to
the south and as far as central WY by the late afternoon.

Heaviest snowfall through the daytime period today will occur pre-
frontal across the western forecast area where more moisture and
higher dew points will be present. A transition from heavier/wetter
snow to light/dry snow west of the Divide will then take place later
tonight as the the colder portion of the upper trough translates
over western WY while the surface cold front pushes into/across the
region. Snowfall, while not completely stopping, will then start to
taper off by Monday morning with much lighter accumulations expected
through the rest of the day. Numerous Winter Weather Warnings are
currently in place.

East of the Divide and across southern Wyoming will see the
beginnings of heavier snowfall synchronized with the passage of the
cold front (along with a nasty wind shift to the north and rapidly
falling temperatures). Ahead of this frontal passage, isolated to
scattered areas of light showers (rain and/or snow) will be
possible. Heaviest snowfall east of the Divide will occur tonight
through the day on Monday with all of this region split between
Advisory (western half) and Warning (eastern portion) level snowfall
by the end of the day. All current Winter Weather Highlights will
expire by Monday evening.

Monday night into Tuesday morning, both the upper trough and
associated surface cold front will push to the south and east with
lingering trapped areas of light snow remaining over areas that
can`t lose the clouds as strong high pressure builds across Wyoming.
Very cold temperatures will be left in the wake of the cold front.
With clearing skies, new fallen snow, and H7 temps dropping to -20
to -25 deg C for both Tuesday and Wednesday mornings, expect
extremely cold temperatures to start each day...ranging from -5 to
-30 deg F with the coldest air trapped in the area`s Basins and
 across Yellowstone NP. With general troughiness aloft continuing
 through the end of the forecast period, low chances for light
 snow or flurries will continue west of the Divide...but with
 little additional accumulation expected through Tuesday night.

.LONG TERM...(Wednesday through Sunday)

Overview...Surface Arctic high pressure will slide east across
northern and central Plains on Wednesday, allowing temperatures to
recover from single digit highs across the area Monday and
Tuesday into mostly teens across the valleys and basins Wednesday
afternoon. However, a cold upper trough will remain across the
west through next weekend keeping temperatures below normal.
Weather systems moving through the larger trough will bring
occasional chances of light snow across west and south, but
specific timing of these systems remains uncertain.

Discussion...General upper air pattern to persist through the
extended period, with strong ridges in the eastern Pacific
centered near 150W and near Bermuda in the Atlantic. Broad
longwave trough will remain across the western and central U.S.
between the two ridges. Main forecast challenge continues to be
with individual systems diving down front side of eastern Pacific
ridge and reloading the trough, Previous runs of the GFS were
tracking these systems further north and east than ECMWF, and the
latest GFS run has trended toward the more consistent ECMWF. This
preferred storm track takes shortwaves south along the west coast,
reloading trough across the SW U.S., then lifting systems out of
the southern/central Rockies into the central Plains, with Wyoming
on the outskirts of more robust precipitation. Thus expecting
occasional light snow showers to brush the west and south, but
deterministic guidance has yet to show any consistency with the
timing of these systems.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday night)

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

Snow will occur over most of the west through 06Z Monday, then snow
will be decreasing after 06Z Monday. MVFR to IFR conditions will
occur with frequent mountain obscuration. Some improvement in
conditions will occur after 06Z Monday. A gusty wind will occur in
places today. KJAC will see snow and a gusty wind through 02Z, then
conditions will improve there. KRKS will see a gusty wind through
04Z Monday with VFR conditions, then the wind shifts to the
northeast after 04Z Monday with snow and IFR conditions occurring by
07Z Monday at KRKS.

Please see individual terminal forecasts for more details.

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

A gusty wind at most of the terminal sites through 15Z. A cold front
will continue to move south across northern Wyoming this morning, then
push south into central WY 16z-19Z, then along the continental
divide by 21Z. Snow will be developing over the north this
morning with snow developing over the central this afternoon. Snow
will occur over most areas through 12Z Monday. Expect MVFR to IFR
conditions in the snow with mountains frequently obscured.

Please see individual terminal forecasts for more details.

&&

.FIRE WEATHER...

Sometimes strong gusty winds will taper off this morning while a
large and brutally cold winter weather system takes hold of the
forecast area, continuing through the day Monday. Almost all of the
region will receive precipitation of one kind or another by the end
of the day on Monday, including possible heavy snow at times. Then,
extremely cold conditions will overtake the region in the wake of an
exiting cold front for both Tuesday and Wednesday. Smoke dispersion
will be poor to fair (east of the Divide) to very good (west of the
divide) both today and Monday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 2 PM this afternoon to 11 PM MST
Monday for WYZ029-030.

High Wind Warning until 5 AM MST early this morning for WYZ003-
005-011-015-016-018>020-022.

Winter Storm Warning from 11 AM this morning to 5 PM MST Monday
for WYZ019-020-022.

Winter Weather Advisory from 2 PM this afternoon to 2 PM MST
Monday for WYZ017-018.

Winter Storm Warning until 5 PM MST this afternoon for WYZ001-
002-012>014-023>025.

Winter Storm Warning from 8 AM this morning to 2 PM MST Monday
for WYZ011.

Winter Weather Advisory from 5 AM early this morning to 11 AM
MST Monday for WYZ010.

Winter Weather Advisory until 2 PM MST Monday for WYZ016.

Winter Storm Warning from 5 AM early this morning to 2 PM MST
Monday for WYZ015.

Winter Weather Advisory until 5 PM MST this afternoon for WYZ026.

Winter Storm Warning from 5 AM early this morning to 8 AM MST
Monday for WYZ027.

Winter Weather Advisory from 2 PM this afternoon to 11 PM MST
Monday for WYZ028.

Winter Weather Advisory from 5 AM early this morning to 2 PM MST
Monday for WYZ008-009.

Winter Weather Advisory from 8 AM this morning to 11 AM MST
Monday for WYZ003-004.

Winter Weather Advisory from 8 AM this morning to 2 PM MST
Monday for WYZ005>007.

&&

$$

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



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