Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 200530 AAA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 PM PST Fri Jan 19 2018

.SYNOPSIS...An active weather pattern will be in place through
next Thursday. First, a strong occluded front will cross Western
Washington Sunday morning, preceded by strong south-southeast wind
and heavy mountain snow on Saturday night. Showers will follow the
front on Sunday, with the possibility of thunder along the coast.
Showers will continue Monday and diminish Tuesday. Another strong
front should cross Western Washington on Tuesday night and
Wednesday, with a cool showery upper trough following Thursday.

&&

.SHORT TERM...A cold and unstable air mass remains over the Pacific
Northwest. Shower activity is increasing on radar across much of Wrn
Washington except the typically shadowed area just northeast of the
Olympics. The lowlands will just see rain at times tonight. In the
Cascades, Snoqualmie pass has picked up about 3 inches of snow and a
couple more inches may fall overnight. Over 10 inches fell at Mount
Baker and Stevens with little accumulation so far could still
snowfall with some convergence possible overnight. The main snow
level remains around 2500 feet, and the winter weather advisory
remain in effect. Amounts in the Cascades of King and Snohomish
county will probably be just shy of advisory amounts, perhaps up to
4 or 5 inches. Travel through the major passes could still be
difficult. Call 5-1-1 before departing through the Cascade passes.
Snow showers should decrease by Saturday morning as the 500 mb
trough axis just offshore finally pushes across.

A lull in shower activity Saturday will be short-lived. There will
probably be some afternoon sunbreaks with just a few scattered
showers around. However, a much more potent frontal system will be
approaching the region.

The newer 00z Models show little change over previous runs in
bringing strong south to southeast winds to the coastal waters by
early Saturday morning. Southeast pressure gradients will increase
through the day into Saturday night. The north interior will usually
get high winds in this type of strong southeast gradient pattern as
will the coast. The high wind watch looks good for sustained winds
up to 30-40 mph and gusts to at least 60 mph possible. Winds this
strong are likely to cause downed trees and power outages. The rest
of the interior will not see nearly the wind speeds up north. The
Everett area could be prone to borderline advisory winds. Will
evaluate this again for the 3 am early morning forecast package.

Models are showing a progressive system with decent rain/snowfall
rates but clearing the area Sunday afternoon. Advisory amounts of
snow look likely for the Cascades and the Watch is OK for the
Olympics. No updates needed to the forecast.

An unstable air mass will follow on Sunday afternoon, with
abundant showers everywhere and thunder possible along the coast.

Monday will not be as unstable, but moist onshore flow will still
lead to showers.  Mercer/Haner

.LONG TERM...Could be a short break (about 12 hours) in precip
early Tuesday.

Then a sharp shortwave will move up in fast southwest flow on
Tuesday night to push a negatively tilted front quickly through
the area. Another round of stronger east-southeast wind will
accompany the front, as well as moderate to heavy mountain snow.

Another trough and front will pass on Thursday, for continued
active showery cool weather. In fact, the post-front air mass will
be quite a bit cooler, with the GFS showing snow levels coming
down below 1000 feet next Friday morning. However, precip will
also be lightening up and tapering off. Still, the forecast
reflects a less than 50% chance of a rain/snow mix over much of
the lowlands next Friday morning. Haner

&&

.AVIATION...An upper level trough extending south from a filling
low that is moving inland over Central British Columbia will move
across the area tonight. Light SW flow aloft will become westerly
late. Low level flow will be southwesterly. Expect areas of MVFR
CIGs this evening to become a little more widespread overnight as an
area of showers pushes inland with the upper level trough. Expect
MVFR VSBYs with the heavier showers.

Some improvement can be expected Saturday with the area being
between systems. Albrecht

KSEA...MVFR conditions will continue tonight as a band of showers
over the Olympic Peninsula moves inland. Improvement appears likely
starting around 12Z early Saturday as the trough moves away from the
area and showers diminish. South winds 9-14 knots through Saturday.
Albrecht

&&

.MARINE...A trough extending south from a filling low that is
moving onto the Central British Columbia coast will move east
through the waters tonight. Small craft advisories are in effect for
all waters with this trough passage.

A warm front to move across the coastal waters late Saturday and
will be followed by a strong cold front Sunday morning. Winds are
expected to be strong with this front with storm force winds
possible late Saturday night into Sunday morning over the coastal
waters and at the east entrance to the strait. The other waters will
possibly see gale force winds during this period. Storm and gale
watches remain in effect for the area waters late Saturday night
into Sunday morning.

Active weather will continue into next week as a series of frontal
systems move across the area waters. Albrecht


&&

.HYDROLOGY...No updates. River flooding is not expected for the next
7 days. Against the background of abundant, long duration rain in
recent days, forecast rainfall amounts this weekend (especially
Saturday night- early Sunday) will push landslide indices close to
thresholds for shallow landslides. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 6 AM PST Saturday for Cascades of
     Pierce and Lewis Counties.

     Winter Weather Advisory until 10 AM PST Saturday for Cascades of
     Whatcom and Skagit Counties.

     High Wind Watch from Saturday evening through Sunday morning for
     Admiralty Inlet Area-San Juan County-Western Skagit County-
     Western Whatcom County.

     High Wind Watch from Saturday evening through Sunday morning for
     Central Coast-North Coast.

     Winter Storm Watch from Saturday evening through Sunday morning
     for Olympics.

PZ...Small Craft Advisory for rough bar until 8 PM PST Saturday for
     Grays Harbor Bar.

     Storm Watch from Saturday evening through Sunday morning for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until 6 PM PST Saturday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Storm Watch from Saturday evening through Sunday morning for
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 AM PST Saturday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Gale Watch from Saturday evening through Sunday morning for
     Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 6 AM PST Saturday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

     Gale Watch from Saturday evening through Sunday morning for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Saturday evening through Sunday morning for
     Puget Sound and Hood Canal.

&&

$$

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