Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 200005
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
405 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...In the big picture, a longwave trough will persist
over the eastern Pacific in some form or fashion for the next week.
This will maintain active west-southwest flow aloft over Western
Washington with ejecting systems crossing the area with
regularity. We will generally remain under or poleward of the jet
stream, keeping showery weather in play between organized systems
with seasonably low snow levels, i.e. at or below 3000 feet.

A negatively tilted occluded front is spreading into Western
Washington with another dose of rain and mountain snow. Southerly
gradients have increased too, with gusts this afternoon near 40
mph over the north interior. Mount Baker already had 5 inches of
snow today, with another 4-10 inches tonight at Mounts Baker and
Rainier, so issued a Winter Weather Advisory for those two Cascade
zones. The major highway passes should also get a few inches, but
not enough for an advisory.

Next up will be a sharp and vigorous occluded front that will
cross the coast early Sunday morning. East-southeast gradients
will really tighten and strengthen in advance of this feature,
peaking in strength around 12z (4 am) Sunday. UIL-BLI will peak
near -6 mb, PDX-BLI near +7 mb, AST-UIL near +6 mb, and SEA-YKM
near -8 mb. Southerly 925 mb wind speeds will peak near 60 kt
along the coast and 55 kt over the north interior. GFS-based 18z
marine MOS guidance peaks wind speed at Destruction Island near 53
kt and Smith Island at 44 kt. GFS MOS for Whidbey NAS peaks at 31
kt, while the usually more conservative NAM MOS peaks Whidbey at
33 kt. Have issued a High Wind Watch for the coast and the
standard 4 north interior zones. Besides wind, this front will
come with an enhancement of precip. With snow levels topping out
only near 3000 feet, moderate to heavy snowfall amounts are
expected in the mountains. Have gone with a Winter Storm Watch in
the Olympic Mountains. In the Cascades, 6 or more inches on
Saturday night-Sunday morning is a safe bet, with over a foot
possible at the volcanoes.

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.  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...delayed

KSEA...

&&

.MARINE...delayed

&&

.HYDROLOGY...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.

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

     Gale Warning until 10 PM PST this evening for East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     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.

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

     Small Craft Advisory until 6 AM PST Saturday for Admiralty Inlet-
     Puget Sound and Hood Canal-West 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.

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

&&

$$

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