Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 180455
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Wed Jan 17 2018

.SYNOPSIS...A cold front will move east of the Cascades this
evening. Large waves generated by a deep low pressure offshore will
impact the Washington coast late tonight through Thursday evening.
Thursday through Saturday will be showery with an upper trough over
the area. Additional fronts will move through the area Sunday and
around Tuesday night.

&&

.SHORT TERM...
Coastal Flood update....Strong southwest winds offshore is
generating a large and energetic swell that will reach the coastline
late tonight. Buoy 46005 that sits about 250 nm W of the mouth of
the Columbia River reported seas of 41 ft at 17 seconds at 7 PM this
evening, and about 32-34 ft of the sea is swell energy that will
propagate all the way to the coastline between 28 and 30 ft at about
18 or 19 seconds.

A coastal flood warning is in effect for the coast. This sort of
coastal flooding mainly affects the coastal communities as waves
run far up beyond where they usually do. The high seas will
combine with tides that are running about 1.5 ft above predicted
values and about 1 ft above the value that usually produces
saltwater flooding in places like Ocean Shores and Westport.
Larger waves will likely crash over over breakwaters and jetties
and collect in low lying areas.

General weather update...A deep sub 970 mb surface low outside 130W
tonight will stall off the B.C. coast Thursday and weaken through
Friday. Southeast pressure gradients will persist tonight into
Thursday and produce breezy winds, and possibly locally windy at
times on the north coast/north interior. The main front is already
through most the interior and will push east of the Cascades over
the next couple hours. The strongest winds have likely peaked and
should ease a bit over the next couple hours. A few gusts to 40 or
45 mph could occur, mainly on the north coast tonight and mostly
associated with thunderstorm squalls in the area. Models do show
near or sub zero LI`s with some marginal CAPE which explains the
lightning activity around the north coast. Models maintain this
instability through Thursday, so the forecast will be updated this
evening to reflect a chance of thunderstorms. Typically,
thunderstorm squalls in these patterns produce brief heavy rain,
numerous lightning, and brief gusty winds.

Colder air aloft arriving tonight and Thursday will drop snow levels
down to 3000 feet later tonight and Thursday. A couple inches of
snow has already fallen at Stevens as the front passed. However,
precipitation rates will drop off behind the front later tonight and
Thursday. Higher passes and Ski resorts above pass level could
potentially get 3 to 6 inches or locally higher amounts. But the
showery pattern does not seem to warrant an advisory for snow given
the spotty accumulations. Persons planning travel through passes
should still call 5-1-1 for road conditions.

Another frontal wave embedded in the relatively cold air mass will
move through the region Friday. This system could produce several
more inches of snow, possibly even down to the lower passes. The
westerly flow is not very strong which may limit orographics, but
the cold air mass in place should give decent ratios for another 3
to 6 or more inches. This system could warrant an advisory, stay
tuned. Mercer/Albrecht

.LONG TERM...Previous discussion...A frontal system will bring a
shot of rain late Saturday night and Sunday along with some breezy
to windy conditions. Showers will linger into Monday but they should
be tapering or even ending by Tuesday as a weak upper ridge builds
over the area. Another front should reach the area Tuesday night and
Wednesday for more rain. Schneider

&&

.HYDROLOGY...A wet frontal system this weekend could push the
Skokomish river in Mason county above flood stage around Sunday.
Otherwise, river flooding is not expected for the next 7 days.

&&

.AVIATION...A cold front has moved across the area this evening
and is now moving east of the Cascades. Strong southwest flow at
the surface and aloft will continue tonight through Thursday. The
air mass is moist and unstable with numerous showers expected
across the area. Thunderstorms are possible at times along the
coast. Ceilings and visibilities will be variable with VFR to MVFR
conditions between showers and conditions locally dropping to IFR
in heavier showers. Albrecht

KSEA...Expect showers with gusty south winds tonight through
Thursday. South winds generally 12-15 knots with gusts 22-25
knots. Ceilings 050 will drop to 2500 ft at times in heavier
showers. Visibilities P6SM falling to 3-5SM in heavier showers.
Albrecht

&&

.MARINE...A deep low centered near 50N 134W will gradually fill as
it slowly moves to the north tip of Vancouver Island early Friday.
Strong southwest flow will continue through Thursday night with
gales over the coastal waters and widespread small craft advisory
conditions over the inland waters through Thursday night.
Conditions will slowly improve late Thursday night and Friday.

A weak ridge of high pressure will brush the area on Saturday.
Then a series of rather strong fronts will move across the waters
Sunday through early next week. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Coastal Flood Warning from 4 AM to 10 PM PST Thursday for
     Central Coast-North Coast.

PZ...Small Craft Advisory for rough bar until 4 AM PST Friday for
     Grays Harbor Bar.

     Gale Warning until 10 PM PST Thursday 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 4 AM PST Friday for East Entrance
     U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     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.

     Small Craft Advisory until 4 AM PST Friday for Admiralty Inlet-
     Puget Sound and Hood Canal-West Entrance U.S. Waters Strait
     Of Juan De Fuca.
&&

$$

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