Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 180354 CCA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Tue Oct 17 2017

.SYNOPSIS...A strong front will move into the area Wednesday with a
period of heavy rain and windy conditions. Showers are expected
Thursday and Friday. A strong warm front will arrive for the weekend.

&&

.SHORT TERM...No updates this evening, forecast is in good shape. A
strong front will move over Western Washington on Wednesday. Heavy
rain will fall in the mountains as a moist plume of air in strong
southwest winds aloft collides with the Olympics and mainly the North
Cascades Wednesday afternoon and evening. A high wind warning and a
wind advisory are in effect for the windy weather. A flood watch
is also in effect. See the HYDROLOGY section below for more info.
The snow level will be high as the front moves over the area and will
fall to around 4000 feet behind the front as the rain turns to
showers
later Thursday through Friday.

.LONG TERM...A strong warm front will move into the area on
Saturday and may result in another period of heavy rainfall and
strong winds. The GFS takes the brunt of the energy with this next
system into northern Oregon, while the ECMWF is a bit farther
north and gives more a prolonged period of heavy rain. This
system will need to be monitored closely for the threat of
flooding.

Later Sunday or Monday flat ridging aloft will build to the south
of the area taking systems more into western portions of British
Columbia. The forecast will call for drying and high temperatures
warming to near 60 by next Tuesday. Albrecht

&&

.AVIATION...A vigorous frontal system will move through the area
Wednesday. Ceilings will gradually lower tonight. Low clouds will
become widespread Wednesday morning.

KSEA...High clouds tonight. MVFR low clouds and rain will develop
Wednesday morning as a front arrives. Southerly wind 5-10 knots
will rise to 10-20 knots later Wednesday. Schneider

&&

.MARINE...A vigorous frontal system will bring gales to most
waters Wednesday. A period of onshore flow will follow the front
Thursday and Friday. Another strong front will move through the
area Saturday. West swell 20-24 feet are expected to affect the
Coastal Waters later Thursday through Friday morning. Schneider

&&

.HYDROLOGY...Heavy rainfall is expected Wednesday through
Wednesday night. The main focus of the precipitation will be on
southwest facing slopes of the Olympics and North Cascades.
Precipitation amounts of 5 to 8 inches can be expected in the
Olympics, while the North Cascades see 4 to 6 inches and the
lowlands see up to 3 inches. The heavy rainfall rates are
accompanied by high snow levels well over 7000 feet. Models do not
show flooding on area rivers, but bring the river stages close
enough (within a foot) on the Skokomish, Nooksack, Stillaguamish,
and Skagit that flood watches have been issued for Wednesday
afternoon through midday Thursday for Snohomish, Skagit, Whatcom
and Mason counties in case rainfall is any heavier than currently
forecast. Lowland locations may see urban and small stream
flooding due to the combination of high rain rates and drainage
issues due to leaves clogging drains.

Rivers will recede later Thursday through Friday night as snow
levels drop to around 3500 feet and precipitation becomes more
showery.

Rivers could quickly rise later Saturday into Sunday with the
weekend system. The ECMWF is warmer and heavier with precipitation
amounts and shows a more classic atmospheric river pattern for
about 24 hours. With rivers starting from a higher point, minor
flooding is possible on some rivers across the area.

Conditions will dry out starting later Sunday or Monday of next
week allowing all rivers to recede. Albrecht.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory from 11 AM to 11 PM PDT Wednesday for Bellevue
and      Vicinity-Bremerton and Vicinity-Hood Canal Area-Seattle and
     Vicinity-Tacoma Area.

     Flood Watch from Wednesday afternoon through Thursday morning
     for Bellevue and Vicinity-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties-East Puget
     Sound Lowlands-Everett and Vicinity-Hood Canal Area-Lower
     Chehalis Valley Area-Olympics-Western Skagit County-Western
     Whatcom County.

     High Wind Warning from 11 AM to 11 PM PDT Wednesday for
     Admiralty Inlet Area-Central Coast-Everett and Vicinity-
     North Coast-San Juan County-Western Skagit County-Western
     Whatcom County.

PZ...Small Craft Advisory for rough bar until 11 PM PDT this evening
     for Grays Harbor Bar.

     Small Craft Advisory until 4 AM PDT Wednesday 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.

     Gale Warning from 4 AM to 9 PM PDT Wednesday 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 11 PM PDT this evening for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Gale Warning from 4 AM to 6 PM PDT Wednesday for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 11 PM PDT Wednesday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 PM PDT Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning from 11 AM to 9 PM PDT Wednesday for Puget Sound
     and Hood Canal.

&&

$$

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You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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