Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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059
FXUS66 KSEW 172255
AFDSEW

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

.SYNOPSIS...Mainly dry conditions are expected across the area
tonight. A strong warm front on Wednesday will bring heavy rain
and locally strong winds to the area. A cold front on Thursday
will push the heavy rain out of the area, but it will be followed
by numerous showers later Thursday into Friday night. Another
strong front will pass across the Pacific Northwest on Saturday
and Saturday night. Drier weather may develop early next week as a
ridge of high pressure strengthens to our south.

&&

.SHORT TERM...With exception to the west slopes of the Cascades
and around the Puget Sound Convergence Zone over King and
Snohomish Counties, skies have cleared somewhat behind this
morning`s front. Isolated showers and a clap or two of thunder can
be expected until about 6 PM in the convergence zone. Dry, cool
and tranquil conditions will continue through tonight.

Latest models continue to show rainfall rapidly spreading east
across the area by about 8 AM Wednesday as a warm front moves
through the area. The warm front will stall across northern
portions of the area Wednesday afternoon then pivot and push
slowly southeast across the area later Wednesday night. Winds
at 5000 ft aloft in the warm air mass are forecast to be WSW 50-65
kt Wednesday and Wednesday night. These strong winds aloft
interacting with the terrain, combined with a somewhat unstable
air mass and moisture originating from the tropical Western
Pacific, will produce impressive rainfall rates Wednesday
afternoon and evening. The interaction of the flow aloft with the
terrain will also form a low level coastal jet and a mesolow in
the lee of the Olympics. The end result will be windy conditions
across the area from about Olympia and Westport northward and a
period of heavy rainfall. We now have high wind warnings in effect
for the Washington coastal zones and for the interior from
Everett northward from 11 AM til 11 PM PDT Wednesday. Wind
advisories have been issued for the same time frame from south of
Everett to southern Pierce and Kitsap counties. The expected wind
speeds are about 10-15 mph higher than what we experienced with
this morning`s weaker front. We would not typically issue wind
warnings and advisories for the expected level of wind, but trees
have been stressed from the dry summer and there are many dead
branches that need nature`s pruning. The falling of larger
branches may result in some power outages and local damage as they
hit wires, and outages will likely be quite a bit more extensive
than this morning.

Heavy rainfall will primarily fall in an 18 hour period Wednesday
afternoon through Wednesday night, while snow levels will rise to
well over 7000 feet. This will result in river rises and the
threat for street and small stream flooding during the evening
commute on Wednesday. See the HYDROLOGY section below for more
information.

Precipitation will taper to numerous showers later Thursday
through Friday night and snow levels will fall back down to 3500
to 4000 feet. Albrecht

.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...Onshore flow and surface winds will ease this evening
across Western Washington. Skies have cleared behind a cold front
except for a band over Snohomish and King counties where there`s
a convergence zone. Otherwise, there will be a lull in the weather
tonight. Wednesday will be wet and windy again as another strong
Pacific system impacts the region. The coast and north interior
will see gusty S winds to 35-45 kt. Rain will be moderate to heavy
at times along the coast and mountains. Rain will increase in
coverage Wednesday evening as a frontal band shifts SE through the
area. Expect MVFR to IFR conditions through much of the day. 33

KSEA...Showers from a convergence zone remaining north of the
terminal tonight. S/SW wind easing. Increasing clouds and S winds
on Wednesday as the next system arrives. Gusty S winds to 30kt by
21z.

&&

.MARINE...Onshore flow will ease tonight with winds turning S/SE
after midnight. Expect another round of gales on Wednesday as a
second strong Pacific system impacts the region. Strong onshore
flow will continue through Thursday and Friday with seas building
to 20-24 feet. A third vigorous storm system will reach the area
on Saturday. 33

&&

.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 Whatcom, Skagit, Snohomish, and Mason counties.

     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.

     Small Craft Advisory until 5 PM PDT this afternoon for Admiralty
     Inlet-Puget Sound and Hood Canal.

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

&&

$$

www.weather.gov/seattle

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



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