Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181714
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PST Sat Nov 18 2017

.SYNOPSIS...Weak high pressure will give way to a vigorous front on
Sunday. Weak high pressure will be over the area Monday. A strong
warm front will arrive Tuesday. The middle of the week will be mild,
rainy, and perhaps windy at times.

&&

.SHORT TERM...High pressure is centered over Idaho and a frontal
system is hung up around Queen Charlotte sound. The front will shift
southeast into Western Washington Sunday afternoon and evening.
Winds are likely to be strong enough for wind advisories in the
typically windy spots which are the coast and for the Admiralty
Inlet/Whidbey Island area north through the San Juans and over
through Bellingham Bay. The 12z GFS shows a peak PDX-BLI gradient of
+11mb early Sunday afternoon with the strongest gradient around
Admiralty Inlet and Whidbey Island.

There will be heavy rain in the Olympics and of course a decent rain
shadow in the lee of the Olympics. The north Cascades ought to get
some heavy wet snow--and perhaps Paradise on Mount Rainier and the
higher peaks will get heavy wet snow--but it could easily change
over to rain for awhile. The front will still be hung up over
southwest Washington and the Cascades through Sunday night--that is
slower than previous runs. A 1001mb low rides up the front Sunday
evening--but the UW WRFGFS shows the fropa wind shift on a line from
Astoria to Tacoma before midnight and the precip should finish up by
daybreak Monday. Most of the day on Monday will be dry and then rain
starts up again Monday night--faster than yesterdays 12z GFS.


.LONG TERM...A strong warm front moves through Western Washington
Tuesday and Tuesday night. The heavier rain shifts to the B.C. coast
and Vancouver Island Wednesday and Wednesday evening and then shifts
back down through the area early Thursday--but then breaks up and
decreases. It will be a little windy at times--mainly in the
Admiralty Inlet area north through the San Juans and on the coast--
and perhaps in the gaps through the Cascades. There is a more
easterly gradient than southerly at times. The latest GFS has
continued the trend of looking less dramatic: Periods of rather mild
wet and breezy weather are certainly on tap Tue-Thu, just not much
in the way genuine storminess that I can find. The highest PDX-BLI
gradient I can find is only +8mb 12z Thu as a 983mb low reaches
Haida Gwaii.


&&

.AVIATION...A flat upper ridge axis will cross Western Washington
this afternoon, then exit east and flatten a bit tonight. A cold
front will spread down the B.C. coast tonight and move southward
through Western Washington later Sunday, preceded by heavy rain and
strong southerly surface wind. Moderate westerly flow aloft today
will become strong on Sunday. The air mass is stable. Clouds below
050 will diminish today. Then moisture and clouds will thicken
rapidly starting late this evening.

KSEA...Gradual drying of the air mass below 100 will continue until
early this evening, causing clouds below 100 to dissipate between
now and 21z. Clear below 100 will continue through at least 08z
(midnight) tonight. Cloud cigs around 040 will rapidly develop
overnight, with rain developing later Sunday morning. Cigs and vsbys
lowering Sunday afternoon in occasionally moderate rain. Southerly
surface wind for the next 30 hours. Haner

&&

.MARINE...A strong cold front extending WSW from the Queen
Charlotte Sound into the Pacific will slowly drag southeast down the
British Columbia coastline today then will push southeastward
through the Washington coastal and inland waters on Sunday. Winds
will increase in advance of the front. The wind increase will
progress from north to south through tonight with gales expected
over all of the waters except for Puget Sound and Hood Canal and the
Central Strait of Juan de Fuca late tonight into Sunday ahead of the
front. Winds will rapidly ease behind the front later Sunday into
Sunday night.

High pressure will bring light winds to the waters on Monday. A
strong warm front will push northward through the waters on Tuesday.
A strong low pressure center developing well offshore will push
northeast into Haida Gwaii on Wednesday. Haner

&&

.HYDROLOGY...The Skokomish river in Mason county is a shoo-in to get
over flood stage and stay up around flood stage for several days.
The 12z GFS does not show especially heavy precip over the Cascades
and for Wednesday and Wednesday evening shifts the heavy rain up in
British Columbia.


&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...Flood Watch from Sunday morning through Monday morning for Hood
     Canal Area-Lower Chehalis Valley Area-Olympics.

     Winter Storm Watch from Sunday afternoon through late Sunday
     night for Cascades of Pierce and Lewis Counties.

     Winter Storm Watch from late tonight through Sunday afternoon
     for Cascades of Whatcom and Skagit Counties.

PZ...Small Craft Advisory until 10 PM PST this evening 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 Point Grenville To Cape
     Shoalwater 10 To 60 Nm-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Gale Warning from 10 PM this evening to 3 PM PST Sunday for
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm-Northern Inland Waters Including The San Juan
     Islands.

     Gale Warning from 10 PM this evening to noon PST Sunday 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-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 AM PST Sunday for Coastal Waters
     From James Island To Point Grenville Out 10 Nm.

     Gale Warning from 3 AM to noon PST Sunday for Coastal Waters
     From James Island To Point Grenville Out 10 Nm.

     Small Craft Advisory until 3 AM PST Sunday for Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm.

     Gale Warning from 3 AM to 3 PM PST Sunday for Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until 3 PM PST this afternoon for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Gale Warning from midnight tonight to 3 PM PST Sunday for
     Admiralty Inlet-East Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory from 6 PM this evening to 9 PM PST Sunday
     for Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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