Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182324
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
324 PM PST Sat Nov 18 2017

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

&&

.SHORT TERM...A 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. There is a winter
storm watch up for parts of the Cascades, you would probably have
to be up pretty high for this front to seem much like winter--above
5000 feet. There will be some heavy rain in the Olympics and also
a decent rain shadow in the lee of the Olympics. The front is mostly
southeast of the area by late Sunday night and most of the day Mon
should be dry before a warm front nears the area Monday night.

.LONG TERM...The strong warm front will be over the region Tuesday.
The heavier rain shifts to the B.C. coast Wednesday and Wednesday
night before shifting south over the area Thursday and breaking up
somewhat. It will be mild, rainy, and breezy and the rivers will
be running high, with the Skokomish River in Mason county flooding.
There is a chance of flooding on the other rivers, but the fact
that this pineapple express is more southerly probably means the
best orographics are limited to the Olympics. But it will still
make a mess of our early season snowpack which is a shame for the
skiers. Seattle ought to be up around 60 for a high Wednesday.

&&

.AVIATION...A flat upper ridge axis will 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 this evening will become strong on Sunday.
The air mass is stable. Moisture and clouds will thicken rapidly
starting late this evening, then lower on Sunday.

KSEA...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. Late Sunday
afternoon, SW wind at 040 will peak near 70 knots. 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 tonight and then push southeastward
through the Washington waters on Sunday afternoon. In advance of
the front, winds will rise. The wind increase will progress from
north to south through tonight with gales expected over most
waters ahead of the front. Winds will rapidly ease and shift out
of the northwest behind the front later Sunday afternoon and
evening.

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. Possibly this pineapple express or atmospheric
river or whatever you want to call it is too southerly for the
Cascades--more westerly flow and stronger orographics might increase
the chance of flooding for the Cascade rivers--but it will still be
wet and warm and this bears watching.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...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.

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

     Wind Advisory from 6 AM to 3 PM PST Sunday for Admiralty Inlet
     Area-San Juan County-Western Skagit County-Western Whatcom
     County.

     Wind Advisory from 6 AM to 5 PM PST Sunday for Central Coast.

     Wind Advisory from 4 AM to 3 PM PST Sunday for North Coast.

PZ...Small Craft Advisory for rough bar from 6 AM Sunday to noon PST
     Monday for Grays Harbor Bar.

     Gale Warning from 3 AM to 7 PM PST Sunday for Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm.

     Gale Warning from midnight tonight to 7 PM PST Sunday for
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.

     Gale Warning from 3 AM to 3 PM PST Sunday for Admiralty Inlet-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Gale Warning from 10 PM this evening to 3 PM 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.

     Small Craft Advisory until 6 PM PST this evening 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 West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from midnight tonight to 6 PM PST Sunday
     for Central U.S. Waters Strait Of Juan De Fuca.

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

     Gale Warning from 6 AM to 3 PM PST Sunday 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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