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Forecast Discussion for Seattle/Tacoma, WA

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000
FXUS66 KSEW 212130
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
230 PM PDT Sat Apr 21 2018

.SYNOPSIS... The Puget Sound convergence zone will dissipate this
evening, and showers over Western Washington should end. An upper
ridge will build offshore Sunday for a sunny afternoon. The ridge
will move inland Monday, bringing a stretch of sunny warm days
that should continue through Thursday. An upper low moving into
Oregon on Friday should result in cooler weather and more clouds.

&&

.SHORT TERM...Westerly flow aloft continues over Western
Washington this afternoon, ahead of a weak upper trough
approaching from well offshore. Low level onshore flow continues
as well, but it is gradually weakening. The Puget Sound
convergence zone that formed early this morning more or less
dissipated as it reached the Snohomish/King County line, then
another one developed further north, and it is moving south into
the north Seattle area. Outside of the convergence zone there have
been sun breaks and spotty showers. Temperatures at 2 pm were in
the 50s.

Low level onshore flow will weaken further during the next few
hours and switch to northerly by midnight, so the convergence
zone should dissipate this evening. The shower activity elsewhere
will end this evening as well. The offshore trough, currently near
135W, will move to the coast late tonight and cross the Cascades
around midday Sunday. The upper trough will bring some cloud cover
but probably no precipitation. Then dry northwest flow aloft will
prevail over the forecast area ahead of an upper ridge building
over the offshore waters. It will be fairly cool tonight, with the
overnight lows mainly around 40. Sunday will be mostly sunny in
the afternoon with highs a couple degrees above normal, in the mid
50s to lower 60s.

The ridge will continue to build as it moves into the Pacific
Northwest on Monday. At the same time a thermally induced surface
trough will extend northward along the Pacific Northwest coast
for low level offshore flow. This will result in sunny and warmer
weather, with highs Monday in the mid 60s to lower 70s.

The upper ridge will strengthen on Tuesday, with 500 mb heights
rising to around 5730 meters over Western Washington. Offshore
flow will increase too, and highs Tuesday should be in the upper
60s and 70s under sunny skies. McDonnal

.LONG TERM...The upper ridge will remain over the region Wednesday
and Thursday, with light offshore flow continuing, and the lower
part of the atmosphere warming. On Thursday, the 850 mb
temperatures should be around +14C; that should be the warmest day
over the interior lowlands, with highs ranging from the upper 60s
in the north to the lower 80s in the south. The pattern will
probably break down Friday and Saturday, as an upper low begins
moving northeast from the California offshore waters into Oregon.
That should at least induce a switch to onshore flow, resulting in
cooler weather with highs in the upper 50s and 60s. It could
produce some showers as well. McDonnal

&&

.AVIATION...At 230pm showers are mainly in the PSCZ over
King County. The PSCZ will move south and break up later this
afternoon or evening. There are a few showers elsewhere and areas of
clearing. A patch of high clouds is also moving through the area.
Expect dry weather on Sunday as an upper level ridge builds and the
air becomes dry and stable.

KSEA...The PSCZ is weak but over King county at 230pm. There will be
a period of variable winds and showers and then a north breeze and
probably clearing. After some high clouds, skies will clear tonight
as high pressure builds in. There could be morning low clouds, but
the UW wrfgfs didn`t have them in the metro area, so I left them out
of the Sea-Tac TAF.

&&

.MARINE...Onshore flow behind the front continues to ease this
afternoon and evening as high pressure builds into the area.
Thermally induced low pressure will develop along the coast early
next week with offshore flow and warm sunny weather.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 11 PM PDT 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 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 for rough bar until 3 AM PDT Sunday for
     Grays Harbor Bar.

     Small Craft Advisory until 9 PM PDT this evening for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

Previous Forecast Discussions may be found at
NWS Seattle/Tacoma, WA (SEW) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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