Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 172230

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PDT Sat Mar 17 2018

.SYNOPSIS...Upper level low pressure over the Pacific Northwest
will bring scattered light showers to Western Washington through
Monday, but for the most part the weather will be dry. An upper
ridge will be over the region Tuesday for mostly sunny weather.
Rainy weather will return Wednesday and Thursday as a weak system
moves through, with cool showery weather Friday and Saturday.


.SHORT TERM...A 5420 meter upper low is over southwest Idaho this
afternoon, weakening as it continues to move slowly northeast.
Another upper low of around 5400 meter has formed over Vancouver
Island. Western Washington is in the region of weak flow aloft
between the two lows, with weak onshore low level flow bringing
some moisture into the lower 5000 ft or so of the atmosphere. The
air mass has become weakly unstable with daytime heating, and
spotty light showers have developed across the forecast area.
Except in the north interior of Western Washington, sun breaks
are few and far between.

The upper low to our southeast will continue to weaken and move
further east, becoming a non-player in our weather by this
evening. The upper low over Vancouver Island will drift slowly
south over our offshore waters tonight and Sunday, then move into
the Oregon offshore waters on Monday. The cool air aloft, low
level moisture, and daytime heating will lead to weak instability
and scattered showers through Monday; showers will be most
numerous during the afternoon and evening hours, and over the
mountains. Much of Western Western will miss out on the showers,
and where precipitation does occur it will be mostly light. There
will be some sun breaks as well, and afternoon temperatures will
be near normal -- mainly in the lower to mid 50s. Lows will be in
the mid 30s to lower 40s.

The upper low will probably dissipate along the Oregon coast (if
the GFS is more correct) or drift well off to the southwest (if
the ECMWF is more correct) on Monday night. Either way, a weak
upper ridge should be over Western Washington on Tuesday for a
mostly sunny and slightly warmer day. McDonnal

.LONG TERM...The models agree that a frontal system will move
northeast across Western Washington Wednesday and Thursday, with
the deep upper trough following the system bringing showery and
cool weather Friday and Saturday. However, the models disagree on
details, show considerable spread in ensemble members, and poor
run-to-run consistency; therefore, the long term forecast for
Wednesday through Saturday is rather broad brushed. We can be
reasonably confident, though, that any high impact weather --
such as high winds or really heavy rain -- is unlikely during the
period. McDonnal


.AVIATION...Low pres aloft will remain over the region thru Sunday
morning. Light and variable flow aloft will become light sly
overnight. The low level flow will be weak onshore. Expect areas
of generally MVFR CIGs to become more wdsprd overnight.

KSEA...VFR conditions are anticipated to continue until 0900 UTC
(2 AM PDT, Sunday). After 0900 UTC, MVFR CIGs (020-030) are
anticipated. VFR conditions should return by 2100 UTC (2 PM PDT).
Winds will be southerly, generally less than 10 knots thru early
Sunday afternoon.


.MARINE...Higher pressure offshore with lower pressure inland
will keep the flow onshore but it will weaken overnight. Expect
weak onshore flow to prevail across the area Sunday through


.HYDROLOGY...River flooding is not expected in the next 7 days.


PZ...Small Craft Advisory until midnight PDT tonight for East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PDT tonight for Central U.S.
     Waters Strait Of Juan De Fuca.


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