Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210418

Area Forecast Discussion
National Weather Service Seattle WA
915 PM PDT Sat May 20 2017

.SYNOPSIS...An upper level ridge will remain over the region Sunday
and Monday, then weaken Tuesday as an upper trough reaches the
Pacific Northwest. The trough will weaken but linger through the end
of the week.


.SHORT TERM...Sunday and Monday will be warm and sunny as a strong
upper ridge builds over the region. On Sunday highs will be in the
70s near the water and in the low 80s over the southwest interior.
The only clouds are likely to be short lived morning stratus along
the coast and part way down the strait.

Models show some weak offshore surface flow Monday, which will give
a boost to temperatures. Heights will be in the mid 580s which
should give 80-85 highs over most inland areas Monday. Areas near
the water will be at least in the mid 70s. If the offshore surface
flow gets strong, places like HQM and UIL get warmer than that.

As an upper trough approaches, a thermally induced surface trough of
low pressure will move inland Tuesday. This will bring a strong
onshore gradient which should lower temperatures dramatically,
especially on the coast. Highs will not get out of the 70s inland,
possibly not even out of the 60s. The Euro MOS gives a high of 73 on
Monday for KUIL, but only 52--the present temperature of the sea--on
Tuesday. That is some serious onshore flow. Burke

.LONG TERM...Previous discussion...With marine air pushing into the
area through Tuesday night and an upper trough sliding in through
Wednesday, the forecast returns to mostly cloudy with a chance of
showers. The trough will probably weaken through the end of the
week, but it it is too early to say with any confidence which days
might still have a good chance of showers and which are likely to be
dry--perhaps showers will be limited to the Cascades and the
convergence zone area.


.AVIATION...The upper level ridge over the offshore waters will
slowly build eastward and onshore Sunday through Monday. At low
levels, high pressure over the offshore waters will maintain onshore
flow through Monday with light northerlies through the interior
south of Admiralty Inlet and southwesterlies north of the inlet. The
air mass will remain generally dry and stable, with some low level
moisture reaching the immediate Washington coast and into the Strait
of Juan de Fuca during the late night and morning hours. IFR or even
LIFT conditions possible early Sunday morning in those locations.
Elsewhere, expect clear skies and VFR conditions to dominate across
the area through Sunday.

KSEA...VFR conditions through Sunday. North to northwest wind 8 to
12 knots will become light northeasterly after midnight.


.MARINE...High pressure over the offshore waters will drive
northwesterly onshore flow through Monday night. As is typical with
this pattern, the strongest flow is expected during the late
afternoon and overnight especially through the central and east
Strait. A Small Craft Advisory is in effect for those areas through
much of tonight, and may be required again for Sunday evening as
well as Monday evening.

A strong upper level disturbance is expected to drop southeastward
from the Gulf of Alaska into Eastern Washington Tuesday. Gale
conditions appear likely across portions of the Strait of Juan de
Fuca starting Tuesday, while Small Craft Advisory conditions are
likely over the remaining waters. Onshore flow will continue
Wednesday onward, but will relax somewhat as temperatures in the
interior cool.


PZ...Small Craft Advisory until 5 AM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.



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