Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 220359

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Sun May 21 2017

.SYNOPSIS...An upper level ridge will give one more sunny and warm
day Monday. An upper trough will move rapidly toward the region
Tuesday, spreading clouds and bringing cooler conditions. Strong
onshore flow will develop Tuesday. The upper trough will weaken and
shift east Thursday and an upper ridge will arrive next weekend.


.SHORT TERM...Other than some high clouds and areas of low marine
stratus on the coast, skies will be fair tonight and sunny on
Monday. High temperatures will rise into the 80s across the inland
portion of Western Washington Monday as 850mb temps peak under a
ridge of high pressure.

An upper trough will move rapidly toward the area Tuesday and there
will be a strong push of marine air. The air will also cool aloft,
with the freezing level falling sharply Tuesday night into Wednesday
morning. The trough will not have much rain, but scattered showers
are likely in the Cascades and maybe in a convergence zone through
Wednesday or Thursday as the trough lingers. Inland highs will be
mostly in the 70s Tuesday and 60s Wednesday. On the coast the drop
will be more dramatic--into the 50s. The Euro MOS for KUIL
continues to show a high of 52 on Tuesday. Burke

.LONG TERM...Previous discussion...There might be enough troffiness
aloft to keep a few showers in the forecast Thursday and maybe for
Friday, but the trough will move east and a ridge is likely to build
in by the weekend. Temps at the least ought to be well into the 70s
for next weekend, but it is too early to promise--but at least the
GFS and ECMWF seem to agree for now that another ridge will be on
the way and maybe we will see 80s again a week from now.


.AVIATION...Rising upper level heights associated with a
ridge over the coastal waters will continue to build through Monday
night. At low levels high pressure over the offshore waters will
drive onshore flow through Monday with light northerlies through the
interior south of Admiralty Inlet and southwesterlies north of the
Strait of Juan de Fuca. The air mass will remain generally dry and
stable. VFR conditions will persist overnight throughout the
interior, with low stratus and patchy fog expected to develop again
overnight along the Pacific beaches and coastal waters.

KSEA...VFR conditions tonight with north winds 9 to 12 knots
becoming northeasterly 6 to 8 knots after 04Z, then northwesterly 8
to 12 kt around midday Monday.


.MARINE...High pressure over the offshore waters driving
northwesterly onshore flow through Monday night. Strongest flow is
expected during the evening hours over southern two-thirds of the
coastal waters today, then again on Monday. Onshore gradients will
further increase Monday evening through Tuesday as a disturbance
over the Gulf of Alaska approaches from the northwest. Gradients
peak Tuesday afternoon highlighting a very strong marine push will
take place during the late afternoon through overnight hours on
Tuesday. At this time we are anticipating 30 to 40 knot inflow
through the eastern two-thirds of the Strait of Juan de Fuca,
marginal gale conditions in Admiralty Inlet and in portions of the
Northern Inland Waters, and Small Craft Advisory conditions in the
remaining waters. A steep fresh swell will develop off Vancouver
Island and push into the coastal waters later Tuesday into Wednesday.

Conditions across the waters will gradually improve Wednesday night
into the later part of the week as temperatures across the interior
cool and high pressure offshore builds into the area.


PZ...Small Craft Advisory in effect until midnight PDT
     tonight for 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.



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