Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201059
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Sun Aug 20 2017

.SYNOPSIS...A system passing to the north of Washington will bring a
few high clouds and patchy morning stratus to the coast. High
pressure will build over the region Monday and Tuesday with warmer
temperatures. An upper trough will bring a chance of showers and
cooler temperatures late Wednesday and Thursday. Dry and warmer
weather returns Friday through the weekend.

&&

.SHORT TERM...A weak system passing through British Columbia today
will bring a few high clouds. Light onshore flow will also result in
some patchy morning stratus, mainly along the north coast. Mostly
sunny skies will prevail by the afternoon along with warmer high
temperatures. The dry weather pattern will also continue as high
pressure builds offshore today and moves over the region Monday and
Tuesday. Light flow and rising 500 mb heights will result in warmer
temperatures Monday and Tuesday, reaching the lower 80s across
greater Puget Sound.

The cloud cover forecast remains generally favorable for eclipse
viewing Monday morning. However, models continue to show the highest
chance for morning low stratus near the immediate coast of
Washington. Also, the UW WRF indicates some patchy low clouds will
be possible from the lower Chehalis valley to some spotty areas
around the southwest interior. Some patchy fog may also be possible
over the waters of Puget Sound but very little of this should extend
to the land areas. Any clouds or fog that do develop will likely
begin dissipating around mid morning (9AM), and the vast majority of
the area will have clear viewing.

.LONG TERM...A trough will approach the region on Wednesday. 500 mb
heights will fall and onshore flow will start to increase. Expect
high temperatures to cool off several degrees with a spotty light
showers by afternoon, mainly near the coast and mountains. Models
bring the main 500 mb trough axis across the area Thursday with the
best chance of showers at that time. Orographics should produce
light measurable rainfall or showers along west facing slopes of the
Cascades and Olympics. Measurable rain will be more hit and miss
across the lowlands and amounts will generally be under a tenth of
an inch.

High pressure builds Friday through the weekend. Dry weather returns
along with a gradual warming trend. Highs will reach the low 80s by
Saturday and possibly the mid 80s by Sunday. Mercer

&&

.AVIATION...Northwest flow aloft over Western Washington through
tonight with upper level ridge offshore moving to the coast Monday
morning. At the surface, onshore flow will continue with high
pressure offshore and lower pressure east of the Cascades.

Shallow marine layer along the coast with the stratus moving down
through the Strait of Juan de Fuca early this morning. Ceilings with
the stratus near 1000 feet. The stratus will burn off this morning.
Just some high clouds over the rest of the area through tonight.

KSEA...Just some high clouds at times through tonight. North
northeast wind 5 to 10 kt backing to northwest again midday. Felton

&&

.MARINE...A typical August pattern will prevail through the
middle of the week. Expect onshore flow of varying strength to
persist due to high pressure offshore and lower pressure east of the
Cascades. A weak system will approach the coastal waters on
Wednesday and move inland Thursday. Small craft advisory west winds
are likely in the central and eastern Strait of Juan de Fuca in the
evening and early morning hours each night. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT Monday
     for Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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