Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 180434
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT TUE MAY 17 2016

.SYNOPSIS...A weak upper level low pressure system will produce
increasing clouds tonight and a chance of showers Wednesday. Showers
are more likely Wednesday night and Thursday. The upper level low
will remain in the area through early next week, keeping cooler
weather and a chance of showers in the forecast.

&&

.SHORT TERM...High pressure aloft over the Pacific Northwest will
weaken tonight as a weak upper trough approaches. Satellite imagery
shows one band of high clouds pushing east of the Cascades this
evening with additional mid/high level clouds offshore with the
approaching system. Models agree that the initial open dissipates
as it reaches the area later tonight and early Wednesday morning.

Onshore flow will strengthen Wednesday with a higher stratus deck
spreading inland across Western WA in the morning. A much deeper upper
level low will dig south but lacks a well defined surface front.
Models show enough upper level forcing due to a cooler and unstable air
mass to produce some showers over the area. The leading edge of
spotty light rain or showers will reach the coast mid morning and
spread inland during the afternoon. Given the lack of deep
moisture...chance pops still look appropriate.

By Wednesday night and Thursday...the surface pattern becomes somewhat
favorable for a Puget Sound convergence zone in the usual corridor
north of Seattle to Everett. Showers are likely in this zone but
just a chance of measurable rain elsewhere with a cool and unstable
showery air mass. Highs Thursday will be quite cool...struggling to
reach 60 with mostly cloudy skies expected to persist through the day.
The snow level will fall to at least 4000 feet...although light
accumulations will likely remain above pass levels.

The large closed low dives southward Thursday night and Friday.
Models are all in good agreement showing wrap around showers
especially in the afternoon with the added diurnal instability.
Showers will be most frequent over the cascades and more scattered
across the lowlands. Highs will stay cool but there is a better shot
at partial afternoon sunshine. Mercer

.LONG TERM...From previous discussion...Models keep an upper low in
the region through the middle of next week at least. This means a
broad brush forecast of mostly cloudy with a chance of showers
applies to each day. Temperatures will be near normal each day.
Burke

&&

.AVIATION...A weakening upper level ridge over the region will
move east tonight. Upper level trough will develop offshore
Wednesday with weakening cold front through the area. Westerly flow
aloft will strengthen and become southwesterly on Wednesday. The low
level flow will remain onshore.

Just some high clouds overnight. Increasing clouds Wednesday morning
with the ceilings starting out in the 4000-6000 foot range lowering
to 2000-3000 feet by 19Z.

KSEA...scattered high clouds overnight. Increasing clouds around
5000 feet 12-15Z with ceilings lowering to near 2500 feet around
19z. Northwest wind 4 to 8 knots switching to southwest around 12z.
Felton

&&

.MARINE...High pressure offshore with lower pressure inland will
result in onshore flow through Wednesday. Onshore flow strengthening
late Wednesday in the wake of a cold front with gale force winds
likely in the central and eastern Strait of Juan de Fuca. The flow
will weaken Thursday due to falling pressure over the coastal
waters. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory in effect until 3 PM Wednesday for the
     central and eastern Strait Of Juan de Fuca.

 Gale Watch in effect from Wednesday afternoon through late
     Wednesday night for the central and eastern Strait Of Juan
     de Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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