Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 290430
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 PM PDT Wed Sep 28 2016

.SYNOPSIS...A low pressure system will remain offshore through
Friday for south or southwest flow aloft. The low level flow will
remain onshore for night and morning stratus mainly over the
coast. A stronger low pressure system passing to the west will
bring an increased threat of precipitation to the area on
Saturday.

&&

.SHORT TERM...
A trough of low pressure centered offshore with a ridge of high
pressure centered over the central USA will result in southwest
flow aloft over the region through Thursday. The low level flow
will remain onshore; therefore, expect late night and morning
stratus mainly over the coast. Temps on Thursday will be near
normal for this time of the year.

The upper level flow will back to southerly Thursday night as the
upper level Low drifts south to a position off the WA coast. The
Low will continue drifting south on Friday. The low level flow
will remain onshore; thus, expect low clouds Friday morning. The
clouds might end up being more widespread across the lowlands
than on Thursday morning. Overall, it looks dry for Thursday night
and Friday but there might be an isolated light shower over the
Olympic range and far north Cascades, near the crest, in the
afternoon.

Another deeper upper level Low will drop southwest off the
British Columbia coast Friday night for a risk of showers over
mainly the coast and Olympic range. This new Low will drift south
well off the WA coast during the day Saturday. Expect an increase
in the precipitation threat across the CWA on Saturday due to
moisture being advected northward and weak diffluence aloft
developing over the area. Temperatures will be below normal on
Saturday.

.LONG TERM from the previous discussion...
An upper level low will move into northern California and Oregon
Sunday. Models are in reasonably good agreement. The low is
perhaps far enough away on Sunday that showers will stay in
Oregon, or at any rate over southwestern parts of the CWA. There
is a hint of offshore surface flow on Sunday in the models, which
would tend to keep things dry. Have changed the forecast a bit to
give chance pops on Sunday. It will be cool, with highs in the
lowlands in the low to mid 60s, and the snow level around 5000
feet in the mountains.

Models all have moist northwest flow aloft beginning Monday after
the upper low has passed to the southeast. There could be another
front about Wednesday (the GFS) or Tuesday (the Canadian and
Euro). For now have broad brushed a chance of showers each day.
Conditions will be on the cool side with highs in the low to mid
60s and the snow level 5000-6000 feet. Burke

&&

.AVIATION...Southwest flow aloft will prevail through Thursday
with an upper trough offshore. At the surface, onshore flow will
prevail with high pressure offshore and lower pressure inland.
The air mass is dry and stable. Patchy low clouds and fog are
likely again for Thursday morning. The best chances for this are
coast and Southwest Interior.

KSEA...Clear until early morning. Low clouds are possible again
13-17Z but it looks hit and miss. Northeast wind 3-6 knots
tonight becoming northwest 5-10 knots late Thursday morning.
Schneider

&&

.MARINE...High pressure offshore and lower pressure inland will
bring marginal small craft advisory west winds to the Central and
Eastern Strait of Juan De Fuca the next couple of evenings.

A frontal system offshore will affect the waters Friday through
the weekend with varying degrees of onshore flow. Schneider

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisories are in effect for the coastal waters
 and central/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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