Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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763
FXUS66 KSEW 240347
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
845 PM PDT Sat Sep 23 2017

.SYNOPSIS...High pressure aloft will produce a dry mild day
Sunday. Then an incoming weakening weather system is expected to
bring clouds and a chance of rain later Sunday night and Monday.
Higher pressure aloft is expected to build off the coast Tuesday and
move onshore mid week for a period of sunshine and above average
late September temperatures.

&&

.SHORT TERM...
The positive tilted upper ridge was extending SW from northern
Vancouver Island this evening. The ridge should offer considerable
sunshine Sunday with seasonable temperatures after areas of morning
clouds and fog.

A shortwave near 135W this evening was dampening out as it was
moving into the ridge. Upstream, another stronger shortwave near
160W is forecast to track into British Columbia Sunday night and
Monday. This system should also dampen out as the upper ridge begins
to rebuild Monday, but likely have enough dynamics left for clouds
and a threat of light rain mainly for the coast, Olympics and closer
to the Canadian border. It should behave like a weak warm front.

The upper ridge is anticipated to build significantly offshore
Tuesday for clearing skies and the start of a warming trend
following a brief cooler day with the warm front`s clouds Monday.
Buehner

.LONG TERM...from the previous edition...
A strong upper ridge will be in control Wednesday and Thursday for
dry and warmer weather. Low level flow will become offshore which
should help highs warm well into the 70s - maybe even lower 80s in
some locations.

The upper ridge will shift inland and an upper trough will reach
the coast Friday. There might be spotty rain near the coast but
the main impact will be to bring some clouds and a few degrees of
cooling.

Westerly flow aloft will prevail Saturday with low level onshore
flow. Highs should be near normal and no fronts or weather systems
are expected. Schneider

&&

.AVIATION...An upper level ridge will bring dry and stable weather
to the region tonight and Sunday. The flow aloft is northerly.
Skies have been clearing this evening with just SCT deck around
10,000 ft. May see patchy low clouds or stratus overnight for brief
MVFR conditions. Otherwise, VFR conditions expected through Sunday
afternoon. A warm front will spread more clouds inland Sunday night
into Monday with a chance of light rain, mainly along the coast. 33

KSEA...VFR and light N winds at the surface. Patchy low clouds
possible early Sunday for brief MVFR conditions. 33

&&

.MARINE...High pressure over the NE Pacific will maintain light
onshore flow through Sunday. A warm front will clip the area on
Monday with possible Small Craft Advisory winds over the Coastal
Waters. The flow will turn offshore by midweek as a thermal trough
forms along the coast. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

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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