Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 161600
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT Sat Sep 16 2017

.SYNOPSIS...Weak high pressure aloft over the area today will
shift inland tonight. A front will bring rain later Sunday. An
upper trough and additional weather systems will keep the weather
cool and showery into the middle of next week. Drier weather is
likely late in the week.

&&

.SHORT TERM...A weak upper ridge over the area will bring one
last dry day today. High clouds will give a partly sunny day.
Highs will be in the 70s. Some smoke from fires to the east will
filter into the Puget Sound lowlands today from about Seattle
south but most of this will probably be aloft. A frontal system
will bring some rain later Sunday. Models have trended a little
slower with the arrival of the rain - late morning for the coast
and mid to late afternoon for the interior. Highs will cool into
the 60s Sunday. Monday will be cool and showery behind the front
with an upper trough over the area. Snow levels will fall to
5000-5500 feet. Highs will be about 10 degrees below normal and
struggle to reach 60 degrees for some spots. Schneider

.LONG TERM...Cool northwest flow aloft will prevail Tuesday for a
continued chance of showers and highs several degrees below
normal. Snow levels will remain unseasonably low - around 5500
feet. The GFS shows a vigorous frontal system bringing another
shot of rain late Tuesday and Tuesday night. The ECMWF and GEM
have the same general idea but are much weaker and a little
slower with this system. There is the potential for some windy
conditions with this system Tuesday night as a surface low tracks
inland over the south part of the forecast area but confidence in
the details is low for now. The upper trough will shift a bit
inland Wednesday with drier northerly flow aloft developing over
Western Washington Thursday and Friday. Schneider

&&

.AVIATION...A weak disturbance has brought mid and high clouds to
the area. There is also low stratus over the coastal waters and into
Hoquiam this morning. The air is stable. Areas of low clouds over
the coastal waters will likely spread back onshore this evening and
inland overnight. Areas of morning low clouds will give way to
increasing clouds all levels from a cold front Sunday afternoon.

KSEA...Mid and high clouds today from a weak disturbance and then
some low marine stratus could push in overnight. Any morning low
clouds that make it into Seattle Sunday morning will be mixed into
the increasing clouds with the cold front Sunday afternoon.

&&

.MARINE...Pressure gradients will remain light today before onshore
flow increases tonight. Winds will come up pretty quick over
the coastal waters Sunday morning--with 20-30kt southerlies
developing by mid to late morning. The winds over the coastal water
will shift to northwest with the frontal passage in the afternoon.
Over the inland waters southerly winds up to around 20kts will be
followed by a typical Puget Sound Convergence Zone wind pattern with
onshore flow through Sunday night behind the front. There will be
additional weather systems moving through the area Monday through
Wednesday and the GFS is stronger than the ECMWF with a deep low
moving through Western Washington Tuesday night and windy weather is
not yet in the forecast for that possibility.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 3 AM PDT Sunday
     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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