Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 170325 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 825 PM PDT Sat Sep 16 2017 .SYNOPSIS...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 next week. && .SHORT TERM...
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A pattern change will bring a period of cool and wet weather to Western WA starting Sunday. Strong west winds aloft will help push wildfire smoke out of the region. Precip timing with a cold front on Sunday has not changed much in the models. Expect rain on the coast by mid morning then rain spreading into the interior during the afternoon. Widespread measurable rain is anticipated with 0.25-0.75" coast and mountains and between 0.10-0.25" across the interior. Temperatures will trend below climo with highs in the 60s. An upper level low will keep the air mass over the region cool and unstable through Tuesday. Expect more showers with snow levels in the mountains around 5,000 to 6,000 feet. Temperatures Monday and Tuesday will remain on the cool-side with highs near 60 degrees. Overnight lows will range from the mid 40s to lower 50s. 33 .LONG TERM...Previous discussion...A vigorous upper short wave will drop down from the northwest Tuesday night and swing inland Wednesday. This will bring a period of rain Tuesday night followed by showers Wednesday. An associated surface low is expected to track across the south part of the area. Models disagree with the track and strength of the low. For now, it looks like it will be far enough to the south that most of the area will not get much wind but this could change. Stay tuned. Drier northerly flow aloft will develop over Western Washington Thursday. It looks like most of the area will be completely dry by Friday and Saturday. Schneider
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&& .AVIATION...
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An exiting weak disturbance will allow for a slight amount of clearing tonight as smoke over the area from Cascade wildfires will also clear out. This window will be limited as clouds associated with the approaching front will start to filter in during the overnight or early morning hours...bringing in not only lower clouds...but also rain to the area by Sunday afternoon. Cigs generally VFR for most locations tonight and into early Sunday with the exception of potentially IFR cigs along the coast...much like Friday night. Cigs area-wide expected to deteriorate somewhat through the day Sunday...likely leveling off at high-end MVFR conditions for most of W WA. SMR KSEA...SCT high clouds overnight tonight as smoke thins out. Some low level marine stratus may work its way into the area after 12Z...just in time to mix with increasing clouds from approaching cold front. FROPA expected in the early afternoon with - RA...increased winds and MVFR cigs. SMR
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&& .MARINE...
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Onshore flow will develop tonight with westerlies in the Strait of Juan de Fuca expected...potentially resulting in SCA winds...as such...advisory remains in place. Southerly winds will come up pretty quick over the coastal waters Sunday morning--with 20- 30kt south winds developing by mid to late morning...thus inherited SCA will remain in place there as well. The winds over the coastal waters 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 for more onshore flow. MM/SMR
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory from 6 AM to 10 PM PDT Sunday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From Cape Flattery To James Island Out 10 Nm- Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Small Craft Advisory until 3 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.
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&& $$ 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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