Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 210411 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 PM PDT Wed Sep 20 2017 .SYNOPSIS...
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An upper level low will maintain a chance of showers and cooler temperatures over Western Washington Thursday. Drier north flow aloft from an offshore upper level ridge will bring sunshine and a little warming Friday and Saturday. The weakening ridge will move over the area Sunday and Monday but could allow transient weak weather systems to bring some spotty light rain.
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&& .SHORT TERM...
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An upper level low oriented northeast to southwest over the Pacific Northwest will slowly move southeast tonight and Thursday. Flow aloft will become northeast Thursday and more northerly Friday. The air mass will remain weakly unstable, giving mostly cloudy skies and a chance of showers into Thursday evening. The flow aloft will still be drier than the air mass the past couple of days so there will probably be some breaks in the clouds at times on Thursday. The northerly flow will dry on Friday with areas of morning clouds giving way to a mostly sunny afternoon. The clearing will also allow radiational cooling Friday night so that patchy fog will be around Saturday morning. The upper level ridge will move inland Saturday. There will be quite a bit of flow through the ridge, perhaps enough that mid and high level moisture from a weak warm front moving inland over British Columbia could brush the area with some clouds in the afternoon. The air mass will still be a little cooler than normal. Kam/Burke .LONG TERM...Previous discussion...The upper level ridge will remain over the area with a positive tilt Sunday and Monday night. This has the ridge axis extending NE from the WA offshore waters across central B.C. There will still be quite a bit of flow through the ridge with mainly mid and high level clouds crossing W WA. Both the GFS and ECMWF have one or two systems embedded in the flow aloft brushing mainly the N part of W WA with some light rain. Unfortunately, the models don`t really agree on which systems could bring the area some light rain, so the Sunday-Monday night forecast is more of a broad brush with slight chance POPs. Both models briefly build the ridge around mid-week with 500 MB heights rising to around 5820 meters. This should allow temperatures to warm back to near normal. Unfortunately, once again the models don`t quite agree on whether this will be Tuesday or Wednesday, so confidence is not very high. Kam
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&& .AVIATION...
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Thu upper level trough over the Pacific Northwest will dig southward into Northern California overnight. Light northerly flow aloft will become northeasterly as the trough shifts southward. Weak low level onshore flow tonight with some partial clearing this evening. Clouds will fill back in with MVFR ceilings by around 10-12Z over most of the interior and coast and linger into the afternoon before lifting and scattering out to VFR levels. KSEA...South to southeasterly winds 4 to 6 kt with MVFR ceilings developing around 10 to 11Z.
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&& .MARINE...
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Light onshore flow with small craft northwesterlies over the outer coastal waters this evening. Weak low pressure over the area Thursday will be replaced with weak high pressure for the end of the week as an upper ridge replaces the upper trough over the region.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory outer coastal waters this evening.
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&& $$ www.weather.gov/seattle An illustrated version of this forecast discussion can be seen at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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