Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 202152 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 252 PM PDT Wed Sep 20 2017 .SYNOPSIS...
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A large upper level low will maintain a chance of showers and cooler temperatures over Western Washington through Thursday. Drier north flow aloft from an offshore upper level ridge will bring more sunshine and a little warming Friday and Saturday. The weakening ridge will move over the area Sunday and Monday and but could allow transient weak weather systems to bring some spotty light rain. The ridge will strengthen on Wednesday for dry weather and more sunshine.
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&& .SHORT TERM...
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A large upper level low remains over the PacNW with the main low center over Alberta. Somewhat moist and weakly unstable northerly flow coming around the low and S over W WA will maintain a 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 out on Friday with areas of morning clouds giving way to a mostly sunny afternoon. The clearing will also set up good radiational cooling Friday night so that patchy fog will be around Saturday morning. The upper level ridge will tilt inland over W WA on Saturday but there will be quite a bit of flow through the ridge, enough so that mid and high level moisture from a weak warm front moving inland over B.C. may brush the area with some clouds in the afternoon. The air mass will still be a little cooler than normal. Kam .LONG TERM...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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Scattered showers will dry up this evening and there should be some partial clearing. Clouds set up again overnight as the air becomes more stable and the area of cloudiness over British Columbia moves down over Western Washington in northerly flow aloft. KSEA...Winds look like they will be variable in some light convergence through this evening. There could be some clearing this evening and then clouds will set up again overnight.
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&& .MARINE...
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Weak low pressure over the area through 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 and Strait of Juan de Fuca 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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