Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 172140 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 240 PM PDT Sun Sep 17 2017 .SYNOPSIS...
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A front will move inland tonight. An upper trough and additional weather systems will keep the weather cool and showery into the middle of the week. Dry weather is likely late in the week.
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&& .SHORT TERM...
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A front will move inland tonight with some lingering showers and perhaps a Puget Sound Convergence Zone. An upper trough will move over the area Monday with snow levels lowering to 5000 to 5500 feet. It will be a showery day. The higher locations in the mountains could pick up a few inches of snow. There will also be a slight chance of thunderstorms near the coast with cold unstable air aloft. Highs will be about 10 degrees below normal. Models show an upper short wave moving through the area Monday night causing an uptick in showers, especially over the south. This could bring 6+ inches of snow to Paradise which would meet winter storm criteria for the first event of the season. Snow levels and temperatures will be right on the edge but it seems worth a winter storm watch for Pierce and Lewis counties above 5000 feet. Showers will linger into Tuesday but probably not as prevalent as Monday as the upper trough weakens over the area. Highs will be about 10 degrees below normal again, similar to Monday. A vigorous upper short wave will move onshore Tuesday night, clipping the south part of the forecast area. This should bring steadier precipitation with heavier amounts south of Seattle. An associated surface low will track inland just to the south of the area. This should keep windy conditions south of the area but it is close enough that it needs to be monitored. The upper trough over the area will shift inland Wednesday for decreasing showers. Schneider .LONG TERM...Drier northerly flow aloft will develop over Western Washington Thursday. It looks like things will dry out entirely around Friday and remain dry into the weekend with northerly flow aloft continuing. Schneider
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&& .AVIATION...
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A cold front over Western Washington will move inland this evening. Onshore flow will ease overnight. A Puget Sound convergence zone is possible this evening. The air mass will become unstable and there will be scattered showers through Monday--and there is a chance of thunderstorms on Monday. KSEA...Cold front will give way to scattered showers and a chance of a PSCZ this evening. Scattered showers for Monday. The wind could become variable for a few hours later this evening if the PSCZ develops, but most of the time it will be southerly.
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&& .MARINE...
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Onshore flow in the wake of the cold front will ease overnight. More breezy weather is possible Monday through Wednesday at times--as disturbances move through the upper level trough over the area. It looks like the strongest gradients and more blustery weather will head into Oregon, but advisories will probably be up at times--mainly for the coastal waters.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter Storm Watch from Monday afternoon through Tuesday morning for Cascades of Pierce and Lewis Counties. PZ...Small Craft Advisory until 3 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca-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-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 11 PM PDT this evening for Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 9 PM PDT this evening for Admiralty Inlet-Puget Sound and Hood Canal-West 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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