Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 182141 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 241 PM PDT Mon Sep 18 2017 .SYNOPSIS...An upper trough will be over the area through Wednesday for cool and showery weather. Drier northerly flow aloft will develop late in the week as an upper ridge offshore moves toward the area. && .SHORT TERM...
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An upper trough will be over the Pacific Northwest tonight and Tuesday for cool weather and showers at times. An upper short wave will move through the area tonight, enhancing showers, especially to the south. There could also be isolated thunderstorms this evening, mainly towards the coast. Another short wave will push onshore Tuesday night. It looks like the heavier shower activity will be south of the area. The upper trough axis will shift inland Wednesday and Thursday with drier northerly flow aloft developing and a decreasing shower threat. Highs the next few days will be several degrees below normal. Snow levels will be around 5500 feet the new few days and the highest elevations could get some snow accumulations. Paradise is the main threat for heavy snow - either tonight or Tuesday night as short waves bring increased showers - but temperatures look just a little too warm for snow to really pile up. Schneider .LONG TERM...An upper ridge over the northeast Pacific will give dry northerly flow to Western Washington Friday through Monday. Highs will warm back to near normal. Schneider
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&& .AVIATION...
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An upper level trough will continue to bring scattered showers to the area. The air is moist and unstable with onshore flow. There will be a chance of thunderstorms through this evening with the best chance near the coast. The mountains and higher terrain will be obscured much of the time. KSEA...Scattered showers and clearing periods with a southerly breeze.
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&& .MARINE...
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Low pressure will remain over the region through midweek. A trough will move through the area tonight and another trough will reach the area Tuesday night and Wednesday morning--but the strongest pressure gradient will be down on the Oregon coast. The winds over Western Washington will be onshore tonight, light and variable Tuesday and then light northerly on Wednesday as low pressure passes to the south.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory until 11 AM PDT Tuesday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm- Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. Small Craft Advisory until 11 PM PDT this evening for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm- Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.
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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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