Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 201530 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 830 AM 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 upper level ridge will bring more sunshine and a little warming Friday through Tuesday.
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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. An upper level shortwave trough rotating around the main low was over the S WA coast this morning, and was dousing the central/south WA coast, the SW interior, and the S Cascades with rain. Models slide the shortwave trough over the S Cascades by 21Z, allowing the area of rain to break up. Otherwise the rest of W WA will have scattered showers. NAM bufr soundings show the lower air mass unstable up to a little above 700 MB. As the shortwave trough slides E tonight, somewhat drier N flow aloft from an approaching offshore upper level ridge will develop over W WA. The lower air mass will still remain fairly moist on Thursday, so not a lot of clearing is expected. The air mass should be stabilizing for just a slight chance of showers. Dry N flow aloft ahead of the approaching offshore upper level ridge will strengthen on Friday for a dry day with more sunshine. High temperatures will warm a bit but should still be a couple of degrees below normal. Kam .LONG TERM...Previous discussion from the 326 AM AFd...The medium range solutions were suggesting that the tail end of frontal systems passing to our north might bring a threat of light rain to the northwest part of the CWA this weekend into Monday. The GFS was starting to come more in line with the wetter ECMWF solution. For now, decided to keep the inherited dry forecast but will need to introduce rain if the 1200 UTC cycle continues with the idea of wetter conditions. At any rate, expect the ridge to be a bit dirty (mid and/or high clouds streaming across the area) through Monday. Anticipate the upper ridge to rebound the end of the period for dry weather with near normal temps.
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&& .AVIATION...
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There are areas of rain or showers from about Hoquiam south and south of Seattle this morning--and there were some showers up over the San Juans. An upper level trough is over the region with light flow over Western Washington. Drier northerly flow will develop slowly and northerly flow aloft will develop so that will help drive the system over Oregon slowly away. There is some clearing north of Seattle this morning and overall more clearing from the north is likely through this evening before low clouds redevelop overnight due to the light flow, stabilizing air, and low level moisture. KSEA...Winds should come around to light nw today and most of the showers should stay south of the area now.
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&& .MARINE...
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A weather system is over the region but the blustery pressure gradients are south of the WA/Oregon border--Western WA has rather light gradients. Weak low pressure will remain over the WA coastal waters through Thursday and then high pressure will build Friday and Saturday.
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&& .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory from 6 PM this evening to 2 AM PDT Thursday 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. && $$ 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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