Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 150420 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 820 PM PST Thu Dec 14 2017 .SYNOPSIS...
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A weak cold front will spread light rain across Western Washington late tonight through Friday morning as it moves southeast across the area. An upper level trough trailing the front will bring a few showers Friday afternoon and evening. A frontal system will bring light rain to the area this weekend. After a minor lull on Monday another system will bring rain and possibly windy conditions on Tuesday. Drier cool weather may return around mid week.
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&& .SHORT TERM...
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IR imagery shows a ragged looking cold front just offshore and moving slowly toward the WA coast. No echoes yet on RADAR. Models have been quite consistent in bringing light rain to the N coast and N interior around midnight, then spreading it slowly SE across the area. Rain may just be moving over the Seattle area around 12Z/4 AM. The front should be through the interior by around midday Friday. Precip amounts should be modest, only up to a quarter inch over the N interior and up to two tenths elsewhere. The lower air mass has a bit more moisture this evening as dewpoints are a little higher. Dense fog in the Olympia area lifted briefly this evening before starting to return at KOLM. Patchy fog has also developed at KPWT. However, highway cams don`t show any fog around. Mid-level cloud cover will begin spreading across the area after midnight, which should pare back the threat of fog. After the cold front moves E midday Friday, the trailing upper level shortwave trough will follow Friday afternoon. It doesn`t look very convective offshore behind the front, but never-the-less there could be a few showers around. The GFS and ECMWF are in good agreement that a slow moving wetter frontal system will bring rain to the area, likely through the weekend. An approaching warm front will bring light rain to the area on Saturday, but accumulations should be 0.15 or less. Rain should pick up a bit Saturday night as the warm front moves inland. The trailing cold front, sliding S along the B.C. coast will approach the area on Sunday. A strong westerly jet stream will slide down over the region on Sunday, causing the cold front to slow to a crawl as it crosses the area. This should keep the rain going pretty much all day Sunday. The coast, N interior, and mountains should get plenty of rain, but low level W flow will put the central Puget Sound region in the Olympic rain shadow, so precip amounts there may end up being lighter. The front should slide S of the area Monday night. Kam .LONG TERM...Monday appears to be a transition day, with a chance of showers. A strong weather system is forecast to arrive Tuesday and impact the region through Tuesday night. This could bring heavy rain to the mountains, along with windy conditions in the lowlands. After that, cool northerly flow aloft sets up, giving dry conditions with highs in the low 40s and lows possibly below freezing. Burke
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&& .HYDROLOGY...
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River flooding is not expected in the next 7 days.
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&& .AVIATION...
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High clouds and patchy fog this evening, but a weak low pressure system and cold front will move over Western Washington Friday morning. The air will become moist with areas of rain developing on the coast after midnight and spreading inland a little after daybreak. Southwest flow aloft. Onshore flow Friday afternoon and evening will keep the air moist with areas of light rain or showers tapering off into Friday night. KSEA...Cirrus this evening and then clouds fill in and rain develops 12Z-18Z Friday. MVFR conditions will likely continue in onshore flow behind the front Friday afternoon and evening. Light winds will become southerly and then southwest Friday afternoon-- but only up to about 10kt.
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&& .MARINE...
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A 1020mb low and cold front will move over Western Washington Friday morning. Small craft advisory northwest winds will develop over the coastal waters behind the front and then spread into the Strait of Juan de Fuca by evening. A warm front will move through the waters late Saturday night into Sunday. Some small craft advisory conditions are likely across the waters, especially Sunday, as flow aloft interacts with the area terrain. A deepening low will move onto Central Vancouver Island on Tuesday then move rapidly eastward along the Canadian border. Widespread small craft advisory conditions are expected with this system and gale force winds are possible over some waters as it moves by to the north of the waters.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory from 6 AM Friday to 3 AM PST Saturday for 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. Small Craft Advisory from 4 PM Friday to 3 AM PST Saturday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of Juan De Fuca.
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