Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 120457 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 857 PM PST Mon Dec 11 2017 .SYNOPSIS...
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A strong upper level ridge will maintain dry and stagnant conditions across most of Western Washington through Thursday afternoon. A very weak front will bring a chance of rain to the coast tonight before dissipating Tuesday morning. The ridge will begin breaking down on Thursday allowing a cold front to bring rain Thursday night and Friday. Another frontal system will continue the wet weather pattern next weekend.
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&& .SHORT TERM...
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Water vapor imagery shows a small upper level low moving northward offshore along roughly 130W through this evening, as it is steered up and eventually over the large upper level ridge over the region. The associated weak front has been moving steadily toward the WA coast this afternoon. At 04Z/8 PM RADAR was picking up some weak echoes about 25 NM W of the N coast, but that`s it. The front doesn`t look very strong on IR imagery and it may struggle to even bring measurable rain to the coast and Olympics. Still the the N coast and Olympics have the best chance of rain tonight. The front will spread clouds across the interior by sunrise Tuesday morning. However, it may take a little longer for the clouds to reach the area tonight. There was some expectation that the overnight cloud cover would keep temperatures up a bit tonight, but as of 04Z/8 PM quite a few mesowest stations have already dropped into the lower to mid 30s. There still isn`t a whole lot of moisture in the boundary layer, but enough for some areas of fog and patchy fog overnight. With clouds arriving late tonight at least they may limit the expansion of fog late early Tuesday morning. The small upper level low cruising over the top of the ridge will weaken and flatten it on Tuesday, and the clouds leftover from the front should hang around for most of Tuesday. High temperatures will be about the same and pretty close to normal. The large upper level ridge still sitting over the W coast will re-strengthen a little Tuesday night and Wednesday which should allow sunny skies to return Wednesday and Thursday. The models stay on target...stay on target... with weakening the ridge rapidly on Thursday, thereby allowing an approaching cold front to bring rain to the area late Thursday night and Friday. As long as the strong upper level ridge remains over the area, the strong surface based inversion will maintain the stagnant low level air mass presently over W WA. Weak offshore flow will probably not supply much in the way of mixing. The 00Z GFS shows the inversion breaking by 12Z Friday morning. For now, the air stagnation advisory for the interior lowlands of W WA will remain in effect through 2 PM Thursday. Kam .LONG TERM...Previous discussion from the 3 PM AFD...The upper ridge will sag into zonal flow beginning Thursday night. A series of weather systems will reach the area Friday through Monday for a wet weekend. The models all show a dry period Saturday, to some extent. Have played it that way in the zones and grids. Temperatures will still be a bit above normal, with highs near 50 and lows in the 40s. Burke
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&& .HYDROLOGY...
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River Flooding is not expected for the next 7 days.
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&& .AVIATION...
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The ridge has weakened a bit as a disturbance passes just offshore. The HRRR shows some marine stratus making it to the coast tonight, but otherwise just some mid and high clouds will pass by the area. Some fog patches will again set up in the foggy bottoms and river valleys and near Puget Sound overnight. KSEA...Some mid and high clouds ought to show up at some point, but the weak weather system just off the coast wont amount to much.
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&& .MARINE...
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A weak system is passing by just offshore, and the flow over the coastal waters has turned to the southeast 10- 20kt. The flow will turn back to light northerly flow again by midweek. A cold front will reach the area late Thursday night into Friday. A warm front will approach the coastal waters on Saturday.
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&& .SEW WATCHES/WARNINGS/ADVISORIES... WA...Air Stagnation Advisory until 2 PM PST Thursday for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and Vicinity- East Puget Sound Lowlands-Eastern Strait of Juan de Fuca- Everett and Vicinity-Hood Canal Area-Seattle and Vicinity- Southwest Interior-Tacoma Area-Western Skagit County- Western Whatcom County. PZ...Small Craft Advisory until 3 AM PST Tuesday 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. && $$ www.weather.gov/seattle

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