Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 111317 AAA AFDSEW Area Forecast Discussion National Weather Service Seattle WA 515 AM PST Mon Dec 11 2017...updated aviation section .SYNOPSIS...A strong upper ridge will maintain dry and stagnant conditions across Western Washington through Wednesday. A weak system will dissipate over the Coastal waters Tuesday morning. The ridge will break down by Thursday with rain returning Thursday night and Friday. Wet weather will continue through the weekend with strong moist flow over the area. && .SHORT TERM...Satellite imagery shows clear skies over Western Washington early this morning. Areas of fog have formed in the last few hours but most of the visibilities in the foggy areas are above a half mile. With the clear skies temperatures are on the cold side with mid 20s to mid 30s at 11z/3am. Air quality readings remain in the moderate or unhealthy for sensitive groups category at most locations. Little change in the forecast today with the upper level ridge firmly in place over the area. Inversion still intact with 3 am temperatures at Hurricane Ridge ( 5250 feet ), Mount Baker (4200 feet ) and Paradise on Mount Rainier ( 5400 feet ) all in the 50s. Sandpoint profiler shows light winds up to 2000 feet. There is a good chance the air stagnation advisory, which currently expires at noon today, will get extended. The fog that has formed in the area is shallow so expect another morning burnoff leaving sunny skies this afternoon. Highs in the 40s to lower 50s will be common. The exception will be along the North Coast where downslope easterlies off the Olympics will push temperatures into the upper 50s. Upper level ridge remaining in place tonight with a dissipating negatively tilted system moving up from the southwest along the back side of the ridge. This will add a little bit of cloud cover to the area overnight especially along the coast. Lows will be in the mid 20s to mid 30s. System dying along the Coast early Tuesday morning with the upper level ridge rebuilding in the afternoon. By 00z Wednesday the ridge axis will be west of the area. The inversion, which weakens Monday night into Tuesday morning, will strengthen again Tuesday afternoon. Cloud cover from the dying system will help limit the fog formation Tuesday morning with just patchy fog if any forecast for most locations. Highs Tuesday will be in the 40s and lower 50s. Upper level ridge remaining over the area Tuesday night and Wednesday. Low level offshore flow much weaker Tuesday night into Wednesday so have added more late light and morning fog to the forecast but coverage will still be less than 50 percent of the lowlands. Light flow in the lower levels will allow the fog to hang on a little longer but still have most areas in the sunshine by Wednesday afternoon. Highs once again will be in the 40s and lower 50s. With the clear skies Tuesday night lows below freezing will be common with the colder locations dropping into the mid 20s. .LONG TERM...Extended models remaining consistent with the ridge breaking down and a series of systems moving through Western Washington. Timing on the last few runs for the first system has been a little slower than the current forecast. Will go ahead and dry the forecast out for Thursday. With the models showing good consistency have raised the pops into the likely or higher category for Friday through Sunday. Rainfall amounts in the central Puget Sound might not amount to much for the middle of December with model solutions showing the area somewhat rain shadowed over the weekend. Felton && .AVIATION...
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A strong upper level ridge axis will persist over Eastern Washington through tonight, though it will flatten some. A dissipating front will brush the coast tonight. Light southerly flow aloft today will become moderate southwest tonight. The air mass will be dry and very stable. Shallow dense fog will be the main forecast challenge this morning over the Puget Sound lowlands. It will mainly favor sheltered valleys and the South Sound Region. In contrast, dry east wind off the Cascades will minimize fog over King County. Looking ahead to tonight, conditions for fog development will be less favorable due to thicker cirrus and a little more mixing in the boundary layer. KSEA...Airport on the northern edge of an area of fog that extends south to near Tacoma and east over to Auburn. The layer is shallow and does not go much further north than Sea-Tac so a midd morning breakout is expected. Clear skies and light south wind by late morning. Tonight appears unfavorable for fog development at SEA due to thicker cirrus and better dry air entrainment from aloft into the surface layer. Felton/Haner
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&& .MARINE...A 1042 mb high over Eastern Washington and lower pres offshore will result in moderate offshore flow through Monday evening. A dissipating front will brush the north coast on Monday night, causing easterly gradients to weaken. Light northerly offshore flow will prevail on Tue and Wed. A cold front will sweep through the waters on Friday, leading to a turn to onshore flow. Haner && .HYDROLOGY...River Flooding is not expected for the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Air Stagnation Advisory until noon PST today 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 9 PM PST this evening 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 3 PM PST this afternoon for Coastal Waters From Cape Flattery To James Island Out 10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca. && $$ www.weather.gov/seattle

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