Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 211125 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 325 AM PST Tue Nov 21 2017 .SYNOPSIS...
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A subtropical plume of moisture will bring heavy rain to Western Washington today and tonight, with the snow level near 8000 feet. After a relative break in the weather on Wednesday, another wet frontal system will move through the region Wednesday night and Thanksgiving. There will be another break on Friday, followed by more wet weather this weekend and into Monday.
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&& .SHORT TERM...
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An upper ridge has been moving inland across the Pacific Northwest overnight, but satellite imagery and radar show the leading edge of the next plume of subtropical moisture moving into Western Washington early this morning. Rain began at Hoquiam just before 3 am. Precipitation will spread from southwest to northeast and cover our entire forecast area within the next few hours, and then continue into tonight as the southwesterly conveyor shifts slowly eastward. The heaviest rain will probably occur from late morning through afternoon, with the warm front passage, as 850 mb south- southwest winds rise to around 70 kt. Precipitation should decrease gradually later tonight. Snow levels will rise to around 8000 feet behind the warm front and remain near that level tonight. Models have been fairly consistent with regard to precipitation amounts; storm totals for today and tonight should be roughly 2 to 5.5 inches over the Olympics and Mount Rainier, with 1 to 4 inches over the Cascades north of Mount Rainier. Warm moist southwest flow aloft will continue Wednesday, as an upper trough digs into our far offshore waters outside of 135W. There will be more rain on Wednesday, especially at the coast and over the mountains, but it will be a relative break in the action. Precipitation in the mountains will amount to another half inch to inch, with the snow level still near 8000 feet. The next frontal system will develop offshore on Wednesday, ahead of the upper trough, and progress across Western Washington Wednesday night and Thanksgiving. The snow level will be around 7000 feet as the event starts, then lower to around 4000 feet behind the front as precipitation decreases Thursday night. This system doesn`t look quite as wet, but it should bring another 1.5 to 3.5 inches of precipitation to the mountains with the heaviest amounts over the Olympics. This will be a period of wet, but not especially windy, weather for the lowlands. Three day rain totals of 2 to 4 inches are likely at the coast with roughly 1 to 2.5 inches in the interior, and according to USGS studies this could result in some landslides. The warm air mass that will drive the snow level to around 8000 feet will also be felt in the lowlands, especially on Wednesday when high temperatures will be around 60 degrees. McDonnal .LONG TERM...The models agree fairly well on the big picture in the long term period, Friday through Monday. They show a short- lived, relative break in the weather on Friday, then two frontal systems in quick succession on Saturday and Sunday, followed by cooler moist westerly flow on Monday. Confidence in model details isn`t very high, but I will increase POPs somewhat for Saturday through Sunday night. McDonnal
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&& .HYDROLOGY...
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A warm front and the plume of subtropical moisture with it will bring 2 to 5.5 inches of rain to the Olympics and Mount Rainier today and tonight, and 1 to 4 inches of rain to the Cascades north of Mount Rainier. The snow level will be near 8000 feet. A second frontal system will bring another 1 to 4 inches on Wednesday night and Thursday, with the snow level falling from 7000 feet to 4000 feet as the system moves through. A flood watch is in effect for most of the forecast area. The Skokomish River, which only fell below flood stage Monday evening, will almost certainly flood again late today or tonight. In addition the Northwest River Forecast Center models show several rivers cresting near or just above flood stage including the Cowlitz, Nisqually, White, Snoqualmie, Stillaguamish, Skagit, and Nooksack. This doesn`t mean the possibility of flooding is limited to these rivers. Flooding on these rivers could begin tonight or Wednesday. It looks like the threat is for a relatively minor flooding episode. In addition, the threat of landslides will increase across the lowlands of Western Washington. The USGS landslide guidance shows one or two stations near or just above the threshold of concern, and of course the additional rain today and tonight and again Wednesday and Thursday will drive that higher. McDonnal
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&& .AVIATION...
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Strong southwest flow aloft today through tonight. A strengthening warm front extending westward from the Central Oregon coast into a 990 mb low near 42N 134W will lift northward across Western Washington this afternoon. Expect moisture to rapidly spread into the area from the south this morning ahead of the warm front, and for the moisture to remain over the area behind the front tonight into Wednesday. The air mass will be stable through tonight. Low level northeasterly flow ahead of the front combined with strengthening southerly flow aloft (south 30-40kt at 2000 ft) will give low level wind shear this morning until the front passes during the afternoon hours. With the exception of KCLM, all terminals are expected to see LLWS ahead of the warm front. Conditions are VFR at most locations early this morning with some LIFR in fog at KOLM. Expect all areas to become MVFR in ceilings and precipitation -except for local IFR conditions at places like KPWT- as the warm front approaches the area this morning. MVFR conditions with rather low ceilings will continue behind the warm front tonight, but rainfall rates and low level wind shear will decrease from the south. Albrecht KSEA...Good VFR conditions early this morning will rapidly deteriorate to MVFR with cigs 015-020 starting about 14Z. Southerly winds aloft will increase rapidly with the moisture with flow below about 2000 ft MSL remaining northeast to easterly. Will continue to indicate LLWS in the terminals through mid afternoon ahead of the warm front. Behind the front, low level winds will become southeast to south 10-13kt and precipitation will become lighter. Ceilings behind the front will likely stay around 020. Albrecht
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&& .MARINE...
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A strengthening warm front extending westward from the central Oregon coast to a 990 mb low near 42N 134W will lift north across the region this afternoon. Expect strong offshore flow to develop this morning in advance of the warm front that will give gale force winds to the West Entrance to the Strait of Juan de Fuca and to the adjacent coastal waters. Small craft advisory conditions can be expected in the central strait and at locations around the east entrance in advance of the warm front. Behind the front, strong gale force southerly winds can be expected over the coastal waters this afternoon. A frontal system west of the waters will give rather strong southerly flow to the waters Wednesday and Wednesday night. Flow will also be rather strong over the northern half of the inland waters as flow aloft interacts with the Olympics forming a mesolow over or near the northeastern tip of the Olympic Peninsula. Rather windy southerly flow will continue on Thursday as a trough south of an offshore low pressure system brushes the area. Latest model solutions show a developing low offshore moving by to the west of the waters on Saturday. This will give windy conditions to the area on Saturday. While it will be windy over the next several days, confidence in the timing and strength of individual features in this active pattern is rather low. Widespread small craft advisory conditions will be dominant while some features may produce gales at times, especially over the coastal waters and at the entrances to the Strait of Juan de Fuca. Albrecht
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Flood Watch from this afternoon through Thursday afternoon for Admiralty Inlet Area-Bellevue and Vicinity-Cascades of Pierce and Lewis Counties-Cascades of Snohomish and King Counties-Cascades of Whatcom and Skagit Counties-Central Coast-East Puget Sound Lowlands-Eastern Strait of Juan de Fuca-Everett and Vicinity-Hood Canal Area-Lower Chehalis Valley Area-North Coast-Olympics-Seattle and Vicinity- Southwest Interior-Tacoma Area-Western Skagit County- Western Strait of Juan De Fuca-Western Whatcom County. PZ...Small Craft Advisory for rough bar from 4 PM this afternoon to 4 PM PST Wednesday for Grays Harbor Bar. Gale Warning until 4 PM PST this afternoon 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-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from noon today to 8 PM PST this evening for Admiralty Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 4 PM PST this afternoon for Central U.S. Waters Strait Of Juan De Fuca.
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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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