Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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002 FXUS66 KSEW 211657 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 857 AM PST Tue Nov 21 2017 .SYNOPSIS...
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A strong warm front, fed by a plume of warm subtropical moisture will bring heavy rain to Western Washington today and tonight, with the snow level rising to 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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RADAR has been lit up this morning by rain from the incoming warm front. Strong SW flow aloft has created a tiny rain shadow NE of the Olympics around Port Townsend. IR imagery shows the tap into subtropical moisture E of Hawaii. The heaviest rain is still expected late this morning through this afternoon. Precipitation is expected to diminish, but not end, this evening as a slight shift in the flow aloft directs the weakening moist plume over Oregon. The subtropical air mass will push snow levels up to around 8000 feet this afternoon 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 warm air mass will push temperatures up to around 10 degrees above normal on Wednesday, with highs near 60. 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. McDonnal/Kam .LONG TERM...Previous discussion from the 325 AM AFD...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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&& .AVIATION...
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Strong southwest flow aloft will continue today and tonight. A warm front will move across the area from south to north today. Rain today, tapering tonight. The air mass will be stable through tonight. At the surface, strong easterly gradients will ease tonight and become more southeasterly. LLWS is possible at most terminals today. Conditions are generally MVFR today with pockets of IFR. Low end MVFR conditions will continue behind the warm front tonight, but rainfall rates and low level wind shear will decrease from the south. KSEA...Discussion above applies. Northeast or east winds 4-8 kt will become southeast 8-12 kt after 00Z. CHB
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&& .MARINE...
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A warm front will move south to north across the area today. Strong offshore flow will give gales to the coast and west entrance, with small craft advisory conditions elsewhere. Another frontal system west of the waters will give strong southerly flow Wednesday and Wednesday night. Windy southerly flow will continue on Thursday as a trough brushes the area. Gales are possible with this system. 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. CHB
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&& .HYDROLOGY...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 && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Flood Watch from 4 PM PST 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 until 10 PM PST this evening for 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. Small Craft Advisory until 4 AM PST Wednesday for Admiralty Inlet.
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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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