Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
855 FXUS66 KSEW 180005 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 405 PM PST Sat Feb 17 2018 .SYNOPSIS...
-- Changed Discussion --
The low center which brought today`s active weather will exit east this evening. A modified arctic cold front will move southward through Western Washington late tonight and early Sunday, bringing the possibility of lowland snow followed by strong north winds. Dry conditions are expected from late Sunday through Tuesday with cold temperatures remaining in place. Another system could slide in from the northwest around next Wednesday, with light snow possible. More systems are possible on Friday and Saturday, with light precipitation and low snow levels.
-- End Changed Discussion --
&& .SHORT TERM...
-- Changed Discussion --
Deepening 996 mb surface low is now over northeast Washington and exiting east. Therefore, strong westerly wind over the forecast area will ever so slowly start to ease over the next few hours. Heavy snow over the Cascades will also ease, though a Puget Sound Convergence Zone may keep Stevens Pass going strong for a while longer. With today`s heavy snow and strong wind, an Avalanche Warning is also in effect for a few more hours in the Cascades. In the wake of today`s low center, a a continental polar air mass will quickly spread south through interior B.C. and come pouring out of the Fraser later tonight. The BLI-CYWL gradient is forecast to peak near -20 mb on Sunday morning, making this a strong Fraser Outflow event. Strong north- northeast winds will ensue, most notably near Bellingham, the San Juans, and around to Admiralty Inlet. Other places will be breezy as well. The windiness from the less common northerly direction will have greater tree impacts than a similar south wind. A modified arctic front will spread south and southwest through Western Washington early Sunday morning, with localized frontal convergence and orographic lift leading to some lowland snow accumulations. The average forecast snowfall up and down the I-5 corridor will be about 1 to 2 inches. However, all meso models show a distinct hole in the snowfall pattern from Poulsbo over to Seattle, likely from precip shadowing downwind of the Olympics in northwest flow aloft. So that is why the Bremerton and Seattle forecast zones are excluded from the Winter Weather Advisory over the lowlands. Some local enhancement of snow is expected just away from the water near Port Angeles due to north-northeast wind upsloping against the north slope of the Olympics, so the forecast there is for 2-4 inches of snow. The front will be followed by the coldest air mass of this winter. A hard freeze is expected areawide on Sunday night, with Monday night likely being the coldest night of the upcoming cold spell. At least the cold will come with drier weather. The last showers will taper off over the Cascades on Sunday night or Monday, with dry weather expected Monday and Monday night. The NAM and GFS keep Tuesday dry as well. The ECMWF tries to develop a warm advection nose of precip over Western Washington on Tue. Since only two of the GFS ensemble`s 20 members show precip at HQM on Tuesday, have kept PoPs on the low side. If any precip does occur on Tue, it would still be cold enough for snow, even at the coast. Haner .LONG TERM...A cool northwest flow pattern will continue from Wed through Sat. A system diving down from the northwest on Wed could very well spread light precip across Western Washington, with light snowfall accumulations. A lull in precip is possible Thu. More precip with more weak shortwaves on Fri and Sat. The 12z ECMWF was colder than the 12z GFS, but the 18z GFS seems to be coming around to a cooler solution. This would keep snow levels at or below 1000 feet, a situation of the snow threat continuing on higher lowland hills but less so at sea level. Haner
-- End Changed Discussion --
&& .AVIATION...
-- Changed Discussion --
The front is moving inland and strong onshore flow and a showery air mass will prevail this evening. A heavy band of PSCZ showers were up over Snohomish county this afternoon and a squall moved through the Strait. The showers overnight will turn over to scattered snow showers as the air cools and then they will move south around daybreak with clearing from the north. KSEA...Scattered showers and blustery weather with SW winds will give way to a north wind late tonight as the colder air pushes south. Scattered snow showers around daybreak will probably give way to pretty rapid clearing so much in the way of accumulation is not expected in Seattle--although a quick inch of snow in a shower is possible.
-- End Changed Discussion --
&& .MARINE...
-- Changed Discussion --
A 995mb low over southern B.C. will sag south as the cold high pressure system over B.C. sags south. The modified arctic front will be moving south through Western Washington before daybreak Sunday--in fact cold Fraser outflow northeasterly gales should develop pretty quickly overnight--spreading down into the Port Townsend to Port Angeles area well before daybreak. The switch to northerly winds in Puget Sound will probably be around daybreak for a raw brisk day. Gradients ease Monday and Monday night and by Tuesday pressure gradients will be quite light.
-- End Changed Discussion --
&& .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
WA...Winter Weather Advisory from 10 PM this evening to 9 AM PST Sunday for Admiralty Inlet Area-Bellevue and Vicinity- Central Coast-East Puget Sound Lowlands-Everett and Vicinity-Hood Canal Area-Lower Chehalis Valley Area-North Coast-Southwest Interior-Tacoma Area-Western Skagit County. Wind Advisory until 8 PM PST this evening for Bellevue and Vicinity-Bremerton and Vicinity-Hood Canal Area-Seattle and Vicinity-Southwest Interior-Tacoma Area. Coastal Flood Advisory from 4 AM to 8 AM PST Sunday for Bremerton and Vicinity-Hood Canal Area-Seattle and Vicinity- Southwest Interior-Tacoma Area. Winter Storm Warning until 6 AM PST Sunday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and King Counties-Cascades of Whatcom and Skagit Counties. Winter Weather Advisory from 8 PM this evening to 4 AM PST Sunday for San Juan County-Western Whatcom County. Wind Advisory from midnight tonight to 6 PM PST Sunday for Western Skagit County-Western Whatcom County. Winter Weather Advisory until 10 AM PST Sunday for Olympics. Wind Advisory until 6 PM PST this evening for Central Coast- North Coast. High Wind Warning until 9 PM PST this evening for Admiralty Inlet Area. Wind Advisory until 6 PM PST Sunday for San Juan County. Winter Weather Advisory from 10 PM this evening to 8 AM PST Sunday for Eastern Strait of Juan de Fuca-Western Strait of Juan De Fuca. Wind Advisory until 9 PM PST this evening for Eastern Strait of Juan de Fuca. PZ...Small Craft Advisory for rough bar until 4 PM PST Sunday for Grays Harbor Bar. Gale Warning until 4 PM PST Sunday for Admiralty Inlet-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-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Gale Warning until 3 AM PST Sunday for Central U.S. Waters Strait Of Juan De Fuca-Puget Sound and Hood Canal-West Entrance U.S. Waters Strait Of Juan De Fuca.
-- End Changed Discussion --
&& $$ www.weather.gov/seattle

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.