Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 182324 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 324 PM PST Sat Nov 18 2017 .SYNOPSIS...Weak high pressure will give way to a vigorous front on Sunday afternoon and evening. Weak high pressure will be over the area Monday. A strong warm front will arrive by Tuesday. The middle of the week will be mild, rainy, and perhaps windy at times. && .SHORT TERM...A front will shift southeast into Western Washington Sunday afternoon and evening. Winds are likely to be strong enough for wind advisories in the typically windy spots. There is a winter storm watch up for parts of the Cascades, you would probably have to be up pretty high for this front to seem much like winter--above 5000 feet. There will be some heavy rain in the Olympics and also a decent rain shadow in the lee of the Olympics. The front is mostly southeast of the area by late Sunday night and most of the day Mon should be dry before a warm front nears the area Monday night. .LONG TERM...The strong warm front will be over the region Tuesday. The heavier rain shifts to the B.C. coast Wednesday and Wednesday night before shifting south over the area Thursday and breaking up somewhat. It will be mild, rainy, and breezy and the rivers will be running high, with the Skokomish River in Mason county flooding. There is a chance of flooding on the other rivers, but the fact that this pineapple express is more southerly probably means the best orographics are limited to the Olympics. But it will still make a mess of our early season snowpack which is a shame for the skiers. Seattle ought to be up around 60 for a high Wednesday. && .AVIATION...A flat upper ridge axis will exit east and flatten a bit tonight. A cold front will spread down the B.C. coast tonight and move southward through Western Washington later Sunday, preceded by heavy rain and strong southerly surface wind. Moderate westerly flow aloft this evening will become strong on Sunday. The air mass is stable. Moisture and clouds will thicken rapidly starting late this evening, then lower on Sunday. KSEA...Clear below 100 will continue through at least 08z (midnight) tonight. Cloud cigs around 040 will rapidly develop overnight, with rain developing later Sunday morning. Cigs and vsbys lowering Sunday afternoon in occasionally moderate rain. Southerly surface wind for the next 30 hours. Late Sunday afternoon, SW wind at 040 will peak near 70 knots. Haner && .MARINE...A strong cold front extending WSW from the Queen Charlotte Sound into the Pacific will slowly drag southeast down the British Columbia coastline tonight and then push southeastward through the Washington waters on Sunday afternoon. In advance of the front, winds will rise. The wind increase will progress from north to south through tonight with gales expected over most waters ahead of the front. Winds will rapidly ease and shift out of the northwest behind the front later Sunday afternoon and evening. High pressure will bring light winds to the waters on Monday. A strong warm front will push northward through the waters on Tuesday. A strong low pressure center developing well offshore will push northeast into Haida Gwaii on Wednesday. Haner && .HYDROLOGY...The Skokomish river in Mason county is a shoo-in to get over flood stage and stay up around flood stage for several days. The 12z GFS does not show especially heavy precip over the Cascades and for Wednesday and Wednesday evening shifts the heavy rain up in British Columbia. Possibly this pineapple express or atmospheric river or whatever you want to call it is too southerly for the Cascades--more westerly flow and stronger orographics might increase the chance of flooding for the Cascade rivers--but it will still be wet and warm and this bears watching. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Storm Watch from Sunday afternoon through late Sunday night for Cascades of Pierce and Lewis Counties. Winter Storm Watch from late tonight through Sunday afternoon for Cascades of Whatcom and Skagit Counties. Flood Watch from Sunday morning through Monday morning for Hood Canal Area-Lower Chehalis Valley Area-Olympics. Wind Advisory from 6 AM to 3 PM PST Sunday for Admiralty Inlet Area-San Juan County-Western Skagit County-Western Whatcom County. Wind Advisory from 6 AM to 5 PM PST Sunday for Central Coast. Wind Advisory from 4 AM to 3 PM PST Sunday for North Coast. PZ...Small Craft Advisory for rough bar from 6 AM Sunday to noon PST Monday for Grays Harbor Bar. Gale Warning from 3 AM to 7 PM PST Sunday for Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Gale Warning from midnight tonight to 7 PM PST Sunday for Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. Gale Warning from 3 AM to 3 PM PST Sunday for Admiralty Inlet- Coastal Waters From James Island To Point Grenville Out 10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca- Northern Inland Waters Including The San Juan Islands. Gale Warning from 10 PM this evening to 3 PM PST Sunday 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. Small Craft Advisory until 6 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. Gale Warning from midnight tonight to 3 PM PST Sunday for West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from midnight tonight to 6 PM PST Sunday for Central U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 9 PM this evening to 6 AM PST Sunday for Puget Sound and Hood Canal. Gale Warning from 6 AM to 3 PM PST Sunday for Puget Sound and Hood Canal. && $$ 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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