Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 190545 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 PM PST Sat Nov 18 2017 .SYNOPSIS...
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A vigorous front will bring rain and windy conditions to Western Washington Sunday and Sunday night. Weak high pressure over the area on Monday will provide a brief lull in the active weather. The next in a series of fronts arrives Tuesday and Wednesday with rain and windy conditions. Wet weather will persist through through the end of next week.
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&& .SHORT TERM...
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Light warm advection rainfall has reached the north coast and extreme north interior this evening. Radar returns further offshore are not very impressive and IR imagery showing brighter clouds tops lifting northward. This matches up well with most models holding off on the bulk of rainfall until later Sunday morning. Warmer air aloft will infiltrate the area causing snow levels to rise as precipitation arrives. The north Cascades may stay cool enough for some decent snow accumulations but some rain will probably mix in at times. Will plan on dropping the winter storm watch for the Cascades of Pierce/Lewis counties and just go with an advisory for the north Cascades above 5000 feet for up to 10 inches Saturday afternoon and night. Will need to adjust the timing on this. The new 00z models are little changed from earlier runs and shows south/southeasterly gradients increasing later tonight and Sunday ahead of a strong front. A meso low will also develop northeast of the Olympics which should induce some stronger wind gusts around the Admiralty Inlet/Whidbey Island area. Southerly winds of 20 to 35 mph with gusts 50 to 55 mph still appear reasonable for the north interior and central/north coast. Wind advisories remain in effect for these locations Sunday. There is some questions as to whether the stronger winds could extend a bit further south into parts of Everett or other areas around Puget Sound. The latest UW WRF4km only shows spotty gusts close to 40 mph around the Sound. This is still sub-advisory but it will still be a windy day across all of Western Washington. Models are also fairly progressive with the front with heaviest rain falling Sunday afternoon through Sunday evening, then tapering overnight. Heaviest rainfall will occur along the south slopes of the Olympics where strong orographics will enhance rainfall. Even then, amounts may struggle to reach much above 2 to 3 inches. The Skokomish river will still be at risk of minor flooding and the flood watch remains in effect. Refer to the latest flood watch statement and hydro section below for details. The system sags south on Monday with a brief lull in wind and rain over the area. Another warm front arrives Monday night and Tuesday with another round of rain and locally windy conditions. This system will bring warmer air into the region and high temperatures could be well above average by Tuesday and Wednesday. Mercer .LONG TERM...From previous discussion...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.
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&& .AVIATION...
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A flat upper ridge aloft will exit to the east tonight. A strong cold front will spread down the British Columbia coastline tonight and move southward through Western Washington later Sunday. The front will be preceded by heavy rain and strong southerly surface wind. Moderate westerly flow aloft this evening will become strong on Sunday while strong south to southwest winds develop between 1500 and 6000 ft MSL midday Sunday. The air mass is stable and will become increasingly moist late tonight and Sunday. Expect mainly VFR conditions across the area today with conditions lowering to IFR in precipitation and low ceilings from the northwest on Sunday. Albrecht KSEA...Only FEW-SCT035 with moisture above 120 through most of tonight. CIGS around 035 will rapidly develop about 12Z late tonight with rain developing later Sunday morning. CIGS and VSBYS will lower Sunday afternoon to 008-012 into the evening hours in occasionally moderate rain. South wind at the surface 6-10 kt through Sun morning will increase to 15G25KT about 18Z. Winds at 025 will increase to 19060kt 20Z-02Z Sunday with peak winds between 040 and 060 of 19070kt. Albrecht
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&& .MARINE...
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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 Sunday afternoon through the early evening hours. 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/Albrecht
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&& .HYDROLOGY...
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The Skokomish river in Mason county will likely rise to near or over flood stage and could stay up around flood stage for several days. For all other rivers, the new 00z models do not show especially heavy precip over the Cascades mid week. The flow will be mostly southerly through the week which is not usually not conducive for the heavier rain amounts needed to cause river flooding in most Cascade basins. Regardless, the system still bares watching with a warm air mass in place and a long fetch of available moisture.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter weather advisory Sunday morning through late Sunday night for the Cascades of Whatcom and Lewis counties above 5000 feet. 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 until 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. 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 until 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.
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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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