Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 210549 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 920 PM PST Mon Nov 20 2017 .SYNOPSIS...
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A strong warm front will bring periods of moderate to heavy rain to Western Washington Tuesday and Tuesday night, with the snow level rising above 8000 feet. A break between weather systems on Wednesday will allow rain to diminish a little. Another wet frontal system will reach the area Wednesday night and Thanksgiving Day. After a short break in rain on Friday, the next wet system will probably arrive Saturday.
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&& .SHORT TERM...
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There is a break between systems tonight and the convergence zone earlier this evening has dissipated. Radar shows the leading edge of the next round of warm frontal rainfall reaching the south/central Oregon coast. Brightest cloud tops are still well offshore but the system has an extensive tap to warm moist air from the tropical Pacific. Satellite imagery has the look of an significant atmospheric river type event. However, model rain amounts still are not as impressive as one would expect. This might be partly due to the more southerly component to the flow rather than westerly which would produce much stronger orographic enhancement along the west slopes of the Cascades. See the hydro discussion below for details on potential flooding and refer to the latest flood bulletins for current flooding. Most models show a lull in rainfall around Wednesday with moisture arriving with two systems rather than one continuous stream of heavy rain. This will hopefully help keep river flooding in the minor category. The air mass will be quite warm and temperatures will be above average Tuesday and especially Wednesday. Clouds should hinder high temperatures somewhat but some mid/upper 50s are likely and some near 60 degree highs are possible, close to record highs. .LONG TERM...Previous discussion...The flow aloft will make a brief shift to weaker to weaker W flow aloft on Friday, for a short mainly dry period. Meanwhile, the large longwave upper level trough offshore will be strengthening again, eventually re-establishing moist SW flow aloft over W WA. An upper level shortwave trough embedded in the SW flow aloft will produce another wet period on Saturday. Both the GFS and ECMWF have this system, but the ECMWF timing is a little slower. Both models also have a surface low crossing W WA Monday morning or afternoon (ECMWF slower). The ECMWF solution could be pretty windy but Monday is much too far out to count much on model details. Kam
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&& .AVIATION...
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Westerly flow aloft will become southwesterly on Tuesday as a ridge of high pressure over the region weakens and moves east. A strengthening warm front extending from Western Oregon westward to a 990 mb low centered near 42N 134W will lift northward across Western Washington on Tuesday. Expect mid and high level moisture late tonight to deepen early Tuesday and the air mass will be stable. Low level northeasterly flow ahead of the front combined with strengthening southerly flow aloft (south 35kt at 2000 ft) will give low level wind shear 14Z Tuesday until the warm front passes during the afternoon hours to most of the terminals. VFR conditions tonight will lower to MVFR as precipitation begins Tuesday morning. MVFR conditions with rather low ceilings will continue behind the warm front through Tuesday evening. However, rainfall rates and low level wind shear will decrease from the south behind the warm front. Albrecht KSEA...Expect VFR conditions through about 12Z. Moisture and southerly winds aloft will increase rapidly Tuesday morning and continue until the warm front passes the terminal mid afternoon. Expect increasing rain with cigs 015-020. Low level winds will be northeasterly Tuesday morning while southerly winds will increase at 020 giving low level wind shear during the morning hours. Behind the front, low level winds will become southeast to south 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 on Tuesday. Expect strong offshore flow to develop 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 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 later Tuesday while winds elsewhere weaken. A second system will reach the waters around Thursday while a third one will move through the waters Saturday or Sunday. Each of the systems will provide gale force winds to some waters, especially the coast, entrances to the strait, and the northern inland waters. Albrecht
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&& .HYDROLOGY...
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The Skokomish river at Potlach in Mason county fell just below flood stage this evening. Some residual minor flooding is still possible downstream the next couple hours. The flood warning will likely be allowed to end at midnight as the river continues to slowly fall. The White river shot up quickly due to water releases but appears to have peaked around 5,000 cfs earlier this evening which is shy of flood stage. River flows on the White river at R street should fall through Tuesday before possibly rising above flood stage Tuesday night. The threat of flooding remains possible on several area rivers running off the Cascades from Whatcom county south to Lewis county Tuesday night through Thursday. A couple of warm wet frontal systems move across the area. The first and wettest system will impact the area Tuesday and Tuesday night, also pushing the snow level up above 8000 feet Tuesday through Wednesday night. The second will reach the area Wednesday night/Thursday. Snow levels will lower to 6000-7000 feet on Thursday. The first system looks more significant. Rainfall amounts on UW4km WRF still point rain amounts in the 2-5 inch range for the Olympics with 2-4 inches over the Cascades. The coast may see a couple inches and the lowlands could get around an inch except in the rain showed area northeast of the Olympics. The Wednesday night/Thursday system will not be as wet, with 2 or 3 inches over the Olympics, and close to the same in the Cascades. A flood watch remains in effect to address potential river flooding from these systems. This does not look like a particularly widespread flood event, but several rivers in the Cascades and Olympics could still exceed flood stage the next 2-3 days. The Skokomish will almost certainly flood again by Tuesday night. Landslides could also become a threat with the lowland rains Tuesday and Tuesday night. The USGS landslide guidance shows one or two stations near or just above the threshold of concern, and of course the additional rain with that front will drive that higher. Mercer/Kam
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Flood Watch from Tuesday afternoon through Thursday afternoon for all counties except San Juan and Kitsap. Refer to flood bulletins for current river flooding. PZ...Gale Warning from 4 AM to 4 PM PST Tuesday 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 to 8 PM PST Tuesday for East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory from 4 AM to 4 PM PST Tuesday 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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