Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 180520 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 PM PST Fri Nov 17 2017 .SYNOPSIS...Weak high pressure will be over the area tonight and Saturday. A strong front will reach the area Sunday--heavy rain is likely in the Olympics, and windy weather is possible for some areas. Periods of wet weather with rather mild temperatures are likely next week--with windy conditions possible at times. && .SHORT TERM...
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Current radar loop shows some spotty light showers around. Models show warm air advection tonight and Saturday ahead of the next system. This will keep some light rain around but should lift north into B.C. by Saturday morning. Winds will pick up over the coastal waters Saturday, and windy conditions are expected across the entire area Sunday. Models differ on the strength of the cold front approaching Sunday. The GFS20 shows much more surface development along the front but other models such as the 12Z ECMWF are not as intense. Wind advisory criteria winds are likely over the north interior and probably the coast. Puget Sound may not be as windy but it will depend on where or if a meso low forms in the lee of the Olympics. Strong flow will produce heaviest rain amounts over the south slopes of the Olympics. See the hydro discussion below and refer to the latest flood statement. The lowlands could get up to an inch but very little rain will fall where rain shadowing occurs in the lee of the Olympics. Rain should ease off by Monday which is reflected in the current forecast. Mercer .LONG TERM...From previous discussion...The front that moves through Western Washington Sunday afternoon will be over Northern California and Oregon later Sunday night and Monday morning. The GFS shows a dry day for Western Washington on Monday. Rain moves back into Western Washington Tuesday afternoon and evening. The heavier rain shifts to the B.C. coast and Vancouver Island for Wednesday and Wednesday night then back down through the area Thursday. It will become unusually mild--the 700mb temp will rise to around 0c and 850mb temps will peak around +9c. That does not bode well for the ski areas by the middle of next week. The good news is that the models do not have especially heavy rain, but that might change again and bears watching.
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&& .AVIATION...
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A flat upper ridge centered over the Washington coastal waters will shift into Eastern Washington Saturday afternoon and evening. A strong and wet cold front will sag southeast through British Columbia on Saturday and will enter the area from the northwest Saturday evening. Moderate westerly flow aloft. The air mass is moist and somewhat unstable, except somewhat drier over the interior lowlands to the lee of the Olympics and terrain of Vancouver Island. The air mass will become increasingly stable later tonight into Saturday. Scattered convective showers are affecting the Washington coastal waters and coastal zones and the interior lowlands from about KSHN and KOLM southward. The showers will gradually decrease in both coverage and intensity overnight. Conditions are generally VFR, with MVFR ceilings in showers. The low level air mass is moist enough that MVFR ceilings with bases 010-020 will become more widespread after 06Z with diurnal cooling. The lower levels are expected to dry somewhat Saturday morning as the upper level ridge shifts overhead then to the east of the area. Albrecht KSEA...South wind 9-13 kt this evening will ease to 7-10 kt late tonight and Saturday morning then rise to 9-14 kt with gusts 18-22 kt midday Saturday. Good VFR this evening with cigs near 030 developing about 06Z and continuing through early Saturday morning. There is a 30-40 percent chance that CIGS will remain better than indicated in the current TAF as westerly flow aloft interacts with the Olympics. Albrecht
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&& .MARINE...
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A strong cold front extending WSW from Haida Gwaii into the Pacific will slowly drag southeast down the British Columbia coastline tonight through Sunday, then will move southeast through the waters Sunday evening. Winds will gradually increase from the south and southeast ahead of the front late tonight into Saturday night with gales expected over most of the waters Saturday night. Winds will rapidly ease behind the front Sunday night. 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. Albrecht
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&& .HYDROLOGY...
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Heavy rain will fall in the Olympics on Sunday driving the Skokomish river in Mason county to near or above flood stage. A flood watch remains in effect for Mason County. Other rivers should not experience flooding with the Sunday system. Mild wet weather is likely from about Tuesday afternoon through about Thursday of next week. The Skokomish river will again be at risk of flooding. Other rivers need to be watched but the threat of flooding is low.
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&& .SEW WATCHES/WARNINGS/ADVISORIES... WA...Flood Watch from Sunday morning through Monday morning for Hood Canal Area-Lower Chehalis Valley Area-Olympics. PZ...Small Craft Advisory from 3 AM to 3 PM PST Saturday 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. Gale Warning from 3 PM Saturday to 3 PM PST Sunday for Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. Gale Warning from 3 PM Saturday to noon 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-Coastal Waters From James Island To Point Grenville Out 10 Nm. Small Craft Advisory from 9 AM to 6 PM PST Saturday for Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Gale Warning from 6 PM Saturday to 3 PM PST Sunday for Admiralty Inlet-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. Small Craft Advisory from 3 AM to 6 PM PST Saturday for East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from midnight tonight to 6 PM PST Saturday for Northern Inland Waters Including The San Juan Islands. Small Craft Advisory from 6 AM to 6 PM PST Saturday for Admiralty Inlet-West Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning from 6 PM Saturday to noon PST Sunday for West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 9 AM to 3 PM PST Saturday 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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