Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 201722 CCA AFDSEW Area Forecast Discussion...corrected typo National Weather Service Seattle WA 916 AM PST Mon Nov 20 2017 .SYNOPSIS...
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A cold front will move east across the Western Washington interior this morning. Convergence zone showers will develop behind the front between Seattle and Everett through this afternoon. A strong warm front will bring periods of moderate to heavy rain Tuesday and Tuesday night, with the snow level rising above 8000 feet. Wednesday will be less wet, then another front moves through Wednesday night and Thanksgiving Day. Friday looks relatively dry, then another front should arrive around Saturday.
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&& .SHORT TERM...
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RADAR shows a cold front moving across the W WA interior this morning was moving over the Cascades at 9 AM. Some areas of light rain were still present over the interior lowlands but these should be done with by 11 AM. Onshore flow behind the front will produce a convergence zone, likely between Seattle and Everett, that should finally dissipate early this evening. The rest of the W WA lowlands should remain dry this afternoon. There will be a brief break in the precipitation this evening through most of tonight. A large deep upper level longwave trough is forming over the NE Pacific around 145W. This is far enough offshore for moist SW flow aloft to develop over W WA. A strong warm front will begin spreading rain northward over W WA before 12Z/4 AM Tuesday with the rain becoming heavy at times late Tuesday morning through Tuesday afternoon before slowly tapering off. Snow levels are expected to rise to 8000-9000 feet Tuesday afternoon through Wednesday night. The warm front will bring heavy rain to the Olympics and Cascades, which could drive some of the rivers flowing off the mountains above flood stage. Refer to the Hydrology section below. At least the warm front will push temperatures up above normal Tuesday and Wednesday, highs Tuesday should be in the mid to upper 50s, warming to the upper 50s to lower 60s on Wednesday. Moist SW flow aloft will continue on Wednesday with a little light rain around for wet warmth. Accumulations over the lowlands will be light. However this will be just a short break since another wet system will arrive later Wednesday night. Kam LONG TERM...The next frontal system will bring another round of rain to Western Washington late Wednesday night and Thanksgiving Day. The rainfall doesn`t look as heavy with this feature as with the Tuesday and Tuesday night system, but the snow level will still be 6000 to 7000 feet. Fairly moist southwest flow aloft will continue over the region Friday and Saturday; it doesn`t look dry for these couple of days, but rather less wet. Models agree generally that another frontal system - also with fairly high snow levels - will arrive Saturday night or Sunday. McDonnal
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&& .AVIATION...Westerly flow aloft will continue today and tonight as an upper level low moves east and weak high pressure builds. A cold front will move southeast out of the area today. At the surface, light to moderate northwest flow will develop. Flow will become offshore later tonight as a strong warm front approaches the area from the southwest. The air mass is moist and stable. Over the interior, ceilings are still MVFR or even IFR as the cold front remains nearby. West of Puget Sound there has been significant improvement, to good VFR mainly, as skies start to clear. A clearing trend will be apparent over all areas today, except a Puget Sound convergence zone will likely form around the King-Snohomish county line this afternoon. Conditions are expected to be VFR tonight over all areas. KSEA...IFR until 18Z or so, then improvement to MVFR and then VFR this afternoon. Northerly wind less than 6 KT this morning will become more variable this afternoon, then northeasterly tonight. CHB && .MARINE...A cold front will move east of Puget Sound this morning, with light to moderate northwesterly wind behind the front. Small craft advisory westerlies are forecast today in the strait, along with residual swells above 10 ft on the coast. A strong warm front will move north through the waters on Tuesday. Gale force easterly winds are likely on Tuesday at the west entrance and adjacent coastal waters. High end small craft advisory strength winds are likely over most other waters, with gales possible on some. The warm front will be followed by southerly flow on Wednesday. Strong low pressure over the northeast Pacific will maintain southerly flow on Thursday and Friday. CHB && .HYDROLOGY...In the very short term, the only river above flood stage is the Skokomish River in Mason County. It should crest this morning just above flood stage then fall below this afternoon. The threat of flooding will become more widespread, though, as warm wet frontal systems move through Tuesday/Tuesday night and again Wednesday night/Thursday. The first system looks more significant with 2 to 5 inches of rain over the Olympics and Mount Rainier and 1 to 4 inches further north over the Cascades, and with the snow level 8000 to 9000 feet. The second system won`t bring as much rain, but the snow level will be 6000 to 7000 feet. We issued a flood potential outlook yesterday, and I will update it this morning. The Skokomish will almost certainly flood Tuesday or Tuesday night. Northwest River Forecast Center guidance also shows several other forecast area rivers cresting near or just above flood stage including the Cowlitz, Nisqually, Snoqualmie, Skagit, Nooksack, and Dungeness. 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. McDonnal/Kam && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Storm Warning until 10 AM PST this morning for Cascades of Whatcom and Skagit Counties. PZ...Small Craft Advisory for hazardous seas until 3 PM PST this afternoon 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. Small Craft Advisory for rough bar until 3 PM PST this afternoon for Grays Harbor Bar. Gale Warning from late tonight through Tuesday afternoon for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 3 PM PST this afternoon for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory for hazardous seas until 9 AM PST this morning for West Entrance U.S. Waters Strait Of Juan De Fuca. && $$ 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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