Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 151206 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 406 AM PST Fri Dec 15 2017 .SYNOPSIS...
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A weak cold front will spread light rain across Western Washington this morning as it moves southeast. An upper level trough trailing the front will bring a few showers this afternoon and evening. A warm front will spread more rain and mountain snow across Western Washington Saturday, then a slow moving cold front will keep Sunday wet at times. After a minor lull on Monday, a deepening low center should pass across southwest B.C. on Tuesday, possibly bringing windy conditions and heavy mountain snow. Drier high pressure will build across the Pacific Northwest next Wednesday and Thursday.
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&& .SHORT TERM...
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A weak cold front will gradually move southeast through Western Washington this morning. This front will be most notable for producing the first rain since early last week, albeit light rain in places. Radar already shows light bands of rain spreading southeast through Northwest Washington early this morning. The front will exit by early afternoon. Some onshore flow behind the front will keep a few showers going into this evening, mainly in the Cascades and in a diffuse Puget Sound Convergence Zone. In terms of cooling, the mountains will see the most cooling after such a long stretch of warm air aloft. Some flat upper ridging will rebound over the coastal Northwest on Saturday. This will push a warm front onshore. Rain and mountain snow will spread onto the coast on Saturday morning then spread inland later in the day. As the ridge flattens and retreats to the south on Sunday, an east-west oriented cold front will slowly sag in from the north. The tail end of a shortwave in strong west flow aloft will enhance lift along the front. With the slow movement of the front and strong west flow aloft, heavier precip amounts are likely with moderate snow accumulation in the mountains. The front will eventually reach Southwest Washington on Monday evening before turning up stationary. Haner .LONG TERM...The weather on Tuesday bears a watchful eye. A strong shortwave will travel east along the leftover front just to our south. Cyclogenesis is expected along the front on Monday between 130W and 140W. Under the influence of strong upper divergence, the low will rapidly deepen while it tracks east on Tuesday to central or southern Vancouver Island. The 00z ECMWF deepens the central pressure by about 20 mb in 24 hours, and the 06z GFS is not far behind. GEFS ensemble means show similar trend, so confidence is good at this point. The 00z ECMWF shows the low continuing to rapidly deepen below 995 mb as it crosses the B.C. Coast Range and passes our longitude. The continued strengthening is key, because such lows bring stronger wind and more wind impacts than lows that are weakening. Again, this setup bears watching for wind impacts. In addition to wind, heavy mountain snow appears likely thanks to a sharply defined trailing cold front that will move across the area concurrent with the large- scale forcing and lift of a deepening parent low. Very cold air aloft will follow Tuesday PM`s front, with 500 mb temps falling to -34C on Wed morning. So plenty of post-frontal convective showers and snow levels down to 2000 feet or less will follow. Later Wednesday, an upper ridge axis around 135W will strengthen, amplify, and spread toward the Pac NW. The air mass should quickly stabilize. By next Thursday, northwest flow aloft with 500 mb heights nearing 570 decameters along the coast should lead to dry weather. All 20 members of the GEFS ensemble system show no precip at SEA next Thursday, so dried out the forecast for all but the Mount Rainier area. Haner
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&& .AVIATION...
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A weak low pressure system and cold front will move over Western Washington Friday morning. The air will become moist with areas of light rain or showers developing. Southwest flow aloft will become northwest later today. Areas of light rain or showers tapering off by evening except for showers in the PSCZ. KSEA...Clouds will thicken up with lowering cigs as light rain or showers develop this morning. A southerly breeze could shift to northerly around 22z with a weak fropa but variable is a good bet too through evening as convergence sets up with some shower activity.
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&& .MARINE...
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A 1020mb low and cold front will move over Western Washington this morning. Small craft advisory northwest winds will develop over the coastal waters behind the front and then spread into the Strait of Juan de Fuca by evening. A warm front will move through the waters late Saturday night into Sunday. Some small craft advisory conditions are likely across the waters, especially Sunday, as flow aloft interacts with the area terrain. A deepening low will move onto Central Vancouver Island on Tuesday then move east more or less along the Canadian border. Widespread small craft advisory conditions are expected with this system and gale force winds are possible over some waters.
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&& .HYDROLOGY...River flooding is not expected in the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory from 6 AM this morning to 3 AM 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-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Small Craft Advisory from 4 PM this afternoon to 3 AM PST Saturday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of Juan De Fuca.
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&& $$ www.weather.gov/seattle

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