Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 220526 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 PM PST Tue Nov 21 2017 .SYNOPSIS...
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A moist frontal system will bring rain to Western Washington tonight. The heaviest rainfall will occur in the mountains and should begin to gradually decrease from northwest after midnight. The snow level will be near 8000 feet. After a lull in the rain on Wednesday, another wet frontal system will move through the region Wednesday night through Thanksgiving morning. After another break on Friday, another system will reach the area Saturday afternoon and night. This active weather pattern will continue into early next week.
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&& .SHORT TERM...
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Water Vapor and IR imagery shows a long fetch of warm moist air extending from just east of the Hawaiian Islands northeast into the Pacific Northwest. This looks like a classic atmospheric river pattern. The freezing level at Forks is over 10K feet so the air mass is capable of holding a lot of moisture. Rain amounts the past 24 hours range from roughly 3-5 inches in the Olympics to 2-4 inches in the Cascades. With the heaviest amounts over the Olympic Peninsula it not surprising that a couple rivers are flooding. Amounts are somewhat lighter in the Cascades, but still respectable. See the hydrology section below and refer to the latest flood bulletins for current and detailed river information. Models generally show a gradually decrease in rainfall rates from the northwest after midnight. The heavier rain has arrived in batches and not the typically focused stream of heavy rain that usually is observed. Pretty much all models show a lull in rain across the area on Wednesday. It will still be a bit wet but rain amounts will not be nearly as heavy. The next wave of rain arrives Wednesday night and Thursday ahead of a front approaching from the west-northwest. Models show this next system to be more progressive in nature, so rain amounts should not be quite as heavy. However, there will still be some flood risk on area rivers and forecasts should be monitored through the week. Another lull or break on some of the rain is expected on Friday. Mercer .LONG TERM...From previous discussion...The GFS and ECMWF are actually in fair agreement with the large scale pattern through this weekend. Another large upper level trough will deepen offshore along roughly 140W Friday night and Saturday, which will re-invigorate the SW flow over the PacNW. This will allow another shortwave trough embedded in the SW flow to move NE across W WA Saturday afternoon and evening. The weakening main upper level trough should follow on Sunday. The models show a warm front arriving on Monday, but day 6 is a little far out for much confidence in this active pattern. Kam
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&& .AVIATION...
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Moist southwest flow aloft will continue through Wednesday night. A warm front near the Canadian border will move north of the area this evening, but rain will continue at times in the moist southwest flow. The air mass will be stable. At the surface, easterly gradients are becoming more southeasterly behind the warm front. While low level wind shear has come to an end behind the warm front, expect strong winds 19040-55 kt to continue at times between 030 and 050 through Wednesday night. There has been some improvement in conditions behind the warm front, but the high levels of moisture are producing rain at times and low ceilings in places. Overall, expect MVFR conditions with pockets of IFR conditions to dominate later tonight through Wednesday morning. Strong flow aloft interacting with the terrain will give pockets of clearing at times to the lee of the Olympics, especially midday Wednesday through Wednesday evening. Albrecht KSEA...CIGS have improved to just over BKN030 this evening, but expect them to lower to 015 08Z-20Z with some diurnal cooling and a continued influx of tropically moist air from the southwest. Expect periods of rain to reduce visibilities to 5SM at times through tonight. Winds will be south 8-11kt through Wednesday morning and S 12G20 kt late Wed afternoon through evening. Winds aloft will generally be strong at 19035-55 kt between 2500 and 5000 ft MSL through Wednesday night. Albrecht
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&& .MARINE...
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A warm front near the Canadian border will lift north of the waters this evening. Another frontal system approaching the coastal waters from the west will give rather strong south to southwest flow to the waters tonight through Wednesday night behind the warm front. Small craft advisories are in effect for the coastal waters, and Admiralty Inlet overnight. The Small craft advisories will continue over the coastal waters through Wednesday. Winds are expected to develop ahead of the offshore front Wednesday night as a wave of low pressure moves through the offshore waters. Gale force winds are possible over the coastal waters and at the entrances to the strait as the wave moves by, so a Gale Watch continues for these areas for Wednesday night. Windy southerly flow will continue on Thursday as a trough brushes the area. Gales are likely on the coast and over the north inland waters with this system. Another system will arrive around Saturday, with gales again possible. Albrecht
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&& .HYDROLOGY...
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The axis of heaviest rain associated with today`s warm front and its plume of subtropical moisture is moving east of the area this evening, and precipitation is generally decreasing across Western Washington. Already, though, 2 to almost 5 inches inches of rain has fallen over the Olympic Mountains, with 1 to 4 inches at Mount Rainier and around 1 to 3 inches further north over the Cascades. The snow level is around 9000 feet. There are a number of rivers that will crest close to flood stage tonight, and a flood watch remains in effect for most of the forecast area (all but Island, San Juan, Kitsap, and Mason Counties). With precipitation decreasing, the upper reaches of some rivers are starting to crest - especially the Olympic Peninsula rivers - and those crests will be moving downstream tonight. The rivers that face the most imminent threat of reaching flood stage tonight include the Nooksack, Stillaguamish, Skykomish, and Snoqualmie; they are still too close to call at this point, but warnings could be issued this evening. If flooding does occur on these rivers, it will be minor. On the Olympic Peninsula, the Skokomish River is flooding and the Bogachiel is just reaching flood stage. Flood warnings are in effect for both rivers. With regard to rivers flowing off of Mount Rainier, the Northwest River Forecast Center model shows reaches of the Nisqually and the Cowlitz Rivers reaching flood stage, but the threat there does look as imminent. In addition, the threat of landslides will increase across the lowlands of Western Washington. The USGS landslide guidance shows one or two stations near or just above the threshold of concern, and of course the additional rain through Thursday will drive that higher. McDonnal
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Flood Watch through Thursday afternoon for all of Western Washington from Grays Harbor and Lewis counties northward (Except for Mason, Kitsap, San Juan, Island counties). PZ...Small Craft Advisory for rough bar until 4 PM PST Wednesday for Grays Harbor Bar. Gale Watch from Wednesday afternoon through late Wednesday night 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-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 4 PM PST Wednesday 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 until midnight PST tonight for East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 4 AM PST Wednesday for Admiralty Inlet-West Entrance 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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