Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 222322 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 322 PM PST Wed Nov 22 2017 .SYNOPSIS...A cold front will bring more rain to Western Washington tonight through Thanksgiving. Scattered showers will continue through Thursday night as an upper level trough follows the front. A weak warm front will bring a little light rain Friday night. The next system to bring widespread light rain should arrive Saturday afternoon. This active weather pattern should continue into early next week. && .SHORT TERM...RADAR shows areas of rain mainly over the mountains and coast this afternoon. A weak cold front was over the Washington offshore waters this afternoon, and was slowly approaching the coast. The front is expected to strengthen tonight as an upper level shortwave trough near 40N/135W at 22Z/2 PM and embedded in the strong SW flow aloft, moves NE across the front. Models have been consistent in indicating this will be a good rain producer tonight and Thursday. However, the latest 12Z models, have trended lower on how much rain will fall. In addition, precipitable water values in the band just offshore were only around 1.2 inches at 20Z/12 PM. The latest rainfall estimate for the Olympics is for 1 to 3.5 inches of rain in the 24 hours ending at 4 PM Thursday. The Cascades should get lower amounts, with 1 to 3 inches through 4 PM Thursday. These estimated amounts could be a little high but with flooding already in progress from Tuesdays rain, a conservative forecast is better. The timing of the cold front should bring the heaviest rain to the Olympics later this evening, then to the Cascades after midnight. The front should be through the interior lowlands by midday Thursday, but it will probably take the whole afternoon for the rain to taper off over the Cascades. I will keep scattered showers in the forecast behind the front Thursday afternoon, but there is not a lot of convection showing up offshore behind the front. The upper level shortwave trough to the W and trailing the cold front could generate some showers through Thursday night as it crosses the area. A slow moving cold front sliding SE along the B.C. coast will bring some light rain to the area on Friday, particularly the northern part of W WA. On Friday, another deep upper level longwave trough will develop offshore along 140W, which will rev up the down stream SW flow aloft over the PacNW. The models have been consistent the past couple of days, showing an upper level shortwave trough embedded in the flow moving NE across W WA late Saturday. This system should produce widespread light rain. The past few GFS runs as well as the latest 12z ECMWF have the rain moving N across W WA Saturday afternoon. The latest 18Z GFS has sped that up a little and now shows some light rain in the morning. QPF should be less than an inch. The warm air mass from Tuesday`s warm front has ended up breaking several high temperature records for the date. Temperatures in the 60s were common and Puyallup and JBLM-McChord even broke 70. We`re still looking at a record high minimum temperature for Sea-Tac. If the temperature doesn`t cool below 56 degrees before midnight, 56 will be the warmest minimum temperature for Seattle in the month of November going back through the entire climate record back to the 1890`s. Kam .LONG TERM...The GFS and ECMWF show the offshore longwave trough weakening on Saturday then moving E across W WA Sunday night. The GFS is a little faster than the ECMWF. This will keep the weather active with rain at times. The GFS has stuck with bringing a warm front E across the area Monday night, but the ECMWF, after showing the same solution for the past 2 days now significantly weakens the warm front offshore. The models diverge even more for Tuesday and Wednesday. The GFS shows a dry period with a moderate amplitude upper level ridge over the area Tuesday night and Wednesday, but this will probably end up being a discarded solution by next week. Kam && .AVIATION...Moist southwest flow aloft will continue tonight and Thursday morning, then become more westerly Thursday afternoon as an upper level trough passes. At the surface, southeasterly pressure gradients will ease to moderate tonight, then become strong southwesterly Thursday. Rain at times will continue in the stable air mass. Mostly VFR conditions continue, with patchy MVFR/IFR in rain. There are some breaks in the clouds. More clouds and rain are forecast tonight, and this will probably bring widespread MVFR ceilings. Rain will spread from the northwest tonight, and turn showery Thursday as gusty southwest winds develop. KSEA...MVFR tonight, improving to VFR late Thursday afternoon. South winds 8-12 kt tonight, becoming southwest and increasing to 15g25 kt Thursday afternoon. CHB && .MARINE...A front will arrive from the northwest late tonight. Low end gale conditions are likely on the coast and over inland waters from Admiralty Inlet north this evening. Small craft advisory winds are likely elsewhere. Winds will ease somewhat late tonight, then gradients will increase again Thursday. Most areas will have small craft advisory strength winds, but local gales are possible, especially after the front passes Thursday afternoon and winds become gusty southwesterly. Southerly gradients will ease on Friday. Another system will arrive around Saturday night. Strong southerly gradients are possible over the entire area Sunday. Gales are possible over most waters with small craft advisory conditions over Puget Sound and Hood Canal. CHB && .HYDROLOGY...Just waiting for the next round of precip to kick in and assess where all the rivers are at the time. Some rivers have crested, some have fallen below flood stage, some are still rising. Warnings and watches change as needed. For the current list, please check: www.wrh.noaa.gov/sew/get.php?pil=RVA&sid=SEW The next front will move through tonight and Thanksgiving Day, with a forecast of generally 1 to 3.5 inches over the mountains. This is slightly less than previously forecast as the models are all trending down in amounts. River forecast updates will likely show some lover crests with the new QPF numbers. The snow level will be around 7500 feet as this system arrives, falling to 4000 feet late Thursday as precipitation decreases. With this second round of rainfall tonight and Thursday, there is a possibility that flooding will become more widespread and/or prolonged. There is at least a minor threat that any flood-prone river in our area reaching flood stage. In addition, there is a threat of landslides across the lowlands of Western Washington. The USGS landslide guidance shows soil wetness above the threshold of concern, and additional rain through Thursday will drive those numbers higher and act as the trigger for landslides. JBB/McDonnal && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Flood Watch through Thursday afternoon for Admiralty Inlet Area- Bellevue and Vicinity-Cascades of Pierce and Lewis Counties- Cascades of Snohomish and King Counties-Cascades of Whatcom and Skagit Counties-Central Coast-East Puget Sound Lowlands- Eastern Strait of Juan de Fuca-Everett and Vicinity-Hood Canal Area-Lower Chehalis Valley Area-North Coast-Olympics- Seattle and Vicinity-Southwest Interior-Tacoma Area-Western Skagit County-Western Strait of Juan De Fuca-Western Whatcom County. PZ...Small Craft Advisory for rough bar until 4 PM PST Thursday for Grays Harbor Bar. Small Craft Advisory from midnight tonight to 4 PM PST Thursday 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. Gale Warning until midnight PST tonight for Admiralty Inlet- 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 Thursday for West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until midnight PST tonight for Central 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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