Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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249 FXUS66 KSEW 202320 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 320 PM PST Mon Nov 20 2017 .SYNOPSIS...A strong warm front will bring periods of moderate to heavy rain to Western Washington Tuesday and Tuesday night, with the snow level rising above 8000 feet. A break between weather systems on Wednesday will allow rain to diminish a little. Another wet frontal system will reach the area Wednesday night and Thanksgiving Day. After another small break between weather systems on Friday, another front will probably arrive on Saturday. && .SHORT TERM...This mornings cold front has completely dissipated and RADAR is only picking up spotty showers over the area at 2 PM. There seems to be a tiny bit more shower activity over central Puget Sound where a convergence zone is trying to form this afternoon, but the increase is subtle. The wind field over central Puget Sound supports convergence but at this point, the winds are not very strong. Whatever does form will probably dissipate early this evening. A large upper level longwave trough has set up offshore between 145W-150W this afternoon. Moist SW flow aloft downstream of the trough is in place offshore, with strong warm advection ready to move inland over the PacNW. Models remain consistent in spreading warm frontal rain northward across W WA after midnight tonight. Rain is still expected to become heavy at times Tuesday afternoon before diminishing Tuesday evening. Mountain snow levels will rise to above 8000 feet. This looks to be the heaviest period of rain this week. Moist SW flow aloft will continue on Wednesday, but with a relative tiny break between systems, rainfall amounts will be quite light. The next system should affect the area Wednesday night and Thursday. Both new GFS/ECMWF models show the main precip period running through Thursday morning with precip diminishing Thursday afternoon. An upper level trough approaching from the W is what pushes the front east of the area in the afternoon. The upper level trough will move E across the area Thursday evening. Rainfall amounts with the tonight/Tuesday system (30 hours, 06Z Tue-12Z Wed) are still looking to be in the 2-5 inch range for the Olympics with 2-4 inches over the Cascades. The interior lowlands will probably range from 0.25-1.00, with 1-2 inches on the coast. The Wednesday night/Thursday system will not be as wet, with up to 3.5 inches over the Olympics, 1.5-3.0 inches Cascades, up to 0.50 interior lowlands, and 1.0 to 1.7 inches coast. A flood watch has been issued this afternoon to address potential river flooding from these systems. The heavy rain over the Cascades and Olympics 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. Kam .LONG TERM...The flow aloft will make a brief shift to weaker to weaker W flow aloft on Friday, for a short mainly dry period. Meanwhile, the large longwave upper level trough offshore will be strengthening again, eventually re-establishing moist SW flow aloft over W WA. An upper level shortwave trough embedded in the SW flow aloft will produce another wet period on Saturday. Both the GFS and ECMWF have this system, but the ECMWF timing is a little slower. Both models also have a surface low crossing W WA Monday morning or afternoon (ECMWF slower). The ECMWF solution could be pretty windy but Monday is much too far out to count much on model details. Kam && .AVIATION...Westerly flow aloft tonight will become southwesterly on Tuesday as a warm front moves from south to north off the coast. At the surface, strong easterly gradients will develop tonight and continue Tuesday. The air mass is moist and stable. Although western sections have cleared out nicely today, a Puget Sound convergence zone has formed over southern Snohomish and northern King counties. Terminals in the convergence zone have MVFR conditions and these will persist into the evening. Elsewhere conditions are mainly VFR. The convergence zone will likely dissipate after 03Z, with VFR ceilings eventually developing everywhere. KSEA...The terminal appears to be just south of the convergence zone, and conditions are currently VFR. The TAF will keep VFR ceilings tonight, then bring MVFR ceilings by 15Z as rain develops. Wind will remain southerly 8-12 kt. CHB && .MARINE...A series of strong weather systems will impact the area over the next five days. The first system will be a warm front that moves south to north mainly off the coast on Tuesday. The second system will reach the area Thursday, and the third system will arrive Saturday. Each system is likely to bring gales to the coastal waters with small craft advisory strength winds inland. On Tuesday the winds will be easterly, and gales are likely at the west entrance as well. For the systems on Thursday and Saturday, the winds will be more southerly. CHB && .HYDROLOGY...In the very short term, the only river above flood stage is the Skokomish River in Mason County. It is expected to fall temporarily below flood stage this evening. The threat of flooding will become more widespread, Tuesday through Thursday, as a couple of warm wet frontal systems move across the area. The first and wettest system will impact the area Tuesday and Tuesday night, also pushing the snow level up above 8000 feet Tuesday through Wednesday night. The second will reach the area Wednesday night/Thursday. Snow levels will lower to 6000-7000 feet on Thursday. The first system looks more significant. Rainfall amounts with the tonight/Tuesday system (30 hours, 06Z Tue-12Z Wed) are still looking to be in the 2-5 inch range for the Olympics with 2-4 inches over the Cascades. The interior lowlands will probably range from 0.25-1.00, with 1-2 inches on the coast. The Wednesday night/Thursday system will not be as wet, with up to 3.5 inches over the Olympics, 1.5-3.0 inches Cascades, up to 0.50 interior lowlands, and 1.0 to 1.7 inches coast. A flood watch has been issued this afternoon to address potential river flooding from these systems. The heavy rain over the Cascades and Olympics could drive some of the rivers flowing off the mountains above flood stage. 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 but not limited to the Cowlitz, Nisqually, Snoqualmie, Skagit, Nooksack, and White river. 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. Kam && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Flood Watch Tuesday afternoon through Thursday afternoon for the Western Washington lowlands except Kitsap county, San Juan County, and Island County. PZ...Gale Warning from 4 AM to 4 PM PST Tuesday 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-West Entrance U.S. Waters Strait Of Juan De Fuca. 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. Small Craft Advisory until 7 PM PST this evening for East Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning until 4 PM PST this afternoon for East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 7 PM PST this evening 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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