Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 201124 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 324 AM PST Mon Nov 20 2017 .SYNOPSIS...
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A front will move slowly east today, with rainfall decreasing across Western Washington this afternoon. A strong warm front will bring lots of rain Tuesday and Tuesday night, with the snow level above 8000 feet. Wednesday will be less wet, then another front moves through Thanksgiving Day. Friday and Saturday look relatively dry, with a front arriving late in the weekend.
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&& .SHORT TERM...
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A southwest-northeast oriented front continues to move slowly eastward across Western Washington early this morning. The front has deep moisture and a warm air mass associated with it, which is being carried into the forecast area by strong southwest winds aloft. Not surprisingly, it has dropped 2.5 to 4+ inches of rain over the southwest facing slopes of the Olympic Mountains. The slow eastward progression of the warm air and easterly flow across the Cascades kept precipitation there in the form of snow until early this morning, with 10 inches of snow at Paradise and 13 inches of snow at Mount Baker since Sunday afternoon. Radar showed rain still falling over much of the area at 3 am, with lowland temperatures in the 40s to lower 50s and winds still gusting into the 20 to 30 mph range in a few spots. The front will continue its slow eastward progression today. The parent upper low of the frontal system is offshore of Vancouver Island near 49N/128W early this morning, and a shortwave is rounding the base of the trough; this feature will move northeast across the area through midday today, which should help to prolong precipitation along the back side of the main frontal band. Precipitation will generally decrease across the forecast area this afternoon, though, as the upper low weakens into an open trough and moves into southwest British Columbia. The Puget Sound convergence zone could form this afternoon as the upper trough moves inland and low level flow veers to westerly. A minor upper trough will progress quickly from the offshore waters across Western Washington tonight. The leading edge of the next round of rain - also a region of deep warm moisture - will spread quickly into the forecast area late tonight and early Tuesday morning. This system will bring significant precipitation to the area Tuesday and Tuesday night, with the heaviest precipitation likely in the afternoon and evening as the warm front lifts north across the area. It will drive the snow level quickly to 8000 to 9000 feet, with storm total precipitation totals of 2 to 5 inches likely over the Olympics and at Mount Rainier, and roughly 1 to 3.5 inches over the Cascades further north. Breezy conditions are likely especially at the coast and in the north interior. Warm moist southwest flow aloft will continue Wednesday. The snow level will be around 9000 feet, but precipitation amounts look lighter. This will be just a short break, though, before another wet system arrives later Wednesday night. McDonnal .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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&& .HYDROLOGY...
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In the very short term, the only river that faces a threat of flooding is the Skokomish River in Mason County. It should crest late this morning close to flood stage - maybe a little above but maybe just below. At this point, I will sit on the flood watch and issue a short-term warning if needed. 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
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&& .AVIATION...
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A cold front will move slowly southeast across the area early this morning. In its wake, light to moderate northwest flow at the lower levels will develop. Flow will become increasingly offshore later tonight as a strong warm front approaches the area from the southwest. An upper level low west of Vancouver Island early this morning will move eastward along the Canadian border today. Moderate southwest flow aloft this morning will become westerly as the upper low passes by to the north. The air mass is moist and stable. It will gradually dry later this afternoon through this evening, then mid and upper level moisture will rapidly increase late tonight. The cold front extends from about KBLI to KHQM at 11Z. Radars show a band of rain extending about 75 miles north of the frontal system as it pushes slowly southeastward. Ceilings and visibilities are MVFR to locally IFR in the rain band and in rain to the southeast of the front. Expect conditions to improve to VFR by later this afternoon as showers become increasingly confined to the terrain and to a Puget Sound Convergence zone from around KPWT to north of KSEA. Conditions will become VFR this evening with clouds above about 050. Albrecht KSEA...CIGS between 020 and 025 will fall to around 015 this morning as the front moves slowly southeast across the terminal. Conditions will only slowly improve this afternoon as convergence zone activity hangs up north of or near the terminal. There may be periods of light opposing winds around the terminal area associated with the convergence zone. Ceilings will slowly lift and become mainly VFR later this afternoon through tonight. Winds at the terminal will be mainly light southeasterly today and will shift to light northerly tonight. Albrecht
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&& .MARINE...
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A cold front from the Northern Inland Waters southwestward into the South Washington Coastal Waters will move slowly southeastward this morning. Behind the front, expect light to moderate northwesterly winds with Small Craft Advisory wind conditions limited to the central and eastern portions of the Strait of Juan de Fuca. Small craft advisories will be maintained over the coastal waters for today as steep short period swell 14 to 16 feet gradually subsides. A weak high pressure center will pass across the waters this evening. Then a strong warm front will push northward through the waters on Tuesday. Gale force easterly winds are possible on Tuesday at the West Entrance to the Strait of Juan de Fuca and the northern portions of the coastal waters. 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. Albrecht
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Flood Watch until 10 AM PST this morning for Hood Canal Area- Lower Chehalis Valley Area-Olympics. 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 Watch 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. Small Craft Advisory until 7 AM PST this morning for Admiralty Inlet.
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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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