Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 200533 RRA AFDSEW Area Forecast Discussion National Weather Service Seattle WA 915 PM PST Sun Nov 19 2017 .SYNOPSIS...A vigorous front will move slowly through Western Washington late tonight and early Monday morning. Winds will ease Monday with some lingering light rain gradually ending. A strong warm front will arrive Tuesday with more rain and breezy winds. The middle of the week will be mild, rainy, and perhaps windy at times. && .SHORT TERM...WV imagery and MSAS surface analysis show a closed 998 mb low off the central Washington coast this evening. Buoy obs match up exactly with the pressure center. Gradients have turned more southeasterly and winds are gradually easing even though the low that formed along the front is still not onshore. Some models hinted at this development but other models missed it and would have pushed the front through quicker. Wind advisories will probably be allowed to expire later this evening, although some gusts are still close to 45 mph in a couple spots around Admiralty Inlet. These winds should ease shortly. One big mystery is the lack of warming given the strong southerly flow over 50 kt that was expected to penetrate the interior. It appears the strong flow made it over lowland areas as a few spots up north hit high wind or close to it. Models were terribly inconsistent with the amount of warm air expected to reach the Cascades today and tonight, which should have caused the snow levels to rise. The problem now is the closed low off the coast moving so slowly and possibly cutting off some of the warm advection. It is pretty clear the north Cascades will get a lot of snow by Monday morning with nearly a foot already at Mount Baker. The passes are still surprisingly cold and models are too fast in warming the air mass. However, I still think there is a good chance that snow levels will finally rise above 5000 to 6000 feet across the Cascades of Lewis/Pierce counties with a change over to rain in the passes still expected but later than was forecast. Went with a winter weather advisory, except up north where 1-2 feet are possible by morning. Pass travel could be very difficult the next several hours will heavy snow at times before the transition to rain hopefully occurs not long after midnight. A few inches have already fallen at Snoqualmie and Stevens pass. 00Z models show more precipitation lingering into Monday, but still tapering off. There could be a convergence zone in the afternoon in the typical north Seattle-Everett corridor. Then a brief break or lull in rain is possible from afternoon to the evening hours. The next warm front will bring rain back to the area late Monday night through Tuesday. This system should bring much milder air into the area with highs possibly close to 60 Tuesday and Wednesday. Models have trended somewhat lower with rain amounts in the Cascades. The flow aloft is not typically conducive for rain amounts that cause river flooding. However, a lot of uncertainty still exists and the river forecasts should be followed closely this week. See the latest flood statements and hydro section below for further details. Mercer .LONG TERM...From previous discussion...After a pretty decent round of rain Tuesday and Tuesday night--with some heavy rain in the mountains, the heavy precip will be north of the area on Wednesday. There will still be rain at times over Western Washington--but this lull on Wed will help a great deal with the rivers. Then on Thursday the rainfall picks up again--but precip amounts in the models are actually pretty modest. Still, the RFC forecasts show pretty high rivers midweek, so a hydrologic outlook was issued, but it merely highlights the uncertainty in the forecast at this time. && .AVIATION...A cold front is gradually moving southeast across the area this evening, but a wave moving northeast along the frontal band will slow the front somewhat through late tonight. At the upper levels, a developing upper low offshore will move eastward along the Canadian border on Monday. Strong southwest flow aloft tonight will ease and become moderate to light westerly on Monday. Rain will ease to showers from the northwest late tonight into Monday morning and will be followed by mountain showers and a Puget Sound Convergence Zone on Monday. The air mass will be moist tonight with some drying along the coast and in the lowlands away from the convergence zone during the day on Monday. Conditions will gradually degrade to mainly lower end MVFR tonight and Monday morning as the winds aloft ease, then will become mainly VFR with MVFR limited to showers and the convergence zone on Monday. Conditions will dry further Monday evening as weak high pressure moves across the area. Albrecht KSEA...CIGS near 040 early this evening will generally deteriorate to BKN015 later tonight into Monday morning as pressure gradients across the area weaken with the incoming front and low pressure trough. Winds 19050kt at 020 and 21060kt at 050 will ease rapidly around 08Z as the front weakens over the area. Convergence zone activity is expected to hang up north of the terminal (near or just north of KBFI) during the day Monday giving light opposing winds around the terminal area. Ceilings will slowly lift and become mainly VFR Monday afternoon or evening. Albrecht && .MARINE...A cold front will move southeast through the Washington waters this evening. The front is being preceded by southerly gales. Winds will ease quickly following the frontal passage. The front has slowed its forward progress somewhat as a wave of low pressure moves northeast along the frontal band. This has prompted the continuation of the gale warnings for some of the inland and southern coastal zones until midnight tonight. Seas will slowly subside late tonight into Monday. A small craft advisory for hazardous seas will continue in effect over the coastal waters into Monday. A high pressure center will pass across the waters late Monday or Monday night. A strong warm front will push northward through the waters on Tuesday, preceded by strong easterly flow with gales possible near the West Entrance. The warm front will be followed by southerly flow on Wednesday. Strong low pressure over the northeast Pacific will maintain southerly flow on Thu and Fri. Albrecht/Haner && .HYDROLOGY...Around a couple inches of rain fell along the south slopes of the Olympics the past 12 hours. Additional rain tonight will probably not be significant. The Skokomish river has started rising sharply but it is questionable if the amount of rain will be enough to cause minor flood stage. The flood watch remains in effect and with so much uncertainty, will monitor trends closely the next few hours before deciding on whether or not a warning is needed. If it does reach flood late tonight/Monday morning, it probably will not remain above flood stage as long as previously thought. Rainfall will be heaviest Tuesday and Wednesday with the next warm frontal system. A lull is expected on Wednesday, and then it picks up a bit Thursday but is not heavy. We will have to watch the rivers with the very high snow level around mid-week, but the Skokomish River in Mason county is the only one that is an obvious threat --- if not tonight and Monday, then certainly on Tuesday. A Hydrologic Outlook was issued today for all counties except Mason, Island, and San Juan -- highlighting the uncertainty of the forecasts next week but the unusually high freezing level and periods of rain in the mountains. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Weather Advisory until 3 AM PST Monday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and King Counties. Winter Storm Warning until 10 AM PST Monday for Cascades of Whatcom and Skagit Counties. Flood Watch through Monday morning for Hood Canal Area-Lower Chehalis Valley Area-Olympics. Wind Advisory until 10 PM PST this evening for Admiralty Inlet Area-San Juan County-Western Whatcom County. Wind Advisory until midnight PST tonight for Central Coast. PZ...Small Craft Advisory for rough bar until 3 PM PST Monday for Grays Harbor Bar. Gale Warning until midnight PST tonight for Admiralty Inlet- 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 for hazardous seas from midnight tonight to 3 PM PST Monday for 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 hazardous seas until 3 PM PST Monday for Coastal Waters From James Island To Point Grenville 10 To 60 Nm. Small Craft Advisory for hazardous seas until 3 PM PST Monday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From Cape Flattery To James Island Out 10 Nm. Small Craft Advisory for hazardous seas until 9 AM PST Monday for West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 9 AM PST Monday 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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