Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 171243 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 443 AM PST Sat Feb 17 2018 .SYNOPSIS...
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A frontal system will sweep across the area today for heavy snow in the mountains and locally windy conditions over the lowlands. A cold low pressure system aloft will move over the region later tonight and then slowly east on Sunday. Expect snow levels to plummet to near sea level later tonight for a threat of lowlands snow. Strong northerly winds will also develop late tonight over the far northwest interior and continue through Sunday. A drier and chilly air mass will prevail on Monday.
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&& .SHORT TERM...
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Precipitation overspread the CWA overnight. The precipitation was associated with a frontal system that was still offshore or to the northwest of the CWA at this time. Expect this system to sweep across the CWA during the day today and be followed by a surface trough this evening. The main message is that the Cascades will receive total snow accumulations of 1-3 feet and it will become locally windy across the lowlands. Expect a cold upper level low to move over the area tonight for plummeting snow levels. By Sunday morning, the snow level will be at or near sea level across the CWA. Also, by this time, the precipitation will be showery and occurring in bands. The question is where the heaviest bands will set up because it will be within these bands where the heaviest snow will fall. It is a given that the heaviest snow fall on the mountains, where there are winter weather hazards in effect. The question is where the heaviest snow will fall on the lowlands since predicting exactly where the heaviest convective showers will occur takes a high degree of skill. It does look like the land area adjacent to the eastern Strait of Juan de Fuca will probably receive significant snowfall late tonight through Sunday morning due to strong upslope or northerly flow. The upslope flow would increase precip rates. The models do not always pick up on this; therefore, went on ahead and issued a Winter Storm Watch for this area. This Watch also includes the north slopes of the OLympic range. Elsewhere, across the lowlands, most places should pick up less than an inch of snow (if at all). The exceptions will be the Cascade foothills and the area east of the Admiralty inlet where up to 3 inches of snow could fall. Strong northerly low level flow will develop later tonight and continue through Sunday, resulting in gusty north winds of 45 to 55 mph over the far northwest interior. The strong winds combined with temperatures near or slightly above freezing will make for wind chill readings in the teens and lower 20s. Brr! The upper level trough will move east of the area Sunday night and will be followed by high pressure aloft. Northerly flow will persist and heights/thicknesses will remain low. Therefore, expect max temps to be mainly in the 30s on the lowlands with min temps Sunday night in the mid teens to mid 20s. The mountains will have lows near zero or in the single digits. More brr! .LONG TERM... No major changes to the inherited forecast. There were some difference in the details concerning the medium range solutions. The operational ECMWF indicated an upper level low dropping into the area from the north about the middle of next week while the GFS kept us more under the influence of the offshore ridge. Beyond this period, the models began to diverge even more. At any rate, it looks, for the most part, temps and precip will be below normal during this period.
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&& .AVIATION...
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Strong northwest flow aloft today will weaken and become southwesterly tonight and variable on Sunday morning. A deepening low pressure system near Haida Gwaii early this morning will move southeast to along the Canadian border this afternoon and evening. Rain out ahead of the system will spread over the area this morning with MVFR and areas of IFR ceilings into the afternoon hours. Some improvement later in the afternoon with a majority of the ceilings in the MVFR category by 00z Sunday. More improvement later Saturday night. Visibilities lowering to 3-5sm at times into the afternoon hours before the rain changes over to showers. Windy conditions developing later this morning and continuing into the early evening hours with southerly winds 20 to 30 knots with some gusts to 40 knots in the forecast. Winds will ease overnight and begin to come out of the north Sunday morning. KSEA...IFR or low end MVFR ceilings with some slight improvement this afternoon into the evening hours. Southerly winds 10 to 15 knots increasing quickly 15z-18z to 20 to 30 knots with gusts to 40 knots. Strong southerly winds continuing into the evening hours before easing overnight. There will be a sharp wind shift from southerly to northerly around 18z Sunday with the northerly winds quickly increasing to 15 to 25 knots. Felton
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&& .MARINE...
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A strengthening low will move southeast across northern portions of the area this afternoon...resulting in gales developing from west to east starting early this morning. Gale warnings remain in place for all waters, beginning over the coastal waters early this morning and spreading into the inland waters by late morning. Strong northeast Fraser outflow wind will spread out across the waters starting late tonight and continuing into Sunday night. At this point...some of the waters such as the inner coastal waters, the western strait, Admiralty Inlet and Puget Sound look to fall below gale speeds...although SCA winds likely to be an issue. The remaining waters...the outer coastal waters, the central and eastern strait and the northern inland waters will retain gale speeds and as such will extend those gale warnings with the morning forecast package. Gradients will ease Sunday night and become light Monday into Tuesday. SMR
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&& .HYDROLOGY...
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River flooding is not expected the next 7 days.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Wind Advisory from 9 AM this morning to 6 PM PST this evening for Bellevue and Vicinity-Bremerton and Vicinity-Hood Canal Area-Seattle and Vicinity-Southwest Interior-Tacoma Area. Winter Storm Warning until 6 AM PST Sunday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and King Counties-Cascades of Whatcom and Skagit Counties. Wind Advisory from midnight tonight to 6 PM PST Sunday for Western Skagit County-Western Whatcom County. Wind Advisory until 6 PM PST this evening for Central Coast- North Coast. Wind Advisory from 11 AM this morning to 6 PM PST Sunday for San Juan County. Winter Storm Watch from this evening through Sunday morning for Eastern Strait of Juan de Fuca. Winter Weather Advisory until 6 PM PST this evening for Olympics. Winter Storm Watch from this evening through late tonight for Olympics. Wind Advisory from 11 AM this morning to 6 PM PST this evening for Admiralty Inlet Area-Eastern Strait of Juan de Fuca. PZ...Small Craft Advisory for rough bar from noon today to 4 PM PST Sunday for Grays Harbor Bar. Gale Warning until 3 AM PST Sunday for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Gale Warning until 4 PM PST Sunday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. Gale Warning from 10 AM this morning to 4 PM PST Sunday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Gale Warning from 10 AM this morning to 3 AM PST Sunday for Admiralty Inlet-Puget Sound and Hood Canal-West Entrance U.S. Waters Strait Of Juan De Fuca.
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