textproduct: Seattle/Tacoma

SYNOPSIS

A colder, wet, and windy pattern will prevail over the next 7 days. Several systems will bring an extended period of heavy mountain snowfall, in particular through the middle of next week. Monday and Tuesday will bring the chance for a rain/snow mix in the lowlands and light accumulations in the foothills.

SHORT TERM /TONIGHT THROUGH TUESDAY/

Radar imagery continues to show light drizzle and showers occuring across the northwestern portions of the area this afternoon. This will continue until a frontal system moves in from the north and traverses the area Sunday morning. This will mark the start of a much cooler and wetter pattern as snow levels fall throughout the day. Snow levels will start at around 5000 ft at present, down to around 1500-2500 ft by Sunday afternoon/evening, then to around 500 ft by early Monday morning. This will bring quick round of snowfall to the mountains, particularly across the northern Cascades, for which a Winter Weather Advisory is in effect. Precipitation will then taper off for the afternoon/early evening hours except for the development of a post-frontal convergence zone that will looks to set up around Stevens Pass. Elsewhere, showers will return across the area early Monday morning and look to continue through Tuesday. With such low snow levels, a rain/snow mix will be possible across the lowlands, as well as light accumulations across the foothills. Daytime temperatures well above freezing will help to limit and melt accumulations. All in all, snowfall amounts in the mountains will be in the 6 to 12 inch range, except up to 18 inches around Stevens Pass and through parts of the North Cascades. 1-3 inches will be possible in the foothills. No measurable snow is expected elsewhere.

Highs will be around 50 on Sunday, falling into mid 40s Monday and Tuesday. Lows Sunday AM into the mid to upper 40s, falling to mid to lower 30s Monday and Tuesday AM. Winds will also be breezy throughout this time period, with gusts in the 20 to 30 mph range.

LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/

The active pattern continues as a surge of increased moisture reaches the area Tuesday night and continues through late Thursday. Snow levels will bounce back up to around 2000 to 3000 ft through the long term period. This will be a longer period of heavy snowfall across all area mountains. Several feet will be likely by the end of the week. In addition to snow, windy conditions are expected on Wednesday. The strongest wind gusts will be along the Pacific coast and through the north interior. Precipitation looks to begin to taper off Thursday into Friday but the longer range models are hinting that the active pattern will continue. Lastly, with the rising snow levels late week and continued rain, the potential will increase for the Skokomish river to rise into flood stage. See more details below in hydrology. Temperatures look to remain fairly steady, with highs in the upper 40s and lows in the mid 30s.

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AVIATION

West-northwestly flow aloft will become more westerly into Sunday as a frontal system slides southwards. Widespread MVFR this afternoon with areas of IFR with continued stratus. MVFR cigs will continue for the interior terminals through Sunday morning, with localized IFR at times. Predominantly IFR with patchy LIFR along the coast. Steadier rain will drift southwards tonight through Sunday morning, which may result in brief vsby reductions towards 4- 6 statute miles during this period. A convergence zone will then develop over northern King County and southern Snohomish County Sunday afternoon. Increased south winds through Sunday morning will transition to more northerly for central Puget Sound in the vicinity of the convergence zone by late morning.

KSEA...MVFR cigs will continue through Sunday morning. There remains a low chance of IFR cigs tonight into Sunday AM, with probabilities peaking around 25 percent. Rain will increase around 12z as a front moves southwards, with nearby showers after 18z. MVFR cigs will lift into VFR around 20 to 22z Sunday. South winds will remain elevated into Sunday morning, with gusts peaking around 25 kt. Winds are expected to transition to more N/NE late Sunday morning (between 17 to 20z) as a convergence zone develop in the vicinity of the terminal. JD

MARINE

Onshore flow will continue through tonight, with flow being further enhanced into Sunday as a weather system moves across the waters. Small Craft Advisories have been issued for the northern inland waters due to increased south to southwest winds through tonight. West winds will also increase through the Strait of Juan de Fuca tonight and continue through Sunday night, resulting in SCA wind gusts. Southwest winds will also remain breezy for Puget Sound through Sunday morning, with the strongest winds in the vicinity of West Point. Winds will ease a bit Monday into Tuesday, but remain elevated, particularly through the Strait of Juan de Fuca. South to southwest winds will increase again on Wednesday ahead of a stronger weather system. Additional headlines are likely Wednesday into Thursday, with the potential for gales, especially for the Coastal Waters, eastern Strait of Juan de Fuca, Admiralty Inlet, and northern inland waters. Onshore flow will then continue late week.

Seas will build to 8 to 10 feet tonight into Sunday, with 10 ft seas mainly over the central and northern outer Coastal Waters. Seas then build further for all Coastal Waters to 10 to 12 feet Monday and Tuesday, and build towards 15 ft by midweek. JD

HYDROLOGY

Heavy precipitation over the Olympics, combined at times with snow levels rising will force rises on the Skokomish River with the potential for flooding late next week. No other river flooding is expected during the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory from 5 AM to 1 PM PST Sunday for Cascades of Snohomish and Northern King Counties-Cascades of Whatcom and Skagit Counties.

Winter Storm Watch from Sunday afternoon through Monday evening for Cascades of Snohomish and Northern King Counties-Cascades of Whatcom and Skagit Counties.

Winter Weather Advisory from 5 AM Monday to 5 PM PST Tuesday for Cascades of Pierce and Lewis Counties-Cascades of Southern King County.

PZ...Small Craft Advisory until 11 AM PST Sunday for Puget Sound and Hood Canal.