textproduct: Seattle/Tacoma

SYNOPSIS

A colder, wet, and windy pattern will remain across Western Washington through the week ahead. Several weather systems will bring an extended period of heavy mountain snowfall, especially during the middle part of the week. A mix of rain and snow in the lowlands, along with light snow accumulations in the foothills, develops early Monday and Tuesday.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

Radar imagery continues to show scattered showers across southwestern Washington, and a hefty convergence zone over the King-Snohomish county line. Showers will depart but the convergence zone is expected to stick around for much of tonight and into Monday. Showers will begin to fill back in across the area tonight. Snow levels will continue to fall tonight. At present they remain just above pass level, but will continue to fall tonight as cooler air moves in. With temperatures tonight in the low to mid 30s, expect snow in areas above 500 ft, with a rain-snow mix possible elsewhere. The areas with the best chances for seeing snowfall in the lowlands will be the foothills, especially under the convergence zone, and also the northwestern foothills of the Olympics including Lake Crescent. Temperatures will warm up during the day on Monday into the low to mid 40s, which will help keep rain in the forecast for much of the lowlands below 1000 ft. Slightly cooler temperatures Tuesday night may allow for more widespread areas in the lowlands to see light accumulations, but outside of the areas under an advisory, it remains unlikely to see more than a few tenths of an inch of measurable snow through the lowlands.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

The active pattern really begins to ramp up on Wednesday as a moisture plume reaches the area that will persist into early Friday. This will bring a longer-duration event of heavier lowland rain and very heavy mountain snow. There may be a quick bump upwards in the snow levels up to around 3000 ft or so Wednesday night before going back down--which may briefly limit snow at Snoqualmie Pass. Otherwise, steady snow into Friday will be on the order of several feet. Low snow levels by Friday into the weekend will begin to re-introduce the potential for a rain/snow mix for portions of the lowlands. However, precipitation looks to start to back off by Friday. A dry period is hard to find at this point, however, as the active pattern looks to continue through the weekend into early next week.

In addition to precipitation, winds will become a concern on Wednesday as widespread gusts in the 30-50 mph range will be possible across the area, with the strongest winds across the coast and the north interior. Winds ease late week.

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AVIATION

Zonal flow aloft will continue through Monday. Post- frontal onshore flow continues at the surface. A mix of VFR/MVFR cigs this afternoon with a convergence zone located across King and Snohomish Counties. The convergence zone will continue to linger in this region into tonight. Additional scattered showers are then expected tonight through Monday elsewhere. Snow may mix in with rain at times tonight into Monday AM, however, no accumulations are expected at the terminals. Predominantly VFR cigs into tonight, with lingering MVFR at times near the convergence zone. Low end VFR and high end MVFR Monday AM with BKN/OVC VFR in the afternoon. W/SW surface winds remaining breezy into tonight with N/NE surface winds through this evening in the vicinity of the convergence zone.

KSEA...High-end MVFR cigs continue this afternoon with a convergence zone located north of the terminal. Some improvements to cigs towards low-end VFR expected between 22-00z, with VFR through tonight. Cigs may fluctuate at times into MVFR between 07 to 15z on Monday, with probabilities of MVFR cigs peaking around 35%. NE surface winds are expected to remain elevated through this afternoon, with gusts ranging 20 to 25 kts through 00z. NE winds are then expected to slowly ease and transition back to S/SW early tonight (current estimates around 04z). However, some additional fluctuations to VRB or light N cannot be ruled out into early Monday AM. Otherwise, SW winds expected for the majority of Monday. JD

MARINE

Onshore flow will continue at varying intensities through Tuesday. This will result in Small Craft wind gusts for the Strait of Juan de Fuca and Admiralty Inlet into Monday. Brief gale force wind gusts also can't be ruled out for the central and eastern Strait of Juan de Fuca through this evening. Otherwise, west winds will remain elevated for the Coastal Waters before easing briefly late Monday. South winds will remain breezy for portions of the interior waters on Tuesday. A stronger weather system will move across the waters Wednesday into Thursday, resulting in additional headlines. Confidence is increasing in gale force winds on Wednesday for the Coastal Waters, with the probability of gales wind gusts ranging between 50 to 70% for the interior waters on Wednesday. Winds will remain elevated, but slowly ease Thursday into late week.

Seas will range 9 to 12 feet through Tuesday. Seas will then build Wednesday to 14 to 17 ft over the Coastal Waters, and then remain above 10 feet into late week. JD

HYDROLOGY

Heavy precipitation, combined with rising snow levels, will bring increased flows on the Skokomish River on Wednesday into late week. The Skokomish River is currently forecast to rise above flood stage for this reason. Elsewhere, river levels will rise, however, additional river flooding is not expected over the next seven days. JD

SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Storm Warning from 5 PM this afternoon to 5 AM PDT Tuesday for Cascades of Snohomish and Northern King Counties-Cascades of Whatcom and Skagit Counties.

Winter Storm Watch from late Tuesday night through Friday morning for Cascades of Pierce and Lewis Counties- Cascades of Snohomish and Northern King Counties-Cascades of Southern King County-Cascades of Whatcom and Skagit Counties.

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

Winter Weather Advisory from 5 AM Monday to 11 AM PDT Tuesday for Foothills and Valleys of Snohomish and Northern King Counties.

Winter Weather Advisory from 11 PM this evening to 11 AM PDT Tuesday for Lake Crescent Area Including US 101-Western Strait of Juan de Fuca.

Winter Storm Watch from Tuesday afternoon through Friday morning for Olympics.

PZ...Small Craft Advisory until 5 PM PDT Monday for Grays Harbor Bar.

Small Craft Advisory until 11 PM PDT this evening for West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 11 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 2 AM PDT Monday for Admiralty Inlet.

Small Craft Advisory until 5 PM PDT 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.