textproduct: Seattle/Tacoma

SYNOPSIS

Active weather continues, with a certain increase in activity as a strong atmospheric river arrives early next week. Heavy rain, river flooding, wind and high elevation snow are in the forecast for the foreseeable future. Coastal flooding due to high astronomical tides will remain minor into the first part of the week.

SHORT TERM /TODAY THROUGH MONDAY/

This morning, winds have begun to ease with gusts now peaking at around 25-35 mph across the area. Winds will continue to be breezy through this afternoon but should slowly ease later today. Additionally, while moderate to heavy precipitation continues to persist across the mountains and portions of southwest Washington, temperatures at Stevens and White Pass sit just above freezing, indicating that snow levels are hovering just above pass level. Winter headlines currently remain as is but will likely need to be downgraded/expired this afternoon as water vapor imagery shows the shortwave trough axis currently crossing the Cascades as of 9AM this morning.

Coastal flood advisories remain in effect for the Grays Harbor coast, San Juan Islands, Western Whatcom county and portions of Snohomish county through the weekend. Only minor coastal flooding is expected for the high tide cycles through Sunday. Higher waves along the Grays Harbor county coastline (generally 12-16 ft) will need to be monitored as it could compound effects from the tides today. More on this can be found in the marine section.

The strong atmospheric river will arrive beginning on Monday, bringing with it a very impressive moisture source (750-1000 kg/m/s) for heavy rain over the next several days. In addition to the attention-grabbing moisture source values, the duration of the event will compound impacts in the region. Rain will be continuous on Monday, with a secondary pulse arriving Tuesday and lasting into early Wednesday. In the long term section, a Key Points format will be outlined to discuss all hazards for the week associated with the atmospheric river.

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LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

KEY MESSAGES:

* RAIN: High QPF amounts are forecast to accompany this potent atmospheric river Monday through Wednesday. 3 to 6 inches of rain are forecast from Seattle south to Olympia and 2 to 4 inches from around Everett north. Cascade and adjacent foothill locations are still on track to receive around 10 inches of rain as snow levels climb again. Forecast indications at this point are for additional atmospheric rivers in the wake of the strong one to bring even more rain in the second half of the week. QPF at this point suggest another 1 to 2 inches in the lowlands, and 2 to 5 inches more in the mountains. There will be hydrologic implications of this much rain that will be detailed in that section of this discussion.

* SNOW: True to an atmospheric river, snow levels will climb to 6000-7500 ft by Monday afternoon. Snow amounts will be confined to peaks and volcanoes above 7500 feet through Wednesday but could be longer into the second half of the week.

* WIND: Breezy to windy conditions are expected with the onset of the atmospheric river event. The winds will start on the coast Monday morning, picking up further inland into the afternoon and evening. Gusts of 35 to 40 mph are in the forecast. As the event continues into the middle of the week, the wind threat diminishes, with only breezy conditions at times through Wednesday.

* LANDSLIDES: Continued heavy rain will increase the threat of landslides. Area burn scars will be monitored throughout the event, but slides are possible elsewhere as well.

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AVIATION

West-northwest flow aloft will continue through today in a post- frontal airmass. Primarily VFR/MVFR cigs this morning with breezy onshore flow at the surface. VFR/MVFR cigs will continue through this evening before lowering late tonight into Sunday morning as another frontal system moves into Western Washington. Shower activity will be around through this afternoon as well, with drier conditions around SEA to PAE due to shadowing from the Olympics. Breezy SW surface winds through midday before winds slowly taper later this afternoon, and further taper off tonight. Gusts between 20 to 30 kts will remain possible through this morning, with stronger gusts possible at times through 18Z around SEA/BFI.

KSEA...MVFR cigs continue this morning in a post-frontal airmass and are expected to continue through 21Z, although some improvements to VFR may occur at times. Some further improvements towards VFR expected by this afternoon. Cigs will then lower again later tonight into Sunday morning ahead of the next frontal system. Gusty SW surface winds through 18Z, with gusts ranging 25 to 35 kts. Winds will slowly subside this afternoon, but remain breezy before winds really ease this evening. JD/14

MARINE

Strong onshore flow continues early this morning across the waters in a post-frontal airmass. Gales will ease over the next few hours through the Strait of Juan de Fuca. Small Craft Advisory winds will follow the gales this afternoon as winds subside, but overall conditions will remain elevated over the majority of waters. Winds will then briefly subside below SCA thresholds tonight. South winds will increase Sunday morning associated with the next weather system that will move across the waters on Sunday. SCA wind gusts are expected for the Coast Sunday morning. SCA southerly winds may also occur on Sunday for the interior waters, and will need to monitor this for further headlines. A stronger weather system will then move into the waters Monday into Tuesday for increased winds, and likely additional gale and SCA headlines. Additional weather systems expected mid to late next week.

Seas over the coastal waters generally range between 14 to 20 feet this morning will slowly subside to 9 to 11 feet by late tonight into Sunday. Seas will then build above 10 feet on Monday and remain above 10 feet through midweek. JD/14

HYDROLOGY

A series of wet weather systems moving across the region will increase the threat of river flooding early next week. While there's still some uncertainty in the precipitation amounts with an atmospheric river moving into the region the first part of next week, confidence continues to increase in heavy rainfall occuring across western WA, especially over the Olympics and Cascades.

Confidence also continues to increase that a widespread river flooding event will commence late Monday and continue through much of the week, with multiple rivers flowing off the Cascades and the Olympics forecast to reach Minor, Moderate, and Major flood stage. Additional precipitation is also possible Thursday into Friday, which will keep rivers elevated through the second half of the week.

Uncertainty does remain in regards to the exact flood levels and stages through next week, and as such, it will be critical to monitor the forecasts through the next seven days.

The series of wet systems will also start to elevate the landslide risk across the region. As soils become more saturated next week, there will be an increased potential for landslides and debris flows on burn scars.

Urban and small stream flooding, as well as areas with poor drainage, should be alert to the potential for localized nuisance flooding. Low water crossings may become impacted or impassable.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...Coastal Flood Advisory until 10 AM PST this morning for Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County-San Juan County.

Coastal Flood Advisory from 5 AM to 11 AM PST Sunday for Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County-San Juan County.

Flood Watch from late Sunday night through Friday afternoon 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-City of Seattle-Downtown Everett / Marysville Area-Eastern Kitsap County-Eastern Strait of Juan de Fuca-Eastside- Foothills and Valleys of Central King County-Foothills and Valleys of Pierce and Southern King Counties- Foothills and Valleys of Snohomish and Northern King Counties-Foothills and Valleys of Thurston and Lewis Counties-Foothills and Valleys of the North Cascades- Foothills of the Western and Southern Olympic Peninsula- Grays Harbor County Coast-Island County-Lake Crescent Area Including US 101-Lower Chehalis River Valley- Lowlands of Lewis and Southern Thurston Counties-Lowlands of Pierce and Southern King Counties-Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County-Middle Chehalis River Valley- Northern Hood Canal-Northern Washington Coast-Olympia and Southern Puget Sound-Olympics-Port Townsend Area-San Juan County-Shoreline / Lynnwood / South Everett Area-Southern Hood Canal-Western Strait of Juan de Fuca-Willapa and Black Hills.

Winter Storm Warning until 10 PM PST this evening for Cascades of Snohomish and Northern King Counties.

Winter Weather Advisory until 7 PM PST this evening for Cascades of Pierce and Lewis Counties.

Coastal Flood Advisory until 3 PM PST this afternoon for Grays Harbor County Coast.

PZ...Small Craft Advisory until 10 PM PST this evening for Grays Harbor Bar.

Small Craft Advisory until 1 PM PST Sunday 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-West Entrance U.S. Waters Strait Of Juan De Fuca.

Gale Warning until 10 AM PST this morning 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.

Small Craft Advisory until 7 PM PST this evening 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.

Small Craft Advisory until 4 PM PST this afternoon for Puget Sound and Hood Canal.