textproduct: Seattle/Tacoma

SYNOPSIS

A strong atmospheric river has moved into the region, and will bring periods of moderate to heavy rainfall and gusty winds to western Washington through Wednesday. There remains high risk of widespread and significant river flooding across the region through much of the week. Minor coastal flooding due to high astronomical high tides will also remain possible for the Salish Sea coastlines through the first part of the week. Wet and unsettled conditions will continue through the end of the week.

SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/

Precipitation associated with a strong atmospheric river has moved into the Pacific Northwest this afternoon, and is expected to affect the region directly through Wednesday, and indirectly through the remainder of the week. The key messages regarding this system are highlighted below:

* HEAVY RAIN: Extended periods of moderate to heavy rain to western Washington through Wednesday. Latest forecast models have remained fairly consistent, with the heaviest rainfall arriving today. The moisture plume still looks to briefly shift southward early Tuesday, easing accumulations for some areas before lifting back northward late Tuesday through Wednesday. While areas east of the Olympics will likely see some shadowing at the onset of precipitation, conditions across all of western Washington will be very wet through Wednesday. Areas generally south of Seattle will see 3 to 6 inches of rainfall through Thursday morning in a 72 hour period, with the Olympics and Cascades on track to see 8 to 12 inches of rain in this timeframe. Locally higher amounts are possible over the southern and central Cascades, where amounts could reach 15 inches or more in 72 hours. This is expected to bring significant river flooding to western Washington, with many area rivers forecast to reach moderate or major flood stage by mid-week. A Flood Watch remains in effect for all of western Washington through Friday morning. For more details, see the HYDROLOGY section below.

* WIND: Conditions will be windy area-wide today, with southwesterly winds picking up throughout the day. Winds will peak late in the afternoon and evening, with the strongest gusts along the Pacific Coast reaching 50-55 mph. Gusts elsewhere across the interior will reach 40-45 mph at times. Winds will gradually ease into Tuesday, but conditions will remain breezy at times.

* COASTAL FLOODING: Minor coastal flooding remains possible along the Salish Sea coastlines through the first half of the week with high astronomical tides. While the tides will generally remain below or nearing thresholds, heightened river flows or wind effects could exacerbate inundation in some areas around high tide.

In addition, snow levels will briefly lower to near 3500-4000 feet over the mountains early Tuesday, resulting in minor accumulating snow generally above 4500 feet. The higher mountain passes, including Stevens Pass and White Pass, will likely see a few inches of snow accumulation throughout the day Tuesday which could cause brief slippery travel conditions.

The returning surge of moisture later Tuesday afternoon will bring warmer temperatures and higher snow levels through the week. Temperatures will peak well above normal most days this week, with temperatures in the low to upper 50s across the lowlands.

LONG TERM /THURSDAY THROUGH MONDAY/

Unsettled conditions continue to be expected into the weekend, though precipitation amounts will start to taper across western Washington by Thursday. It is likely that impacts like lingering river flooding and elevated landslide risk will linger through Thursday for many areas. Ensembles a system likely to brush the area Thursday, with extended long range ensembles hinting at a break Friday, followed by another stronger system likely focused to the north over British Columbia early Saturday and then possibly Western Washington Saturday into Sunday.

15

AVIATION

Majority of the terminals IFR this evening as a weather system continues to push through the region. Precipitation is slowly pushing southward away from the terminals and may lead to some high- end MVFR or low-VFR improvement. Already seeing northerly flow at PAE and expecting BFI and SEA to shift NE shortly after 06Z. Expecting winds to shift southerly again after 12Z. Winds will gradually subside overnight to around 5-10 kt, but are expected to increase again Tuesday evening, with sustained speeds 10-15 kt and gusting to 20-30 kt.

KSEA...Currently IFR at the terminal and conditions should improve slightly to MVFR overnight after 12Z. May see some brief improvement to low-end VFR conditions around 21Z, but ceilings are expected to deteriorate late Tuesday evening into the overnight hours to IFR. Southerly flow at the terminal is expected to switch NE around 06Z tonight, gusting to 25-30 kt. SW winds return after 12Z and winds will gradually subside late Tuesday morning to 5-10 kt. Winds will increase again late Tuesday evening to 10-15 kt, gusting to 20-30 kt.

29

MARINE

The first frontal system from an atmospheric river continues to impact area waters today. The areas with the breeziest SW winds this afternoon are the coastal waters and the interior waters. As the cold front kicks in, strong west winds will pick up in the Strait of Juan de Fuca through tonight. The wind threat will end early Tuesday morning across all waters as winds decrease behind the cold front. Hazards were adjusted to account for the extended duration of winds behind the front, and also seas of 10-15 ft dropping to 9-12 ft Tuesday. Heavy rain will continue tonight into Tuesday morning, which may reduce visibilities at times. The next system arrives late Tuesday night through Thursday, with more moderate to heavy rain, breezy winds southwest winds (most likely remaining below gale force at this time, but most hazardous to small craft), and seas remaining above 10 ft through Thursday at 9-11 ft, decreasing to 4-6 ft Friday through Saturday. Another system will increase seas and winds early next week.

HPR

HYDROLOGY

A strong atmospheric atmospheric will move through the Pacific Northwest through Wednesday, bringing significant heavy rainfall across the area. 72-hour precipitation amounts between ending Thursday morning will likely reach 2 to 6 inches across the lowlands, with 6 to 10 inches over the Olympics and Cascades with locally higher amounts up to 12 inches over the central and southern Cascades.

Rivers continue to rise across the region, with many flood warnings in effect. Consult weather.gov/seattle for the latest. While some uncertainty does still remain in regards to the exact flood levels and stages through the next week, many river forecast points are projected to reach major flood stage in the days ahead. Therefore, it will be critical to monitor the forecasts now and through the next seven days. Additional precipitation is also possible Thursday into Friday, which may keep rivers elevated heading into the weekend. A Flood Watch remains in effect across all of western Washington through Friday.

With the wet antecedent soil conditions, the landslide threat across the region will increase with this system. Potential has also increased for debris flows over area burn scars, which will continue to be monitored.

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

SEW WATCHES/WARNINGS/ADVISORIES

WA...Flood Watch 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.

Coastal Flood Advisory from 7 AM to 1 PM PST Tuesday for Lowlands of Western Skagit and Northwestern Snohomish Counties-Lowlands of Western Whatcom County-San Juan County.

PZ...Small Craft Advisory until 7 PM PST Tuesday for Grays Harbor Bar.

Gale Warning until 1 AM PST 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-West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 1 AM to 7 PM PST 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-West Entrance U.S. Waters Strait Of Juan De Fuca.

Gale Warning until 7 AM PST Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Gale Warning until 4 AM PST Tuesday for Admiralty Inlet- Northern Inland Waters Including The San Juan Islands.

Gale Warning until 2 AM PST Tuesday for Puget Sound and Hood Canal.