textproduct: Seattle/Tacoma

UPDATE

Convergence zone activity remains over Snohomish county this evening with scattered showers elsewhere. Isolated wind gusts upwards of 40-45 mph have been observed within the last few hours, resulting in a short-fused Wind Advisory being issued for the Seattle/Tacoma metro for tonight. Rain showers are favored to remain overnight with snowfall expected for elevations near 4,000 ft and above.

SYNOPSIS

The weather pattern has transitioned into an active phase as a series of system bring rain, high elevation snow and wind into the region through the end of the weekend. A strong atmospheric river is still on track to region the area by Monday and last into at least the midweek time frame. This system will bring very significant rainfall and likely hydrologic impacts. Coastal flooding due to higher astronomical tides will remain minor into early next week.

DISCUSSION

KEY MESSAGES...

* RAIN: a few relatively minor systems will sweep across the area tonight, and on Sunday, before a strong atmospheric river system will bring heavy precipitation the area Monday through at least Wednesday. Models ensembles are beginning to show another round of heavy precipitation Thursday into Friday as well, now. The hydrologic implications will be detailed in the HYDROLOGY section.

* SNOW: lowering snow levels tonight to just below 4000 ft will allow for significant accumulating snow for Stevens Pass and White Pass. Heavy precipitation through tonight into early Saturday, as well as the potential for a convergence zone late tonight will allow for 1-1.5"/hr+ snowfall rates, which will make travel very difficult. Wind gusts up to 30-35 mph will limit visibility at times as well. Check WSDOT for the latest pass conditions and for closures.

* WIND: winds will generally be elevated through the next 7 days, but the time periods for the strongest winds will be tonight, as well as on Monday, where gusts up to 45 mph will be possible along the Pacific coast, the Strait/Northwest Interior, as well as through the Cascade Mountains.

* COASTAL FLOODING: tides have now peaked for the Pacific Coast, through with larger waves coming in tomorrow, the potential for minor coastal flooding impacts will be possible again on Saturday, mainly for the beaches of Grays Harbor County. For the northern interior waters, tide levels this weekend will rise, allowing for mostly minor coastal flooding impacts. Going into next week, active weather looks to keep tide levels high and may create additional impacts for Puget Sound coastlines as well.

As of this afternoon, radar imagery is showing the strong influx of moisture, creating showers across the Pacific coast and through the Cascade mountains. As the supporting shortwave trough aloft begins to move across the area, that will begin to push in even more moisture, as well as increase westerly winds across the area. Most locations will see winds gusts between 25-35 mph, but Island County may see gusts up to 45 mph, which warrants the wind advisory through tonight. This will also bring some cooler air, which will allow for snow levels to drop below 4000 ft, where impacts will start to be felt at Stevens and White Passes. With the increased westerly winds, we will likely see the development of a convergence zone across central/southern Snohomish County, which will be flirting closely with Stevens Pass. Regardless, moisture will continue to filter in across the area through tonight into Saturday, primarily focused on the Cascades. Latest hi-res guidance is showing the potential for 1-1.5"/hr+ snowfall rates over the passes tonight, and as such, total accumulations through tomorrow afternoon will likely surpass a foot at Stevens Pass, perhaps as much as 16-18 inches. Locally higher amounts are expected at the higher peaks, and at locations such as Paradise and Mt. Baker Ski Area. As such, a winter storm warning is in effect for Stevens Pass, while slightly lower amounts are forecast for White Pass, lending to a winter weather advisory. These products are in effect through Saturday evening.

There will likely be a brief lull in the precipitation overnight on Saturday, but it won't be long until the next system arrives Sunday morning. Precipitation amounts with this systems will be relatively light--a quarter to a half inch across the lowlands, with up to an inch or two for the coast and mountains. Rising snow levels back up to around 4500-5000 ft will allow for rain at the passes. No major hydrologic impacts are expected with this system, but it will prime the soil and the rivers for what will come next week.

A strong 100-125 kt jet (at 500 mb) will become aimed at western Washington starting on Monday and continuing through Wednesday. Accompanying that will be a plume of significant moisture; precipitable water values will reach up to 1-1.5", which is 250-300%+ above normal for this time of year. Models are showing integrated vapor transport values reaching or exceeding 750-1000 J/kg during this time period. Needless to say, a lot of water will accompany this atmospheric river headed this way. Heavy rain will begin to arrive early on Monday and continue through early Tuesday. The main moisture plume looks to briefly sink southward on Tuesday, allowing for a brief lull in the heavy rain before another punch of moisture brings back the heavy rain on Wednesday. In this period, the latest rainfall totals put around 3 to 5 inches across the interior lowlands, 4 to 6 inches along the coast, and 8 to locally 12+ inches across the mountains. Flooding is likely; not just the rivers, but also urban/small stream flooding as well. More details on the hydrologic impacts are below in HYDROLOGY. While the forecast trend in QPF continues to inch upward, things can change, so it is pertinent to stay tuned to the latest updates.

Lastly, models are starting to show indications of another weather system bringing significant precipitation late Thursday into Friday. While it may not be as much as we'll see from Monday-Wedensday, with saturated soils and rivers already running very high, this rain may prolong or exacerbate the hydrologic impacts expected by mid-week.

As typical in a rainy/atmospheric river setup, temperatures through the week will remain slightly above normal, with highs generally in the low to mid 50s and lows in the 40s.

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AVIATION

Northwest flow aloft will continue tonight as a frontal system pushes inland across western Washington. Conditions are primarily a mix of IFR to MVFR across the majority of the area terminals this evening with a convergence zone continuing over Snohomish County, which will bring additional post- frontal showers and gusty winds across the region tonight (mainly impacting KPAE). Forecast models also show areas of low level wind shear east of the Olympic Mountains and over the Cascades, but is not expected to impact the western Washington terminals. Localized brief improvements to VFR will still be possible this evening in the post-frontal airmass, however expect more widespread MVFR conditions for most terminals into Saturday morning. Shower activity will increase again overnight into Saturday morning. S/SW surface winds will peak tonight, but remain breezy into Saturday, with gusts generally ranging between 20 to 30 kts for most areas, though localized gusts to 35 kts will be possible in some spots as well.

KSEA...MVFR cigs this evening, likely persisting into Saturday outside of any drizzle/shower activity. A convergence zone will continue over the northern Puget Sound, but will remain well north of the terminal. S/SW winds persisting at 10-20 kts will peak tonight, with gusts between 25-35 kts possible at times. Winds will ease somewhat into Saturday, but will remain breezy with gusts to 25 kts at times through the day.

MARINE

A weather system moving inland tonight will continue to bring gusty winds to the area waters into Saturday morning. Westerly gale force wind gusts will remain possible into early Saturday for the Coastal Waters and western Strait of Juan de Fuca in its wake. A strong push of westerly winds is also likely along the Strait of Juan de Fuca tonight, where localized gusts to 45 kts will be possible at times.

Stronger southwesterly winds are also expected along the southern portions of the northern inland waters and near Bellingham Bay tonight, where gusts to 35 kts remain possible. Gusts may also peak briefly around 35 kts for Puget Sound as well tonight, though expect predominantly SCA winds to prevail for the majority of the area. Winds will slowly subside area-wide on Saturday. Additional weather systems are expected over the next week - the next being on Sunday morning, with a stronger system following on Monday and Tuesday. Further headlines will likely be required, with potential gales again with the Monday/ Tuesday weather system.

Seas will build to 13 to 18 feet tonight into Saturday morning, before subsiding back towards 8 to 10 feet by late Saturday night. Seas will build again starting late Monday as the next round of weather systems move into the region.

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 possible 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, throughout Tuesday and into Wednesday, with multiple rivers flowing off the Cascades and the Olympics reaching 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 throughout the weekend, with conditions expected increase rapidly early next week with the arrival of an atmospheric river.

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 from 4 AM to 10 AM PST Saturday 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 Saturday for Cascades of Snohomish and Northern King Counties.

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

Wind Advisory until midnight PST tonight for City of Seattle- Lowlands of Pierce and Southern King Counties.

Coastal Flood Advisory from 9 AM to 3 PM PST Saturday for Grays Harbor County Coast.

Wind Advisory until 7 AM PST Saturday for Island County.

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

Gale Warning until 4 AM PST Saturday 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 4 AM to 10 PM PST Saturday 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 Saturday 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 AM PST Saturday for Admiralty Inlet-Puget Sound and Hood Canal.