textproduct: Seattle/Tacoma

SYNOPSIS

A period of very active weather will dominate the week ahead as a series of frontal systems produce cascading impacts across Western Washington. In addition to ongoing river flood concerns, another period of windy conditions is expected tonight into early Wednesday. Snow levels will tumble to the passes by early Wednesday with heavy snowfall expected in the mountains. Cool, unsettled conditions will remain in place through the remainder of the week with substantial additional snowfall in the mountains.

SHORT TERM /TODAY THROUGH THURSDAY/

After a brief lull between systems early today, a secondary stronger system arrives tonight. Wind potential with it remains a concern. When combined with saturated soils already in place, the threat of fallen trees increases. Models show a sharp surface mesolow forming to the lee of the Olympics by 09Z Wednesday. This creates a very strong pressure gradient Seattle southward. Isallobaric analysis shows impressively strong pressure rises behind it...as much as 7 to 10 millibars in just three hours early Wednesday morning. So, it's not just wind speeds alone, but the rapid manner in which they could arrive. A very strong westerly surge will occur through the Strait of Juan de Fuca. In the period between around 11 PM tonight and 6 AM Wednesday, gusts of 40 to 50 mph are expected for portions of the the interior lowlands with 50 to 60 mph along the coast, strait, and around Hood Canal. Snow levels will tumble to the passes by early Wednesday with significant snowfall expected. Totals could approach 10+ inches at Snoqualmie Pass by Wednesday afternoon, 18 to 24 inches at Stevens, and 2 to 3 feet at Mount Baker/Rainier. The snowfall will be accompanied by strong winds as well. A combination of post-frontal convergence and upslope flow will keep the snow piling up in the Cascades into Wednesday night. Quick on the heels of that system, another arrives on Thursday for additional lowland rain, breezy winds, and impactful mountain snowfall.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

There is no significant change in thinking for the extended portion of the forecast. A parade of systems will arrive nearly daily with additional lowland rain and mountain snow. Snow levels will remain near or below the passes through the period with significant additional accumulations. 27

AVIATION

Strong west to southwest flow aloft will turn westerly late tonight as a vigorous frontal system moves onshore into Western Washington. Ceilings will lower to MVFR later this morning in increasing rain with a few pockets of IFR. These conditions will persist into tonight. Surface winds will increase by early afternoon and become strong overnight as the system moves onshore. After 03Z tonight, southwest surface winds gusting as high as 35 to 45 knots can be expected along the coast and across interior terminals around Puget Sound. These winds will persist into early Wednesday morning before gradually diminishing.

KSEA...A mixing of IFR and MVFR ceilings in scattered showers are expected to solidify into mostly low MVFR later this morning as stratiform rain associated with the next frontal system spreads into the area. MVFR ceilings in rain will continue into tonight. Southerly surface winds of 8 to 14 knots this morning will steadily increase this afternoon and become quite gusty from the southwest tonight. Gusts of 25 to 40 knots, potentially higher, are expected between 05Z tonight and 12Z Wednesday morning. 27

MARINE

Frontal system moving into Western Washington late tonight into early Wednesday morning. Gale force winds developing ahead of the front over the coastal waters. Sharp pressure rises behind the front will produce a round of strong winds over all the waters. Gale warnings are in effect for all the waters tonight with storm warnings up for the Central and Eastern Strait of Juan de Fuca for a few hour window in the early morning hours Wednesday. Winds easing later Wednesday morning into Wednesday afternoon with small craft advisory winds continuing over the coastal waters and in the Strait of Juan de Fuca. Another system arriving Thursday will keep small craft advisory winds going over the coastal waters.

Seas in the 10 to 14 foot range today building to 18 to 24 feet tonight into Wednesday. Seas subsiding back down to 10 to 14 feet Wednesday night into Thursday. Seas remain just above 10 feet Thursday night falling back into single digits Friday. Felton

HYDROLOGY

Another frontal system will bring one to three inches of precipitation to the mountains through Wednesday with an inch or less in the lowlands. Cooler air moving into the area later tonight adds some uncertainty to the forecast river levels with snow levels dropping to 2000 to 2500 feet Wednesday morning. The front tonight has more moisture than the system Monday but the cooler air will keep some of the moisture in the mountains in the form of snow. This combination makes for tricky river forecasting.

Major flooding is possible on the Skagit river beginning Wednesday morning in the upper reaches with the flood crest reaching Mount Vernon Thursday morning. Moderate flooding is forecast for the rivers flowing out of the Central Cascades like the Skykomish, Snoqualmie and Snohomish. Could also see some moderate flooding on the Cowlitz river. Most of the remainder of the river forecast to reach flood stage will experience minor flooding.

A flood watch is in effect through Thursday afternoon across the lowlands.

The landslide threat remains very elevated will continue to be high through at least tomorrow. Felton

SEW WATCHES/WARNINGS/ADVISORIES

WA...Wind Advisory from 7 PM this evening to 6 AM PST Wednesday for City of Seattle-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 the North Cascades-Lowlands of Western Whatcom County-San Juan County-Shoreline / Lynnwood / South Everett Area.

Flood Watch through Thursday 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-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- 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-Olympia and Southern Puget Sound-Olympics-Shoreline / Lynnwood / South Everett Area-Willapa and Black Hills.

Winter Storm Warning from 4 PM this afternoon to 10 PM PST Wednesday 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-Olympics.

High Wind Warning from 7 PM this evening to 6 AM PST Wednesday for Downtown Everett / Marysville Area-Eastern Strait of Juan de Fuca-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 Pierce and Southern King Counties- Lowlands of Western Skagit and Northwestern Snohomish Counties-Middle Chehalis River Valley-Northern Hood Canal- Northern Washington Coast-Olympia and Southern Puget Sound-Port Townsend Area-Southern Hood Canal-Western Strait of Juan de Fuca.

PZ...Small Craft Advisory until 4 PM PST Wednesday for Grays Harbor Bar.

Small Craft Advisory until noon PST today 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 from noon today to noon PST Wednesday 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 from 4 PM this afternoon to midnight PST tonight for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Storm Warning from midnight tonight to 6 AM PST Wednesday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Gale Warning from 6 PM this evening to 6 AM PST Wednesday for Admiralty Inlet-Northern Inland Waters Including The San Juan Islands-Puget Sound and Hood Canal.