textproduct: Seattle/Tacoma

SYNOPSIS

A broad low pressure system over the NE Pacific will move southeastward through Thursday, keeping scattered rain and overnight snow showers in the forecast through then. Cooler temperatures follow Thursday and Friday with Fraser River outflow. A warmer pattern returns Friday into early next week, accompanied by a more significant system bringing gusty winds, as well as additional rain and mountain snow.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

No major changes have been made to the forecast this evening. Showers are moving north through Puget Sound and portions of the coast as the low saunters off to the southeast. The rest of the previous discussion can be found below, with an update to the aviation sections.

This system will continue to track to the southeast tonight into Thursday, and bring the showers with it. Across the northern half of the area, showers will begin to taper off tonight, through some offshore flow could enhance shower activity on the western slopes off the Strait of Juan de Fuca for a bit this evening, which could cause some light accumulations there and along US-101. The main bulk of the activity tonight will be across southwest Washington. Tomorrow should be mostly dry across the lowlands, with the best chances for additional showers staying over the Cascades. On Friday, as systems start to develop over the NE Pacific, that may send some moisture up our way to get some chances for rain on the coast.

Temperatures will be below normal across the area through Friday, but Fraser River outflow winds will help to keep the north interior cooler. Lows tonight in the low 30s to mid 20s. Lower 20s possible in Whatcom County, as well as wind chills in the teens by early Thursday morning. Lows Thursday night/Friday morning slightly cooler, in the mid to low 20s. High temperatures each day will be in the mid to lower 40s, except upper 30s on Thursday in the northern interior and Strait.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

This weekend and early next week will signal a warmer but more active pattern as a large upper level trough digs over the NE Pacific. With several shortwaves moving through it, these will will generate several stronger but fast-moving low pressure systems that will sweep along the Washington coast. These systems will push snow levels up to around 2000-3000 ft. While QPF and snow amounts don't look to be noteworthy at this point, these will be windier systems with more widespread breezy to gusty conditions. A few inches of snow will be possible at the passes will be possible each day. Temperatures will warm with highs going up into the mid to upper 40s Friday and Saturday, reaching the low 50s Sunday and onward. Lows return to the mid to upper 30s.

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AVIATION

Westerly to southwesterly flow aloft continues as low pressure moves south of the area. Surface flow across the terminals will begin to shift northerly overnight with cool air lingering. Expect MVFR to VFR cigs across the area tonight as showers begin to taper off. Gustier conditions will be present overnight at KBLI as Fraser outflow continues, keeping the northern areas generally drier. IFR cigs possible in south Puget Sound and in through the Chehalis Valley.

KSEA...MVFR cigs and rain showers now at the terminal, likely to end in the next hour or two. Guidance is indicative of MVFR cigs lingering throughout the night, becoming VFR tomorrow afternoon. Winds are expected to become ENE by 06Z, becoming more northerly early tomorrow morning. Speeds will be between 7-9 kts after 14Z.

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MARINE

A surface trough over the Washington coastal areas will shift southward tonight into the Oregon waters. A surface ridge will strengthen tonight over the interior of British Columbia for a brief increase in Fraser River outflow and easterly flow through the strait. Small craft advisories have been issued for the strait and adjacent waters of the north interior. Light offshore flow will continue into Friday with a broad surface trough in place across the coastal and offshore waters and a weak ridge east of the Cascades. Low pressure at the surface and aloft will deepen offshore over the weekend for increasing offshore flow and likely wind related headlines for the coastal waters, strait entrances, and northern inland waters. This pattern will persist into early next week.

Coastal seas are expected to remain below 10 feet into Thursday, but increased activity offshore this weekend and beyond will drive seas back into hazardous double digits once again as additional large swell trains arrive.

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HYDROLOGY

No river flooding is expected over the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Cold Weather Advisory until noon PST Thursday for Lowlands of Western Whatcom County-San Juan County.

Cold Weather Advisory from 1 AM to 10 AM PST Thursday for Eastern Strait of Juan de Fuca-Island County-Lake Crescent Area Including US 101-Port Townsend Area-Western Strait of Juan de Fuca.

PZ...Small Craft Advisory until 10 AM PST Thursday for East Entrance U.S. Waters Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 10 AM PST Thursday for Central U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands.