textproduct: Seattle/Tacoma

SYNOPSIS

Northwesterly flow aloft continues today with high clouds and an otherwise pleasant day. Additional weather systems will begin to move through the region tomorrow for rain, mountain snow and potential convergence zone. Dry weather returns Monday and Tuesday before a stronger system arrives Tuesday night into Wednesday. Unsettled weather and precipitation chances linger through the end of the week.

SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/

The remainder of this afternoon should be very pleasant as mostly sunny skies prevail. Area temperatures have been in the mid to upper 50s. A weak trough will move through the region Sunday for light lowland rain and mountain snow. Hi-res models are suggestive of a post-frontal convergence zone in the afternoon along the King and Snohomish County border, which will progress south as westerly winds force through the Strait of Juan de Fuca and shift surface winds to northerly through Puget Sound. This convergence zone could sink as far south as Sea-Tac and Tacoma as the northerly winds continue. Snow levels drop below pass level tomorrow, leading to potential snow accumulations of 4-6 inches at both major Cascade Passes.

Rain and snow taper off into Monday morning, with snow levels dropping even further to around 500 feet or less. There could be a brief period of time late Sunday night into early Monday morning for snowflakes in the Cascade foothills and valleys, but no meaningful accumulations are expected. Monday morning lows will be at freezing or below. Expect a clear, but chillier afternoon with highs only getting into the upper 40s to right around 50.

LONG TERM /TUESDAY THROUGH SATURDAY/

Tuesday will also be dry and a few degrees warmer in the mid 50s. This slight increase in temperature will be in part due to a relatively short period of offshore winds during the day. Interestingly, minimum relative humidity values on Tuesday will be much lower than what is typical for this time of year, between 35% to 45%. The dry and mild conditions won't linger past Tuesday as another weather system is forecast to arrive late Tuesday night into Wednesday. Snow levels will drop again late Wednesday into Thursday for another round of mountain snow. Both the Cascades and Olympics will see snow, and while the amounts are subject to change between now and then, early estimates range from around 6-8 inches at Hurricane Ridge, 3-6 inches at Snoqualmie Pass, and 6-9 inches at Stevens Pass. This period of snow into Thursday will be monitored for headline potential over the next several days.

The ridge rebuilds late Thursday into Friday over the offshore waters. With western Washington remaining in the northwesterly flow, slight chance PoPs linger in the forecast through the end of the week. Extended range forecasts suggest more drying over next weekend.

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AVIATION

VFR at the terminals this afternoon with high clouds passing through. Expecting VFR conditions to prevail today. A cold front has pushed through the area, expecting winds to shift W/SW with for terminals behind the front. However, terminals within central Puget Sound are expected to remain northerly due to a convergence zone forming this evening as westerlies push through the Strait of Juan de Fuca. A stronger system is expected to move through on Sunday, with rain reaching the coast around 13Z and interior terminals around 14-15Z. This system will be accompanied by MVFR ceilings and increased winds around 8-12 kt.

KSEA...VFR conditions expected today. The front has pushed through the terminal, which will bring a brief period of SW winds starting around 22/23Z. Winds are expected to shift back northerly this evening after 02Z and continue through Sunday morning. Latest guidance suggests southerly winds return as early as 13Z Sunday. Southwesterly winds will increase Sunday afternoon to 8-10 kt. Ceilings will gradually lower overnight to MVFR after 18Z with the next system. Improvement to VFR possible Sunday evening.

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MARINE

Weak high pressure will remain over the waters through early next week. Today's cold front has pushed through area waters, which will lead to post-frontal westerlies through the Strait of Juan de Fuca. A Small Craft Advisory has been issued for the Central Strait due to increased confidence (70-80% chance) in seeing small craft winds through tonight. There wasn't enough confidence to issue an advisory for the East Strait; however, at times small craft winds are possible. High pressure will weaken slightly on Sunday as another frontal system pushes across the waters. This system will bring elevated winds and seas across the coastal waters and West Strait starting Sunday afternoon. Advisories have been issued for the coastal waters and western portion of the Strait. The central and eastern portions may see small craft winds at times (30-50% chance). Northwesterly winds will also increase across the northern interior waters Sunday evening. Current probabilities are around 50-70% and small craft winds are not looking to be widespread across the area. The strongest winds will be confined near the international border. High pressure will quickly rebuild behind the system. A stronger system arrives on Wednesday with elevated winds and seas. Currently there is a 60-80% chance of small craft winds for the coastal waters, East Strait, Northern Interior Waters, and Admiralty Inlet.

Seas will build Sunday evening 8-12 ft then decrease below 10 ft by Monday afternoon. Seas will rebuild on Wednesday to 10-13 ft before decreasing below 10 ft late Wednesday night.

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HYDROLOGY

No river flooding is expected in the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 11 PM PDT this evening for Central U.S. Waters Strait Of Juan De Fuca.