textproduct: Seattle/Tacoma

SYNOPSIS

Broad upper level troughing and onshore flow will continue to bring cooler temperatures and morning clouds and afternoon sun breaks through midweek. A Puget Sound Convergence Zone is favored to develop tonight into Tuesday morning, with additional chances for light rain Wednesday morning as a weak system comes through. A stronger system will bring widespread rain later Thursday through Friday, with additional chances for precipitation throughout the weekend.

SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/

No planned updates to the forecast tonight. 33

Previous discussion...As a series of storm systems develop over the Gulf of Alaska, southwest flow aloft will continue over the Pacific Northwest with onshore flow at the surface. A weak system rippling through the upper level flow will bring increasing high clouds tonight, with potential for Puget Sound Convergence Zone showers to develop and continue into Tuesday morning. Onshore flow will also increase overnight, bringing another round of low stratus across western Washington Tuesday morning. The marine layer is expected to erode midday, revealing mostly sunny skies with temperatures peaking in the mid to upper 60s for most lowland areas.

Wednesday will bring more of the same as another weak disturbance crosses the region. This will bring another round of low clouds in the morning, with increased chances for some light rain in the morning as the weak system moves through. While low clouds clear up once again around midday, higher clouds are expected to linger, bringing temperatures down a few degrees with highs in the mid 60s for most areas.

LONG TERM /THURSDAY THROUGH MONDAY/

Models continue to show good agreement over a frontal system moving into the region on Thursday, bringing widespread rain and this fall's first taste of high elevation snow. Rain is expected to spread inland Thursday afternoon and into Friday, though ensembles show varying timeframes of how long the precipitation will linger. The ECMWF suggests the upper trough may become slightly negatively tilted on Friday, which could prolong the duration of rainfall over the region. Snow levels will also gradually lower as this system brings in cooler air, dropping to around 6000 feet on Friday. Nearly all of the precipitation will fall as rain, but the higher peaks will see the first dusting of snow of the season. Cooler temperatures and chances for additional precipitation continue through the weekend, though a large degree of uncertainty remains between ensemble solutions.

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AVIATION

Southwest flow aloft is expected across Western Washington into Tuesday. Low level onshore flow will increase behind a front tonight for increasing low level moisture across the lowlands this evening through most of Tuesday morning. Widespread MVFR ceilings is currently developing and will persist across the lowlands into early afternoon on Tuesday. Low level convergence will lead to some spotty light rain or drizzle in central Puget Sound Tuesday morning.

KSEA...VFR will continue into this evening then MVFR ceilings will develop later tonight and likely hang around through at least midday on Tuesday. Surface winds will veer light N/NW for a period this evening before backing once again to light southerly Tuesday morning.

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MARINE

Increasing onshore flow will lead to Small Craft Advisory westerlies in the central/east strait tonight. Another push of westerlies through the Strait of Juan de Fuca is forecast Tuesday evening, but guidance remains confident in the strongest winds remaining on the Canadian side of the strait. A weakening front then moves across the waters on Wednesday with a more significant weather system moving through late Thursday into Friday.

Seas of 5 or 6 feet will build to 7 to 9 feet tonight as a longer period northwesterly swell arrives to the coastal waters. Seas then subside to 6 to 8 feet Tuesday afternoon and remain like so through the remainder of the week.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 3 AM PDT Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.