textproduct: Seattle/Tacoma

SYNOPSIS

An upper level trough will continue to move east of Western Washington today. Drier and warmer weather Monday and Tuesday as an upper level ridge builds over the Pacific Northwest. An upper level low will approach the area around midweek, bringing increasing onshore flow and cooler temperatures to Western Washington for the second half of the week.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

An upper level trough continues to push across Washington this afternoon. Partly cloudy skies this afternoon with high temperatures in the low to mid 70s across the interior and mid 60s along the coast. An upper level ridge will build over the Pacific Northwest on Monday, with the ridge axis east of the Cascades on Tuesday. This will bring warmer and drier conditions. The warmest day will be Tuesday. High temperatures are expected to peak in the mid 60s/low 70s along the coast. Across the interior, temperatures will climb into the 80s and low 90s in the Cascade valleys. This will reintroduce widespread areas of Moderate HeatRisk for the Everret-Seattle-Tacoma Metro and Cascade valleys.

Additionally, surface flow will transition more northerly to easterly Monday and Tuesday, resulting in wildfire smoke being transported from nearby fires in British Columbia and Eastern Washington into Western Washington. 29

LONG TERM /WEDNESDAY THROUGH SUNDAY/

There is good agreement between ensemble guidance that an upper level low will approach the Pacific Northwest on Wednesday and an upper level trough will settle over the region through late in the week. This pattern will reinstate onshore flow across Western Washington and bring cooler temperatures. Latest guidance has high temperatures trending slightly below normal in the the low to mid 60 along the coast and low to mid 70s across the interior. Low temperatures by the end of the week will trend towards the low to mid 50s. Chances for precipitation look to return on Wednesday and continue into the weekend. However, the latest QPF amounts indicate precipitation will be light. While thunderstorm chances remain at 10% or less, they could be a possibility on Wednesday and Thursday as the upper level low swings north into British Columbia and brings southwesterly flow aloft over Western Washington. 29

AVIATION

An upper ridge begins to build into the area tonight through Monday, which will result in westerly flow aloft becoming southwest on Monday. VFR cigs this afternoon with lingering areas of MVFR mainly from PAE northwards. Clouds will slowly clear into this evening. LIFR stratus will develop along the coast tonight. LIFR cigs are also expected across the Strait of Juan de Fuca by early Monday. Some uncertainty exists as to the extent of stratus around central Puget Sound Monday AM. At this time, LIFR cigs are expected to be confined to PAE and nearby river valleys. Otherwise, mainly VFR elsewhere with a few high clouds on Monday with haze increasing aloft late Monday with more of an E flow in the mid levels. Winds at the surface will transition more north this evening, increasing from the north Monday afternoon.

KSEA...VFR cigs this afternoon with continued mid level clouds. Cigs will slowly clear into this evening. LIFR stratus will push inland along the Strait of Juan de Fuca into early Monday. VFR conditions are expected on Monday as the stratus is anticipated to remain north of the terminal. There is approx. a 10% of LIFR cigs between 13-17z. W/SW surface winds will transition to N/NW around 23-00z this afternoon. Winds remain NE tonight at 5 to 9 kts. JD

MARINE

High pressure continues to build over the Coastal Waters today and will remain in place through Tuesday. North winds will increase on Monday, particularly for the Coastal Waters and Puget Sound. Will need to monitor for SCA wind gusts late Monday afternoon through Monday evening for the Coastal Waters, with current probabilities of SCA gusts around 40 to 70 percent during this period. Onshore flow then increase Tuesday night through Wednesday as a weak frontal system moves across the waters midweek. This may result in SCA west winds through the Strait of Juan de Fuca beginning Tuesday night. Onshore flow then continues late in the week.

Seas around 3 to 5 feet may briefly peak towards 7 feet for the southern Coastal Waters late Monday with increasing north winds. This may lead to brief choppy waters with a period around 6 seconds. Otherwise, 4 to 5 foot seas are expected Tuesday through much of the week. JD

FIRE WEATHER

An upper-trough has progressed off and away from western Washington this afternoon. Conditions have trended drier but marine stratus has remain well entrenched as shown by recent satellite imagery. Guidance is in lock-step regarding a ridge building into the region on Monday into Tuesday. A thermal trough will also snake along the coast during this time, leading to a warm-up and drier conditions as light offshore flow develops. Isolated areas of min RHs reaching near critical thresholds are possible during this time in the Cascades. Onshore flow then returns Tuesday night into Wednesday as another upper-trough enters the region. Showers as well as the chances for more thunderstorms are possible around the midweek timeframe.

Less certainty in how the pattern will evolve towards the second half of the week. Solutions are hinting at an upper-trough swinging a pair of upper-lows into the region but there is disagreement in exact timing and track. Nonetheless, below normal temperatures, onshore flow and the potential for precipitation appears to be in the offing towards the weekend.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...None.