textproduct: Seattle/Tacoma

SYNOPSIS

An upper-trough will swing a couple of disturbances into the region through Saturday. Brief ridging is in the offing to begin next week but the pattern is forecast to reopen by Tuesday as additional troughing takes aim. A well-ogranized system looks to arrive around midweek with the potential for widespread rainfall. Cooler than average temperatures are favored throughout this time.

SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/

We're in between systems here across western Washington as the next disturbance looms off the BC coast. Conditions are mostly dry but that'll change later tonight into early Saturday morning. A convergence zone is slated to develop this evening, mainly Snohomish County northward before meandering southward into Saturday. The aforementioned upper-low will also track across the area during this time as rain showers become widespread. Overnight lows will range throughout the 50s with a few spots in the upper 40s.

Active weather into Saturday as the upper-low not only bring showers but the potential for thunderstorms. HREF has 200-400 j/kg of surface based CAPE analyzed across western Washington Saturday afternoon. Mean 700-500 mb lapse rates are also around 7 C/km, indicative of some degree of instability. These parameters in place with dynamics provided by the upper-low has allowed for parts of the CWA to placed in the SPC's convective outlook for general thunderstorms. The threat will end Saturday night as an upper-level ridge enters the fray with conditions drying out. Can't rule out areas of fog developing into Sunday morning with lingering surface moisture and lighter winds. Ridge axis is modeled to pass directly overhead on Sunday as morning clouds give way to mostly clear skies in the afternoon. However, the flow aloft will trend more out of the southwest on Sunday night as the next upper-low digs over the NE PAC. High temperatures in the 60s to lower 70s throughout the short term with overnight lows in the mid 40s to lower 50s.

LONG TERM /MONDAY THROUGH FRIDAY/

Conditions will remain drier on Monday as the upper-ridge exits the region. Then, the next upper-trough will continue to dig over the NE PAC and sit right off the PNW coast by Monday evening. Given latest guidance, it's associated upper-low will track south of the area into Oregon but we could get side swiped with showers on Tuesday. Then, a more organized disturbance is slated to arrive Wednesday and possibly stalling over the region into Friday. This feature has the potential to bring widespread rainfall and well-below average temperatures as the region will begin to get a taste of fall with highs only in the 60s.

AVIATION

Southwesterly winds will become cyclonic Saturday evening as an upper level low moves directly overhead of western Washington. Ceilings have mostly risen to VFR with clouds expected to continue to lift and break up through this evening. VFR conditions will continue through this evening before a weak frontal system moves across the area overnight, bringing a return to MVFR to IFR ceilings across the area as well as chances for scattered showers. IFR cigs are most likely along the coast, with chances for IFR cigs elsewhere under 20%. Expect cigs to slowly lift and break up through the late morning into the early afternoon on Saturday.

S/SW winds will continue around 5-10 kt, with some gusts to 15-20 kt possible this evening through the Strait of Juan de Fuca and the Chehalis gap. Winds look to switch to northerly this evening through Puget Sound for the Seattle area terminals. Winds then switch back to southerly overnight and remain southerly into the daytime Saturday.

KSEA...VFR conditions will continue through late this evening. A weak front moving through will lower ceilings to MVFR after around 08-12Z Sat and will again slowly lift and scatter into Saturday afternoon. S/SW winds up to around 6-8 kt will turn more NW to N this evening, around 00-02Z Saturday. Winds switch back to southerly after around 09Z Sat.

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MARINE

A weak frontal system will move through the area waters tonight, which will maintain northwesterly winds through the coastal waters through Saturday, as well as bring pushes of westerlies down the Strait of Juan de Fuca tonight and Saturday night as well. A Small Craft Advisory is in effect for the Strait tonight, and another will likely be needed tomorrow. Systems will pass by well to the north on Sunday and Monday, keeping winds fairly light across the area. A stronger low pressure system look to move through the area waters Tuesday into the middle of next week, which may bring increased winds to much of the coastal waters and higher seas through the middle of next week.

Seas remain 4 to 6 ft through Tuesday, then increasing to around 7 to 9 ft by the middle of next week.

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FIRE WEATHER

Onshore flow, cooler temperatures, and increasing RHs will persist through next week. An upper level low pressure system will bring chances for showers this evening through Saturday evening. In addition, there still remains an isolated threat for thunderstorms Saturday afternoon mainly over the Northern and Central Cascades. If a thunderstorm develops, wetting rains are unlikely and lightning may pose a hazard that could lead to new fire starts. Erratic and gusty outflow winds over existing fires could also be problematic. Weather system next Wednesday has a good chance of producing wetting rains along the coast and over the mountains. JBB/Felton

SEW WATCHES/WARNINGS/ADVISORIES

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