textproduct: Seattle/Tacoma

SYNOPSIS

Gradual cooling over the weekend to near-normal temperatures as onshore flow persists. The backside of an upper level ridge will remain over Western Washington today before upper level flow becomes zonal on Sunday. This will continue through at least the first part of the week. Uncertainty in the forecast increases towards the second part of next week.

SHORT TERM /TODAY THROUGH MONDAY/

A shallow marine layer made similar progress inland this morning to that of yesterday. With a little more of a marine influence and high cloud coverage this morning, much of the interior should be able to shave 3 to 5 degrees off the high temperature over yesterday. The forecast looks on track overall. Previous discussion follows. 27

Latest satellite imagery shows a similar picture to 24 hours ago with marine stratus moving in over the southern half of the W WA coastline, getting as far inland as Shelton while the remainder of the CWA sees only high clouds. Even temperatures this early morning are not seeing that much variation from yesterday at this time, with only 1 to 2 degrees of cooling...if any.

That will be the defining feature of the weekend and start to next week: gradually cooling temperatures. Most locations will only see about 2 to 3 degrees of cooling compared to highs yesterday. The interior lowlands are expected to still range from the upper 70s to lower 80s while mid 60s to around 70 is expected along the coast and in water adjacent areas. Sunday continues this trend, about another 2 to 3 degrees cooler, putting daytime highs for the interior in the mid 70s to around 80, mid to upper 60s along the coast and Strait. Getting into Monday, this trend levels off, with daytime highs in the same ranges as Sunday.

Persistent onshore flow will also continue to alleviate smoke concerns over W WA as any smoke aloft will be pushed back east via zonal flow aloft while surface smoke will be limited to the Cascades and locations near wildfires. Any smoke present there will mainly be of concern overnight and into the morning hours each day. By the time afternoon westerlies kick in, that will be enough to give ventilation a boost, improving air quality.

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LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

Zonal flow is favored to continue through at least into mid-week with daytime highs remaining static, staying near seasonal normals. Long-range models remain in one of two camps for Wednesday and beyond: either general troughiness with a closed low over the eastern Pacific, cycling moisture into W WA or a strong ridge of high pressure, keeping the area dry. The first scenario would likely keep temps where they are as well as bringing the prospect of much needed rain and not-so-needed thunderstorms to the CWA while the latter would bring another warm-up to the area, boosting temps back up into the mid 80s to near 90. Operational NBM largely leaning toward this latter scenario, but again, with absolutely zero consensus, it is difficult to have any degree of confidence in this solution. Thus, highly advertising a 'wait-and-see' approach.

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AVIATION

Zonal flow aloft will take shape across western Washington today as an upper level ridge flattens over the region. Low-level onshore flow has brought marine stratus into portions of the interior early this morning, with satellite imagery showing stratus already making it into KSHN. Stratus will continue to push inland into the interior over the next few hours, though expect stratus to mainly result in some scattered clouds for the majority of the interior terminals east of the Sound. Expect stratus to retreat back to the coast around 17Z, bringing mostly VFR conditions to the interior terminals through the remainder of the day. Another round of stratus is expected to push inland late tonight into Sunday and may bring some MVFR to IFR conditions to the inland terminals for a few hours.

KSEA...VFR conditions continue early this morning, with winds out of the SW at 5 kt or less. Latest probabilistic guidance has started to back off on a stratus deck making it into the terminal and only indicates a 10 percent chance of IFR conditions developing at the terminal this morning and a 15 percent chance of MVFR conditions developing. Any stratus that does make it into the terminal looks to be more likely SCT to FEW in nature. SW winds will pick up to 4-8 kt and shift to the NW after 00Z.

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MARINE

Broad surface high pressure will remain centered over the offshore waters with lower pressure east of the Cascades into next week. This will maintain onshore flow over the next several days and bring diurnally driven small craft advisory strength westerlies to the central and east Strait of Juan de Fuca daily. The push tonight will be a bit stronger than this morning, with latest HREF guidance hinting at about a 35-45 percent chance of gusts briefly reaching gales this evening.

A series of weak systems passing to the north of the area next week will strengthen onshore flow at times for additional risks of gales along the central and eastern Strait.

Seas across the coastal waters will primarily persist at 4-6 ft into early next week and may become steep at times this weekend under locally driven winds. Seas will then subside to 2-4 ft by Tuesday, before increasing back to 4-6 ft by midweek.

FIRE WEATHER

Onshore flow will keep marine air over western Washington at least for the short term period. Very isolated areas of 20-25% min RHs are possible in the Cascade valleys on Saturday but widespread concerns are not expected - alleviating the threat for elevated fire weather concerns. More benign fire weather conditions will emerge on Sunday and lead into into the middle next week. Cooler temperatures and increased moisture with much better humidity recoveries are expected across the region.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None.

PZ...Small Craft Advisory from 1 PM this afternoon to 5 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca.