textproduct: Seattle/Tacoma

SYNOPSIS

Dry and warm conditions will peak this afternoon ahead of a pattern change tonight that will bring the return of onshore flow, cooler temperatures, and chances for precipitation to the area. Thunderstorm chances increase Thursday across the Olympics, northern Interior, and North Cascades. Upper level troughing will continue to influence the region over the weekend, maintaining cooler and cloudier conditions across western Washington.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

Upper level ridging over the Intermountain West will maintain southerly flow aloft across western Washington today, bringing the warming and drying trend to a peak. Temperatures across the region are well on their way to reaching the mid to upper 80s, with some areas in the Cascade foothills and valleys reaching the lower 90s. This corresponds to widespread Moderate (orange) HeatRisk across the lowlands from the Puget Sound metro areas northward through Bellingham, with some pockets of Major (red) HeatRisk in the Cascade valleys. Along the coast, low level flow has switched to onshore, bringing in cooler and more moist air under the marine layer with highs closer to the mid 60s to lower 70s.

Onshore flow will continue to increase throughout the afternoon and evening today, bringing in a push of cooler and more moist air. This will also help to scour out any lingering haze from wildfire smoke. The onshore flow will bring a return to cooler and more seasonable temperatures, morning clouds, and afternoon sunshine to most areas over the next several days across western Washington, though areas along the coast may have a hard time breaking out of marine stratus. Continued troughing will also bring increased chances for light precipitation and isolated thunderstorms, though rainfall amounts look to be minimal through the week.

An upper level trough will swing inland to the north on Thursday, bringing the highest chances for thunderstorm activity over the next week. The Olympic Peninsula and North Cascades will see the best chances for isolated thunderstorms on Thursday with potential rain shadowing over the northern interior and islands, though storm chances peak at around 10-20%. Storms and rain showers with this system are expected to bring light rainfall amounts of generally up to a tenth of an inch of rain to the region.

LONG TERM /FRIDAY THROUGH TUESDAY/

Ensembles continue to show high chances for troughing to remain over the Pacific Northwest into next week, bringing continued chances for light rain and isolated thunderstorms over higher terrain. The details remain fairly uncertain at this time, but seasonal temperatures, cloudy mornings, and afternoon sun breaks are expected through early next week.

15

AVIATION

Southerly flow aloft will persist as upper level ridging continues to move eastward. Marine stratus will persist along the coast through this afternoon with MVFR to IFR conditions continuing. It is unlikely that the coast will fully clear out this evening. Smoke and haze will continue aloft, mainly affecting slant- wise visibility in the mid and upper levels. The back edge of the smoke looks to move east of the Cascades tonight. Winds remain SW 5 to 10 knots, with gusts up to 20 knots through around 06Z Wed for terminals west of the interior.

Tonight, after 06Z, the marine push will make farther progress inland. An extensive mix of IFR/MVFR ceilings and patchy fog is expected to reach peak coverage around sunrise, then gradually erode during Wednesday. Southwesterly winds will slowly east this evening to around 5 kt or less by early Wednesday morning. Winds increase again to around 5-10 kt during the day Wednesday.

KSEA...Patchy smoke at around 6000 ft agl is expected to push eastward tonight. VFR will persist through this evening with SW winds remaining at 5 to 10 knots. Onshore flow is expected to bring a marine push late tonight into Wednesday morning, with stratus beginning to develop around 10-12Z. MVFR ceilings are expected from 13Z to around 17Z, with clouds lifting and scattering to VFR, which will follow through Wednesday evening.

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MARINE

Surface high pressure will continue over the coastal waters today with a thermal trough along the coast shifting inland this afternoon. Increasing onshore flow tonight and into Wednesday will result in small craft westerlies through the Strait of Juan De Fuca. Weak systems crossing over the coastal waters will continue the onshore flow pattern through the remainder of the week with continuous pushes each afternoon and evening down the Strait of Juan De Fuca. Chances for isolated thunderstorms over the waters increases on Thursday with the passage of a frontal system, which may locally produce gusty outflow winds and lightning.

Seas generally remain around 4 to 6 feet through the weekend.

Mazurkiewicz/15

FIRE WEATHER

Elevated fire weather concerns will continue into the afternoon today as easterly flow promotes additional drying and very low relative humidities in the mid to lower 20s along the western Cascade gaps.

A pattern change on track for later today into Wednesday will bring the return of moist onshore flow, excellent overnight relative humidity recoveries, cooler temperatures, and chances of light precipitation to western Washington at times through much of the second half of the week. Overall, expect upper level troughing to help mitigate fire weather concerns for most areas. The exception will be in any isolated thunderstorm activity through the week, particularly on Thursday across the mountains and northern interior. While the exact details on thunderstorm activity remain uncertain at this time, lightning from any storms that do develop may lead to new fire starts mainly over the mountains and gusty, erratic outflow winds could impact ongoing fires within the vicinity of any storms.

14/15

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Wednesday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca.