textproduct: Seattle/Tacoma

UPDATE

Satellite imagery shows mostly clear skies over Western Washington this evening with some thin upper level smoke/haze. High in Seattle today of 85 degrees was only 6 degrees off the record high and the tied for the 7th warmest July 14th in 82 years of records.

Low level flow weakly onshore tonight. This will produce some stratus along the coast later tonight. The stratus will try and move inland but with the onshore flow so weak not expecting the stratus to get east of Puget Sound, just mainly along the coast and through the Strait of Juan de Fuca. Stratus dissipating before noon making for another sunny day across the area. Temperatures pretty similar to today with highs in the 60s on the coast and mid 70s to upper 80s over the interior.

Changes still in store for Wednesday night and Thursday. First look at 00z models shows two possible waves of convection. The first over the southern portion of the area Thursday morning. The second over the Cascades, especially the North Cascades, in the afternoon. A fire weather watch is up for the mountains from late Wednesday night through Thursday evening for lightning and gusty outflow winds. Increasing low level onshore flow Thursday combined with possible convection cooling high temperatures over the interior into the 70s. Current forecast on track. No update this evening. Previous discussion follows with updated aviation section. Felton

SYNOPSIS

Warm and dry conditions through Wednesday with high pressure over Western Washington. Widespread showers and the threat for thunderstorms returns Thursday with a low pressure system. The low pressure system will exit the region this weekend, allowing high pressure to build over Western Washington next week, bringing drier and warmer temperatures.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

Latest satellite imagery shows mostly clear skies across Western Washington this afternoon with high pressure continuing to keep the region warm and dry. Expect these conditions to continue through Wednesday. High temperatures through Wednesday will be in in the mid to upper 60s along the coast and 80s in the interior. A low pressure system spinning off the Pacific Ocean will slowly start to move inland late Wednesday night/early Thursday morning. Southwesterly flow aloft will introduce the the threat for thunderstorms to the region. On Wednesday, thunderstorm potential will be mostly limited to the higher terrain of the Cascades with a 10-20% chance for thunderstorms and highest chances in the vicinity of Mount Rainier. As for Thursday, the threat for more widespread showers and thunderstorms continues to increase. Latest model guidance suggests a 20-25% chance for thunderstorms. However, some uncertainty remains regarding thunderstorm coverage and exact rainfall amounts. Any thunderstorm that does develop may produce dangerous lightning, gusty/erratic winds, small hail, and brief heavy downpours. Thunderstorm activity will slowly subside late Thursday night. 29

LONG TERM /FRIDAY THROUGH TUESDAY/

The aforementioned low pressure system will slowly begin to push out of the region Friday into Saturday. This will allow a broad upper level ridge to start building over Western Washington with ensemble guidance suggesting the ridge will be over the region early next week. This will set up a dry and much warmer weather pattern. Guidance suggests temperatures will peak in the mid to upper 60s to low 70s along the coast and 80s to low 90s along the interior. As a result, this will bring areas of moderate HeatRisk to most of the region and a 20-50% chance for major HeatRisk by Tuesday across portions of Western Washington.

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AVIATION

Widespread VFR conditions will continue throughout the evening hours for all terminals. The exception is KHQM, where a mix of MVFR/IFR early Wednesday morning starting at around 07z-09z. Marine stratus looks to hang around along the coast throughout the morning and early afternoon, before burning off late afternoon/early evening. N/NE winds 5 to 10 knots shifting more SW this evening before transitioning to NW by 15z-17z Wednesday morning.

KSEA...VFR conditions will continue throughout the TAF period. N/NW Winds 5 to 8 knots shifting more SW late tonight into early Wednesday morning, staying light. Winds will then return to NW and stay around 4 to 8 knots through Wednesday afternoon.

Mazurkiewicz/21

MARINE

Surface high pressure offshore will be replaced by an offshore low approaching the waters. Diurnally driven westerlies will continue down the Strait of Juan De Fuca, with winds remaining under any small craft advisory thresholds for the next couple of days. The coastal waters may see a few wind gusts up to 20 kts on Thursday and Friday as the low pressure system tracks through the waters. A more summertime pattern will return this weekend and early next week, with more westerly pushes down the Strait of Juan De Fuca.

Coastal seas remain at 4 to 6 feet through midweek, with seas slightly increasing to 7 to 8 feet with periods of 6 to 8 seconds.

Mazurkiewicz

FIRE WEATHER

Warm temperatures are expected to continue through Wednesday, with minimum relative humidity values dipping into the upper 20% to low 30% throughout Western Washington. An elevated fire weather pattern will set up late Wednesday and likely continue through early next week. Showers and thunderstorms return to the forecast as early as late Wednesday night through Thursday. The main concerns with these storms will be dry lightning and the potential for strong outflows between 35-50 mph. Storms that develop on the west slopes of the Olympics are more likely to produce a wetting rain (0.25 inches), while storms along the Cascades will produce sub-wetting rain amounts of 0.10-0.20 inches. Fire weather concerns linger into early next week as a hotter and drier pattern sets up over Western Washington, potentially increasing fire activity from any new lightning starts from Thursday's convection.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...Fire Weather Watch from late Wednesday night through Thursday evening for East Portion of the Olympic Mountains-West Portion of the Olympic Mountains-West Slopes of the Central Cascades Generally above 1500 Feet-West Slopes of the North Cascades Generally above 1500 Feet.

PZ...None.